Gasoline Distillation Curves

[Robert D.]

Re: Gasoline distillation curves

John,
thats exctly what i think is happening. as i said before im going to try the fuel with a higher boiling point just to see if it stops and insulate the carb bases.
Steve maybe i just havent found whats wrong but if 50% of today's fuel vaporizes just below 200 degrees and turns to a gasseous state (granted i dont know what the distillation curves were in the 60's and 70's) the laws of physics states that it should expand and when you expand something in a closed space or behind a column of liquid it greatly increases the pressure which can force it through the carb. on top of that when it cools it turns back into a liquid which condenses on the butterfly's and shaft and wicks through

[Steve G.]

Re: Gasoline distillation curves

You're not interpreting the distillation curves properly. At any given temperature, the greater the percentage of the fuel's components that are near, at, or above their boiling points, the more rapid the rate of vaporization from the surface of the liquid as I illustrated at a carburetor temperature of 160F in my initial post.

Fuel vapor is heavier than air, so it will displace air in the carburetor and continue to displace air from the air cleaner interior until it escapes to the surrounding area and creates the "fuel stink" that owners report after bringing the car back to the garage from a drive on a warm to hot day. If a hot restart is attempted after a few minutes the engine just ingests fuel vapor and considerable cranking time is required to purge excess vapor and ingest enough oxygen to form a combustible mixture, and then rough operation will usually occur for several seconds before the excess vapor is purged and a more normal A/F ratio is established.

Violent percolation that ejects liquid fuel from the bowl vents and/or booster venturi nozzles is the extreme case, but a high rate of vaporization from the fuel's surface without actual boiling in the bowl is all that is required to create the problems.

The long-dash line of the summer non-oxygenated gasoline distillation curve is representative of average distillation curves from decades ago includilng the sixties.

The first link you posted is a good tutorial on gasoline, but it does not address the difference in distillation curves between traditional non-ethanol gasoline and current E10, nor does it discuss the driveability problems that can arise from E10 on vintage cars with non pressurzied fuel systems.

The second link is a good tutorial on refining, but does not address difference distillation characteristics of modern E10 compared to traditional non-ethanol gasoline blends.

Both are good references for someone who wants to take a deep dive into the technology, but neither helps in understanding current driveability problems on vintage cars with non-pressurized fuel systems.

That's the beauty of the Chevron graph that compares the distillation curves of modern E10 with traditional non-ethanol gasoline, like what was typical in the sixties. There's no need to complicate it. The answer is there at a glance in a couple of simple curves, and the archives are rich will simple methods to mitigate the problem.

Duke

If you're smelling gasoline standing beside the car it's because the gasoline is beside the car. That is fuel vapour for sure. It's there, and not down the throat because the hot idle vent is allowing it out there instead of down the throat. That's why they put them there. And they called them hot idle compensators because that was the problem they were mostly responsible for. Yes, you might have a mildly rich mixture at startup, and it may have to crrank an extra half a revolution, but it was never much of a hot start issue. Or you are smelling the fuel that is evaporating off the manifold because it dripped off the end of the throttle shaft onto the manifold, outside the carb. You won't smell the vapors captured under the air cleaner in the throat. Any time I've smelled fuel standing beside a car I've found liquid fuel, or the evidence of where it dripped then evaporated.

We're still at this diagnosis of percing fuel and dripping carbs. And that's how I started into this. If your carb is dripping internally, it's not boiling fuel. You've said it yourself, it would have to shower out the bowl vent and that would be an extreme case. Yet the diagnosis for these "non extreme cases" with liquid fuel drip is still fuel boil. And what makes an extreme case? If you went to a meet on a 100 degree day are all the cars dripping fuel down their throats? Because they likely all have the same fuel.

The claim has been made over and over that this is a problem that has become much more prominent today because of current fuels, high volatility rate. The first article I linked categorically denies that. Volatility in pump gas peaked in the 80's. Their bent is absolutely on ethanol laced fuel The article is written by Ethanolrfa.org, a trade association for the US ethanol industry.

