"Heat Soak"

[Robert D.]

Re: "Heat Soak"

i tried lowering the floats and there was no improvement when i changed the fuel it improved fuel leakage about 50% and resolved the hot start issue

[Steve G.]

Re: "Heat Soak"

i tried lowering the floats and there was no improvement when i changed the fuel it improved fuel leakage about 50% and resolved the hot start issue

Where do you see the fuel leaking? Is this down the throat?
Steve

[Robert D.]

Re: "Heat Soak"

It leaks onto the manifold after the car is shut off a while when it's hot if run for only a little bit there is nothing on the manifold I can't tell where it's from

[Steve G.]

Re: "Heat Soak"

Pictures are apparently worth a thousand words. Why percing in the bowl can not cause a liquid fuel drip in the throat.
holley2.jpg

As this diagram illustrates (which is common to all of our down draft, fixed venturi, bowl type carbs), fuel always exits the near the bottom of the bowl. Regardless of which circuit, idle, main, power or accel, it must travel back up next to the bowl to a height above the level of the fuel in the bowl then turn around and flow downwards. If it did not gravity would do it's job and the fuel would continually flow of it's own accord. So in order for the liquid fuel to flow from the bowl and out whichever outlet in the throat, the fuel level in the bowl has to climb above the highest point in the vertical channel where the fuel heads back down. Percing, or boiling, of the fuel in the bowl does not raise it's level. It lowers the level as the liquid fuel becomes vapour. The vapour is then vented our the vent tubes or out the idle compensator vent.

The exception is the Holley carbs that use a metering block. If the block is warped or the gasket between the block and carb body is compromised, the fuel can leak across between where it leaves the bowl and the idle discharge orifaces (area with the red rectangle in the 2nd photo.
holley3.JPG

This leak is not tied to temp and is pretty easy to diagnose. If you see fuel dripping, pull the fuel level site plug. If it's at or below the site level you have a metering block issue. If it's well above the site hole, lower it and recheck.

With all of the other carbs we frequently see in these cars, the AFB's, WCFB,s, and Q-Jets, the bowl and vertical transfer tubes are all integral to the body of the carb so there is no route to the throat except up and over. And percing does not raise the fuel up and over.

Percing in the bowl was an issue in earlier cars where the vapour that was boiling off the fuel entered the airstream via the vent tubes and overly richened the idle mixture. It was corrected by adding a second vent to the bowl that opened at idle or at idle in high temps. They were called idle compensators, or hot idle reliefs, or idle vents. The were never named "hot start compensators" by any of the manufacurers, because that is not the complaint they corrected.

If I get a chance in someday I will mock up a fuel line needle seat set up and illustrate why the fuel line, with all it's air in the filter can not build up enough steam or pressure to unseat the needle.
Steve

[Leif A.]

Re: "Heat Soak"

I just want to thank Steve and Duke for their thoughtful, intelligent and informative back and forth here in this thread. Without a doubt, one of the most worthwhile threads I have read and followed in quite some time addressing an issue that comes up often and affects many of us. Thanks, again, gentlemen.

[Duke W.]

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

You need to download that Chevron pdf I reference previously and study it. (This goes for everyone who is interested in this subject or wants to comment on it.) It does a better job of answering your question, however, I will point out a couple of important issues. Gasoline does not have a "boiling point" like water. It's made up of hundreds of hydrocarbon species that have boiling points ranging from below zero (dissolved butane) to nearly 400F. This is expressed in the distillation curves illustrated and explained in the Chevron publication.

Reid vapor pressure as I explained before is measured at 100F. The higher the RVP the more low boiling point components are in the fuel, and the easier it will start especially in cold weather, but high RVP contributes to percolation.

Ask your fuel supplier for the distillation curve of their products. If digital post them here or scan and post them and compare to the examples in the Chevron publication.

In the absence of any evidence of leakage from the carb, fuel stains on the inlet manifold may be gasoline that condenses on the throttle blades as the engine cools and then runs out the throttle shaft bores. Percolation can sometimes be so violent that liquid fuel literally shoots out of the bowl vents and then drops onto the throttle valves where is can subsequently seep out of the throttle shaft bores.

