"Heat Soak"

[Steve G.]

With apologies to Chester, we can continue the discussion here.

I stand strongly behind my previous statement that "heat soak" does not cause fuel drip. I made my living dealing with driveability complaints and dealing with this stuff on a daily basis back in the day. Other than the Ford flatheads with their vapour lock issues from the fuel pump mounted above the engine, there were no issues of "heat soak". They learned a lot from the cars of the 40's and 50's. These cars were designed to start easily and run smoothly in temps from -40 to +110. And they were tested in those environments b4 they were released for sale. And in the field, that's how they performed.

The expansion of the fuel explanation has some serious flaws. For one thing, today's fuels do not increase in volume related to temp change anymore than old fuels did. Another thing is that if you were to replace the metal can fuel filter with a clear or opaque plastic one you will see that they are never completely full of fuel. In fact, they are usually about 75% air. They can't fill completely without some way to bleed the air off. Standing vertically, there is even less fuel when they are oriented in the direction of the arrow. So as the fuel expands it will then occupy the air space and only compress the air. Additionally, it takes an awful lot of pressure to unseat the needle. Try it sometime when you have a bowl cover off. Turn it upside down so all you have is the weight of the float holding down the needle. If I ever have a day with nothing else I should be doing I'll put a gauge on one. The reason it takes a lot to unseat the needle has to do with the square area of the point of the needle visible through the seat. With fluid pressures we talk in terms of pounds per square inch. How many square inches is the visible point of the needle through the seat?

It wasn't that we didn't see carbs dripping down the throat. There was lots of that. But we didn't send the customer away telling them they would have to live with it. We fixed them. And they can still be fixed today.

Steve

[Frank D.]

Re: "Heat Soak"

I won't debate it further - but I respectfully disconcur with your opinion...

This outfit makes their living restoring carbs and has been around for eons. The fuel expansion into the throttle plates is covered herein:

http://www.thecarburetorshop.com/Troubleshooting.htm#Hardstarthot

[Edward J.]

Re: "Heat Soak"

Steve, What I see happens with the fuel in the float bowl is where the drip comes from,The heat from the cross over and summer heat will tend to perk the fuel and generally does not happen on short trips, it seems to be the just heat of the summer that brings on these start up issues, it will generally stop after the a cool down period. You are correct that the needle and seat will not lift off its seat once shut down and fuel will not be released into float bowl.

[Stuart F.]

Re: "Heat Soak"

The information I posted last night on Chester's Thread (sorry) came from the same source as you eluded to in your E-Mail reference. I did reword it some to make it more specific to our AFB's. I'd like to think they wouldn't mind that I did not give them credit or put it all in quotation marks. This is a good source for information, and they used to sell all the tooling to work on Carter carburetors including the removal and replacement of idle/low speed jet tubes from clusters. They also are the source for a good hot start procedure that I've used successfully for years. They really impress me as knowing their carburetors as that is their specialty.

I too will drop the subject now as I think we have beaten it to death. What works for one might not work for another. And, like I said; the 63 L-76 I have is a beast no matter what the ambient conditions. Zora's old 097 cam is what it is. I love it!

Have a nice day.

Stu Fox

[Steve G.]

Re: "Heat Soak"

I prefer to believe the the information from the guys that designed engines, the fuel systems and tested them before they sold them. By the millions. Because the cars with the highest underhood temps weren't the ones with high performance engines running on race tracks. They were the ones with AC and automatic transmissions running in downtown stop and go traffic on 100 degree days.

I trust my own experiences with hundreds, maybe into the thousand of carbs of all kinds over the almost 40 years I've held a journeyman's ticket. I believe what I was taught in trade school wasn't wrong.

Given that the standards for today's fuels are pretty consistent and have to meet certain engineering criteria, how can it be to blame for a problem that shows up on only some cars?