Fuel volatility is not something that is left to chance or sacrificed in the design of fuels. It is something that is targeted in the formulation. According to this article, it has been lowered in the last 30 years, not raised.

While fuel volatility is not as critical inside the injection system of today's cars as it was in carbs, it has become more important in it's storage. Fuel vapour is recognized as a significant source of air pollution. Capturing it off the fuel in the tank then burning it off while maintaining precise control of the AFR is not easy. It would be better if we had less of it. Fuel under pressure atomizes quite well and will stand a lower volatility rate than fuel at atmospheric. Product loss is another concern. These are the a couple of the reason why they lowered volatility.

So, if the fuel is less volatile than 30 years ago, underhood temps are unchanged, why is this now a bigger problem than it was in the 80's when I drove my 64 across Missouri in 110 degree heat without a "heat soak" hard start, fuel dripping issue?

Steve

[Steve G.]

Re: Gasoline distillation curves

John,
thats exctly what i think is happening. as i said before im going to try the fuel with a higher boiling point just to see if it stops and insulate the carb bases.
Steve maybe i just havent found whats wrong but if 50% of today's fuel vaporizes just below 200 degrees and turns to a gasseous state (granted i dont know what the distillation curves were in the 60's and 70's) the laws of physics states that it should expand and when you expand something in a closed space or behind a column of liquid it greatly increases the pressure which can force it through the carb. on top of that when it cools it turns back into a liquid which condenses on the butterfly's and shaft and wicks through

Robert, that's what I've been getting at. Unless it's leaking across and by-passing the transfer tube, fuel can not flow via gravity from the bowl down the throat. There is a high point in the vertical channel that is higher than the level of the fuel in the bowl. The bowl can not build pressure to push the fuel out, it is vented. So what has to happen is that the fuel level in the bowl has to rise above the high point of the passage. When it does that, fuel spills over the top and out the throat. You have identified that as the problem. YOur fuel level is too high. No question about that.

But the fuel level isn't too high because you have a rolling boil going on in there. It's not caused by excess amounts of vapour. This is nothing to do with percing. This is thermal expansion. There has been no data that I've seen in the discussions on here talking about different rates of thermal expansion.

Fuel expansion has always been there. The design has always been that the height of the spillover point is far enough above fuel level so that when the fuel expands when the engine is shut off it will still fall short of the spillover point.

Lower your bowl level. You don't care where it's at cold, it's not going to operate there. Set it where it will be hot.

Steve

[Duke W.]

Re: Gasoline Distillation Curves

I see a couple of recurring misunderstandings in this thread. They involve the words "boiling point" and "volatility".

Gasoline does not have a boiling point! It's composed of hundreds of different hydrocarbon species that have boiling points ranging from well below zero F (dissolved butane) to large HC molecules that boil at close to 400F.

So the boiling characteristics of gasoline can only be expressed by the distillation curve. It does not have a single boiling point like water. It has a distillation curve. Gasoline's boiling point is a two dimensional characterisitc, not one dimensional like water. The term "boiling point" is meaningless when applied to a gasoline's physical properties. When it comes to gasoline, forget the words "boiling point" and substitute "distillation curve". It's important to understand this.

The term "volatility" refers to a gasoline blend's vapor pressure at a specific low to moderate temperature. It's an important characteristic for cold starts and control of evaporative emissions on modern vehicles, but it says absolutely nothing about the vapor pressure characteristics at engine component operating temperatures.

A common measure of volatility is Reid Vapor pressure, which is measured at 100F, and there are other volatility tests that are measured at different specific temperatures, but it's always vapor pressure measured at a specific low to moderate temperature.

You can see from the Chevron distillation curves that current E10 is less "volatile" than traditional non-ethanol blends, but at temperatures over about 100F a much greater percentage of E10s components are near, at, or above their boiling points, which means that high temperature vapor pressure is much greater than traditional non-ethanol blends.

