Carb percolation and vapor lock

[Pat]

Now that summer has hit and local fuel has gone to a greater ethanol content, my car is starting to have percolation and vapor lock stalling when it sits after a long run. Having attended a show last weekend, a number of other folks reported similar issues with their carbureted cars. Has anyone had any luck with a specific additive to raise the fuel boiling point or used any of the carburetor heat shielding blocks with success? As you know with a '40 there isn't much clearance between the top of the air cleaner and the rear glass (I have 3/4 of an inch) so the big phenolic blocks aren't really an option.

 

[Randy V]

Pat, there area few ways to clear this up, but I think the best way is to kill your fuel pumps a minute before shutting off the ignition.

 

[Pat]

Thanks for the suggestion Randy. It will definitely be something I'll do.

 

[Scott Calabro]

I ran ceramic insulator gaskets (Pierce Manifolds), an insulator shield aft of the carbs, and the biggest single thing in my mind.....avgas.

Although I also would shut off my pumps prior to shutdown and always immediately opened the rear clip.

 

[Pat]

Scott, how thick were the gaskets you used?

 

[Scott Calabro]

3.25 mm according to Pierce website.

40, 44, 48 IDF

 

[Pat]

Thanks!

 

[Randy V]

Which grade of AvGas did you use Scott?

I used to use 100LL in my Corvette with 12.5:1 compression and had one heckuva time with percolation on hot days at the track.. I was told it was something to do with vapor pressure? Don't know, but others in my car club experienced similar issues.. That's when I started shutting down the fuel pump before the ignition.

 

[Scott Calabro]

I used 100LL (Chevron).

With pump gas here in Massachusetts having 10% ethanol, I could actually hear the fuel boiling in the float bowls after engine shut down.

I never heard that with the "juice" and this was before adding the exhaust heat shield but after the installation of the ceramic gaskets.

I know avgas certainly evaporates quickly on a surface due to its high tolulene content, but I can say that the boiling went away.

I've read on forums about what you have described, I just never experienced it on the setup I ran, for whatever reason.

Also the avgas would not go bad like pump gas.

I would let the car sit all winter in the garage and in the spring turn on the pumps, pump the pedal twice to the floor, crack the throttle, turn the key and off she would go.

Plus I could read plugs again!

I could not do that with pump gas.

 

[Dave P]

Pat:

What carb are you using? Edelbrock?

 

[Mark Clapp]

I recently replaced my old Holley carb with a new one. I did have an insulating spacer (1/2 inch thickness) but did not put it back with the new carb. I then had starting issues after shutting down with high outside temperatures (summer). I installed a 1/2 inch phenolic spacer and have had no problems since.

 

[Pat]

Hi Dave,
I do have an Edelbrock on the car. I've ordered the heat shield spacer from them as well as the Mr. Gasket heat shield kit. I'll see if it reflects Mark's experience.
I'm also replacing the fiberglass/foil heat barrier with Lava Mat to increase the under-deck space and hopefully improve airflow. When I get it all together, I'll but the temp gun on it and see if it makes a difference.
Randy's tip about fuel pump shutdown to drain the carb prior to engine shut down has made a HUGE difference.

 

[Randy V]

Glad that's working out for you Pat!

 

[Dave P]

Pat:

A buddy of mine had problems with one of those years ago. He thought it was vapor lock. He ended up figuring it was the float being forced open by fuel pressure. He put a fuel regulator on it and had to crank it down to a real low Psi. it has been a long time but I think it was less than 7 psi. He was using a mechanical pump also. Randy's suggestion may help if that is the case .

 

[Robert May Eng.]

The insulated spacer is a must have but something not touched on here is a fuel return. I know you would need one those six port fuel switch over valves but at least you would have an effective fuel cooler in operation (fuel tanks). I had a lotus on webers years ago that used to boil its fuel, flood and be a pain in the butt to start hot, the fuel return fixed all the problems in one hit. If the carb has no return port Weber/Holley do a fuel pressure regulator with a return on it.

Bob

 

[Tim Kay]

The insulated spacer is a must have but something not touched on here is a fuel return. I know you would need one those six port fuel switch over valves but at least you would have an effective fuel cooler in operation (fuel tanks). I had a lotus on webers years ago that used to boil its fuel, flood and be a pain in the butt to start hot, the fuel return fixed all the problems in one hit. If the carb has no return port Weber/Holley do a fuel pressure regulator with a return on it.

Bob

Bob, I would like to try this solution but curious to the benefit of a fuel return with a Holley regulator. Unlike EFI, the unused fuel is not flowing back into the tank, it continues to sit in the bowls of the carbs until used. How is the fuel "cooling" in the carbs?

The percolation happening on my motor seems to be obviously related to heat from the headers because only the rear two IDA's are percolating, closest to the "bundle" (no isolators, turkey pan or shield barrier) and begins after a minute or two from turning the motor off. Odd thing is, #3 & #8 cylinders are the only two that percolate fuel into the throttle body. Is that normal? It's the same throttle body if you're to orient the carbs the same direction so my assumption is that there is an outlet for the fuel to escape easier at one throttle body than the adjacent one.

Tim

 

[Pat]

Thanks for the insight Bob but given Randy's tip of leaving the fuel bowls empty seems to work indicating to me that the problem on my car is not so much the heat of the fuel supply but the post shutdown heat soak of the carb. This is especially true on the rear of the block on the end near the collectors and away from the coolant entry/exit.
I've read that fuel boils anywhere between 38-200C (100-400F) and given my running engine temp is 80c or 180F. After 20 minutes of heat soak, the temp gun registers the manifold and carb float bowl at 92c (197F).
So I've shopped around and looked at a few products, the Mr. Gasket heat shield, Cool Carb, and the Edelbrock phenolic spacer. For various reasons, including the limited clearance space, I went with a mix of the Mr. Gasket shield and the Edelbrock spacer. When I finish the rear deck conversion to Lava-Mat, I'll take it for a run and see what the temps look like. Here are some before and after pics.

 

[Tim Kay]

Pat, so is the percolation heat soak from the motor up to the carb or is it radiant heat from the bundle of snakes? or a combination?

 

[Pat]

I think it is more soak from the block than the radiant heat from the headers given the temps of the block were identical to the carb only after shutdown and the coolant flow stopped. Additionally, I never have problems until after shutdown and I restart. I put a shield to reflect the header heat to be cautious. I'm sure the header heat doesn't help though.
Here is the before and after the installation.

 

[Jimmy P]

Tim my car is fitted with Webers and like Pat my gas percolates in the back two carbs after shutdown. I try to remember to shut the fuel flow off a couple of blocks from home and that appears to help as my carbs also have the "gaskets" fitted. I did note its always the back two carbs sitting closest to the headers. I feel its a combination of engine heat and header heat. Just my .02.