To mitigate percolation, first wire the heat riser valve wide open. Consider insulation sleeves on the fuel supply pipes and hoses. (There was a lengthy thread on this subject a couple of years ago with before and after temperature data. It works!) If the above does not work, remove the manifold and use a thin stainless steel shim to block the driver's side of the heat riser. This will effectively block it, but allow enough heat to reach the choke stove on the passenger side to keep the choke functioning properly.

Also, your L-71 has ported vacuum advance, which throws a lot of heat into the engine at idle and low speed driving that promotes percolation. If you have not already done so, convert it to full time vacuum advance along with the installation of a B26 VAC. Set initial timing at 8-10, then go through the idle speed/mixture adjustment procedure with a target idle speed of 900. Then buy a spring kit and see if you can get full centrifugal in faster without detonation. The above will reduce heat rejection to the cooling system, improve around town fuel economy, and increase low end torque...lots of threads about this in the archives.

Duke

[Donald O.]

Re: "Heat Soak"

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.

It wasn't the engine's need to warm up but rather the need for catalytic converter to heat up so it could react to the Hydrocarbons, ie unburnt fuel.

New direct injection systems can get away with even less cold start enrichment, which generates even lower emssions on the EPA certification test.Duke

Agreed. It worked wonder for the deisels for NOx, HC, CO2, and particulate emissions, with the added benefit of more HP from the engine. It is the main reason for the use now in gasoline engines.

DonO

[Steve G.]

Re: "Heat Soak"

Robert,

Your answers to my questions earlier came as I was composing the above explanation. So, with that additional info I'll add the following.

Here's my experience. 40 years ago carbs dripping fuel down the throat was as common as dirt. It was the number one reason carbs were overhauled back in the day. We called it flooding. Here we are 40 years later and the same carbs are exhibiting the same symptoms, but the diagnosis appears to have changed to being a fuel formulation issue requiring a re-engineering of the engine management systems and the malfunctions that caused the exact same symptoms 40 years ago are no longer considered valid causes, even though we corrected the issue by repairing those causes back in the day.

Additionally, while fuel formulation changes from supplier to supplier that difference between the suppliers is not that great and certainly not as great as the difference between 1975 fuel and 2015 fuel. Yet not all the cars with the same carbs in the same environment exhibit the same condition. If it's a fuel formulation issue, why don't we all have the problems?

If you have raw fuel dripping down the throat it will make the engine hard to start. Again, that's called flooding. Every street corner mechanic in the days of carbs knew/knows that. No mystery there. So when you reduced the drip, you reduced the starting problem.

There are a number of things that can change how much liquid leaks through a hole. One is viscosity of the liquid. Your fuel change may have/likely changed that. Another thing that can change is the size of the hole it's leaking through. As the carb warms up and expands it changes shape ever so slightly. But it only takes a very small hole to leak fuel. So a very small change in shape can be significant. You may be able to reduce the rate of drip by reducing the heat that reshapes the carb (disabling the heat riser). But you are not addressing the root cause, only the symptom and in doing so you are creating other problems. The important clue here is that you still have a drip after a change of fuel to one that supposedly addresses the issue.

Now let's think about the perc concept. This is going on in the float bowl where the fuel is. Now we have to figure out how this vaporizing fuel, no matter how violently, is getting out of the bowl and manifesting itself as drips in the throat of the carb. What are the holes in the fuel bowl that anything can get out? The only ways out of the float bowl are through the main jets, through the accel pump feed or through the bowl vents. The fuel has to leave from one of those routes, no other way out. Does it leave as a liquid or a vapour? This percing is rapid vaporization of the fuel. Is this vaporization raising the pressure in the bowl to the extent that it's forcing fuel out through the main jets or accel pump outlet (liquid fuel covering these outlets)? Can't build pressure, the top of the bowl is vented, so that's not it. Is vapourizing the fuel raising the liquid level high enough that it's flowing over the high point of the passages. Can't be that, vapourising turns the liquid to gas, lowering the liquid level. And, we can confirm that through the site plug. That leaves the bowl vent. We know it can't be liquid leaving through the bowl vent because we know the level is below the site plug. Is it splashing up and around 2 bends to make it's way to the top of the vent. Pretty unlikely if we don't see violent splashing when we look in the site plug and we don't see liquid dripping from the vent. So it must be vapour. You can see fuel vapour when it is the kind of concentration that would condense anywhere and cause it to drip. Are you seeing whispy vapour coming out of the vent tube? And if you are seeing vapour how is it getting down the throat to condense inside there. Is it rising up out of the vent then falling into the throat? You should be able to see this. But if you think you are getting enough vapour that it will condense on a hot carb and form drips then the easy test for that is to simply stick a snug fitting piece of clear vinyl tubing in the vent and direct the vapour away from the carb as soon as you shut down.