Volatility is only an issue with cold starting and evaporative emissions. The source of hard hot starting or vapor lock is revealed by the gasoline blend's distillation curve at temperatures above the single low to moderate temperature used to measure volatility.

It's absolutely critical to understand the concepts of "volatility" and "distillation curve" as they apply to gasoline in order reasonably understand gasoline physical properties and their effect on engine performance.

Duke

[Steve G.]

Re: Gasoline Distillation Curves

I see a couple of recurring misunderstandings in this thread. They involve the words "boiling point" and "volatility".

Gasoline does not have a boiling point! It's composed of hundreds of different hydrocarbon species that have boiling points ranging from well below zero F (dissolved butane) to large HC molecules that boil at close to 400F.

So the boiling characteristics of gasoline can only be expressed by the distillation curve. It does not have a single boiling point like water. It has a distillation curve. Gasoline's boiling point is a two dimensional characterisitc, not one dimensional like water. The term "boiling point" is meaningless when applied to a gasoline's physical properties. When it comes to gasoline, forget the words "boiling point" and substitute "distillation curve". It's important to understand this.

The term "volatility" refers to a gasoline blend's vapor pressure at a specific low to moderate temperature. It's an important characteristic for cold starts and control of evaporative emissions on modern vehicles, but it says absolutely nothing about the vapor pressure characteristics at engine component operating temperatures.

A common measure of volatility is Reid Vapor pressure, which is measured at 100F, and there are other volatility tests that are measured at different specific temperatures, but it's always vapor pressure measured at a specific low to moderate temperature.

You can see from the Chevron distillation curves that current E10 is less "volatile" than traditional non-ethanol blends, but at temperatures over about 100F a much greater percentage of E10s components are near, at, or above their boiling points, which means that high temperature vapor pressure is much greater than traditional non-ethanol blends.

Volatility is only an issue with cold starting and evaporative emissions. The source of hard hot starting or vapor lock is revealed by the gasoline blend's distillation curve at temperatures above the single low to moderate temperature used to measure volatility.

It's absolutely critical to understand the concepts of "volatility" and "distillation curve" as they apply to gasoline in order reasonably understand gasoline physical properties and their effect on engine performance.

Duke

Now I admit, I am confused.

With this heat soak/hard start thing the explanation that you provided previously covered two points. You referenced the fuel vapour coming off the fuel in the bowl and making it's way to the carb throat via the bowl vent. You identify this as the cause of the hard start issue related to heat soak.


The rate of this vaporization, and consequently it's movement into an area that it will cause a hard start is all about volatility. Except at cold start-up, fuel environment temps are always above 100F. Reid Vapour charts are valuable when assessing what's happening to fuel in storage in the tank, but it obviously is not telling us much about what's happening in the operating environment. Is that charted or is it just left to chance when formulations are made up?


Further, you previously (post #18 here) referred to liquid fuel making it's way to the throat by way of the vent tube was being caused by the violent boiling of the fuel in the bowl and splashing out the vent. Does it reach this state, as you described as "like what you would observe in a nearly full pot of boiling water" and called boiling or not? If you're saying it does not reach a point where this rolling boil occurs, then the splashing liquid out the vent idea doesn't hold water. If there is a point at which this rapid bubbly occurs, what we might mistakenly refer to as it's boiling point, how would we find that point from the distillation curves?

In a nutshell, we're being told that the operational issue is caused by two components. Fuel vaporizing and liquid fuel splashing out the vent. We're told fuel vaporizing is about volatility and that's not what the distillation curves are addressing. The liquid fuel is splashing because of an action that looks like water boiling, but the distillation curves don't tell us anything about that either.

What should we be taking from the chart?

Steve

[Steve G.]

