I just picked up a 1988 Formula 206 and it has the 320HP MPI small block in it.I bought the boat knowing it had a knock,but I can't pin point what it could be.It starts and idles fine,a little rough, ans revs to 2000 rps with no problem.Over 2000 rpms it develops a knock sound but not as loud as a rod knock.It has 70 psi oil pressure and if I rev it up fast there is no noise at all,weird.I pulled the drive off to make sure it was not a bearing or something.I pulled a valve cover off and no loose rockers,motor only has 87 hours on it.Any ideas before I pull motor tomorrow???


Thanks Nordic95

sounds like a rod bearing.

Wouldnt that lower oil pressure??


Thanks Nordic95

Hello,
just went through crap like that. You might consider taking off your oil filter, check the inlet brim or cut it open to see if you can see those little sparkles or a hint of metal dust in your oil. Some times you can pull the dip stick out a see sparkles in the sun light. My oil pressure was pretty good but I didn't let it run very long to get really warm either.

Is there any place like Summit Racing for car parts but for boats parts? Like with Summit you could order a complete rebuild kits with all gaskets and bearings for a specific engine.I would like to find a place for marine engines except the normal places that you have to order individual pieces.


Nordic95

All the gaskets marine to auto are the same, except the head gaskets. and the last time I bought head gaskets for a big block, the fel pro marine and street were the same.

I had teh same issue that you have years ago on a SB, and it too was teh rod bearings. didn't affect oil pressure at all.

I think the closest thing to Summit for marine parts is CP Performance.

Have you done a cylinder kill test to try to link the knock to a specific cylinder?

Cutting the oil filter open to check for metal is not a bad idea, but what about having an oil analysis done? That would tell you what type(s) of metal you have in the oil and help you determine what might be causing the knock.

Dave

I'd do a compression test, a small block marine motor I looked at last year had a strange intermittent knock, it was a head gasket blown from a overheat.

Other thing I have seen is when they have the crank mounted raw water pump. (Volvo type) the rubber bushing was gone from the stay bracket and it made a horrible knock at low rpm's.

I may be going out on a limb here - just going from what I;ve heard - but I think rod / rod bearings issues aren't uncommon in these motors. Still have to find out what your real issue is of course.

Could be a wrist pin