Carb heat soak: causes, related problems, and cures?

Hey -

Recently installed an old Edelbrock tri power set up on my Chevy 283. Im running a set of original tri-power Rochester 2g carbs which had been rebuilt, jetted, etc, for my engine.

But almost right away, I begin having what I now realize is problems with heat soak...fuel boiling in the carbs, making the engine stall and impossible to restart until things cool down. At first I thought it was just vapor lock, and insulated and re-routed fuel lines accordingly, but the problem persists.

My engine is also making less power than I think it should, and only getting about 13 mpg (mild cam, balanced internals, otherwise stock engine in a light '40 Ford pickup with a TH350 auto and 2.75ish freeway gears in back). Im thinking these things are all related.

I never had this problem with the dual quad set up I had on there previously, nor the other Rochester 2x3 set up I had on there even earlier, but with a different motor.

I know phenolic spacers will help, but I'm wondering if there is something else I should be addressing here too? Isnt all this heat a sign that something isn't right? Like Im wondering if my timing is too retarded or something like that (my timing light is busted or I'd have checked by now). I know that, if anything, Im jetted a little rich, and not lean.

Any insights? Suggestions? Thanks in advance - Jones

 

I had a similar problem with a pontiac tri power. I tried insulated spacers under the carbs and while there was some improvement it wasn't a cure.

Eventually I put in a booster electric fuel pump that fed the suction of the mechanical pump and a recirc line from the fuel block back to the tank.

I normally operated only the mechanical pump and let it draw the fuel through the electric pump. If I started to have heat soak trouble I would start the electric pump.

That way I only had to listen to the electric pump when there was a problem.

I ran the engine that way for a number of years and only occasionally had to resort to the booster pump.

 

Contact Dashman,here on the Hamb for some really sweet phenolic spacers and pick his brain while you have him on the phone...

http://www.dashman.net/category.html?id=13

 

We are going to have to do something like the phenolic spacers on the two Edelbrocks on my Son's T bucket. We came back from a long drive on day and it started cutting out. It was in the heat of Summer and the carbs were so hot you couldn't touch them. The fuel was percolating in the carbs.

I think the problem is made worse because he has a polished intake that is reflecting the heat back up to the carbs, because I am running the same setup on a dull finished intake and my carbs are cooler. I am also running one inch spacers under mine, which probably help.

Don

 

Doesnt help that ethanol lowers the boiling point of the fuel.

 

Yep...already got hold of him and considering the options he layed out for me. Im certain Ill be buying one kit or anther from him, but I'm curious if there is something else I should be addressing as a piece to this puzzle, but all I can think of as ways to cool things down are the obvious, like correct engine timing, clean oil and enough of it, correct fuel mixture...

... and then there is that variable, which I am certain is playing a major part in all this, that and the fact that this has only happened on really warm days (90 degrees and up).

This is really frustrating, as this truck is my daily driver. Oh well, this is life with a hot rod, I guess.

 

I dont have tri power experience but i had a bad heat problem on my FE when u went to an aluminum intake. The spacer will help, make sure ur fuel lines are away from the heat and then go over ur tune again. You will have to adjust ur carbs due to the spacers anyway.

 

Funny, I had two rods with tri-power and they both ran fine without spacers. However, the rods were hoodless and got plenty of air. My '52 Chevy with an Edelbrock carb had that problem. I added a spacer under the carb plus Edelbrock carb fuel pressure should be 5.5 lbs. The stock Chevy pump was 8 lbs so I added a regulator. That cured the problem.

I would check your fuel pressure. I believe Rochester carbs like low pressure, 4 lbs or so. Your fuel pressure could be over powering the needles and flooding the carbs when you turn it off. The Edelbrock carb had that problem. It acted like it was flooded once I turned it off and tried to re-start.

 

Does the intake have a heat crossover passage? If it's open to the heads maybe try blocking it off or restricting it. If you're not running headers, is there a heat riser valve in one if the exhaust manifolds? Is it working freely or stuck open or closed? Is there anything you can do to improve air flow over the intake, especially around the primary carb?

 

Fuel volatility or "boiling point" has changed dramatically over the years. Gasoline is made up of many compounds which all boil at a different temperature.

It is harder/more expensive to make less volatile fuel.

The volatility rating sounds techie but it works something like this, you put a sample in a pressure tight container and heat it to 100 degrees, it will build pressure until it stabilizes, that stable pressure is the Reid Vapor pressure or RVP.

Depending on the area summer gasoline is 7 - 9 PSI and winter is 12.

No gasoline can have a RVP over atmospheric or it would pressurize the tank.

Now an infrequently driver car with a lot of winter gas in the tank is going to have a lot of hot soak, or hot fuel handling issues.

We got RVP kits in the 80s because independent distributors would sell gas that was way out of spec. A cheap way to dilute gas is with butane which has a RVP of 52.

In the old days we worked hard to vaporize the gas with heat, this can create a lot of trouble with today's fuels.

So we need to use insulators, etc to keep carb temperature down in high ambient.

Also you can restrict the crossover passage or block it as stated above.

Hoop

 

You're exactly right on the heat issue. It's the best bang for the buck and can remove a lot of headaches while driving your Hot Rod.

Dashman's Hot Rod & Speed Parts www.dashman.net

We have these in 1/4" and 1/2" thick. Here are a few that we carry.

 

Good thinking...I'm pretty sure this manifold does have a heated crossover passage. Ill have to look into that. I am running old exhaust manifolds, but they no longer have heat risers. Thanks for the input.

Dashman, come payday, you are going to be the first guy I call. I appreciate the info you've provided me and I'm convinced insulators are a good place to start. Next will be checking fuel pressure issues, as I know that is part of the equation as well. I've already insulated and moved fuel lines, so that's done, but problem still exists.