To test the theory of the perc in the line opening the needle simply remove the sight plug and have a look. The theorey here is that the expanding fuel in the line is overfilling the bowl to the extent it runs out over the high point in the passages (refer to the pics above).

There's two differing takes on your issue. Maybe others will chime in with more.

Steve

[Steve G.]

Re: "Heat Soak"

You need to download that Chevron pdf I reference previously and study it. (This goes for everyone who is interested in this subject or wants to comment on it.) It does a better job of answering your question, however, I will point out a couple of important issues. Gasoline does not have a "boiling point" like water. It's made up of hundreds of hydrocarbon species that have boiling points ranging from below zero (dissolved butane) to nearly 400F. This is expressed in the distillation curves illustrated and explained in the Chevron publication.
Duke

Duke,

The fuel information makes for interesting reading, but it is approaching the problem backwards. Are the carbs dripping because the fuel graphs say it vaporizes at a lower temp? It's like doing an analysis of nails commonly found on roadways because your tire keeps losing air.

It doesn't matter the characteristics of the fuel unless you have proven that the root cause of the problem is percolating fuel. Not only has that not been proven, it's been shown to be extremely unlikely and may even be at high risk for breaking laws of physics. And in the meantime all the proven, known causes of drips in carbs are ignored.

Show us the evidence that it is percolating fuel causing the drips then we can move on to the fuel analysis.
Steve

[Duke W.]

Re: "Heat Soak"

TDB members have reported observing liquid fuel being ejected from the bowl vents after shutdown - like what you would observe in a nearly full pot of boiling water. This could only be from violent boiling in the carburetor bowls. As you can see from the Chevon distillation curves, modern E10 gasoline sold in the USA has much more tendency to boil due to having a greater percentage of components that are at or above their boiling point at about 180 degrees compared to traditional non-ethanol blends, and carb body temperatures this high have been reported with IR temperature gun measurements.

I see from your profile that you live in British Columbia, and if you are near the coast you rarely see ambient temperatures above about 80F. Having grown up in Seattle, I am familiar with the Pacific Northwest climate, and vintage cars operated in that area are less likely to suffer from the effects of percolation than those operated in hotter climates that often see 90-100+F during the summer months.

Also, what can you tell us about Canadian gasoline. Does it have ethanol? What percent by volume?

Duke

[Steve G.]

Re: "Heat Soak"

TDB members have reported observing liquid fuel being ejected from the bowl vents after shutdown - like what you would observe in a nearly full pot of boiling water. This could only be from violent boiling in the carburetor bowls. As you can see from the Chevon distillation curves, modern E10 gasoline sold in the USA has much more tendency to boil due to having a greater percentage of components that are at or above their boiling point at about 180 degrees compared to traditional non-ethanol blends, and carb body temperatures this high have been reported with IR temperature gun measurements.

I see from your profile that you live in British Columbia, and if you are near the coast you rarely see ambient temperatures above about 80F. Having grown up in Seattle, I am familiar with the Pacific Northwest climate, and vintage cars operated in that area are less likely to suffer from the effects of percolation than those operated in hotter climates that often see 90-100+F during the summer months.

Also, what can you tell us about Canadian gasoline. Does it have ethanol? What percent by volume?

Duke

Duke,
I live in central interior BC, Kamloops to be precise. We are semi-desert, cactus and tumble weeds. Climate is similar to the eastern Washington area around Yakima. Extended periods of 90 - 100 are the norms in the summer. Humidity is usually in the low teens, single digits are not uncommon.

