Hey everyone. I rebuilt my 300 a couple or 3 years ago. I am still running all stock parts such as the regular duty exhaust manifold and the single barrel stock intake and carter carb.
I live in Las Vegas and the truck hates the triple digit heat in the summer. It has vapor locked several times this year already. I normally don't drive it much in the summer since it is without AC and I like my comfort.
I am curious about switching to the FI exhaust manifolds and plating off the bottom of the intake. Will this even help with the problem or will it cause more issues than it's worth.
I read that the intake needs the heat to properly burn the fuel, so am I looking at the wrong thing causing the issue? I have wrapped the fuel line from the mechanical pump at first with hose. Then I got thermal tape and a thermal sleeve.
It didn't help and then I remembered the exhaust porting into the intake. I am guessing that to be the issue. My manifold does have the flapper and it seems to be functioning best I can tell.
Any advise or a nudge in the right direction would be great. Thanks

EDIT: Forgot to add that this is in a 1970 F100 so no emissions equipment.

 

Hi Gembone,
You've covered most of the things I was going to suggest to look at.
Number one issue is your fuel getting heated up when going to the engine. Not such a big issue on the inlines since the fuel comes in on the opposite side of the exhaust.
The other is your exhaust flapper. If that gets stuck open, that'll overheat your carb and cause issues, but you say that's functioning correctly.

A few other quick questions:
Is the exhaust stock? If you've split the exhaust into duals or something, one of the pipes might be too close to the fuel line.
Is the gas tank venting properly? If not, it'll cause a vacuum inside the tank, especially on hot days. This vacuum will make it harder and harder for the pump to pull fuel until it finally goes kaput. I fought this for a long time until I realized what it was. (That flexible air vent hose that goes into the tank can easily kink if the tank has been taken off).

EFI manifolds wouldn't make much difference unless your exhaust heat riser (flapper) is bad, since the temperature of the fuel going in would be the same.
You could try installing an electric low pressure fuel pump back at the tank.

 

Stock exhaust and fuel lines are on drivers side. Manual pump on the block. The tank isn't exactly well sealed so it is venting.
I have always been happy with stock. That is why I don't want to swap the manifolds unless I need to.
Until I take the manifold completely off I can't be sure the flapper is working properly, but I can move it by hand and it returns like I think it should.

 

I had a big problem with vapor lock in my '46 pickup with a built 460 engine. I installed a fuel return line back to the tank with a .080" restriction, similar to some OEM 460 Lincolns, etc. That made it 90% better. I plan to install a carb insulator gasket soon too.

 

EFI exhaust manifolds would make some difference. They don't bolt to the intake manifold like the stock carb exhaust manifold does. Even with the heat riser flapper valve shut, the carb exhaust manifold is going to transfer more heat because it's in direct contact with the intake manifold. If you do swap to the efi manifolds there is no need to plate off the intake unless you plan to run coolant into that area of the intake. It's just a compartment on the bottom of the intake. The efi manifolds will still transfer heat to the intake but it would be slightly less heat.

Like FTF mentioned you will want to isolate the carb from the intake manifold. You can do this a few different ways. The first is a insulator gasket. That is a thick gasket that will block some of the heat from transferring from the intake to the carb. A composite/phenolic carb spacer also helps with this. A heat shield is another option. This is just a sheet of metal that you place between a carb spacer and the insulator gasket. It should be larger then the size of the carb. It just helps block some of the heat radiating/rising off the intake from getting to the carb. But this can be a pain to make depending on how your carb and linkages are situated. I plan on doing all three options on my van along with some ducted outside air.

I know you are a stock kind of guy, but if you still can't fix the issue. Then ditching the mechanical fuel pump and installing a electric pump back by the tank could also be the solution. You might not like that, but it could be a last ditch effort to take care of the problem.