If all that fails me, I look for the science. This thing about expansion in the line. The co-efficient of expansion for today's gasoline is .00095 for every degree C temp change. Calculate the volume of fuel in a 5/16 fuel line 18" long. Double that, hell triple it for what's in the filter. Pick your degree temp rise and multiply them out. That change relative to the amount of air in the filter is insignificant. You couldn't possibly push the needle off it's seat.

But even more important, if that were in fact a problem, the fix is so simple. You vent the line between carb and pump back to the tank, just like the big boy engines with 3/8 fuel lines did.

If the fuel is spilling down the throat when it sits hot it's because the fuel level in the bowl is too high. Yes, temp increases in the engine when it is first shut down. Yes, fuel in the bowl will expand with the rise in temp. All that was anticipated in the original engineering. If on the hottest days it reaches the point that it will spill over the top, it's starting from too high a level, assuming you have confirmed no float/needle/seat malfunction. You have to remember that unlike Holley fuel level adjustments, most carbs rely on an assumption of buoyancy of the float. Making a dry inverted cover float adjustment is not a guarantee of an accurate fuel level. When I am suspecting a high fuel level after a proper float adjustment I remove the top in place and visually check the level. There is no measurement for that level, but over time you get a sense for where it should be. I make small incremental changes until I'm happy with the fuel level, regardless of what the float adjustment ruler says. This deals specifically with the dripping. The carb shop goes on to talk about vapour. Properly operating bowl vent systems take care of that.

I suspect as well, that from the number of posts I've read about heat risers disabled or removed that there may be a large misunderstanding in the entire air/fuel system where heat is involved. Heat risers are not there for proper choke operation. Older cars with manual chokes had heat risers. There's a specific reason exhaust heat is routed right into the base of the carb. So here we have people on one side trying to reduce the heat when the engineers designed a system around that heat being there.

My point in writing this was not to provoke you guys into an argument or offend you in any way. People come to this forum looking for answers. Many seem to put a lot of faith in what they read here, or on the internet in general. I think it healthy and helpful to present another side when it's presented with the reasons for that belief. Let the readers make their own determinations once they've read it all.

Respectfully,
Steve

[Duke W.]

Re: "Heat Soak"

I may be possible that fuel in the line from the pump to the bowl evaporates, which increases pressure, and it may be possible for this additional pressure to open the needle valve allowing liquid fuel to flow into the bowls and susequently spill over the discharge nozzle and into the thottle bores.

However, I believe the most common cause of hard hot starting is percolation. Carburetor temperatures of up to about 180 degrees have been reported here after shutdown, and modern gasoline blends may have close to 50 percent of their components above the boiling point at this temperature, so the air horn and interior air cleaner fills with fuel vapor, pushing out the air, resulting in a mixture too rich to ignite.

Modern gasolines are blended for modern cars. and vaporization from the fuel supply system may be a larger source of HC emissions than the tailpipe. For this reason Reid vapor pressure has been lowered over the years. Reid vapor pressure is measured at 100F, which is typical of fuel tank temperatures in very hot weather, so the lower the vapor pressure at 100F the less fuel vapor will escape from the fuel supply system.

However, the midrange boiling points - the temperature that 20-60 percent of the components are at or above their boiling temperatures is lower than traditional gasoline blends before the advent of ethanol, which boils at 170F. This lower mid range boiling temperature of the distillation curve does not effect modern vehicles since their fuel systems operate at 40 or more psi beyond the pump, which is often inside the fuel tank, so percolation is not an issue as it is on a non-pressurized carburetor.

Any student of fuel problems should download and read the following Chevron publication:

http://www.chevron.com/documents/pdf/MotorGasTechReview.pdf

The distillation curves shown on page 10 of the pdf tells you in a nutshell why hot soak percolation is more of a problem today for carbureted cars than it was with the gasoline blends back in the day. At 155F about 40 percent of the summer blend gasoline components are at or above their boiling points compared to only about 10 percent for a traditional non-ethanol summer blend. Also note the higher [Reid] vapor pressure of the winter blend. This is why percolation is worse on warm to hot days when winter blend fuel is still in the tank.

