Yamaha WR650R Backfires thru Exhaust

[canal_guy]

1993 Waverunner LX. I started the motor with the pump unit removed. It started normally and then died right away, which is not uncommon for it when it is cold. I cranked it over again and instead of starting it began backfiring through the exhaust. So bad that it blew my water box apart. I replaced the spark plugs, checked the index mark on the stator plate, and even put the CDI unit from my 1993 WRB700R in. None of this made any difference and it still will not start, it only backfires LOUDLY through the exhaust. Any suggestions? neil@hctc.com

 

[Jeff Walkowiak]

Re: Yamaha WR650R Backfires thru Exhaust

getting raw fuel in exhaust must have a bad plug or wire or lack of back preasure now it has no exhaust system I had a kaw blow the water box to pieces when I tried to start it with either, one plug was not firing raw either went in the exhaust and the other plug fired up and ignited the either in the exhaust quite an explosion,, repaired the water box and found it was plugged up from sediment and in any event once I fixed the box it ran like a top if you have too much or not enough back preasure the engine will not run or it will back fire like crazy i can tell you where to get a water box for the yamaha if you need one the initial back fire was probably raw fuel in the box

 

[canal_guy]

Re: Yamaha WR650R Backfires thru Exhaust

Thank you for the suggestions Jeff. I put new plugs in, they appear to be sparking evenly and strong. You mentioned lack of back pressure. The motor still has the exhaust system on it, that dumps into a rubber hose which goes to the water box. I think the water box keeps raw water from flowing backward into the exhaust manifold. That water box is what was "blown up"...smile... Do you think that would cause low backpressure? How do I test for low back pressure? Thanks again, Neil

 

[Jeff Walkowiak]

Re: Yamaha WR650R Backfires thru Exhaust

if it blew apart you have low preasure if its plugged up you have high back preasure check it visually and see if its blown apart or still intact, did you use starting fluid when it blew the first time,,thats either.your post said it blew apart, with no load on the engine "pumpremoved" and no back preasure, its probably not going to run, 2 strokes need back preasure to operate properly I would leave it alone and fix the box and put the pump back in before messing with it any more. it will probably run fine once its back together.

 

[canal_guy]

Re: Yamaha WR650R Backfires thru Exhaust

Thanks again Jeff but I just answered my own question. I did a compression test and it was 55 & 40. Put oil in cylinders and compression came up to 155 and 45. Pulled cylinder and found broken rings and 1 broken piston. Just a coincidence that it happened after I took the pump out I guess, I think it was going to go next time I started it no matter what. Thanks to all who thought about this.

 

[Jeff Walkowiak]

Re: Yamaha WR650R Backfires thru Exhaust

that explains the fuel in the exhaust.