I think my bird is suffering from vapor lock. I've had the car out twice this spring and each time after running the car for about 20 minutes, it starts hesitating and bucking. Thankfully, I've been able to coast a good distance and change lanes until I can get it stopped on the shoulder of the road. Looking through the little window of the carb, I didn't see any fuel in the primaries. So I unhooked the fuel line at the filter, between the metal fuel line and the metal lines going to the carb. I wanted to see if the fuel was flowing, and it was. The fuel line had pressure to it, since the fuel came spitting out, but I notice it was very hot, and as it spilt onto the manifold, it was bubbling. I'm not sure it was bubbling because it was hot already, or because it was puddling on a hot manifold.

Anyway, I was wondering if the fuel is typically hot when its pumped from the tank to the motor. I'm going to look to see the route of the fuel lines to see that the lines is run the proper route to eliminate the possibility of the lines being too close to the motor, and being exposed to too much heat.

Would a carb spacer or carb heat shield also help?

Any other suggestions would be appreciated. Thanks

Seth

On 67 4bbls a SS carb spacer was used to reduce boiling. If you have a 68 this spacer should still help but was not originally used. Your fuel should not be hot going into the carb, so checking your fuel lines/routing seems like a good idea.

If he sees no gas when looking through the sight glass (we know this is not a Q-jet now), then I would be looking at the float level, may be set wrong or the float could be shot. But I don't know what kind of carburetor he's running. Holley?

Here is another idea. I have several old cars and installed a flow through Electric fuel pump to correct this as I was using gas with Ethanol and that tends to take on water and boil especially down here in Florida when the temperature goes up like its doing now. What seems to be avoiding this issue is using RV gas which is 90 Octane and contains no Ethanol.
Doug

Where does the fuel pick up the heat? I would think most of the fuel line is in a highly cooled/vented area of the car? I suppose it does get hot around the metal line as it goes up from the pump to the carb even though it is getting the full force of the fan? Is the carb itself that gets hot? Fuel pump?

What about using some insulation on the fuel lines?

Heat is everywhere under the car. The fuel pump is obviously hot, when the engine is hot, as is the intake & carb.

Then under the car, where the fuel line runs, is the exhaust system.

In the summer, you also have the added heat from the pavement/concrete

Insulating the lines from any heat source, would obviously reduce fuel temps in those lines.

Several ways to keep the carb cooler. There are all sorts of insulating gaskets & spacers you can use under your carb.

You can also block off the heat crossover of the intake manifold.

An alum intake will also transfer slightly less heat to the carb.

Some use a fuel pump with a vapor return line, in order to keep the unneeded fuel circulating thru the pump & back to the tank.

Then, if you trust an electric fuel pump, you can just not use a mechanical pump. That will prevent the fuel from picking up any heat in the mechanical pump.

I just keep the petal to the metal! Gas has no time to heat up!



Craig always talks favorably of the front spoiler and all the cooling it does for his engine!

Yep, they work great keeping your engine cool when plowing that foot of snow off your driveway.

OK interesting subject and I've been looking into it because over the years it seems my FB gets vapor lock sooner, generally what happens to me is when it was hot out, 85F and up when I stopped somewhere the fuel would boil off and I would have a hard start, generally typical Pontiac issue beause the fuel does get hotter due to the longer line going around the block and then out the fuel pump up the front of the engine, the fuel line on Pontiac V8s is exposed to more heat, add to that the stupid exhaust crossover in the intake and you're vaporizing the fuel.

Over the past several years this problem has gotten worse and I believe it's due to the increased use of Ethanol. Big oil is not always following the rules about the % but even 5-10% will make a huge difference in how much sooner at lower temps fuel will vaporize, higher altitudes with increase vapor lock too. Newer cars running electric pumps aren't as affected.

Currently I am tossing around several fixes in my mind, Carter electric fuel pump, Ram Air manifold with NO crossover but a homemade block off plate out of cast or steel, perhaps both.

Sethmast is obviously having a major problem probably from the fuel line from the pump touching the block, etc .........you're lucky you didn't have a fire, you could have a failing pump too, I have found standard mech pumps and hi perf do not last thanks to Ethanol, in fact the factory replacements from Ames last longer than the hi perf ones.

This is why I fill my tank with race gas these days.
Not because I need it with my current engine, but because I got tired of it vaporizing every time I drove my bird when the temperature was above 85 degrees.
No more hard starts or rotten gas when I let the car sit for a few weeks

We are fighting this same issue with our 69 FB (469 auto 3.55 rear). The return line runs parallel to the passengerside manifold. The fuel line crosses the exhaust at the rear passenger tailpipe tank area, and at the back of the passenger wheel area. We have had our long branch manifolds ceramic coated, wrapped our pipes front to rear in header wrap, and run both the supply and return lines thru insulated fire sleeve. Hopefully we will get it running again by sunday to see if it resolves the issue.

We still will pick up heat from the mechanical fuel pump. I measured the temps during vapor lock under our car. Fuer pump top 189F, lower section 150F, exhaust manifolds 511F, floor pans 189F, fuel line next to passenger exhaust manifold 289F, exhaust pipes 250F, mufflers 150F, outside temp 88F.

wow at those temps todays fuel is definately vaporizing

watch this video, fuels blended with ethanol etc today can boil at 100F or less

https://www.youtube.com/watch?v=cadNfSNi_Oc

There are also fuel pump gaskets made to reduce heat transfer, just have to make sure you don't use one that's too thick. I don't run one yet tho.

I have a mechanical pump with a return but after heat soak (45misn of driving) my AFR gets about a ratio and a half leaner. I have a wooden spacer under my carb and heat shielding around my hardline from the pump up to the carb.

When I do my new heads/cam/intake/pistons this winter I'm just throwing in the towel and going Holley EFI.

Yeah I was looking at the new quad sniper set up, but then I figure I drive 500 miles a year, is it worth several thousand for a complete FI set up? so I'm looking at a Carter fuel pump and filter, new SS line from tank right up the pass side to the carb away from the engine and exhaust and intake heat passage blocker gaskets, $300 vs $2500.

$300 sounds way better than $2500 for 500 miles a year.


Each to his own, but something just grinds my gizzard about putting fuel injection on these old cars. I have the same problem with LS or any chebby swaps. But then I haven't suffered vapor lock either, don't know why, it gets mighty hot even behind the firewall.

During my next engine refresh, I plan on ceramic coating AND wrapping my headers and I'm also adding air extractors to my hood. I already cut my fenders to make the Trans Am air extractor functional and when the dual Spal fans are full blast on my C&R radiator, it's amazing how much heat comes out of those vents and the bottom of the car. Just a tremendous amount. Stop and go in traffic when it's 120f outside with the Vintage AC running is a lot to ask.

Secret I found out when getting my headers recoated - some shops will coat BOTH the inside and outside. Figure that cuts down on heat even more.

Absolutely. The only time I would advise people to coat only the outside is if they are a GOOD set of proper stainless steel headers. Mild steel headers should always be coated inside and out. I'm hoping when I go to E-heads I can find a good set of SS headers.