Also noteworth is that one of the co-authors of the document (see page 2 of the pdf) is NCRS President Mike Ingham!

You can also mitigate percolation by good management of your fuel supply. In Calilfornia winter blend is in the gas station tanks about November 1 and summer blends by about March 1. These dates vary by area of the country and there is a transition period of a few weeks. So find out the dates in your area. If you drive your car in the winter, fill up with summer fuel before winter blend arrives, and run it to empty before you refuel, so that you might not have to refuel until summer blend is available in the late winter or early spring.

Duke

[Steve G.]

Re: "Heat Soak"

I may be possible that fuel in the line from the pump to the bowl evaporates, which increases pressure, and it may be possible for this additional pressure to open the needle valve allowing liquid fuel to flow into the bowls and susequently spill over the discharge nozzle and into the thottle bores.

However, I believe the most common cause of hard hot starting is percolation. Carburetor temperatures of up to about 180 degrees have been reported here after shutdown, and modern gasoline blends may have close to 50 percent of their components above the boiling point at this temperature, so the air horn and interior air cleaner fills with fuel vapor, pushing out the air, resulting in a mixture too rich to ignite.

Modern gasolines are blended for modern cars. and vaporization from the fuel supply system may be a larger source of HC emissions than the tailpipe. For this reason Reid vapor pressure has been lowered over the years. Reid vapor pressure is measured at 100F, which is typical of fuel tank temperatures in very hot weather, so the lower the vapor pressure at 100F the less fuel vapor will escape from the fuel supply system.

However, the midrange boiling points - the temperature that 20-60 percent of the components are at or above their boiling temperatures is lower than traditional gasoline blends before the advent of ethanol, which boils at 170F. This lower mid range boiling temperature of the distillation curve does not effect modern vehicles since their fuel systems operate at 40 or more psi beyond the pump, which is often inside the fuel tank, so percolation is not an issue as it is on a non-pressurized carburetor.

Any student of fuel problems should download and read the following Chevron publication:

http://www.chevron.com/documents/pdf/MotorGasTechReview.pdf

The distillation curves shown on page 10 of the pdf tells you in a nutshell why hot soak percolation is more of a problem today for carbureted cars than it was with the gasoline blends back in the day. At 155F about 40 percent of the summer blend gasoline components are at or above their boiling points compared to only about 10 percent for a traditional non-ethanol summer blend. Also note the higher [Reid] vapor pressure of the winter blend. This is why percolation is worse on warm to hot days when winter blend fuel is still in the tank.

Also noteworth is that one of the co-authors of the document (see page 2 of the pdf) is NCRS President Mike Ingham!

You can also mitigate percolation by good management of your fuel supply. In Calilfornia winter blend is in the gas station tanks about November 1 and summer blends by about March 1. These dates vary by area of the country and there is a transition period of a few weeks. So find out the dates in your area. If you drive your car in the winter, fill up with summer fuel before winter blend arrives, and run it to empty before you refuel, so that you might not have to refuel until summer blend is available in the late winter or early spring.

Duke

Two areas of interest, distinctly different from one another. Fuel dripping down the throat and fuel vapours entering via the vent tube in the throat.

Dripping fuel. Fuel will not drip into the bowl because fuel is boiling in the bowl. Fuel drip has to be too high a fuel level in the bowl, for whatever reason, for it to spill over into the throat.

Vaporization increasing pressure in the fuel line can only happen after the engine stops running. Every stroke the pump makes regulates fuel pressure. Given the amount of air space in the fuel filter you would have to have a very significant temp increase over a very long time to boil off enough fuel to increase pressure enough to overcome the float, given the square area of the exposed portion of the needle. Additionally, increasing pressure increases boiling point. What is the rate of increase of the boiling point of the fuel with the increase in pressure. And that's exactly what we're talking about. Raising pressure enough to overcome the float. I just can't see it happening. When I was a kid I had one of those model steam engines. Took forever to build enough pressure to do anything. I can't see this happening with the small rise in temp and a diminishing amount of heat.

