overheating 165 mercruiser

At the end of the line here - need ideas on what to look for?

Symtom : with muffs on engine maintains around 140deg, can run at 3000rpms and engine will remain cool with or without the thermostat. Put the boat in the water and below 1500 rpms engine temp will rise to 160deg with and with and with out thermostat. When I take it up to 3000rpm and better the engine temp steadily rises(10 minutes) to 190-200, slow down to below 1500 rpms and down comes the temp to 180, does not fall back to 140-160. While engine was running pulled hose connection between heat exchanger and exhaust riser to check for flow, was getting around 4-5 gpm(used a jug and timed it). Even removed thermostat on closed side of system no change in symtoms -

I have a 1987 merc 165 hp that is a 224 cid inline 4 cylinder with 4" heat exchanger I believe full cooling by way of exchanger. Have replaced points, plugs, cap, rotor, condensor and timed to 8 deg btdc(timing stamped on engine), was timed at 4deg btdc(service manual calls for this).The timeing is an issue for me, I was told that this could cause the symtoms i am seeing, called merc and their tech indicates due to fuel changes that I should go with the service manual settings i would like to here from people in the field on that one. Additionally on the timimg my engine seems to run better at the 8 deg btdc setting as to the 4 deg btdc confused on that one. Also replaced impeller in lower unit, replaced thermostat, verfied that i do not have any blocked passages in exhaust manifold. Pulled heat exchanger and checked it as well, clean.
did not mess with engine water pump at this point, I also heard that I could have a head gasket problem here, not what I want to deal with. i would think that I would be losing water on the closed system side and am not.

Anyway suggestions and comments greatly appreciated

Jim

Re: overheating 165 mercruiser

Hello

How did you rule out the risers? okay at low rpm and bad under way is classic riser sign. Can you put your hand on them? How hot are they getting? Did you check the flappers in the y pipe? I know you said you have good flow going into the risers but what about out of them?
Thats where I would start after all the obvious other places.

While timing may be an issue thats easy to test, overheating at high rpm is usually associated with water flow.

Re: overheating 165 mercruiser

I had one simular,turned out to be gasket located between upper gear housing and upper water tube grommet housing.Housing gets distorted from overheating allowing exhaust in, on plane, but cools when not on plane.