I have a 1971 corvette 350 ci with mild cam. About six months ago I installed a 4150 650 cfm Holley. Now after shutting off for 30 min or so I can smell gas and it's hard to start. I checked wet float levels and they seem to be OK.

 

After you shut off the engine look down both the primary and secondary throats to see if gasoline is dribbling down into the intake. Check immediately after you shut if off and at 10 minute intervals.

 

Yes, I checked and gas is dribbling down into the primaries about 5 minutes after I shut it off.

 

I believe a blown or malfunctioning power valve could cause extra fuel to leak during and after engine operation. There are other reasons too, i'm sure. But, I'm no Holley guy.

Edit: Maybe Vapor Lock? lol. I don't know what all has been considered up to this point.

 

a power valve doesn't cause gas to dribble in after the engine is shut off. Either your floats are too high or you have dirt/wear on the needle and seat.

 

Might have a fuel percolation problem, tends to be more of a issue in spring due to the fact that the ambient temps are rising but there is still "winter blend" at the pumps. Basically the fuel is boiling in the bowls after shut down. Normal symptom is after a hot shut down it will restart fine if restarted in a 5 minute or so window but will have a long crank if let sit longer than that. Some people have had good luck using the intake manifold gaskets with the blocked off heat riser passages and/or a heat shield under the carb.

 

A ruptured or damaged diaphragm in the power valve WILL cause fuel to run into the engine both while running and after shutting it off. Trying to start up right after getting gas or being off for a short period time and the thing starts like its flooded.

This also leads to engine that runs really fat and you get popping from the exhaust almost all the time from un-burned fuel. Greate article on Holleys http://www.hotrod.com/how-to/engine/...-basics-guide/

The ethanol content in our gas today (not E85) is like 9-10%. If the car sits alot like these tend to the gas evaporates quick due to the ethanol and then reeks havoc on rubber. My fuel pump diaphragm was shot and my accelerator pumps as well as my powervalve. Edelbrock tech told me to run Marvel mystery oil in the gas to keep oil on the rubber of my 1722 fuel pump so it doesn't dry out from sitting. Your needle and seats could also be eaten up.

I know because I just went through it. If the power valve is compromised it will leak fuel all the time until the bowl empties out. You cannot see it leaking because it leaks right through the base plate via the vacuum signal port NOT through the venturi or boosters.

I have pen light under the power valve and you can clearly see the light shining through the diaphragm in both pictures below. 3 out of the 4 windows in the power valve shown light.






The smallest of the red circles shows where the power valve gets its vacuum signal from in the main body through the base plate. If the power valve is compromised it will leak fuel all the time until the bowl empties out. Running rich and flooding the engine after being shut off. The bigger circles show where the powervalve is located in the metering plate and main body incase ur unfamilar.


Accelerator pumps are shot (wish i kept pics of my fuel pump diaphragm to show you guys.

 

Thanks to all responders. I lowered the fuel level and it solved the problem. However, when I ran the A/C for an extended period of time and then shut it off the hard starting returned. I lowered the fuel level again and it seems to be OK. What problem will I encounter if I lower the fuel level too low?
Also, after shut off it sound like the gas is boiling in the fuel bowls. Would a phenolic insulating carb spacer help?