Even if it were able to overcome the float, it would then have to release enough fuel to raise the level above the cushion amount designed into the float bowl and fuel level. This is just too improbable.

Percolation.
Fuel boiling in the bowl. The only time this is a problem is if the vapours that are boiling off make it into the intake. Historically this has not been considered a major cause of hard starting, but more a problem with rough idle. And it can be contained entirely by the idle compensator.

As your research points out, there may be a need to adjust the opening temp on the ones thermostatically controlled. But on the WCFB and any of the others using a mechanical idle bowl vent (Holley comes to mind), it's a none issue. Fuel vapour is vented to atmosphere when the engine is off and throttle in the idle position. But again, vapour out the throat vent is not a significant cause of hard starting. It's the dripping ones that cause the issue.

Steve g

[Stuart F.]

Re: "Heat Soak"

A few years back, a number of the members contributed data and reports on our similar experiences with the "hot Soak" and "percolation" related issues. The temperature data, for what it's worth, was all arrived at using hand held IFR guns. We measured all probably points of concerns including fuel lines, carburetor fuel bowl, manifold crossovers (both exhaust and water), etc.. It was interesting, though varied, still relatively consistent for different engines, model years and parts of the country (not sure if Duke compiled the data or not).

The major outcome for me was the basic "common sense" modifications that I made which have contributed greatly to me being the happy camper that I am living the retired life of Riley here in central Florida. Basically, they are as follows and are all of the nature that can be reversed easily:

1) I raised the Carburetor on my aluminum intake using a 3/8" phenolic spacer (from a 63 L-75) and 2 correct base gaskets (as a sandwich). The only correct ones I found are those from Dr. Rebuild.

2) I isolated the carb from points of conductive heat, such as the choke hot air tube (thermo unit controlled electric choke), the steel fuel line from the filter (replaced with 3/8" fuel hose - also needed to adjust for the raising of the carb), and the use of Tomco fuel inlet valves (in place of needle/seats). I also adjusted the floats about 1/16" lower and plugged the fuel vapor vents in both the primary and secondary (little holes above the throttle butterfly's to the out side of the body. The latter item was done as part of my effort to reduce fuel stink in my garage (didn't help much). The result of the heat reduction measures was a consistent reduction of fuel bowl temperatures from 35F to 40F at full warm operating condition at 92F ambient.

3) I use a B28 vacuum advance can, and a Pertronics II electronic ignition with their flame thrower coil. Note; I recently rebuilt my distributor to save it from self destruction of the tach drive cross shaft (The tach needle is now a steady 750 rpm at idle). I'm still playing around with the total advance setting as right now I do not have a dial back timing light, rather I have a degree tape on the vibration damper and I can only see clearly with one eye (going in for Cateract surgery on March 4th. Need a helper too to hold my throttle at a steady 3000rpm).

Have a great day.

Stu Fox

[Steve G.]

Re: "Heat Soak"

Stu,

If those are the fixes you like and they work for you, they were absolutely the right fixes. I would have lowered the fuel level in the bowl and bent the idle compensator up a bit.

Yours are not the fixes I would recommend to people living in places other than Florida tho.

Here's the thing (simplified). Gas burns best in engines when combined with oxygen in a ratio of approx 14 parts oxygen to one part fuel. That's the carb's job, to ensure that under all conditions that ratio is being met. From dead cold to fully warmed up, throttle in the process of opening or closing, heavy loads with high volumetric efficiencies etc.

But here's one of the biggest problems. The ratio is based on molecules of oxygen and gas. And the number of molecules of oxygen in a cubic foot of air varies greatly depending on it's temp (and elevation).

And here's the apparent contradiction. We know that colder air contains more oxygen. More oxygen, we can add more fuel and we'll get a bigger fire. Aftermarket manifolds block off exhaust crossovers, place air gaps under the manifold runners and we run headers with no heatrisers, all to reduce heat in the air so we can add more fuel and get a bigger bang. Cars since the introduction of electronic fuel injections don't have heat risers or hot air stoves on the ex manifolds preheating air into the air cleaners. They don't even have chokes. They sell all kinds of cold air intakes for newer vehicles. Cold air is good!

