Mercruiser 4.2 V6 Petrol - 2002 Carb

[Who]

I have twin:
Mercruiser 4.2 Petrol Carb, not efi.
190bhp each
Via Bravo 2's
Manufactured 2002
Single Fuel tank feeding both engines

Up until now perfect running

However, recently when running at speed, say 25knts, I get a missfire on Port engine ONLY, that sounds like a backfire every minute or so.

If I slow down a bit it seems to go away. Sometimes it can vanish for a mile and then come back on its own.

Any ideas?

 

[timtap]

Sounds like a distributor problem to me. cracked cap , carbon build up inside cap . plug leads, rotor arm....

 

[Who]

Thats what we thought, but I have also been told to do a compression test to see if its a sticking valve. (boat is used every few weeks)

Has anyone heard of this before?