loss of power at certain revs

[Blink]

something weird has been happening to my bike recently. It accelerates smoothly all the way up to 8-9k but then everynow and again it seems to just die. Although if i get off the throttle and try again it'll be fine.

It feels like it hits a rev limiter or something. Its starting to get annoying as when i'm fully leant over round a corner, it certainly gets interesting when it hits this flat spot and i lose power.

My problem is thats its very intermittent and sometimes it doesnt do it at all.

any ideas? or is just a matter of getting it tuned ?

any help would be appreciated :)

[ZorsT]

It could be your power valves, although i have no idea how the Yamaha ones work...

[bugjuice]

have you checked the valves/reeds and carbs (if that's right) for dirt etc? And checked the spark plugs for wear n tear?

[Blink]

have recently stripped carbs, cleaned and put back in. Might do it again this weekend (such a mission to do!) and see if can find anything.

spark plugs look allright... NGK iridiums, so doesnt look like theyre the problem.

[Flyingpony]

Could the engine be starved of fuel?
I've had a 2-stroke do that. Was a dirty carb and a carb screw not correctly adjusted.

[Hitcher]

It's a 250. Most 250-multis have a flat spot somewhere in their rev range. If you have one that is suffers, there is little you can do to mitigate it, other than being aware of where it happens and doing some pre-emptive gear changing.

[vifferman]

It might be a loose fan belt, a worn muffler bearing, or maybe it's just low on blinker fluid.

Either that, or you've worn it out. Send it on a nice vacation to the Islands or somewhere.

Then again - what would I know. It's been....oooh... about seventy bazillion years since I owned a 250, and that was a single, not a multi. And it was a 2-smoke.

And the only thing that I've owned that ever had intermittent power was my body.

[James Deuce]

Was that you I saw at the gas station in Greerton, Wellington anniversary weekend Sunday?

[Dadpole]

Did the problem start after the carb work?
Otherwise, if the problem starts at around 8000 rpm each time, I would suspect an electrical fault. 8000 rpm (or whenever, on your model) is when the power valves kick in, but if they were not working the engine would keep increasing in revs without the usual "power band" effect. It may, however, be related to the servo motor that opens the valves.

PS: 333 posts!! Halfway to the mark of the beast. So far only a small stain of irritation.

[Blink]

Was that you I saw at the gas station in Greerton, Wellington anniversary weekend Sunday?

yeah it probably was! thought it was someone that i knew.

Although ive think ive only riden with you a few times when i was down Wellington, back when you had your TRX ?

[James Deuce]

That's me! :) I just about came up and said hello, but the helmets made me hesitate :)

[Blink]

I starting to think its a carb issue, and that its not getting enough fuel through. If i grab a whole handfull of throttle out of a corner it tends to allways hit this 'flat spot' rather than if i accelerate normally.

Its definately not a normal 250 flat spot as it is a pretty nasty slump in power, and half the time it will quite happily rev through it until hitting the normal flat rev spot up high near red line.

oh well off with the carbs again i guess... if only these reverse cylinders models were easier to work on, have to pull heaps of stuff off just to get started on the carbs.

bring on the sunny weather this weekend...

[Blink]

That's me! :) I just about came up and said hello, but the helmets made me hesitate :)

ahh you should of! we were just gassing up for a KB ride that weekend.

I definately remember chasing you up the Rimutakas when you had your TRX. I'd get close round the corners, but then the big twin would just power away from me out of the corners. Good times :)

definately have to send me a PM next time your up this way

[nsrpaul]

check your power valves are free and working properly and check ya battery , if its flat there can sometimes be a real bad flatspot when the valves are meant to open as there is not enuf power/voltage to power the spark and valves at the same time

[Blink]

hmm not a bad idea.

cheers for all the suggestions and i will see if i can get it sussed out.

thanks