Ye Old Vapor Lock vs Percolation

[meadowbrook]

Hi folks.

 

Now that it is warmer, I am back having the issue with my engine stalling after idling hot for a long time.

 

I have discussed it before and was led to believe that the issue is likely percolation, ie the fuel in the bowl boiling or expanding enough to flood the engine, requiring me to wait 10 min to restart.

 

The issue happens after a long idle in hot weather or after being in the highway running fast and then stopping in slow traffic. Increasing the throttle during the idle helps, I think because the fan cools the carb better.

 

So the last time this happened, I sprayed the air cleaner opening (oil bath) with some starting fluid and was able to restart the vehicle without waiting the 10 min as before.

 

So I am asking, if the stalling was caused by flooding, why would starting fluid help? Wouldn't this indicate a lack of fuel? IE vapor lock before the fuel gets to the engine?

 

I often wonder if an electric fan might help because it would be running at slow engine speeds if it got hot, thus cooling the carb better than  the stock fan.

 

Any comments?

[55 Fargo]

Good question, and what you describe is what happens in my case too. So is it a lack or over abundance of gas.

I firmly believe both are possible.

I firmly believe the gas gets boiled right out of the bowl, and no fuel is getting there, until things cool down, or fuel is eventually pumped up.

The electric fuel pump guys rarely complain about this, must be a reason.

At any rate, hard starts can be a pain, but when things get really hot, ambient temp, very hot underhood temps, come off the exit ramp or highway, and drive slowly through city traffic or wait at a stop light, and the fuel delivery might get impaired.

Very good topic, even if there are those who claim no such thing as "vapor lock"......

[knuckleharley]

I am thinking that vapor lock and "fuel percolation" are related,but different.  Fuel percolation will temporarily prevent a car from starting until the engine,fuel pump,and fuel line to the carb cool down enough the air "bubble" evaporates and the engine starts again. Vapor lock is when the fuel lines and fuel pump get so hot it causes air lock and the engine just shuts down even thought it is running above idle.

 

The reason the electric fuel pump guys don't have problems with this is the electric fuel pump isn't bolted to the block and getting hot enough to "boil away" the fuel before it gets to the carb. When running down the highway the fuel volume is so swift it doesn't have time to get hot in the pump and percolate,so the engine keeps running. When you slow the engine to idle enough of the fuel sits in the pump and lines long enough to get hot and start to boil,meaning it gets air bubbles in it. Part of that has to do with there is no air moving around under the hood like there is when the car is going down the road. Hit it up with starting fluid and the starting fluid ups the octane so it will start,and by then the gas lines and pump may have cooled off enough to keep it running for a while.

 

Some people used to wrap their gas lines under the hood with insulation to help prevent this from happening.

 

Or I may be full of crap and having a senior moment.

[TodFitch]

If the engine dies while idling during hot weather my experience has been that the fuel pump is not operating as desired. Easy check, simply pour some water on the pump. If the engine quickly and easily restarts and then runs after you cool the pump down then it is not the carburetor.

 

In my case, I've only had this happen on hot days after the car has been driven hard. In my case also in those hot running conditions, if I stop for a few minutes, say to refill the gas tank, the car will restart and run for as long as there is gas in the float bowl before dying. Again, cooling the fuel pump will cure the condition. If it was a case of percolation in the carburetor, I'd expect that I would be unable to restart it and cooling the fuel pump would have no effect.

[Merle Coggins]

I had this similar issue with my truck on hot days, until I installed the fuel pump heat shield. I'm quite sure it was the fuel pump getting too hot from the exhaust manifold because I also have an electric pump plumbed inline, to assist in priming after extended parking periods, and if I turn it on when the symptoms would appear the engine would smooth out. The electric pump would force fuel through the pump and keep if from "vapor locking". I know this term is not popular with some here, but I have experienced what the original poster described.

 

Merle

[Jeff Balazs]

I believe there are a few contributing factors that combine to cause this problem....whatever you want to call it.

1) Modern fuel formulation.

2) Proximity to heat source and lack of adequate shielding.

3) Float level.

4) Quality of ignition source.

 

It is hot here quite often. It is over 90 today. I managed to eliminate the problem by ditching the mechanical fuel pump in favor of a full time electric pump and by adding a wired in place home made heat shield below the float bowl. The float level is set a hair below spec and for ignition I use a Pertronix trigger and matched flamethrower coil with NGK BR6S plugs. This combination works flawlessly. Hot starts are no longer a big deal.

