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Gary, Australia Car Mechanic
Category: Australia Car
Satisfied Customers: 916
Experience:  Senior Technician
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350 chev in a boat, was running fine but now runs up to 3000

Customer Question

350 chev in a boat, was running fine but now runs up to 3000 rpm then drops off. It occurs when the secondarys open, there is also a deep throated sound when free revving if all 4 barrels are opened quickly. It has no vacuum advance of course, weights are free, timing is good at 8 BTDC. I put a pressure guage on the fuel pump outlet line and really could not record any pressure other than the 1 - 2 psi from the electric feed pump. I believe the pump may have a small hole or tear in the rubber diaphragm. Your thoughts?
JA: When did you last fill the tank? And do you smell gasoline around the engine?
Customer: last filled with 98 premium very recently and funny you ask about the smell. It never has but during sea trials yesterday my deck hand did state he smelt gas fumes but no signs of any leaks.
JA: Are you fixing your 98 yourself? What have you tried so far?
Customer: gas from the service station direct from the bowser pump. carby overhauled, fuel water additive in tank, new filters, cleaned out all fuel deliver lines. The hesitation on this engine is not present on the other engine fitted next to it and both draw from the same fuel supply. The hesitation when full throttle is applied is not present on the second engine either. New points, leads, cap rotor etc. Engine fully rebuilt 200 hours ago and was running great before 3 month slipping earlier this year.
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 8 months ago.
Category: Australia Car
Expert:  Gary replied 8 months ago.

Hi there,

What carburettor?

What brand and model of fuel pump?

Do you have electric pump only, or does this pump supply a mechanical pump on the engine?

If has a mechanical pump, then you have a ruptured diaphragm, and there could be fuel in the engine sump.

Can you please check the engine oil level to see if it is excessively high?


Customer: replied 8 months ago.
Carby is Rochester 4 barrel quadrajet, fuel pump is AC Delco 40778 mechanical. There is an electric pump in-line that feeds to the filters then fuel is drawn to mechanical pump via suction. Will check sump and get back to you.
Expert:  Gary replied 8 months ago.

Ok, sounds like the mechanical pump is your problem.

Would also explain your fuel smell.

You may smell it more when you remove the dipstick and oil cap.

Do not run the engine any more until we sort this.


Expert:  Gary replied 8 months ago.

Hi again,

How did you go with your problem?

I hope my advice has been of assistance to you.

Kind regards