Yamaha 15SH 1987 backfire/pop

[bb_ed]

I've got a 1987 Yamaha 15hp that pops or backifres when I let off the the throttle sometimes. If I'm not mistaken this particular ignition fires twice per rev. If that is so, would it normally be expected to backfire occaisionally when you let off quickly given that there may be extra fuel in the crankcase? It idles sweet and operates OK otherwise.

 

[Tom Scully]

Re: Yamaha 15SH 1987 backfire/pop

I have a 40hp that had the same symptoms, which slowly got worse.<br />It turned out to be a blown lower crankshaft seal.

 

[jim dozier]

Re: Yamaha 15SH 1987 backfire/pop

2-stroke backfire through the carb is more commonly referred to as "sneezing". It is usually caused by a too lean mixture. They aren't good for the engine and they probably caused the blown crankcase seal referred to above.

 

[bb_ed]

Re: Yamaha 15SH 1987 backfire/pop

OK, Tom. I'll see how it goes...if it continues or gets worse I'll check the bottom seal. Good thing it's an easy one on this particular motor. It is definitely not lean popping (sneezing) as it only occurs when I let off the throttle from high speed. I'll post a message eventually to let you know what happened.