Jeff

[deathbound]

I've had an electric fuel pump since day 1 of owning my car, no heat shield & have NEVER had a hard start problem when the engine is hot. Funny this came up, my electric fuel pump went out a few days ago, luckily at home.

[greg g]

Is the base of the carburetor wet on the outside or near the throttle butterfly shaft? That would indicate perculation, or an improperly set float, or debris in the needle and seat.

[meadowbrook]

Wow, my carb base is often wet. But the float height is to spec.

[CoronetGuy]

I had this issue last year. Glass bowl filter right before the carb. Engine stumbled and shut off. Popped the hood, yup, fuel boiling inside the filter. The car would not start even with some starting fluid. We poured water over the fuel pump and someone fed fuel into the carb while I cranked it. I was told that putting insulation over the lines would actually make things worse. If it's the pump thats getting hot, then the insulation will keep the heat from the hot fuel inside the line. 

 

Car then got a fuel pump heat shield, 1" phenolic carb spacer, and I re routed the fuel line between the carb & pump to keep it as far away from the exhaust as possible. Things seemed ok but that setup was on there for a very short while. I had a exhaust header installed which required the fuel pump to be removed and an electric pump was put in. Now I have the opposite problem, the carb gets real cold

[mopar_earl]

Anyone try running a fuel return back to the tank? Like use a Mopar fuel filter that has the return line connection on it. That would push the hot gas back to the tank. Would be best to install an electric pump with the fuel return. Install the return fuel filter right at the carb. Just pondering out load.

Earl

[mmcdowel]

My Dodge hasn't done this....yet....but my 1950 Chevy 2 ton wheat truck used to do that regularly.  I was told to put a couple of radiator clamps on the "out" fuel line at the fuel pump and two more a few inches out from where the fuel line enters the carburetor and see if that worked.  Didn't have any trouble at all last Summer...so it seems to have worked.  Its a cheap trick, so worth a shot.  (sorry I should have said "fuel line clamps" not "radiator clamps")

Edited April 21, 2016 by mmcdowel

[greg g]

Factory float level was predicated on old gas formulation, back when gas was, you know, mostly gas.  Todays stuff is a coctkail of things  many of which have a much lower vapor point than dinosaur juice, making its boiling point lower as well as the expansion temp.  So I have found that when I was faced with a similar situation, adjusting the floats a couple of 32nds lower, pretty much eliminated the problem while not adding any driveability concerns.  Lower float = less gas in the bowl= more room for expansion before overlowing raw gas into the carb.  

 

I have also seen research that the flame front of modern motor fuel is slower than older stuff and as such can stand a bit more timing advance that the factory recommended also.  Old gas went bang, new gas goes whoomph, providing a slower total ignition and a slightly more prolonged push against the piston, so lighting it a bit earlier is benificial to an enhanced power stroke.

 

In either case, things that may help, and have a low likelyhood of doing harm.

[James_Douglas]

I run my '47 hard.  Sometimes when it is hot out and I have run the car hard and then parked it...

 

It will take an extra 10-15 seconds to start.  But it always does.

 

In my car I have a "racer" big Fram fuel filter.  I run the line to the mechanical pump out to the front apron and have the filter on the right shield.  I then run the line back into the mechanical pump.  I also have a electric booster pump which I only turn on for a few seconds before I start the car in the morning when it is dead cold.

 

I have the heat plate and I have the line from the pump to the carburetor cover with heat reflective cloth.

 

It never stalls in traffic no matter how hot it gets out. 

 

My mechanical fuel pump's are rebuilt by Terrell Machine.

 

I would say to do the following:

 

1. Make sure you have the heat shield.

2. Wrap the fuel line from the mechanical pump to the carb.

3. Check and re-check the float level.

4. Add a pass-through electric pump or near the tank.

5. Mount a large fuel filer ahead of the radiator as I did.  I think it holds a reservoir of cooler fuel that does not heat soak like the fuel in the pump or the carb.  A few cranks and it starts to work its way up and helps.

 

James.

Edited April 21, 2016 by James_Douglas

[Jeff Balazs]

With a full time vane type electric pump back at the tank fuel never get's warm until it enters the carb itself. A heat shield or spacer below the carb helps reduce the percolation affect. If you do this and have your ignition in tip top shape you should not have any issue with this hot start stuff. My truck always fires up right away no matter how hot it gets. And it does get truly hot here.

