Mercury 4hp 2cyl Gnat 1979

by Bruce Hood
(Chelmsford UK)

Starts OK except when I run the tank dry when hot (on the water) Then difficult to start and requires 3/4 choke to run at all for many minutes.
I've done all the usual checks carb cleaned, manifold leaks, reed valves replaced, fuel filters replaced. Gives the impression of low crankcase vacuum, but only when its been allowed to run dry.
I have two of these engines a year apart. They both exhibit the same behaviour! Old engines but only medium usage. I'm stumped by this one!

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Jun 22, 2021
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by: Bruce Hood

Logical to think so. However, I have replaced both on both engines. Pump fuel pressure measures in spec (instrumented on the water). Also a long time ago I added a carb float bowl fuel level indicators so that I am always able to see that the fuel level is correct in the float bowl.
Both engines run badly with correct carb fuel levels only in the circumstance I have described.
Otherwise everything is peachy and very smooth running. As I said, this is a hard one.
Thanks for your suggestion.
Jun 22, 2021
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by: Anonymous

It needs a new fuel pump and filter.

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