Re: Gasoline distillation curves

John,
thats exctly what i think is happening. as i said before im going to try the fuel with a higher boiling point just to see if it stops and insulate the carb bases.
Steve maybe i just havent found whats wrong but if 50% of today's fuel vaporizes just below 200 degrees and turns to a gasseous state (granted i dont know what the distillation curves were in the 60's and 70's) the laws of physics states that it should expand and when you expand something in a closed space or behind a column of liquid it greatly increases the pressure which can force it through the carb. on top of that when it cools it turns back into a liquid which condenses on the butterfly's and shaft and wicks through

Robert,
As I was going through the "heat soak" thread looking for something else I came across your post where you said you had lowered the fuel level.

So a little more information might help. What ambient temp is this happening? Can warm the engine right up and run all day on a 75F day and not drip? Fuel perc has always been a hot weather (not hot engine) issue, like 100f or more. It would be helpful to know at what outdoor temps this is happening for you.

Another thing to consider is the possibility that the needle and seat are not sealing completely. In a small leak, as long as the engine is running it is consuming as much fuel as the leak is letting through. Probably more. When you shut off the engine the drain from the bowl is gone so the pump pressure trapped in the line continues to fill the bowl through the small leak at the needle. Hot temps will fill more than cool days because of the thermal expansion factor.

I know you said the carb was rebuilt, but we have warranties because sometimes things go wrong. Your bowl is definately over filling. If you have fuel running out the hole when you remove the sight plug, it's extremely likely it's high enough to flow down the throat. That is the most logical explanation for liquid fuel down the throat of your carb. The question is, why is the bowl over filling. You have already gone to a fuel that, if I understand correctly, vaporizes at a much lower rate than pump gas. And still you have a drip that others running pump gas don't experience. Just makes the fuel theory somewhat unlikely.

In the trade back in the 70's with Q-jets that used the small paper filters right at the fuel inlet on the carb, we had an issue with the filters. Poor quality paper filters. In the process of a tune-up you would replace the filter. Start it up and go through all your set-up and checks on the scope and shut it off. Occasionally, shortly after shut down you would experience this hard start/dripping fuel syndrome. What was happening was that a small piece of paper fuzz was coming off the filter and lodging in the needle and seat. Sometimes it would be enough to flood the bowl to the point of coming out the vent while it was running. But often it would take a minute after shut down to start to drip. A very slow leak. I saw the tiny little paper fibre when I took it apart.

The filter manufacturers advised us to dip them in solvent and blow them off with an air hose b4 installing them.

Point is, high fuel level can have numerous causes. If I couldn't adjust this issue out, i would look to replacing the float, needle and seat. I would take a fuel pressure reading.

If this were a problem that was being universally experienced, I would tap a pressure gauge into the supply line and take readings. I would then run tests to see how much pressure it takes to overcome the float. There is a methodical way to diagnose this.

Steve

Edit: Another potential source of fuel leak into the bowl on a Holley is the lower o-ring on the seat. If it was nicked or rolled when installing any leakage at the bottom ring is into the bowl. Leakage at the top o-ring is to outside. This is unlike most carbs with a gasket and a seat tightened hard against it. Holley seats slide on the o-rings when adjusted.

[Timothy B.]

Re: Gasoline distillation curves

Robert -

There's no mystery here - hot-soak-after-shutdown percolation has been going on for about 80 years, but is more prevalent now due to higher underhood temperatures and the lower boiling point of today's E10 ethanol-laced gasoline. Fuel boiling in the float bowl is forced out either through the vent or the boosters, dribbles on the primary throttle shaft and butterflies, and some wicks out through the throttle shaft bushings and drips on the intake manifold. Anything you can do to insulate the carburetor from convection shutdown heat will help - there are lots of techniques discussed in the archives.

John,

The best approach for me is to wire the heat riser open during the summer months as that will lower the temperature of the plenum so the heat soak at shutdown is not as bad, block the under carburetor slot and run a vacuum advance with approx 21* idle timing.

If I run the engine with the heat riser functioning normal the engine runs fine but heat soaks bad at shutdown in the hot summer months. No doubt in my mind it's the ethonal in the fuel. One other thing that would help is a stainless shield under the carburetor like the LT-1 cars had as this shield was long enough the shield the primary and secondary fuel bowl from the plenum heat.

