Yamaha 150 engine miss

[Key west Tom]

I'm working on a 1987 Yamaha 150 that has a very bad miss at idle and at higher speed. It has good compression on all cylinders and I can't pin it down to which cylinder is missing by pulling off plug wires. Any ideas about how to attack this problem?<br />Thanks,<br />Tom

 

[FHARVELL]

Re: Yamaha 150 engine miss

This sounds distinctly like a carb problem. Has the engine set up? Gas turns gummy after six months or so. Also has the linkage on the Carbs been tinkered with? It has to be set just so to allow for idle and high speed operation. If your carbs are gummed up, it's a good chance you might have problems in the fuel pump and filter too.

 

[Key west Tom]

Re: Yamaha 150 engine miss

All three carbs were removed and cleaned (they were not dirty) also checked fuel pumps they also were clean. also ran from other temp fuel supply. This motor has some type of a "save the motor-limp home" mode that cuts the max RPM my killing cylinders doesen't it? Could it be a problem with that? If so, how do I test? This was a problem that just started suddenly, but I was not in the boat at the time (not my boat)<br />Thanks,<br />Tom

 

[rodbolt]

Re: Yamaha 150 engine miss

if you suspect an alarm then test them. turn the key on. dissconect one of the pink wires on the cyl head. grond the harness side. the alarm horn should sound. if it does the circuits and horn are ok. what kind of compression numbers are you getting?<br />are you getting any visuaal alarms?

 

[Key west Tom]

Re: Yamaha 150 engine miss

The alarm system works, I was wondering if the module that cuts cylinders out could be bad. no visual alarm lights. I can't remember the compression numbers, but they were all good and even, that is they were well within the specs in the book. <br /> I have never seen this motor run with out problems, my neighbor bought it used and with this problem. When I first started working on it (running it with a hose and muffs) it ran hot, no water comming from the pee hole.Alarm system worked then also. We checked pump,t-stats, and poppet valve. Pee hole was cloged w/mud, poppet valve looked like it was held open with corrosion--cleaned that out now no more over heat compression still same(good)