So why, then, do we have this exhaust crossover and heat riser routing exhaust gas of almost 1000 degrees to the base of our carbs? Some actually even route it right into the throttle plate portion of the carb, which is below and directly attached to the float bowl? 1000 degree exhaust gas blowing on the bottom of the fuel bowl.

Other than the chokes that only operate for the first few minutes, carbs meter fuel only by air volume, they don't adjust for temp. Ideally you would have metering orifices that grew when cold and shrank when hot to cover the air temps they will encounter from -40 to +120.

But they don't. They are set and fixed so what must be done instead is to maintain a consistent charge air temp. It doesn't really matter what that temp is, you simply design the jets and metering rods for whatever temp you set the air at. Now, ideally, it would be best to maintain the temp at the lowest possible point, the densest air for the biggest bang. But how do you bring the 110 degree Phoenix summer air down to -40? Refrigerating the air to bring it down wouldn't be practical. But we're got lots of heat, so why not raise the temp above the highest anticipated ambient temps and keep it there and jet the fuel accordingly? Not hard to bring -40 degree air up to 140 when you have access to 1000 degree exhaust temps.

So that's what the heat riser and exhaust crossover in the intake are there for. The base of the carb and the intake area is heated by hot exhaust gas so that it will heat up the incoming air to a consistent temp whether it's -40 or 140 outdoors. The exhaust gas is quite consistent and always above those temps, consequently we get a relatively uniform incoming air temp. Additionally, fuel vaporizes quite poorly at low temps. So when the fuel coming from the tank is well belowing freezing you actually want to be heating it up in the bowl.

Now the engineers developed that carb around that incoming air temp. Those metering rods and main jets meter the precise amount of fuel for the the volume of air at that predetermined temp that's going down the throat. In cold starts the choke richens the mixture until the intake preheat system is able to bring that charge air up to temp.

So, when you deviate from the design temp of the intake air, you no longer have the correct metering of the fuel. Additionally, engines with choke wells in the cross over require a heat riser for their proper operation. So when you alter any of these systems, you throw off everything. and if you drive in an area where temps will vary quite a bit through the course of the day you would seldom be in the correct air/fuel zone,

Coming into the 70's the manufacturers augmented the system with the thermostatically controlled air cleaners with the heat stoves on the exhaust manifolds in order to shorten choke time. Computer controlled cars don't worry about it because the computer monitors ever conceivable condition including outside air and intake air temps and precisely adjusts the injection to suit. However it's donr, air temp and fuel volume must be accommodated.


It's not all wrong for everyone. Your car in Florida likely is driven in a very narrow range of ambient outside air temps. Correct your metering for your present air intake temps and you're fine. Hot rodders with headers, air gap manifolds with no exh crossovers simply jet for the air temp they run in, which is usually nice summer days. Then they fiddle with them at the track to dial in for that day. They are purpose built cars. They aren't expected to start up with 2 pumps of the pedal on a cold fall day and be able to pull away without hesitation immediately.

But for the purist here that wants his car to run properly and consistently through a normal range of temps they are best to stick with the design. The change in fuel formulation is not an insurmountable obstacle.

Steve

[Duke W.]

Re: "Heat Soak"

Prior to emission controls the purpose of the heat riser was to promote fuel vaporization. It had nothing to do with controlling inlet air temperataure. Pre emission engines were set up somewhat rich so that worst case conditions of fuel distribution and high air density would not result in lean misfire. Idle mixtures were typically set around 12.5:1 (about 20 percent excess fuel) 14:1 average at cruise and 12.5:1 for maximum power.

Inlet air heat stoves appeared in the early emission control area and their purpose was to maintain an inlet air temperature of about 100F during operation other than WOT. This yielded a more consistent air-fuel ratio regardless of ambient air temperature, which yielded lower emissions.

