98 yamaha 115 c series overheating

Hello forum

Just recently picked up a yamaha 98 c115 outboard.. compression is 125 across all cylinders.. have had it in the water 3 times and 3 times my high temp alarm has gone off while at high rpm.... checked tstats and it did not have any... tried poppit valve and it was missing also... only had spring and grommet... not sure if s valve melted or it was removed... could poppit be cause of my high temp overheat........

Nowhere near enough information from you yet.
At what RPM does the engine overheat?
Is it a salt water engine or has it ever been in salt water?
What boat is it on? How high is it mounted? Is there anything (transducer) mounted anywhere in-line or close to in-line with the engine?

To answer part of your question...YES....the engine does need the thermostats and pop off valves.
By removing all restrictions there is a chance the block will not completely fill.

You will need to start with the water pump. On inspection, if you find grooves in either the cup or wear plate that you can catch a fingernail in then replace those as well. Yamaha is funny about their pump kits. Basically you can order all but the housing if you want. If the cup is melted into the housing then the complete pump assy will be needed.

Now....on to the fun stuff with that engine. This is a last but not least scenario however you have bought an engine that someone else tried and failed to fix.
There is a good chance you will need to pull cylinder heads and remove the build-up of sediment and whatever "Jelly" feeling crud you may find that builds up in them.
Yamahas for some reason seem to be prone to build-ups and subsequent overheats where other manufacturers are not.

Boat is on a 20 ft jon boat .. cavitation plates its perfextly even with bottom of boat... do have a transducer. Have placed closed attetion to water pickup while on plane... skegss seems low enought for water pickup no to be a problem... i replaced water pump however old one did not show much wear... stats amd poppit housing do show some crud. However i dont think passages are completely clogged.. im really hoping its poppit valve.. if i understand correctly if vavle is stuck open water is by passing block exiting thru back of motor and therefore overheating at high rpm....i would imagine no valve is just as valve stuck open.... i will start by setting vavlve and give it another try. Pee stream seems a little weak. And at idle i do see plenty of water out rhe port holes on top on engine...ive had a 40 hp in the past and i recall stream beeing alot more strong. I have heard on acid flushes thru the tstats with lower unit off.. im still pondering on that idea...

This was the only thing rhat was found in poppit valve... im leaning towards someone just removing it. Unless vavlve just melted away... then again if someone removed valve why not remove spring also

from the looks of things, (even that spark plug) I would say this motor has sat for a long time

I see someone wrote the compression on the head, is this motor from a boat junkyard of some kind?

Might just be all scaled up inside

Not tha i know of

what is the history of the motor?

who wrote the 125 on it

Im not saying that the motor is junk, just that it may have been sitting around a long time and its getting corroded inside

Has anyone ever heard of acid flush

Will be installing a poppit valve this weekend and ibwill try it out... hopefully thats my problem...

Picked up some muriatic acid to try to flush thru stats. With lower off.. i couldnt get myself to do it. Ive seen some videos of how strong this acid can be... would poppit valve missing be the cause of overheating at high rpm?

Installed poppit valve and new spring. Have had it in the water 2 times now.. no alarm....... cost was around.. 20 dlls for parts....