Vapor Lock

Based on my symptoms I think I’m experiencing Vapor Lock. On the 1968 302 2V Cougar, when it’s running for awhile and I stop for longer than 5-10min it’s really hard to start. It starts fine when it’s cold. It even seems to start fine if I stop just briefly (5min). But if it sits for more than 5min I have to crank the starter for 10secs or more before it finally starts to catch. The theory is the heat from the engine I guess builds up while the engine is off and the fuel evaporates in the carburetor before it gets a chance to ignite. (OK that’s my layman’s understanding).

I’ve been told replacing the 195 degree thermostat with 180 degree and a carburetor spacer will help. I tried to do the thermostat myself but I couldn’t get to a bolt (lower right) without removing the A/C which I don’t want to do. I might have the mechanic do that.

Can anyone recommend a compatible carburetor spacer and tips for installing? From what I’ve read here if insulation is the goal, a phenolic is better than aluminum. True/false? I’m not finding any compatible 2 barrel spacers that aren’t aluminum. The carburetor has Holly stamped on it so I’m guessing it’s the original as most of the stuff on the engine seems to be original.

Any installation advice, or parts recommendation would be greatly appreciated!!!


Perfect. Thanks!!! Is it as simple as removing four bolts and slipping underneath the carburetor?

…And the studs, seeing that it’s a 1" spacer. Would be better to find a 1/2" spacer, less of a chance of having to re-jet your carb. The spacer will increase the air volume, so if your mixture is good currently, you may have to move to a larger jet size to prevent a lean mix. If you’re running a little rich on the mixture the spacer will help with fuel/air mixture balance…“And the hits keep on coming!”
Keep in mind air cleaner to hood clearance…don’t want to fix your fuel boil problem, close your hood and put a big dent in it!

I was thinking hood clearance was another reason 1/2" might be better. I found this one: It looks like it is a 1/2" equivalent to the one you recommended from Summit. Unfortunately it doesn’t come with gaskets or stud extenders so I’ll to figure out what that needs.

I’m going to put a 1" piece of Styrofoam on the air cleaner and slam the hood. If it doesn’t get squished I should be good with a 1/2" spacer.

Thanks again!

Did some quick shopping …This looks pretty good, should not be any spacing issues here!

That looks perfect. Ordered and on it’s way. I really do appreciate it. Thanks!!!

Just saw a sweet Trans Am at the gas station. Talked with the owner for a bit, and realized he was just waiting around. Apparently he’s got an ongoing problem with vapor lock, “hot start” as he called it. I asked if he had a spacer between the carb and manifold and he said no. Problem for him is, he’s got a shaker on his 403 so can’t really mess around with height like that. What do you do in that situation?

Here’s the discussion from a few weeks back.
I was having the same symptoms on the '69,… I was thinking all kinds problems were causing excessive heat,; bad valves, head gaskets, rings, vacuum leaks ect… and was actively pricing out engine rebuilders. By happenstance, on a 90+ degree day, when performing my one-hundredth “why is this thing running so hot” inspection, I pulled off the stock air cleaner while the engine was running and noticed the heat riser flap was diverting all of the intake air directly from the exhaust manifold. Removed the spring, so the flap would block the preheated air…problem solved. It’s still a little problematic on hot days after stopping for gas,…but nothing like it was. Before the engine would have to turn over 6-8 times before firing up,… now at worst, 3-4 times.

Thanks for the tips. I installed the carburetor spacer and checked the heat riser flap. Still having vapor lock problems. Any advice on how to insulate the fuel line leading up to the fuel filter? That seems to be the next thing to try.

I had the same issue with my '72 Spitfire with a 240Z engine. In the middle of the Summer (over 100) I had vapor lock on the long commute (not to mention cooking my right foot because the exhaust was too close to the firewall). I had heard the old wives tale of wrapping the fuel line with aluminum foil so I walked down the highway to the gas station that was luckily just a half mile down the road. I picked up a bag of ice and a box of foil and headed back to the car. I tried to start the car, no dice. I put the ice on the intake and wrapped the fuel line as much as possible. After 15-20 minutes the ice was mostly melted so I tried again and she cranked right up. I never had a vapor lock issue for the rest of the time that I owned her.

Whenever the engine sits for 20-40 minutes my car is a bear to start. It cranks but just doesn’t start. Only solution is to put the pedal to the floor and hold there while cranking and after about 10 second it’ll finally catch.

I was convinced it was vapor lock. I added a 1/4" plastic carburetor spacer (underneath an already 1" aluminum one) and no effect. I placed wooden clothes pins on fuel line near fuel filter. No effect. I bought heat resistant tape, taped up some pipe wrap (stuff you’d use to insulate pipes from freezing) and wrapped around the fuel line from the fuel filter to the fuel pump using zip ties. No effect.

Could heat effect the fuel pump? I’m kind of out of ideas. Anyone have a suggestion. THANKS!!!

Actual vapor lock is really very very rare. If you have to hold your foot to the floor, most likely what is happening is that the carb is leaking into the intake manifold and it is flooded. When you hold your foot to the floor what you are doing is clearing a flooded condition. What carb are you running?

It’s a Holly 2 barrel carb. I think it’s stock. I’m beginning to think it’s not vapor lock as well as I’ve heavily insulated the fuel lines and added a 1/4" spacer underneath a 1" aluminum spacer and nothings changed. It should have at least gotten a little better or changed somehow.

Turned out to be the carburetor. I took it a carburetor shop, yes that’s what they specialize in, and he showed me how after the engine was turned off you could see fuel bubbling up into the top of the carburetor. I had the it rebuilt and it seems to be much better now. Thanks for all the advice in helping get to the bottom of this mystery.

I had the same problem with fuel dripping into the carb and then bubbling after the engine was turned off, months back. One of the floats was not adjusted correctly, all I had to do was bend it slightly to get it in spec and the problem went away. Glad to hear you got your issue straightened out!