[Duke W.]

Re: Gasoline distillation curves

Liquid fuel coming out of the bowl vents or venturi nozzles is usually observed after shutdown. Heat soak increases carburetor temperature because the coolant and cast iron block and heads may be 200 or more. So if the carburetor temperature increases to 180F or more 50 percent or more of the E10 fuel components are at their boiling temperature, and this may be sufficient to cause vapor bubbles to form in the bulk liquid that rapidly travel up and spew liquid as they burst on the surface - what is called a roiling boil, like a pot of boiling water. An old fashioned coffee percolator is also a good analogy.

As a pan of water is boiled on a stove burner you can see increasing signs of vapor as the temperature rises. The "white smoke" you see is saturated steam. The water leaves the surface as vapor, but then begins to condense into micro liquid droplets, which is the "white smoke" we see. Not long after big bubbles form in the liquid and burst at the surface when the temperarture reaches 212F at sea level, and if the pan if near full, it will spew boiling water out of the pan.

The same thing can occur with gasoline, but the onset is more subtle because of the difference in component boiling temperatures. The bubbles will be much smaller, but when enough of the gasoline components are at their boiling point it can be like a pan of boiling water.

As I said in post #19 volatility is about vapor pressure at moderate to low temperatures, typically no more than 100F. It's a very narrowly focused parameter that is important for cold starting and evaporative emissions from the fuel supply system on modern cars. The problems with excess fuel vapor or liquid sprewing into the carburetor throat are about the percentage of a fuel's components that are at or above their boiling points when the carburetor is at operating temperature, which in hot weather and low speed driving can be in the range of 150-180F and even greater after a hot shut down heat soak. That's what the distillation curve tells you. A blend that has half of it's components at or above the boiling point at 180F is much more likely to cause hot restart problems due to excess fuel vaporization or percolation - creating a fuel vapor/air ratio that is too fuel rich to ignite - than a blend that has only 30 percent of its components at or above the boiling point at 180F.

Duke

[John S.]

Re: Gasoline distillation curves

Duke
Thanks for the data.
Steve,
i for one can tell you that the ethanol is a problem. I have two C2 Vettes. The 67 300hp car runs ok in the cooler months in So. Cal. As soon as it gets to about 85 or above I along with most others here suffer from boil off at shut down. Have seen the fuel dripping from the boosters myself. I just live with it in this car because it runs fine otherwise on the pump premium. The 65 fuel car is a bit different. At those same temps I will get percolation in the injector lines if I get caught in any type of traffic. The engine coolant temps will climb to 200 or more and then the car will not idle. Same thing happens when it is very hot outside, say 95+. Then it's a bear. All other times the car also runs great on the pump premium. I used some VP 108 leaded fuel about a year ago and it solved this problem somewhat. I say somewhat because I drove the car in ambient temps well above 100 degrees and still had some minor problems. For this reason I will be using the VP fuel when I go for my PV in Tucson in 2 weeks. Better safe then sorry. Just my 2 cents worth.

[Steve G.]

Re: Gasoline distillation curves

Liquid fuel coming out of the bowl vents or venturi nozzles is usually observed after shutdown. Heat soak increases carburetor temperature because the coolant and cast iron block and heads may be 200 or more. So if the carburetor temperature increases to 180F or more 50 percent or more of the E10 fuel components are at their boiling temperature, and this may be sufficient to cause vapor bubbles to form in the bulk liquid that rapidly travel up and spew liquid as they burst on the surface - what is called a roiling boil, like a pot of boiling water. An old fashioned coffee percolator is also a good analogy.

As a pan of water is boiled on a stove burner you can see increasing signs of vapor as the temperature rises. The "white smoke" you see is saturated steam. The water leaves the surface as vapor, but then begins to condense into micro liquid droplets, which is the "white smoke" we see. Not long after big bubbles form in the liquid and burst at the surface when the temperarture reaches 212F at sea level, and if the pan if near full, it will spew boiling water out of the pan.

