427 C2 Percolation or Vapor Lock Problem

[Joe T.]

With the hot temperatures (85 - 95) driving the corvette is fine. It does not over heat. It's when I park for a half hour or so, all indications point to a vapor lock or percolation and it will not start. I was going to install an electric fuel pump in line with the mechanical pump.

Any suggestions or thoughts?

[Mike E.]

[Joe T.]

Re: 427 C2 Percolation or Vapor Lock Problem

I had vapor lock in the past where the car would stall in traffic on hot days. I resolved that issue by wrapping the fuel line from the bottom radiator hose to the fuel pump and increasing the idle. Now the only time I have a problem is after driving on 80+ days, turning the car off for about a half hour and trying to start it again. It will not start while, not touching the gas, holding the pedal to the floor or pumping the pedal. If I pump it rapidly it will try to start and backfire through the carb. I can start it by drifting down hill in second gear. I must say that if I shut the car off and try to start it immediately it will start. It is only after setting for that half hour or so on a really hot day.

[John D.]

Re: 427 C2 Percolation or Vapor Lock Problem

Wire the heat riser valve in the open position and see if that helps.

[Frank D.]

Re: 427 C2 Percolation or Vapor Lock Problem

Vapor lock is the running car suffering from fuel starvation caused by vaporization between the tank and the pump in most cases.

Percolation often occurs when heat soak on a parked car causes fuel to boil out the squirters and collect on the intake runners after 20-45 minutes; so essentially the driver trying to start the car in that period is dealing with a "flooded" car. Restarting after an hour is usually fine as the excess fuel has evaporated. When perc occurs you can sometimes hear the fuel boiling or see it dribbling down the carb throat. Shooting the carb bowls with an I/R temp gun that shows temps north of say 140* are also a trouble indicator.

Blocking off the intake heat risers in the intake manifold and wiring/eliminating the PS exhaust manifold heat riser often helps as does a carb-to-manifold phenolic spacer.

An electric "booster" pump helps with vapor lock but most likely have no effect on percolation. Here is the booster pump on my 63 car. Its only used on the rarest of occasions and only for very brief periods. Its a stopgap and has two issues, there is no rollover/crash protection unless you add an oil pressure based "cut off" switch and if your mechanical pump diaphragm develops a hole, the booster pump will keep injecting gas into your crankcase without any noticeable symptoms.

[Joe T.]

Re: 427 C2 Percolation or Vapor Lock Problem

Frank,
Thank you for the great advice. I will focus more on resolving percolation.
Joe T.

[Joseph S.]

Re: 427 C2 Percolation or Vapor Lock Problem

Joe, Try 1 tank or 10 gallons of VP 110 Race fuel. Make sure that you empty whatever fuel is in the tank now. The VP has a much higher boiling point than current pump gas. It should not vaporize out of the carburetor when the engine is in heat soak. The only way around the pump gas issue is to keep the heat isolated away from the carburetor. Not too easy to do in a BB mid year.

[Joe T.]

Re: 427 C2 Percolation or Vapor Lock Problem

Thanks Joe,
I am currently running 50% VP110 and 50% E-090. I will increase more or all VP110 to give it a try along with a couple other suggestions on this discussion.
Joe T.

[Joseph S.]

Re: 427 C2 Percolation or Vapor Lock Problem

I'm told that if your E-090 has a low boil point, mixing the VP110 will not have any effect on raising that boiling point number. I know (from experience) that the FI cars will not run correctly on a mixture. It has to be 100% race fuel. You might find this to be the same situation with your engine/carb issuses.

[Joe T.]

Re: 427 C2 Percolation or Vapor Lock Problem

Wow! I wasn't aware of that.

[Greg W.]

Re: 427 C2 Percolation or Vapor Lock Problem

Joe,
I have a 66 390hp and hade the same problem. Cold would start fine and would run fine, but let it sit for more than 15 minutes very hard to start. Pulled the front carb bowl and found the float had a small amount of fuel in it causing it to flood when hot. Put a new float and never a problem again. Just a thought. Greg

[Joe T.]

Re: 427 C2 Percolation or Vapor Lock Problem

Greg,
How did it run on the highway with the float problem?

[Greg W.]

Re: 427 C2 Percolation or Vapor Lock Problem

Joe,
When cold always started great and ran great weather hot or cold. But when hot and cut it off for more than 15 minutes good luck starting until cold. Also had a problem where the car started to run like s***. Looked over everthing, no luck . Talked to a friend NCRS member and he told me to spray carb cleaner down the vent tube in the center of the carb, ran great after that. Check the front float. Or pull the sight screw out and start it , if fuel runs out there is an issue. Good luck. Greg

[Greg W.]

Re: 427 C2 Percolation or Vapor Lock Problem

Joe,
Forgot to say if you pull the sight plug, have someone else start it and you watch the plug. If fuel runs out you have a mess. I know. Greg

[Joe T.]

Re: 427 C2 Percolation or Vapor Lock Problem

Greg,
Thanks for the caution advice.