Carburetor heat soak or vapor lock?

coronet_bob · Thursday at 8:08 AM

I read a post here talking about running a return line to solve a fuel starvation/hard start problem when the car has hot and sat for a brief time. No mention of general driving problems. I was told that this is carburetor heat soak to be solved with a carb parobolic spacer and that vapor lock messes with the car just operating right. Can anyone share their experiences and solutions.
My car has an air gap intake for air circulation but is hard starting when hot and it sits for a brief time. Otherwise it runs great. I have no hood clearance for a parobolic spacer. Thanks

snake · Thursday at 8:22 AM

i had just what your going threw, i could see fuel bubbling,i have the clear window on my holley.Went away after installing 1/2 inch parobolic,you shure not enuff hood clearence.

DeltaV · Thursday at 9:03 AM

I've heard of phenolic spacers and I've heard of polymer spacers, but I've never heard of parobolic spacers, nor have I heard of parabolic spacers.

Does anyone have a link for a parobolic spacer?

R/T Boy 67 · Thursday at 9:48 AM

phenolic spacers.

dfrazz · Thursday at 10:01 AM

I installed a couple of things that worked great for this issue. Cool Carb (- Home) is a great spacer because it is wider so it protects the bowls from heat and its super thin (.155) so it will fit even when hood clearance is tight. I also install a fuel filter with dual outlets with one outlet routing a line back to the tank.

68BabyBlue · Thursday at 10:42 AM

I used to have this problem with the slant six Valiants I drove back in the day, was surprised when it didn't happen with the 1970 GTX I bought in 1977. The factory vapor separator with return line in the second generation GTXs and R/Ts worked effectively. Years later, I owned two GTXs, both of which had been restored, and didn't have the vapor separator installed. I had hot start problems with both. Replacing the factory system solved the problem.

Baby Blue had the factory vapor separator, return line, a carb spacer, and a Holley electric fuel pump from the 70s. That car started like it had modern day fuel injection, under all conditions.

coronet_bob · Thursday at 12:22 PM

snake said:
i had just what your going threw, i could see fuel bubbling,i have the clear window on my holley.Went away after installing 1/2 inch parobolic,you shure not enuff hood clearence.

coronet_bob · Thursday at 12:23 PM

My air cleaner lid is already making an imprint on the under hood insulation pad.

coronet_bob · Thursday at 12:27 PM

dfrazz said:
I installed a couple of things that worked great for this issue. Cool Carb (- Home) is a great spacer because it is wider so it protects the bowls from heat and its super thin (.155) so it will fit even when hood clearance is tight. I also install a fuel filter with dual outlets with one outlet routing a line back to the tank.

Do you think the spacer or return line made the biggest difference?

dfrazz · Thursday at 1:53 PM

coronet_bob said:
Do you think the spacer or return line made the biggest difference?

Return line first because I've ran one without the spacer and it helped. Also, make sure your tank in vented.

ckessel · Thursday at 10:17 PM

At the speed shop I used to work at back in the 80's, we sold quite a few of these shields. They were oe GM at the time. Works for any standard flange carb like Holley's, Carters etc. They used to make another version that was for spread bore carbs, q-jet/tq's etc. These units are very effective for dissipating heat and are thin. One gasket, the shield, then one more gasket. You may need to trim in areas to get around stuff since you're crossbreeding. They were especially effective on RV's.

WileE · Thursday at 11:12 PM

DeltaV said:
I've heard of phenolic spacers and I've heard of polymer spacers, but I've never heard of parobolic spacers, nor have I heard of parabolic spacers.

Does anyone have a link for a parobolic spacer?

this is a parabolic curve.

ckessel · Friday at 10:01 AM

coronet_bob said:
I read a post here talking about running a return line to solve a fuel starvation/hard start problem when the car has hot and sat for a brief time. No mention of general driving problems. I was told that this is carburetor heat soak to be solved with a carb parobolic spacer and that vapor lock messes with the car just operating right. Can anyone share their experiences and solutions.
My car has an air gap intake for air circulation but is hard starting when hot and it sits for a brief time. Otherwise it runs great. I have no hood clearance for a parobolic spacer. Thanks

Bob, what are you working on? What's your engine, intake, carb, filter combo?

coronet_bob · 2025-07-27T06:58:02-0500

ckessel said:
Bob, what are you working on? What's your engine, intake, carb, filter combo?

I plan to insulate the fuel line from the pump to carb.

coronet_bob · 2025-07-27T07:01:08-0500

ckessel said:
At the speed shop I used to work at back in the 80's, we sold quite a few of these shields. They were oe GM at the time. Works for any standard flange carb like Holley's, Carters etc. They used to make another version that was for spread bore carbs, q-jet/tq's etc. These units are very effective for dissipating heat and are thin. One gasket, the shield, then one more gasket. You may need to trim in areas to get around stuff since you're crossbreeding. They were especially effective on RV's.

View attachment 1890216

I'll look into this. Thanks

coronet_bob · 2025-07-27T07:04:48-0500

ckessel said:
Bob, what are you working on? What's your engine, intake, carb, filter combo?

I have a 1966 poly 318 with Chrysler Power air gap aluminum intake, 600 cfm Edelbrock carb, standard metal filter and ceramic coated TTI headers.

Don Frelier · 2025-07-27T07:59:17-0500

Do you have the original gear reduction starter on it?
A newer style permanent magnet would help spin it faster.

coronet_bob · 2025-07-27T08:08:51-0500

Don Frelier said:
Do you have the original gear reduction starter on it?
A newer style permanent magnet would help spin it faster.

Got a mini high torque starter to clear headers.

67 WS23 · 2025-07-27T09:09:22-0500

I solved an annoying fuel leakage issue on my 1967 4327S AFB. When the engine was warn, fuel would drip from the area near the primary shaft on the driver's side of the carburetor onto the intake manifold. Noticed only on these hot days (over 85F), even with the engine off, the dripping, a drop every 5 seconds or so, would last for at least 20 minutes until things began to cool off (fanning the carb would speed up the stopping of the drip). I was thinking there was still some suction out of the bowl occurring even with the engine off.

So yesterday I disassembled the carb again and checked the gasket under the driver's side venturi, and it was ripped by the screw hole, or more likely, the screw made its own hole. This had to have occurred during installation during a recent rebuild. I replaced the gasket, operated the engine until warm (it was really warm here yesterday in the NY metro suburbs), and no more dripping. I saw this suggestion in a 2022 thread here, so I gave it a look... sure enough. Thanks to all on that thread!

ckessel · 2025-07-27T09:46:05-0500

coronet_bob said:
I have a 1966 poly 318 with Chrysler Power air gap aluminum intake, 600 cfm Edelbrock carb, standard metal filter and ceramic coated TTI headers.

What does your air cleaner look like? Especially the base.

Carburetor heat soak or vapor lock?

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