I can't find overheating cause....:(

[new2obrepair]

I posted some time back on my overheating problems on a 1984 yamaha 15hp 2-stroke. model #15SN. So far, I have changed and rebuilt the water pump, changed the head gasket and the exhaust gasket, put in a new thermostat, and worked on water jacket, rebuilt the carburetor, had some debris and small stones in water jacket. I just discovered the water inlet strainer has been destroyed, only fragments left. But I thought I would have flushed any debris out of the head and lines. I also found the pick up tube had a rubber boot that was pushed over the end of the metal tubing about 3/8". So I trimmed that off thinking it was folding over under pressure. Put motor back together, and ran in tank of water. good pee stream at idle, sputters and stops by 1/3 throttle. Any ideas?

 

[vinney]

Re: I can't find overheating cause....

If you have good water pressure at idle and it goes away. It sounds like there is an exhaust leak to the water passage some where. The water pressure is greater at idle and when the rpm's come up on the engine the exhaust pressure is greater so the water pressure is stopped. Check Head gasket leaking,base gasket, or hole in water tube.

 

[new2obrepair]

Re: I can't find overheating cause....

head gasket and base gasket both are brand new, torqued properly. old gaskets did not show signs of failure either. So I will remove and check the water tube, which means the head comes off again anyway. I will also remove the t-stat and pressurize the system to 30 or 40 psi backeards to seee if I can find a leak in the head. Wouldn't it be difficult to find a leak to the exhaust? I tried to not take apart the powerhead any more than neccessary. But the water journal is only on the combustion side anyway. I did replace the lower oil seals, there was no water in the cam shaft area, which right now is very re-assuring! Thanks for the suggestion, I will work on it tomorrow. then I must work for 8 days strait. one day off, one on, 1 off, 4 nights. no real time to work then. hope I get it tomorrow.

 

[jim dozier]

Re: I can't find overheating cause....

I reread your earlier post. Does this engine have a removable exhaust cover plate? If yes, when you had the power head off did you remove it and check for leaks underneath? Sometimes a exhaust cover plate can corrode through and cause a leak there. That might be consistent with your description of a change in the exhaust note.

 

[Yepblaze]

Re: I can't find overheating cause....

As you run in a bucket,<br /><br />At idle most of the exhaust comes out the exhaust bypass above the water.<br /><br />But as you rev it up most of the exhaust comes out the prop right next to the water inlet. This can cause the waterpump to ingest copious amounts of air (exhaust) instead of fresh water.<br /><br />Try it on a boat while going forward.

 

[new2obrepair]

Re: I can't find overheating cause....

Hi all, thanks for the suggestions. Here is the latest...<br />I did pull the exhaust plate, replaced that gasket, no leaks there. also replaced the thermostat, no leaks there. first test failed after all work was done. So I backflusshed the motor through the t-stat water journal, foot removed, with my garden hose. Flushed out some more stone-sand-mud...and...3 whole kernels of corn??? Can you believe that? put it back together, ran well in test tank for almost 30 minutes, by then pisser trickled down to near nothing. DAMN, NOW WHAT? I took it back apart, backflushed and nothing came out. Put water up thru the tube without the foot attached, good flow from foot thru the motor. checked the "NEW" water pump rebuild, it looked ok. put foot in tub alone, ran with my drill motor, started with good strong stream, then flow gradually reduced to trickle? This is very aggravating, but I think I may be close to solving the problem. This motor has the water inlet under the cavitation plate above the prop. the cover screen somehow was destryed, and I wonder if the inlet tube has a missing part that should come all the way down to the screen if it was there? I have ordered the replacement screen, but the water inlet may simply be to high in the recess and maybe I am experiencing some cavitation? It makes sense to me, can anyone confirm this for me? The tube that is there stops about 3 inches above the cavitation plate. That seems like a long pull without assistance for such a small pump. THANKS AGAIN for all the help.

 

[new2obrepair]

Re: I can't find overheating cause....

please see my newest post just below this one

 

[rickdb1boat]

Re: I can't find overheating cause....

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