Hey guys - I’ve been fighting a vapor lock / heat related fuel delivery issue with my truck for a bit and I’m looking for help in resolving it.

I’ve got a basic street / strip 440 D150. Current fuel set up is a 3/8 line to a mechanical pump dead headed into a 750dp - no regulator or return line and no insulation or heat shields. You can probably guess where I’m going with this.

The issue rears it’s ugly head on hotter days about 30ish minutes into a cruise. You can steadily watch the AFR gauge go from 13-14:1 cruise to 12:1, 11:1 and finally around 10:1 where it will cough and die while rolling down the road. To me, it seems as if the fuel is literally boiling over out of the carb.

Anyone run into this issue? What are my options to solving this? As to be expected, I’m on a budget so I’m looking for the low hanging fruit vs reworking the whole fuel system.

Thanks in advance!

what is the fuel psi ?

some heat sleve may fix the prob.

You need to keep the heat from boiling the fuel so try insulation for the fuel lines.Also is the radiator keeping the motor cool? What temp t stat?

Still running exhaust manifolds? If so make sure the exhaust flapper isn't stuck shut.

Haven’t checked fuel psi. My guess is 5-6psi - probably should rig a gauge up and check.



I’m running a Flowkooler water pump and 160 degree t’stat. Water temp is 180-185. I recently switched to this setup vs an electric water pump thinking this might help. Hasn’t so far.



No - headers.

Well something is amiss. The intake on my flathead six literally sits atop the exhaust manifold and I don't have these issues, pretty much as the factory delivered it, no heat sleeves, no heat shields, no return line, etc. Been 100+ for days on end here too.

I would closely inspect the fuel line and make sure it is getting no where close to the headers. Maybe fill up at a different gas station to see if it's fuel related. Get an IR gun and next time it happens get some temps from the carb, fuel pump,, fuel filter, etc to see where the problem is.

Yeah something is definitely amiss. This is the second year I’ve had this issue - I parked it w/o troubleshooting it last year. The water pump swap this year was me trying to kill two birds with one stone.

The fuel line is routed in the frame rail which certainly is close to the headers and the carb feed line up from the pump is routed up by the alternator. That’s probably not helping the situation.

I’ve considered adding a return style fuel filter off the pump to return the vapors back to the tank. This certainly would work but I’ve been lazy and not done that yet. And I’m not 100% this would solve the gas literally boiling out the carb. Idk.

The cheap and easy thing to do seem to be add insulation and a heat shield under the carb.

Good idea on the IR gun!

I agree. My 528 Hemi in my Charger, a/c, mechanical pump, no return line, premium fuel, no problems even idling in town. I suspect the poster has a fuel delivery problem and not vapor lock. As said check the fuel pressures.

Is your heat cross-over blocked? There is a port in the intake under the carb that allows exhaust to cross from one side to the other thereby heating the intake and carb base. There are certain weather conditions where this is a good idea, but it was designed in an era with entirely different gasoline. With today's highly volatile gas, it will boil the gas out of a carb. Blocking it is as simple as changing your valley pan. Something we have been doing since the 70s.

Notice that some pans have the smaller hole in the middle for the exhaust and some don't:

www.manciniracing.com/valleytrays1.html

He has headers.

Are you suggesting that without the flapper in the manifold there will be no exhaust flowing through the intake? My experience says otherwise. Certainly less flow, but with today's gasoline, how much exhaust under the carb would it take to cause a problem? Given the OP's issue, it is not something that should be ignored and in fact, would be the first thing I would make sure is done. Today's gas is far more volatile than at the time that system was designed. And ANY extra heat to that fuel is a bad thing.

No I am not suggesting anything. Point is with headers there is very little crossing over. I do agree that if in fact he has vapor lock any possibility for improvement or correction should not be ignored.

The OP said; "To me, it seems as if the fuel is literally boiling over out of the carb." That doesn't sound like vapor lock to me. It sounds as if the fuel is being boiled away in the carb which would point to a super heated intake.

