Strong Smell of Gas from Exhaust

[Jack O.]

Another issue I've had is that when I first start the car the exhaust reeks of gas but only at idle and mostly when it's first started. The engine is an L71 however not with the original cam and with very low compression because it was rebuilt with a 6-71 blower. I removed the blower and put a single 4bbl carb on it. If I start it in the garage with the windows and doors open my clothes and hair will also reek of gas fumes. Could the low compression cause the strong smell due to incompletet burning of the fuel? I've adjusted the mixture to make sure it's simply not too rich. I posted a separate thread about a popping sound from my exhaust and was also wondering if the two could be due to the same issue.
Thanks,
Jack

[Jim T.]

Re: Strong Smell of Gas from Exhaust

With a Holley carb the power valve can ruputre and produce what you describe. Quick check for a failure of power valve, start your Corvette, remove the top of the air cleaner, put a finger over the front fuel vent of the Holley. If your car dies quickly after holding your finger on the vertical front fuel bowl vent, the power valve needs replacing. If your engine continues to run for awhile after putting a finger on the fuel vent, the power valve is OK.

[Gene M.]

Re: Strong Smell of Gas from Exhaust

Jack,
Just suggestions, very high overlap camshaft, leaky exhaust valve, carb leak (as mentioned #2), failed hydraulic lifter, valve set too tight and holding valve open, and bad exhaust valve seat. Is there any smell of gas in the oil?

I'm sure the guys will suggest other options. Assuming power valve is ok as per #2 looking else were. If every thing was fine before I would look for a valve held open as that is the easyiest.

Low compression and big camshaft are not a good combination.

[Duke W.]

Re: Strong Smell of Gas from Exhaust

Cold starts on carbureted engines require a lot of excess fuel, so cold start fuel odor comes with the vintage car territory, but you should look at the choke system.

It sounds like you have a lot of non-OE components on your engine, and getting everything to work together can be nearly impossible for someone who doesn't understand the issues. Back in the day GM had a lot of "calibration engineers" who experimented with distributor and carburetor calibrations to achieve good cold start, drive-off, cold and hot transient response - in general, "good driveability" under all driving conditions over the full range of climate conditions where the cars were driven. This was no easy feat, but those guys had a lot of experience and knew what they were doing.

Assuming you have a basic understanding of how choke systems work, adjusting choke spring tension and/or the choke vauum break to lean the after lightoff mixture should mitigate the fuel odor.

Duke

[Chuck G.]

Re: Strong Smell of Gas from Exhaust

Although I know NOTHING about blower motors per se, I do know that blower motors are built with very low compression, maybe 6:1-7:1-8:1, depending on the boost provided by the blower. Thus, a motor built with 7:1 compression, running a blower with a 2:1 boost would "see" a compression ratio of 14:1...

I can't help but think that this is part or all of your problem.

Chuck

[Jack O.]

Re: Strong Smell of Gas from Exhaust

Thanks everyone. I'll poke around a bit with the suggestions but would not be surprised either by the fact that it's only going to run so good when it was built around having a blower and I simply removed it. No time/money for a rebuild now but that may be my only real soultion. Just trying to get it running as well as possible now.