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.

I’m using 100 octane low lead avgas, as recommended by many experts, as it doesn’t deteriorate. I’ve tried holding the throttle to the floor, choking, etc. but it is difficult to start. If I take off the air cleaner and give it a squirt of ether, it fires right up. Once it warms up, it runs fine, except a slight miss on acceleration, which occurs in a narrow band of rpm.

Restarting is okay, but sometimes a bit worrying. I have retarded and advanced the timing a bit, but that doesn’t seem to help. I’ve checked and rechecked everything and have not been able to find an answer. I’m hoping you might have a recommendation.

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.

Cold-starting isn’t easy for most cars, and perhaps one with a Blue Flame inline-six custom-engineered with sideways-mounted carburetors (ensuring fitment under a C1 Corvette’s sleek hood line) has an even harder go of it. Talk to the same expert that set up those stunning carbs for you; I suspect installing a high-quality fuel pressure gauge in a discreet location is likely your next step.

Your Corvette needs roughly 5–10psi of fuel pressure to get that motor running. There’s a chance that your fuel pump isn’t up to the task. Most (all?) mechanical pumps have rubber diaphragms that fail, just like the stuff inside a carburetor. While aftermarket electric pumps are better for drivability, your Corvette looks too original to consider anything but a factory-style fuel pump.

So check the fuel pressure and consider installing a gauge in a remote location that can also be removed without ruining your Corvette’s originality.

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