My thought is that ANY exhaust through the intake given the OP's problem could be too much. Which is why I asked if you thought that there would be no exhaust flow without the flapper. It would seem to me that the only way to know whether the exhaust cross-over plays any role in the OP's issue would be to make sure that it is blocked completely. And the OP won't even need to pull the intake to find out. A temp gun will tell him what he needs to know. Either the cross-over area is significantly hotter or it's not.

I am focused on this issue because I have seen it so many times. Quicker ignition advance and blocking the heat cross-over are the first two things I always recommend to guys that come to me with any problems like the OP is experiencing.

I hadn’t given much thought to a crossover issue as I have a single plane intake (Holley SD) and headers. I honestly thought a single plane did not have a crossover.

I have considered that I’ve had a fuel issue however. The truck lost 2-3 tenths last year - doesn’t nose over but it lost numbers everywhere. I sort of chalked it up to needing to freshen the engine up. I probably need to rig up a temporary fuel gauge taped to the window to get a read on what my cruise and WOT pressure is. Lower pressure = lower fuel boiling point.

I do have a timing limiter plate on the distributor from 4secondsflat for more initial timing (20deg if my memory is correct) while limiting total timing. That certainly improved drivability.

If the fuel line is located above the Intake cross over that would be a potential problem area. The closer the line is to the manifold the more suspect IMO. Adding an electric pump out back as a "PUSHER" sometimes helps as it creates a positive pressure in the supply line which raises the boiling point of the fuel. Keep in mind with a mechanical only, it is at atmosphere or a negative pressure which has the opposite effect.
I have in some severe cases fabbed up a piece of aluminum to cover the heat crossover area bolting it in place to create an air space between the manifold & line. The larger the piece the more it will deflect and radiate the heat away. One could also add insulation to the bottom side to block even more of the heat. I also prefer phenolic spacers under the carb.
Some of the BB Corvettes had a shield under the carbs directing the heat away from the carb and fuel bowls.

It is baffling why two cars that appear to be the same will act differently. It also P'sMO that this problem had gone completely away for decades until they started playing with the fuel and more so ethanol.

You need to know what the fuel pressure is not assume. If vapor lock was an issue which it could but I doubt nearly everyone would be plagued by it. I had a 360 mechanical pump which drove me nuts. In the end with a gauge it acted up. Fuel problem lost pressure. New lift pump cured it. I took the pump apart as it worked and had pressure then it would act up. Ended being a piece of soft like lead or solder in a check valve. I then ran a filter on the pumps inlet.

How close is the fuel line to the headers? That's where i would put on some insulation. Just slide it over the existing fuel line. There is a velcro version also...
https://www.heatshieldproducts.com/blog/tech/using-thermal-barriers-to-prevent-vapor-lock

you are correct that the crossover is not open through the intake but the passages in the heads if left open will heat the intake.

And Nitrous is 100% correct on the fuel pressure especially when running WOT at the track. leaning a motor out can have expensive consequences.

KDY - I suspect you Do Not have a “Vapor Lock” issue, but a boiling the fuel out the carburetor problem. A vapor lock would cause the fuel pump to be unable to supply fuel consistently, since it is not capable of pumping vapor very well. You indicated the AFR was tipping to rich and not towards lean. I think this supports the fuel boiling out the carb and not vapor lock.

Do you have the carb mounted on a phenolic spacer to reduce the heat transfer? If not, you should install one at least a 1/2” thick if you have the hood clearance. Are you running an Edelbrock or Carter type carburetor? If so, they are more prone to the heat boiling the fuel, because the fuel bowls are closer to the heat source, the intake manifold.
Ron

I'm using a valley pan with a blocked crossover.



This is what I suspect as well. I just haven't seen a lot of info on combating this - most is aimed at vapor lock issues. I do find it strange that my father's stone stock carb'd '85 360 has zero issues with vapor lock or fuel boiling - it does have return off the filter as compared to mine.

My carb is mounted on a 1/2" phenolic spacer. I'm using a Holley 750dp.

I think my plan of action will be 1) adding a return line back to tank using a return style filter w/ orifice, 2) insulating the fuel lines from the headers to the carb inlet. I do plan on rigging up a test fuel pressure gauge taped to the window for testing - little sketched out about this but an electronic gauge is $$$ and I don't really want to mount one to the hood.