Most heat stoves had a vacuum override that opened them up at WOT to aspirate either engine compartment air or ambient air via a cold air induction system. The emission certification test of the era did not require the engine to operate at WOT, so there was no need to control WOT emissions.

Since carburetors meter fuel on the basis of volume, not mass, they will run richer at high altitude, which is why most cars produced for "high altitude areas" (I recall is was over 4000 feet.) had to have differently calibrated carburetors.

Duke

[Steve G.]

Re: "Heat Soak"

Prior to emission controls the purpose of the heat riser was to promote fuel vaporization. It had nothing to do with controlling inlet air temperataure. Pre emission engines were set up somewhat rich so that worst case conditions of fuel distribution and high air density would not result in lean misfire. Idle mixtures were typically set around 12.5:1 (about 20 percent excess fuel) 14:1 average at cruise and 12.5:1 for maximum power.

Inlet air heat stoves appeared in the early emission control area and their purpose was to maintain an inlet air temperature of about 100F during operation other than WOT. This yielded a more consistent air-fuel ratio regardless of ambient air temperature, which yielded lower emissions.

Most heat stoves had a vacuum override that opened them up at WOT to aspirate either engine compartment air or ambient air via a cold air induction system. The emission certification test of the era did not require the engine to operate at WOT, so there was no need to control WOT emissions.

Since carburetors meter fuel on the basis of volume, not mass, they will run richer at high altitude, which is why most cars produced for "high altitude areas" (I recall is was over 4000 feet.) had to have differently calibrated carburetors.

Duke

Not what I was taught and certainly counter to the way computer controlled fuel systems operate today. Chokes were required because the high density air in cold temp req'd more fuel to ignite. No way around that. That same -40 air would continue to require an enriched mixture if you did nothing to warm the air charge. Heat risers accomplished that. That's why you could start your car in Winnipeg at -40 and drive south for 20 hours and maintain the same driveability.

Further, if all you were worried about was vaporizing the fuel it would have been much simpler to heat the fuel only.

Engine management systems today monitor ambient air temp and intake air temp and adjust for those temps. Atomization is in the hands of the injectors. There is no preheating of fuel or air to improve vaporization, but they still care about the other inputs.

If you lived in worked in the extreme cold as I did you would have experienced the effects first hand. It is not an extreme cold only effect, the extreme temps illustrated to an extreme the effect. But it applies to a lesser degree with lesser variation in temp. The idea of the design is to make operation consistent whatever the climate and temps it would be operated in.

The best example of this was the cars with the thermostatically controlled air cleaner pre-heats. In the dead of winter you would have a customer come in with a complaint of a severe hesitation, stumble. Engine fully warmed up and comfortable inside. Step on the pedal and it was as if you shut off the key. Barely driveable. Like a disconnected accel pump. Lift the hood and you would find the hot air tube from the exh manifold to snorkel rotted off (Ford's fibre one), the steel tube not placed properly when the air cleaner was removed, vac hose off etc. Point is, the only thing you did was correct the air cleaner issue and the stumble was completely gone. In this case the heat riser working fine, manifold and carb fully warmed. But the fuel system is calibrated to operate with the charge air at the temp the two systems would bring it to. There certainly were benefits to both emissions and fuel economy by maintaining because they could more precisely meter fuel instead of dumping in excess amounts to cover the range. And the more preceisely the fuel was metered the more evident the effects of temp change on air density.

Steve

[Duke W.]

Re: "Heat Soak"

...don't know what you were taught or when, but my previous post only addressed non-emission controlled engines and early emission controlled engines before the advent of modern electronic engine management systems with O2 sensors.

Chokes are required on carbureted engines because only the most volatile fuel components will vaporize during cranking to provide an ignitable mixture. Liquid gasoline does not burn. It has to be in a gas state and mixed with oxygen in a A/F ratio of about 8:1 to 16:1 to ignite. During the first few seconds to minutes of cold operation, depending on temperature, there is a lot of liquid fuel in the manifold, which evaporates and is consumed (much of it going out the tailpipe unburned) as the engine warms up. This is why 80 percent of emissions occured in about the first two minutes of the 45 minute EPA certification test.

