F25 runs only on one cylinder

[kennethhigh]

My 2003 4 stroke F25elhb lost power suddenly last week. I was running at 3/4 throttle and backed off the throttle to coast into shore and the motor stalled, then would't start. With much fuel bulb pumping the motor finally started but ran only on one cylinder. I have pulled plug wires and the engine starts with the top wire off. I have changed the starting coil but no help. Changed plugs, nothing. The timing belt is loose, but there is no adjustment. At this point I am operating on the theory that the timing skipped a tooth and that is why there is only one cylinder firing.
Anyone have advice on diagnosing the problem, or replacing the timing v belt, like an easy way to accurately locate top dead center on the # 1 piston?

[99yam40]

Dial indicator set up in #1 plug hole is the best way to find TDC

A lot of 4 strokes are non clearance motors, if they jump timing it bends valves. I do not know if yours will do this or not, but you can check compression or better yet do a leak down test on all cylinders to see if they are sealing properly

pick up a Yamaha service manual for your motor so that you can work on the motor yourself it will have all of the steps and spec or take to a Yamaha shop.

[Toulla]

How did you end up fixing the engine? The good news is the F25 is one of the non interference motors made by Yamaha. That means no internal damage to the engine even if you break the timing belt.

[damirdaco]

clean the carb..it will run fine