Random power loss

[dr3ckster]

Hello Everyone,

I have a 1993 115HP Johnson Model #TJ115T attached to a 24' pontoon. It's a 4 cylinder or to say it has 4 plugs. I've only had the boat a few months. I have drained the gas, run only non-ethanol gas, changed the plugs, and replaced the water impeller pump. The boat runs strong most of the time. Really smooth. In neutral it rev's over 4000RPM. And most of the time on the water, and low weight (3 people or less), at WOT it is right at 4000RPM. But, other times its only 3000RPM at WOT. Still runs smooth. Just doesn't have the power. Doesn't want to get out of the water.

It's like it has a governor or a restriction in the leakage. But, you can put it in neutral and it's back to 4000 rpm. But under load it goes back to 3000rpm. It might do this for half an hour and its back to 4000rpm. Does it cold and hot. Temperature doesn't seem to affect.

I'm not sure what to do? Where should I start?

Thanks for your help, Stuart

 

[emdsapmgr]

Possible that the engine is losing spark to one of the cyls. When it bogs to 3000, you could remove the cowling and check the spark on each of the plugwires with an inductive timing light. The flashes from the gun will show you the quality of spark on each wire. (misfires, intermittents, etc.) When the engine loses power, try pumping up the fuel primer bulb. This is a good test for a weak fuel pump. If the engine picks up rpm's as long as you prime the fuel hose bulb, the VRO pump may be weak. Thirdly, as a side note: you are probably running the wrong pitch prop. That engine should run WOT at 5500 rpm's, when it's making normal power. Your max observed rpm's of 4000 is way too low, and you are "lugging" the engine if you keep running it at that rpm's when at WOT. Get a lower pitch prop. I would not keep revving the engine to 4000 rpm's in neutral (under no load.) One of these times a rod may go thru the block.

 

[dr3ckster]

Thanks for the response. Very good information. And now I have somewhere to start.

The last owner had just put on a new aluminum 4 blade prop. Maybe he purchased the wrong pitch. How can I determine the correct pitch? He also gave me the old steel 3 blade prop. I think I will switch it out for my next test run. See if it makes a difference. I'm going to take it out one evening this week. I'll post my findings.

Thanks so much.