2011 RMZ250 Starting/Power Issues

[Jessa27]

Buckle in this is a long one. Any advice is appreciated! I have a 2011 RMZ 250 that has done me very well for 4 years. Has been a very reliable bike with lots of power.

Last year it started to act up a bit, slightly less power & became more difficult to start, especially cold (side note: it’s a Suzuki so it’s never started great cold but usually 3 or 4th kick was the case). Still ran okay though & fiddled with the fuel screw a bit which seemed to help with the starting/power issue for the rest of the season.

This past winter we pulled it apart, put a brand new piston in, checked the valve clearances, moderate amount of carbon build up on the valves which we cleaned off, and put in a new spark plug which is also in spec. Bike still wouldn’t start great cold but was starting on about the 8-10th kick and power seemed to be improved although not 100%. I’ve take it for quite a few rides now and broken the new piston in and am having more issues:

-won’t kick over cold at all but bump starts just fine

-will kick over first or 2nd kick when warm but NOT when the bike starts to get even slightly hot

-very little top end power. Bottom of the gears seem to be full power and the hotter the bike gets the less power I have in the top end

-bike began to backfire quiet a bit when hot

-won’t idle in gear when hot

Any advice on what to replace/trouble shoot next? I had a buddy tell me it could be a worn out injector so that’s what I’m looking at replacing next. Thanks!

[mlatour]

As with 95% of all high performance 4-stroke motocross engines,

your initial 'hard to start when cold' problem was no doubt a worn valve/valve seat problem,

allowing compression to escape past the valves while kicking it over.

Bump starting spins the engine over faster than operating the kick lever so it can overcome the loss and eventually start.

 

While replacing the piston and rings did restore some compression, it did not fix the real problem.

Despite being shimmed to proper clearance, the valves aren't sealing properly around their seats.

A leakdown test will confirm it.

 

At that age, yes you could also suspect some fuel pump and injector issues beginning to arise,

also contributing to hard to start problems either cold or hot.

Edited August 7, 2019 by mlatour