 

I am not completely against non-stock but I prefer the simpleness of the stock 300. any time other than triple digit heat and it runs great. I have been driving it more this summer due to my other ride having issues.
I appreciate the info guys. I think I might try a carb spacer and heat shield before swapping the manifold.
I am planning the under bed mustang tank later this year and might add a small electric pump when I due that for backup.

 

If you want to keep the stock carb ex. manifold. You might want to try and carefully remove the spring off the heat riser valve and wire it shut.

Just make sure you don't run the electric and mechanical fuel pump. If you run the electric pump make sure to unhook the mechanical one. And it's also a good idea to run a regulator with a electric pump. Even if it's a low pressure one.

Also it may not have anything to do with your issue but if you haven't already, make sure to replace all your fuel hose. It can look fine on the outside and break down on the inside.

 

I will give removing the spring a shot. I know about the no manual with electric pump, but thank for noting it. Also all the rubber lines are about 6 months old.

 

You may want to give this a read on this issue https://www.ford-trucks.com/forums/1...g-and-you.html

But if you don't want to read it right now take note anything under a suction / vacuum will start to boil at a lower temperature.
Anything put under pressure will have a higher boiling point. Look at a cooling system it is under PSI and has a higher boiling point.

With that said and all new cars / trucks have electric pumps in the tank puts the whole system in a positive PSI so the fuel boiling point is now higher from tank to motor.
Have you heard of any now stuff having vapor locks?

Your system is under a suction from tank to pump on the side of the motor. I would say your fuel is boiling between tank & pump not from pump to carb.
What have you done to insulate the tank to pump lines?
A hard start after a drive could be fuel boiling out of the carb so don't over look heat at the carb altogether.

Oh the post said you can add stuff to the fuel, like diesel (if you don't have a cat), that helps on vapor locks.
Dave ----

ps I have not tried any of the additives as I do not have my truck on the road and what I drive has a pump in the tank.

 

I don't recommend eliminating the hot spot under the carburetor. I would add insulation between the carburetor and intake manifold (like a phenolic spacer) to keep the carburetor cooler. If your car is vapor locked, I think its more likely that the mechanical fuel pump is adding heat to the fuel and, if it gets hot enough, the more volatile parts of the fuel will flash into vapor on the suction stroke. See Vapor Lock.

You can add a toggle-switch operated electric fuel to prime the mechanical pump if it vapor locks when you restart after a short shutdown. If it happens regularly in traffic, you're probably better off replacing the mechanical pump with an electric one.

 

Another thing you can try is finding non-ethanol gas station. Modern fuels have a lower flash point than older fuels and, as such, turn to vapor at a lower temperature. Perfectly fine for modern autos where the whole fuel system is pressurized, but causes a problem on carb'd engines. Ethanol is mostly to blame, so if you can get gas without it, it might help to alleviate your problem.

 

I have ordered the carb spacer to see if that helps. I have one around here somewhere, and it will probably show up after I install the new one.
Thanks for all the info and suggestions guys. I will update you when I have tested it out.

 

After watching the video, I have another question. Should I run a spacer on the fuel pump also?
I know a lot of people have spacers on them, but mine didn't have one when I got it. I just kept it that way.

 

No spacer on the fuel pump, just a gasket.

 

If you have a clear fuel filter, I would also try running water over the fuel pump to see if that makes any difference. If your fuel pump is actually vapor locking, you should be able to get back on the road quicker by slowly dousing it with water from a couple of jugs.

Since you've ordered a carb spacer, I would install it and see if your truck has better hot weather performance. I would not make any other changes until you've determined that you still have a problem. If you're still having vapor lock, I think an electric fuel pump would be more effective than a fuel pump spacer. If it's just happening after a hot soak, then a priming pump that only runs with a toggle switch or when the starter is engaged would probably suffice. Otherwise, if you're regularly stalling in traffic, then an electric pump that runs continuously with the mechanical pump removed might be a better option. There is a list of suitable electric pumps here: Vapor Lock.