The newest vehicle that the fellow that owned the business I look after was 1980. That was his daily driver, an IH scout with a Holley 4bbl (which he drove until his passing in 13). I have a shop and yard full of carberated vehicles, and I own a 67 442 that I drive on hot summer days. Given the mountainous terrain to get out of our valley with 7 to 11% grades, it's not unusual to have the temp gauge get pretty high with a good climb on a hot day. You are right that I have never seen fuel splash out the vent. Dripping fuel in the throats and hard hot starts, not unusual and fixed them all without a redesign of the management system. Common float/needle and seat/gasket issues.

Ethanol is available here by choice. I don't know anything about it because I don't use it and have no cause to research it or the non ethanol fuels. We are a small market area relative to Vancouver and district so I suspect the fuel formulations we get are the same as is sold in the cooler coastal areas and the high altitude markets of Calgary. This would cause me to wonder if they are even more volatile, not unlike the changes in winter fuels you eluded to earlier.

I have not been coming to this forum for long and did not do a search on the topic, but since I've been here, and as recently as yesterday with Robert's post, the complaints I've been reading about were carbs dripping down the throats and hard hot starts. I hadn't read anywhere the complaint was hard hot start with fuel spray out the vent. That's a pretty easy one to spot. In the event of a hard hot start and raw fuel in the throat at shutdown the first question asked should be "do you see fuel spraying out the vent" Answer, no, it's not fuel perc. If yes, fuel percing and running from the vent into the throat. Even then tho, I would not deal with it by redesigning the engine management system, opting instead to modify the bowl vent system so as to not adversely affect cool temp operation.

For those reading this as an aid to their specific diagnosis, this fuel spray from the vent is not to be confused with fuel overflow from the vent. If the float or needle and seat are faulty the bowl will continue to fill right up to the point it is overflowing the vent.

I can not tell you what this splash from the vents from the boiling fuel looks like, because I've never seen it. Perhaps someone else can chime in and give us a first hand description.

Steve

[Duke W.]

Re: "Heat Soak"

Duke,

"do you see fuel spraying out the vent" Answer, no, it's not fuel perc. If yes, fuel percing and running from the vent into the throat. Even then tho, I would not deal with it by redesigning the engine management system, opting instead to modify the bowl vent system so as to not adversely affect cool temp operation.

Steve

So how would you modify the fuel bowl vents?

Duke

[Steve G.]

Re: "Heat Soak"

So how would you modify the fuel bowl vents?

Duke

Once I had determined that without a doubt that this is fuel splashing from the violent boiling action I would either attach a shield inside the bowl that covered the hole but was open on the sides (picture an inverted table with the legs the attachment points) or I would insert a metalic filter of some kind, similar to the ones found in the bottoms of old downdraft carbs, into the vent tube.

If this is what you contend it is then it is simple splashing, no different than if you held the carb in your hand and shook it. Covering the line of sight hole would stop the splash but allow the vapor. Again, I am very skeptical that this is at all a common or frequent source of liquid fuel down the throat. Picture the length of the vent tube and the angles it takes on it's way to the throat. But if that were the problem, that's how I would deal with it.

If the symptoms were poor hot idle (as opposed to hard hot start with dripping fuel talked about above) and I had determined that it was due to an overly rich mixture from excess fuel vapour venting into the airstream at idle, I would work with the idle vent compensator. If it was not oe equipped with one, I would fab a system. Adapting a carbon storage canister as was used in later emmissions cars would be the easiest.

Steve

[Steve G.]

Re: "Heat Soak"

Here's another little tidbit for the trivia interested.

Back in the day when the phrase "hard start associated with heat soak" was first used it was referring to a completely different problem. The issue was poor cranking when you went to restart a hot engine. It's what led to The General's mid model year addition of a heat shield between the exhaust manifold and the starter on the big block cars. But it happened on lots of vehicles, not just these.

Steve

[Timothy B.]

Re: "Heat Soak"

I will need to go back and read this complete post but I want to add on a Holley carburetor fuel can boil in the main well and push out the nozzle that's located just inside the booster venturi. The main well is located vertically inside the metering block, fuel does not have to go up and back down to exit the booster nozzle like the idle circuit. This fuel usually drips down onto the throttle blades and out the side of the carburetor onto the intake manifold.

The idle circuit fuel is drawn by vacuum so the up and over/down breaks suction where the main circuit fuel is delivered by air flow through the booster venturi.