If you go this route put a large canister filter in ahead of the electric fuel pump as dirt can damage them.

 

Jeff

[knuckleharley]

Factory float level was predicated on old gas formulation, back when gas was, you know, mostly gas.  Todays stuff is a coctkail of things  many of which have a much lower vapor point than dinosaur juice, making its boiling point lower as well as the expansion temp.  So I have found that when I was faced with a similar situation, adjusting the floats a couple of 32nds lower, pretty much eliminated the problem while not adding any driveability concerns.  Lower float = less gas in the bowl= more room for expansion before overlowing raw gas into the carb.  

 

I have also seen research that the flame front of modern motor fuel is slower than older stuff and as such can stand a bit more timing advance that the factory recommended also.  Old gas went bang, new gas goes whoomph, providing a slower total ignition and a slightly more prolonged push against the piston, so lighting it a bit earlier is benificial to an enhanced power stroke.

 

In either case, things that may help, and have a low likelyhood of doing harm.

Thanks for the tip. I would have thought with the lower octane crap we get today that setting the timing back would be the thing to do. It never even occurred to me to think about the flame front and it's effect.

[knuckleharley]

In my car I have a "racer" big Fram fuel filter.  I run the line to the mechanical pump out to the front apron and have the filter on the right shield.  I then run the line back into the mechanical pump.  I also have a electric booster pump which I only turn on for a few seconds before I start the car in the morning when it is dead cold.

 

I have the heat plate and I have the line from the pump to the carburetor cover with heat reflective cloth.

.

 

 

5. Mount a large fuel filer ahead of the radiator as I did.  I think it holds a reservoir of cooler fuel that does not heat soak like the fuel in the pump or the carb.  A few cranks and it starts to work its way up and helps.

 

James.

I would have never though of running a fuel filter out in front of the radiator like a trans cooler. Thanks for the tip!

[Tom Skinner]

Put a Heat Shield on.

Put a few Wood Clothes Pins on the Fuel Line going to the Carburator.

Prestodigitorium - Vapor Lock Gone.

I live in NC - Hot. No Problems with the above - even when 95 Degrees.

Oh and buy Ethanol Free Gas - Not the Un-Leaded regular

Edited April 22, 2016 by Tom Skinner

[T120]

Without meaning to hijack this thread on the much discussed vapour lock and fuel percolation and flooding problem ; it is interesting to note that Chrysler used 

a drain on the intake back in the thirties. The manifold drain was located in the intake at the base of the carburetor with a check valve and drain tube that extended down along the inside fender allowing any fuel that may puddle in the intake at shutdown to drain onto the ground. I think 1936 was possibly the last year Chrysler used this...A bit of trivia.

 

  

[Reg Evans]

Yup, My '33 Plymouth has one. Neat idea...wonder why they quit using it.

[55 Fargo]

Yup, My '33 Plymouth has one. Neat idea...wonder why they quit using it.

Production Cost/expense, cheaper without this feature??

[Jeff Balazs]

I would have never though of running a fuel filter out in front of the radiator like a trans cooler. Thanks for the tip!

If you stop to think about this the only heat source that could affect the fuel temperature is the manifolds and to a lesser extent the block itself.

The run between the tank and the stock fuel pump is going to be relatively cool. From the pump itself up to the filter and carb is another story.

And IMO the biggest part of the problem is really heat transfer between the manifold and the base of the carb. A heat shield can help with this.

I also think that our engines are producing a bit more heat with these modern fuels than they did with the old stuff.

If you factor in how the pump life can be negatively impacted by these modern fuel formulas you may come to the same conclusion as I did. Obviously I felt a bit of a re-design was in order to maximize reliability for regular use today.

 

Jeff

[Don Coatney]

I routed my  pump to carburetors fuel line in front of the exhaust manifold as pictured. I do not use a heat shield and have never had the fuel related no start problem.

 

[Reg Evans]

Gee whiz Don....when are you gonna install that engine in your car ?

[Jeff Balazs]

Don;

That is not really surprising either. Your dual carbs are not located directly above the mating surface between the manifolds. It has to be a bit cooler at the base of your carbs than it is on the standard set up. Could actually be significantly cooler......and I think that is where most of the heat transfer happens.

 

Jeff