The easiest way to heat/vaporize the fuel is to have a hot spot under the idle/off idle discharge ports. This was the reason for the "heat slot" on a lot of carbureted engines, which today most recommend plugging because we usually don't drive vintage Corvettes in very cold weather and the heat can warp carburetors. Heat from the heat riser passage heats the base of the manifold plenum below the carburetor, and the reason this area usually has a wavy as-cast surface is to provide more hot area to vaporize liquid fuel that drops to the bottom of the plenum.

Rapid vaporization of the fuel yields better mixture distribution, which is always a problem in multicylinder manifolds. Dave McClellan, in his book, stated that it's amazing they work at all, and having done research in carburetor/manifold fuel metering and distribution I can assure all that he is absolutely correct. There's a lot of complicated physics going on in a typical carburetor/manifold system, but as long as you run them rich enough, they work well, but at the expense of fuel economy, emissions, and power. The conclusion to my master's thesis at the University of Wisconsin Engine Research Center in 1970 was that manifold/carburetor systems would not meet future proposed emission standards beginning in the mid to late seventies and the solution was electronically controlled fuel injection systems (already in production by some European manufacturers), which would be required to maintain the required precise A/F ratios under most operating conditions to meet future emissions requirements.

I was right, but it took longer than I predicted as a few manufacturers, notably GM and Honda, tried to make manifold/carburetor systems work with all sorts of add on crutches for a few years after they were decidedly obsolete before developing their own electronic fuel injection systems. Always cost conscious GM tried it a half -a...ed way with throttle body injection, but that idea ran out of gas, so to speak, by the late eighties.

Modern high pressure port injection provides better fuel atomization and more rapid vaporization, so a less rich cranking and initial cold operation mixture is required. New direct injection systems can get away with even less cold start enrichment, which generates even lower emssions on the EPA certification test.

Duke

[Steve G.]

Re: "Heat Soak"

...don't know what you were taught or when, but my previous post only addressed non-emission controlled engines and early emission controlled engines before the advent of modern electronic engine management systems with O2 sensors.

Chokes are required on carbureted engines because only the most volatile fuel components will vaporize during cranking to provide an ignitable mixture. Liquid gasoline does not burn. It has to be in a gas state and mixed with oxygen in a A/F ratio of about 8:1 to 16:1 to ignite. During the first few seconds to minutes of cold operation, depending on temperature, there is a lot of liquid fuel in the manifold, which evaporates and is consumed (much of it going out the tailpipe unburned) as the engine warms up. This is why 80 percent of emissions occured in about the first two minutes of the 45 minute EPA certification test.

The easiest way to heat/vaporize the fuel is to have a hot spot under the idle/off idle discharge ports. This was the reason for the "heat slot" on a lot of carbureted engines, which today most recommend plugging because we usually don't drive vintage Corvettes in very cold weather and the heat can warp carburetors. Heat from the heat riser passage heats the base of the manifold plenum below the carburetor, and the reason this area usually has a wavy as-cast surface is to provide more hot area to vaporize liquid fuel that drops to the bottom of the plenum.

Rapid vaporization of the fuel yields better mixture distribution, which is always a problem in multicylinder manifolds. Dave McClellan, in his book, stated that it's amazing they work at all, and having done research in carburetor/manifold fuel metering and distribution I can assure all that he is absolutely correct. There's a lot of complicated physics going on in a typical carburetor/manifold system, but as long as you run them rich enough, they work well, but at the expense of fuel economy, emissions, and power. The conclusion to my master's thesis at the University of Wisconsin Engine Research Center in 1970 was that manifold/carburetor systems would not meet future proposed emission standards beginning in the mid to late seventies and the solution was electronically controlled fuel injection systems (already in production by some European manufacturers), which would be required to maintain the required precise A/F ratios under most operating conditions to meet future emissions requirements.

