HELP 4HP Mercury problem

[joellerich]

I have been working on a Mercury Model 40, 4HP ser # 3303202 I think it is a mid 70’s engine. CANNOT GET THIS THING TO RUN FOR MORE THEN 30 SEC TO 1 MINUTE, Here is some data I have collected and the repairs made to date.

Compression 110 psi
Very good spark
measured intake vac and pressure at the fuel pump connection reads +/_ 10 psi using a drill motor to spin engine w/o plugs
requires full choke to get it to run the 30 sec.

Repairs:
New top and bottom engine seals
Repaired fuel pump
New water pump
Clean and overhaul carb

ANYONE GOT ANY IDEAS WHAT TO TRY NEXT ? TO ME IT SEEMS LIKE A INTAKE VACUUM PROBLEM.

John

[Buccaneer]

If you disconnect the fuel pump hose to the carb,
is it actually pumping gas when you’re spinning over
the engine?
I rigged up a gravity feed tank once to bypass the fuel
pump on one motor, and it ran good, telling me
I had a vacuum / fuel pump problem.

Prepare to be boarded!

[billw]

Check the float needle and seat. If it is still the original set up, it will have a metal point needle and rubber SEAT. (Due, I suspect, to Mercury’s propensity to do EVERYTHING opposite of OMC.) Anyway, the rubber seat can swell to make its effective opening very small. Thus you can fill the bowl, the engine runs but the fuel can’t reenter the bowl as fast as the engine uses it. Solution is to replace them with new ones, that are rubber-pointed with a metal seat, like normal humans would make. Just one thought. I have seen it.

Long live American manufacturing!

[Author]

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