The same thing can occur with gasoline, but the onset is more subtle because of the difference in component boiling temperatures. The bubbles will be much smaller, but when enough of the gasoline components are at their boiling point it can be like a pan of boiling water.

As I said in post #19 volatility is about vapor pressure at moderate to low temperatures, typically no more than 100F. It's a very narrowly focused parameter that is important for cold starting and evaporative emissions from the fuel supply system on modern cars. The problems with excess fuel vapor or liquid sprewing into the carburetor throat are about the percentage of a fuel's components that are at or above their boiling points when the carburetor is at operating temperature, which in hot weather and low speed driving can be in the range of 150-180F and even greater after a hot shut down heat soak. That's what the distillation curve tells you. A blend that has half of it's components at or above the boiling point at 180F is much more likely to cause hot restart problems due to excess fuel vaporization or percolation - creating a fuel vapor/air ratio that is too fuel rich to ignite - than a blend that has only 30 percent of its components at or above the boiling point at 180F.

Duke

There is no disagreement about the temp rise, although I think your numbers are way off. The air blasting through the rad onto the fuel pump, fuel line and carb is at least 180F. Exhaust/combustion temps are always higher than cooling system temps. The fuels probably at least 180 when it reaches the carb. And it always has been.

What's wrong with all this is exemplified perfectly in Robert DeFalco's example here on this thread. He has liquid fuel dripping down the throat. He stares at his carb at shutdown and sees nothing.

No fuel spouting out the vent or the transfer tube air bleed as a result of a roiling boil going on.

No flume of white vapour streaming out the vent from fuel rapidly vaporizing (with or without roiling boil).

He takes the sight plug out and does not see boiling fuel, he sees fuel spill out.

How does this result in a diagnosis of fuel perc?

My contention is that a condition of heat perc can be identified on visual inspection. While I've never seen fuel enter a throat from the roiling boil effect as you contend, you would to see it if it were happening. Excess vaporization of fuel, quite easily visible and identified by the white vapour streaming out the vent.

How is percolation causing liquid fuel to drip down the throat of Robert's carb with no visible spouting out of the vent or the nozzles?

Yet, he pulls the sight plug and fuel runs out. It's not supposed to. He has a high fuel level. No boiling, just a high fuel level. A high fuel level has been known for 100 years to be the cause of liquid fuel dripping down the throats in probably 99 out of a 100 cases, but now it's caused by fuel perc because a graph told you it must.

Fuel, like most things, expands with heat. The heat soak will cause the fuel to expand in the bowl. As I've said before, that's perfectly normal and was allowed for in the original design of the carb. What we, the people working in the trade 40 years ago, discovered way back then is that if the fuel level is too high when cold, when hot it will expand and raise the level enough to spill out into the throat. We fixed that by lowering the fuel level. Of course you can treat it by reducing the heat so that it doesn't expand the fuel as much, but that's treating the symptom, not the problem. And doing that adversely affects AFR consistency through the range of temps the engine will operate.

Now, if you were telling me that the rate of thermal expansion has increased by 20% from fuel 40 years ago, I could get behind that.

Steve

[Steve G.]

Re: Gasoline distillation curves

Duke
Thanks for the data.
Steve,
i for one can tell you that the ethanol is a problem. I have two C2 Vettes. The 67 300hp car runs ok in the cooler months in So. Cal. As soon as it gets to about 85 or above I along with most others here suffer from boil off at shut down. Have seen the fuel dripping from the boosters myself. I just live with it in this car because it runs fine otherwise on the pump premium. The 65 fuel car is a bit different. At those same temps I will get percolation in the injector lines if I get caught in any type of traffic. The engine coolant temps will climb to 200 or more and then the car will not idle. Same thing happens when it is very hot outside, say 95+. Then it's a bear. All other times the car also runs great on the pump premium. I used some VP 108 leaded fuel about a year ago and it solved this problem somewhat. I say somewhat because I drove the car in ambient temps well above 100 degrees and still had some minor problems. For this reason I will be using the VP fuel when I go for my PV in Tucson in 2 weeks. Better safe then sorry. Just my 2 cents worth.