I was right, but it took longer than I predicted as a few manufacturers, notably GM and Honda, tried to make manifold/carburetor systems work with all sorts of add on crutches for a few years after they were decidedly obsolete before developing their own electronic fuel injection systems. Always cost conscious GM tried it a half -a...ed way with throttle body injection, but that idea ran out of gas, so to speak, by the late eighties.

Modern high pressure port injection provides better fuel atomization and more rapid vaporization, so a less rich cranking and initial cold operation mixture is required. New direct injection systems can get away with even less cold start enrichment, which generates even lower emssions on the EPA certification test.

Duke

Duke,
My reference to modern day engines was solely to illustrate the relationship between air temp and fuel mix and how it's handled today. The physics are the same today as they were back in the day. Fuel delivery must be adjusted according to charge air temps for optimum efficiency and smoothest operation. Fuel combines with oxygen, not air and oxygen varies in concentration in air depending on it's temp. That's the physics.

A choke does not aid in atomization. If anything, it impedes it. The only thing there is to break the fuel down into finer particles in an ice cold engine is the air flow. The choke reduces the air flow. What it is doing is richening the mixture. And it's doing that because cold air has more oxygen and needs more fuel for a given volume. And what changes as the engine warms up and needs less and less choke is the air charge temp.

I agree whole heartedly with what your resources say and have said it often myself. Relative to the way fuel delivery is handled today, it's amazing these things ran at all. But they did, and they ran well. And they will still run well with today's fuels. Today's gasoline is not formulated to run properly in fuel injected engines only. It has evolved, yes, but always keeping in mind that it must still support carberated engines still produced to this day and found in the ag, construction and industrial areas. And in supporting those carbs, they support our old ones. Using fuel as an excuse for a poor performing engine is just that, an excuse. What's required to make these old cars run like they did back in the day is to have all the interdependent systems functioning like they did back in the day.
Steve

[Robert D.]

Re: "Heat Soak"

i have a question for you guys i have a 435 car and when i first got the car i had extremely hard hot start problems. the car also "leaked" fuel out of the carbs after it sat for a while most likely perculation. i have been experimenting with some fuels and found with VP vintage fuel i have absolutely no hard start problems and leakage problems have improved. the reid vapor pressure (RVP) on the VP vintage is 6.7 which i believe increased the boiling point of the fuel. i spoke with the tech dept at VP today and he said to use VP c-10 which has a RVP of 1.6. the tech said that this is one of the reasons they developed this fuel it has an octane rating of 100 which would be gret for my setup and should hopefully eliminate perculation. if the RVP is so low which in turn raises the boiling point of the fuel shouldnt this solve the problems that we face with these cars or does using such a low RVP cause other problems? Ignoring the price of the fuel
thanx
Bob

[Steve G.]

Re: "Heat Soak"

i have a question for you guys i have a 435 car and when i first got the car i had extremely hard start problems. the car also "leaked" fuel out of the carbs after it sat for a while most likely perculation. i have been experimenting with some fuels and found with VP vintage fuel i have absolutely no hard start problems and leakage problems have improved. the reid vapor pressure (RVP) on the VP vintage is 6.7 which i believe increased the boiling point of the fuel. i spoke with the tech dept at VP today and he said to use VP c-10 which has a RVP of 1.6. the tech said that this is one of the reasons they developed this fuel it has an octane rating of 100 which would be gret for my setup. if the RVP is so low which in turn raises the boiling point of the fuel shouldnt this solve the problems that we face with these cars or does using such a low RVP cause other problems? Ignoring the price of the fuel
thanx
Bob

I'm the wrong guy to ask. I don't belief there is a need for any special fuel. If it was/is leaking, it's not perc. If the new fuel only improved it and did not stop the leaking I'd say that's not your answer. If the leak is inside, down the throat, lower the fuel level in the bowl.

Steve

Edit I'd still run that fuel for the octane if the engine is internally stock.