Runs rough when warm

[toughguy]

I recently purchased a Yam YZX 250 (Zeal). I'm really enjoying the bike but have a problem with carburation (I think?).

When the bike is cold, it runs like a dream wiith smooth power all through the rev range.

Then, when the engine warms up (after 15-20 mins), it doesn't run well between 2000 and 6000 revs. Response is very poor and it doesn't get going until I pass the 6000 rpm mark. Sounds gravelly too)

I serviced the bike when I got it 2 weeks ago, plugs, air and fuel filter, oil change, & drained float bowls. I use premium grade unleaded fuel.

Any suggestions? My gut feeling is that it's a carby problem but not sure.

Is there a chance that imported bikes such as this are not jetted for the Australian Climate? I'm in Perth.

Appreciate your help peeps.

[poppygene]

Have you pulled the spark plug and checked the color of the insulator? I'd guess it's a bit dark. You may want to check the service fuel level in the float bowl - it could be too high, causing a rich condition. Also, does this bike require high octane fuel? If it isn't specified as such in the owner's manual, it would be best to use regular. Higher octane fuels burn slightly slower so maybe a switch to regular would help.
Good luck, and keep us posted.

[ronboskz650sr]

If you know the service history...When were the valves last checked? As the engine heats up, the parts expand and your valves can "tighten up" as they do. This will cause the affectred valve/valves to be slightly open, due to inadequate clearance at the cam for the spring to close it/them (whew). Then you get rough performance, loss of power, all the stuff that goes with incomplete or non-existent combustion. It's all about the complete burn. Running okay when cold and getting worse when hot is a classic symptom of tight valves. With just carbon fouling, it won't run right when it's cold either. Poppygene hit it on the head about the octane. My vote...check valve clearances. Lots of ways to skin this cat, but one of them will fix it, so don't give up.

[toughguy]

Thanks for the help guys. It was bits and pieces of your information that allowed me to understand the problem. I took the bike apart last evening and figured out what was wrong...

The slide in Carb No 3 was stuck in the open position. Now this was fine when the bike was warming up cos it ran one cylinder rich. But when the bike got up to temperature, the open slide was choking cylinder No 3 with fuel (but no air) at low revs when the butterfly was closed. Of course, when I wacked it open, it cleared itself and ran smoothly again. (above 6000 RPM)

It took me 3 hours to sort the problem out. I've no idea why the slide was sticking. I used emery paper to sand it down and try and remove any edges.

My prize for sorting the problem? A 30 minute rip on empty roads at 11 PM last night. Nice one.

Thanks again folks.

Don

[poppygene]

Well done, Don! Glad you found the problem.

Ride safe!

[ronboskz650sr]

Awesome! Glad you got it.