 

Blocking off the heat crossover helps a bunch. It does however make the engine more cold blooded when it's cold out.

 

That is another good point by blocking off the crossover. Sometimes you have to knock out the gasket in that area and sometimes there isn't a provision for it. Better double check on that, it will help as well.

Dashman's Hot Rod & Speed Parts www.dashman.net

 

Someone above mentioned the heat riser valve;



If it is stuck closed it will force way too much exhaust thru the heat riser passages and restrict power on that bank.

Of course if you have headers this won't apply, however, for that matter, anything that gives you unbalanced back pressure will do the same thing. Any restriction on one side will force excessive exhaust thru the heat riser passages.

The purpose of the valve is to get a shot of exhaust to get the manifold warm and then open up.


Hoop

 

The Y-blocks have a free-standing intake that doesn't seal directly to the lifter valley, don't they? Don't know if it might be more beneficial to try to route more air under the intake or try to add a heat shield or some insulation to the bottom of the manifold, at least around the center carb.

 

51 Norm, I was interested in what you mentioned in this post and did a little research. I found this picture of a '58 Pontiac factory tripower set up and noticed it has provisions for a vapor return on the fuel filter, along with some sort of vacuum doohicky worked in there too.

I'm wondering if this wasn't some sort of factory fix for vapor lock, sort of functioning similarly to your fuel return/booster pump set up?

 

ClayMart, you are right about intakes on Y-blocks having separate valley covers, but I'm running an old Chevy 283 in this truck (but would rather have a 312!).

 

Duh! Well is my face red. I'm mixing this up with another thread!

 

Hello mr jones ,
If you think you have or are chasing a fuel related problem it is imperative to know the fuel pressure going into the carbs before you can even start thinking about anything else. That's the secret trick .

Ok few things stand out to me in your post.
Bear with me for a moment ,,,
You say you've re jetted
You say you're probably jetted a little rich
You say the problem starts almost right away.
You say it stalls and won't restart until it cools off.
You say you're down on power
You say you're down on mileage

Every one of those , every single one is a symptom of running or being rich.
A fuel related heat soak give you lean conditions because there's not enough fuel. What you are describing is an engine that tries to run when it's cold ( needs to be rich) and once the temp comes up it doesn't want to be rich any more. Since you may be rich by the above this could be the cause of the confusion. Heat soak takes awhile to show up at least long enough to raise the engine compartment temps to well over 150* maybe 20 mins in spring . Rich problems show up within 120 seconds of starting no matter what the ambient temp is.

I'd say you should pull your plugs and have a look at what they are telling you.
Black and you should be looking at why you are rich.
White and keep chasing the heat soak problems.

There were a few manufactures that used a return line off the filter to help correct vapor lock. I had a thread on those , let me see if I can find it .

 

No worries at all! I appreciate you trying to help out. Didn't mean to call you out on it or anything.

 

Here it is -
http://www.jalopyjournal.com/forum/showthread.php?t=726747&highlight=fuel+filter

As promised from last post

 

So what's going on with this ?

 

One thing stock intakes have that aftermarket ones don't is a tin shield that keeps hot oil from spraying on the underside of the manifold. There are aftermarket oil shields that snap down under the edges of the cylinder heads to protect the bottom of the intake from a hot oil spray. Something to think about.

 

Or one could get the bottom of the intake ( valley side) treated with one of the ceramic thermal barrier coatings ...

 

Do these carb gasket heat sink shield things work as well as the phenolic spacer deals?

http://contentinfo.autozone.com/znetcs/product-info/en/US/mrg/98A/image/2/

 

When i was troubleshooting a customers car for similar problem i checked the temps with a heat gun. An edelbrock reading about 145deg outside the bowl will be boiling the fuel inside the bowl.
To test, run the engine up to temp, add a fuel pressure gage to the line between the pump and carb. Shut the engine off, if the fuel is boiling in the carb the pressure will remain constant (being trapped between the check valve at the pump and the satisfied float via needle & seat. When the fuel boils off the needle and seat will open to allow more fuel to enter and the pressure will drop, the new fuel will boil and the cycle will repeat itself until the fuel is gone.
The engine won't restart until it pumps enough fuel to refire.

 

Had to wait to get a paycheck, haha!

I'm going to start by getting fuel pressure under control with a regulator set at about 3lbs, then take it from there. Also, I am certain you are right about the carb running too rich, as you indicated in your previous post. I'll have to take care of that, too.

This is a great idea! One more thing to look into, but I'll take on the "external" stuff first, since it's easier and I'm lazy.

This is EXCELLENT info, thanks! I just ordered a fuel pressure regulator and inline gauge that will mount in my fuel block, so I will definitely do this.

Thanks guys, the HAMB is an absolute Godsend!

 

I don't know if you have this under a hood or not. When we had carb heating problems, phenolic spacers helped but the carbs are larger than the spacers. We used to make a heat shield out of aluminum sheet that fit the carb bolt pattern and extended beyond the carb area to get the hot air to go around the carb on the way up. If you are using open air cleaners, they are sucking in the hot air rising off the engine and adding heat to the carb.

 

I have a hood that is louvered, however I usually run with no hood at all.

Here is a picture of my 3x2 intake...same thing as pretty much
any other out there, except Im running Rochester 2gs instead of Holley 94s or Strombergs.

What frustrates the crap out of me is that I have run these same exact carbs on another Edelbrock manifold just like this one on another 283 with this same fuel pump in this truck and never had any problems what-so-ever.

But this time around it has been nothing but trouble. This picture is from one of several times I've been stuck - generally middle of nowhere - with fuel boiling out the carb.