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Hagerty Employee

Piston Slap: More fuel for a C1’s true Blue Flame?

Harold writes:

My ’54 Corvette starts hard. The carburetors have been rebuilt by an expert in the field, using modern rubber materials. The distributor is also rebuilt and checked for operation, including the vacuum advance. The timing is set at the factory mark on the flywheel, which is zero degrees advance, and the spark is strong. I don’t believe there are any vacuum leaks ...


Sajeev answers:

Thank you for your detailed description of the problem. Your diagnosis so far sounds spot-on—you are certainly on the right track … and wow, what a beautiful restoration!


My concerns are threefold: Are the choke lever’s motions translating into choking inside the carburetors? Is a smoke test is needed to verify the absence of vacuum leaks? Most importantly, what’s your fuel pressure? Let’s focus on the final issue ... Read the full article on



I'm not a fuel expert, but I would guess that Av gas would burn much slower than gasoline and require advancing the timing at least a few degrees from the factory setting. Actually, modern gasolines require this also. I'd advance timing 5 degrees and start there. Check that the choke is fully closing when you pull the cable, and also check that the accelerator pumps in the carbs are delivering fuel when you pump the throttle. 3 carburetors are 3x the trouble to get right, so getting them all sync'd to run properly can be tricky.

Community Manager

That is excellent advice.  Didn't even consider that! 


I agree on the Av gas advice.  It can be troublesome on cars, on the ground.  It does burn slower that normal fuel.  Here in Minnesota you can get 91 or 92 octane non-alcohol gasoline for collector cars and other implements of destruction.  That works well and is very stable.


I still use the Sta-bil product when I store vehicles for more than a couple months.  It works well.  Good thoughts, Sajeev, on the fuel pressure!  That is often overlooked, both in carbureted, and fuel injection situations. 


Since this is a cold starting problem, carb engine and mechanical fuel pump, I don't think that's the problem.  Av gas doesn't vaporize as easily as regular automotive fuel.  So, even if you get the full choke, the fuel won't vaporize enough to allow combustion.  In fact full choke might be a hindrance, because not enough air flow is available to help vaporize the liquid fuel.  My recommendation is, back to regular fuel (non-oxy if it's available) and most of the problems will go away.  The acceleration misfire could be a fuel problem also, (my thought is it's too  lean, but let's get it starting first then tackle resetting the a/f ratio and possibly re-jetting the carbs till after the starting is fixed).


There's a quick way to check if atomization is the problem.  If you take a little fuel from the tank, put it into a small squirt bottle or mister bottle, then set the spray nozzle for mist.  Take the air cleaners off the carbs, open the chokes, spray just a stroke or two into the carbs.  Close the chokes to about 90% and see if it starts correctly.  


If that works, the Av gas isn't atomizing enough.  If it kinda works, try the same experiment with fuel from your lawn mower.  If that works then it really is a fuel problem and it's time to update your fuel.