John, I can't comment on the fuelie car, and it may well have an issue with the fuel. As to the carbed car, are you seeing fuel spray out the vent tube when this drip occurs?

I could get behind this fuel thing if someone could explain all the inconsistencies in the theory to me and show me the how they traced the route the fuel took to drip internally. And why the excess vapor isn't venting out the hod idle compensators. And why the exact same problem that I fixed over and over again 30+ years ago , hard hot starts and fuel drip, are now no longer caused by all the things that caused them then. And why some cars have this problem with the fuel today and other near identical cars don't.

Rather than repeat, post 25 on here is a good summary of what makes me so very skeptical.

Steve

[William F.]

Re: Gasoline distillation curves

Robert, Is the problem with primary or secondary or both? Remove the bowl(s); remove the floats and check for fuel inside the floats- shake them; if inconclusive place in a bowl of gas and see if they sink. I bought a new 3367 carb and had continuing flooding from secondaries. Found secondary float had a pin hole in it and was sinking like a rock causing flooding. Check it; doesn't matter if carb has been rebuilt or , as in my case, even new.

[Timothy B.]

Re: Gasoline distillation curves

The spill area between the nozzle tube and the fuel level in the main well is closer than the distance from the fuel in the float bowl to the top of the bowl where the hot idle compensator vent is located on Holley's. That's why fuel will push out the booster tube before it pushes out the bowl vent.. But, have you ever noticed the staining on top of the float bowls where these vents are located.

Depending on how bad the fuel boils, a high float level will contribute to fuel from the booster nozzle because it reduces the spill area BUT it's the fuel percolation that causes the fuel to push up into the nozzle tube.

[Steve G.]

Re: Gasoline distillation curves

The spill area between the nozzle tube and the fuel level in the main well is closer than the distance from the fuel in the float bowl to the top of the bowl where the hot idle compensator vent is located on Holley's. That's why fuel will push out the booster tube before it pushes out the bowl vent.. But, have you ever noticed the staining on top of the float bowls where these vents are located.

Depending on how bad the fuel boils, a high float level will contribute to fuel from the booster nozzle because it reduces the spill area BUT it's the fuel percolation that causes the fuel to push up into the nozzle tube.

That's correct about the distance fuel level to idle comp vent. My reference to the fuel spray out the vent was to do with Duke's contention that the fuel dripping down the throat was coming out the main vent in the air way of the carb, the result of the splashing from the roiling boil going on in the bowl. And I agree, long before it would splash up and around two bends you would see it coming out the idle comp vent.

I'm still very skeptical about the theory that the boiling fuel is causing liquid fuel to drip from the nozzles. The very small surface area in the tube and the absence of fuel spray from the air bleeds which are directly above the tube make me question whether that's happening.

But most importantly, the idea that this is happening in the absence of any other symptoms, like in Robert's case and this appears to have become the standard diagnosis for fuel drip/hard start. No visible fuel vapour coming from any of the vents, no fuel spray out the main vent, no sign of roiling boil viewed through the sight hole.

Fuel perc is real and it did happen and it still happens. But it didn't happen on an 85 degree day summer day. It was a 100 degree day thing. With the fuel industry telling us that fuel is less volatile now than in the 80's I'm skeptical about a suggested change that the fuel has caused that critical temp to move down very much. And fuel perc was evident by the plumes of white mist coming from the vents. It was this excess vapour that caused the problem. It wasn't liquid fuel.

I don't believe you can properly diagnose carb problems with an IR thermometer and a fuel distillation chart.

Steve

[Michael J.]

Re: Gasoline distillation curves

I don't believe you can properly diagnose carb problems with an IR thermometer and a fuel distillation chart.

Steve