I pulled the cylinder head and it looks very clean, same thing with the water jacket around the cylinders. I suppose I could pull the exhaust side to make sure there’s no clog there, but given the fact that the cylinder head isn’t getting hot, but the cylinders are, that is what makes me lean toward the ‘lean fuel-air’ condition.

I was consulting a champion spark plug guide (not saying this is the bible or anything), and it listed air leak at the manifold as a culprit. All the other explanations were things I knew I have gone through already. I’ll pull the exhaust cover because I’m reassembling everything right now so it’s easy. Was getting ready to remount the powerhead after resealing the crankcase – that thing was clearly leaking gas/oil out of it after I split it in half.

If this doesn’t solve the problem and I don’t find anything on the exhaust side, I am thinking maybe it IS running lean at the carb setting. I’ll double check my link n sync job, but I’m pretty sure both are OK. First motor I’ve seen do this, particularly these early 70’s ones which run rich usually, anyway.

no, it was in good condition. I replaced it as this motor is being used for an emergency vessel, so pretty much everything is brand spankin’ new.

ONE THING OF NOTE. I installed a weedless prop on this motor – is it possible that it is running at too high an RPM at WOT causing this overheat condition?

Everything else about it seems to point to a lean fuel condition.

I just got done running the motor sustained at 5500 RPM in my test tank with test prop for 5 mins on 2 separate trials. This is only bout 3/4 throttle in the tank with the wheel. I found that it was still running a bit hot, not on the temp gun (stayed at 145-150° both cylinders), but by pulling the plugs and examining both those and the pistons. The pistons were somewhat wet on the intake side, but nearly ‘white burnt’ dry on the exhaust side. The plugs were pretty dry too. I put brand new NOS J6C’s in for the 1st of the 2 runs.

The 2nd go around, I put J4C’s in. Motor ran a little cooler after 5 minutes of sustained 5500 RPM. I looked at the ’71 service manual and it called for J4C’s. Pulled the J4’s, and the pistons and plugs had a minute amount of fuel moisture. I am guessing initially that the J6’s are too hot for this motor for sustained WOT or high RPM.

I’m going to conduct the test out on my test boat under load in a few minutes. This is a motor that will probably be run at WOT for long sustained bouts since it will be used in an emergency rescue application.

I tried richening up the low speed end, but doubt that is the culprit since the high speed jet is fixed on this unit.

After spending 90 mins on the water with the motor, my explanation for running hot is that it was probably the plugs. The J6’s were 1 range higher than the OEM recommendation of J4’s. Granted, this motor is going to be run at WOT quite a bit due to it’s application, but I guess the J6’s are really just for the put-put trolling fisherman with a few short WOT’s on the day. It was only running hot after sustained, 2-3 min bouts of WOT. And I’m talking 170° not super hot, but it would fall back to 145° reasonably quick. What alarmed me is the J6’s a pulled yesterday after using the motor for 90mins were crusty and had blisters on the tips. The cylinders were bone dry too. That was unusual. I ran a heat gun on it yesterday and today and closely monitors the head, cylinders, and thermostat cover to observe trends.

My boat is not lugging. It’s me on a 14′ aluminum starcraft with 180lbs of ballast at the bow. Gets up on plane quickly and I have the motor trimmed a couple of holes to keep it from sitting too low on the transom (long shaft motor, mini-jacker plate installed, ventilation plate is roughly even with the bottom of the boat give or take an inch).

I did switch to a regular prop and didn’t see a significant change in top end speed or RPM’s. I did remove the thermostat and normally wouldn’t run motors without a thermostat, but in this application, the motor only lost about 10° of running temp without the thermostat (115-135, previously 125-155~170 at sustained WOT). It ran fine at idle around 100-120° while at headway speed. So not too cold.

Maybe I was just chasing a ghost with this one.

Jeff Register: all of those things you brought up I already went through previous to the post.

Side question – the motor did develop a minor issue which I wanted to pick your brains about. The shift lever isn’t engaging the fast start safety lockout either at the rear of the manual starter (the black plastic piece which rotates to stop the manual starter if throttle all the way up – prevents accidental starting at WOT), and the shift lever which runs down through the pan and interacts with the actual shift handle to the gearcase. It isn’t hitting that brass ‘fixture’ that is attached to the mag plate, or just barely rubbing. I had this happen on a 20hp of the same era – any suggestions on how to fix this?