15hp Yamaha strange problem

Rudderman · Mar 7, 2005

Its a 1998 elec. start 15hp four-stroke engine. I took her out yesterday for a spin and she never missed a beat, purred like a kitten. Today I took her out, she started ok, I let her idle for about a minute with the choke in, then I did a short circuit at about 4000rpm around a little island (only a few hundred yards). I throttled back and went to neutral to look over the back to check which hole I had the trim set to, and suddenly the engine conks out. When I tried starting it again, it would only run (roughly) with the choke pulled out and I couldn't shift into gear without stalling. At idle, the minute I put the choke in, she conks out again. I checked and the vent was open on the tank.....any ideas whats wrong? The fuel should be fine since I had stabiliser in it.

JB · Mar 7, 2005

Re: 15hp Yamaha strange problem

You seem to have lost fuel feed to the engine, Rudderman. Could be the fuel pump, carburetor or that insane mad scientist rigamarole that controls mixture. You can confirm this by squirting some fuel into the carb throat while it is bogging. Good luck.

Grant S · Mar 7, 2005

Re: 15hp Yamaha strange problem

If it has a fuel filter it could be blocked.

Rudderman · Mar 7, 2005

Re: 15hp Yamaha strange problem

Thanks I'll try swapping the plugs, squirting fuel in the carb, checking the filters and see what happens.

classic 24 · Mar 9, 2005

Re: 15hp Yamaha strange problem

if it only runs with the choke out, you probably have a dirty carb.

Rudderman · Mar 13, 2005

Re: 15hp Yamaha strange problem

I dismantled the carb, cleaned it with carb cleaner, blew it out with compressed air and reassembled it. Then I dismantled the fuel pump and everything looked new in there with no dirt or wear. The spark plugs are sparking on both leads, colour of the spark plug tip is white/gray/brown. When I start the engine now, I have to give it lots of choke to keep it going, then when I get it revved up to say 2 or 3000 revs, and then go back towards idle, it just wants to die. If I give it more throttle, it will stay going but theres still something not right.

rodbolt · Mar 13, 2005

Re: 15hp Yamaha strange problem

something is blocking the carb passages. by choking all you do is force fuel from the carb bowl to the intake bypassing the normal ciruits in the carb. if it stays running with the choke pulled it means there is fuel in the bowl. if the leakdown test is good on the motor I will bet on a carb problem. what is the model # of that little beast ?

Rudderman · Mar 14, 2005

Re: 15hp Yamaha strange problem

I found the problem.....somehow I managed to swap the plugs with an old set of faulty ones. But when I swapped them for some brand new plugs, the problem is gone. Any ideas why this is happening? This is the second set of plugs I've changed in less than 30 hours engine use (over a 12 month period.)

rodbolt · Mar 14, 2005

Re: 15hp Yamaha strange problem

why do you save faulty plugs ? my grand father did but he was old and even washed and saved soupcans. what do they look like when they go bad?

Rudderman · Mar 15, 2005

Re: 15hp Yamaha strange problem

I dunno, I suppose I could make some sort of spark-plug sculpture out of them some day

They looked pretty new, black coloured but not wet or sticky...more of a light powdery black colour, no big deposits or anything.

Rudderman · Mar 20, 2005

Re: 15hp Yamaha strange problem

Ok I'm still having problems!! Here is a more precise description of the problem: When I start the engine from cold (at idle & choke out), the engine starts and runs ok for maybe 5 seconds and then dies (as if it ran out of fuel or something). If I play with the choke (in--out---in--out--in--out), I can get it to stay running at idle, but the moment I apply throttle, it cuts out. This problem only occurs when the engine is cold. I know because if I reach in and manually push the throttle lever on the carb to rev it up to 4000rpm for about 2 mins, so the engine gets warmed up properly, the engine runs absolutely perfectly at all throttle positions. Tops out at 5000rpm and 26mph and will run all day afterwards with no problems and without missing a beat.

Rudderman · Mar 23, 2005

Re: 15hp Yamaha strange problem

bump^

31900 · Mar 24, 2005

Re: 15hp Yamaha strange problem

Take another look at what rodbolt has said about the carbies. It's odds on that you have gremlins or in your case those green small people in the carbies.

rodbolt · Mar 24, 2005

Re: 15hp Yamaha strange problem

hello try a higher proof fuel

. have you tested the t-stat? and carbed 4 strokes take a few minutes to warm up and stabilize. its just how it is. but I still suspect something is not quite clear in the carb.

Woosh · May 24, 2005

Re: 15hp Yamaha strange problem

Have very similar problem with 2003 Yamaha 8 hp electric start and trim. Just posted in this forum as a "choke problem". I hope rodbolt reads "it" because your problem seems similar to mine

15hp Yamaha strange problem

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