250 2001 YZ250 Idle

Another project, running into issues.

This one had been sitting for years. So once again I cleaned it all up... Drained everything, new fluids, new air filter, carb is beyond clean, ill stress that (so I don't think its a carb issue). Now at this point I will say I hadn't gone into the motor yet. I started it up and it sounded just weird, it was unable to idle correctly. I even took it to the desert with no major problems but the idle sounded like the RPM's were just going up and down and up and down over and over and over (and I mean fairly quick, they would do a full loop from low to high back to low within a second or so repetitively... and then it would get lower and try to die so I always had to be revving the engine even when I was holding in the clutch.

So, I pulled the top end. The cylinder looked very good and the piston didn't look terrible so I had a shop professionally clean up the piston. I put back together what I would consider to be a "new" top end... BTW I BOUGHT NEW RINGS AND CIR CLIPS FOR THAT PISTON...INCLUDING THE CYLINDER GASKET AND O RINGS FOR THE CAP AT THE TOP

Now its all back together and sounding the exact same. Although I technically didn't put in a new top end, it might as well have been. I mean I consider myself knowledgeable enough to know when it actually needs a new motor and id say it all looked in pretty good shape. Could it be the power valve??? If its not the carb and not the top end what could it be? Like I said, the carb is super clean. All the jets are cleared. The air/fuel is at 1 1/2 turned out (even when I try to adjust that + the idle screw it doesn't help)

Ill try to post a vid of the sound. Its almost like after I rev it once it sits there and revs itself over and over and over. I cant just rev it and have it come right back down to a normal idle RPM.
 
99% on the air leak. if your power valve closes then as 2s says, no way that it can be related to your problem. one of the common causes is a leaky seal on the ignition side. you can sometimes confirm that if the rpm tends to go up with the cover off. if it does rise, then you know for sure that seal needs replacement, but of course may not be the only cause.

did you check to see how close to stock the pilot jet is?
 
I'm with these guys. Time to chase an air leak. I would check that crank play while I was there. Should have zero up and down movement.
 
Thanks for the reply guys. I just took a quick glance at everything and noticed the rubber piece that connects the carb and the reed valve is cracked all over from old age. Possibly allowing air to escape. Could it be that simple? As for the cases and other connections everything seems solid. And when I put the carb on I always make sure to tighten up those 2 screws to the cylinder and the air box real good
 
Thanks for the reply guys. I just took a quick glance at everything and noticed the rubber piece that connects the carb and the reed valve is cracked all over from old age. Possibly allowing air to escape. Could it be that simple? As for the cases and other connections everything seems solid. And when I put the carb on I always make sure to tighten up those 2 screws to the cylinder and the air box real good
with it running, spray carb clean or propane around the suspected problem area, if it is leaking then the idle rpm should change and probably settle down to a more stable rpm. this will pinpoint your problem.
 
I did the carb spray test on those cracks in the carburetor joint and it definitely reacted to it. I have no doubt that's the problem area. I'm going to pick up a new one soon and try it out. Ill keep you guys posted. Thanks for the help everyone dirt bike addicts never lets me down!!!
 
We sprayed the front runner to that stuff they sell on tv that seals a screen door well enough to float a boat in order to get a ride in on an old yamaha it. Floyd wants me to remind you not to try that when the motor is running as it can pull enough of it through to glue the reeds shut!!
 
We sprayed the front runner to that stuff they sell on tv that seals a screen door well enough to float a boat in order to get a ride in on an old yamaha it. Floyd wants me to remind you not to try that when the motor is running as it can pull enough of it through to glue the reeds shut!!
Is that based on prior experience?
 
Yep, but while that stuff may keep your boat afloat the same IT sunk just like a rock when it was applied to the intake. When you have a couple of those reeds glued shut you do pick up some low end response. (youlose more on top that you gain on bottom) We decided that we would glue up the reeds on a yz 80 to make it easier to learn on. that one needed a whole bunch of rejetting, but it actually did work. wasn't worth it for the short time to ruin a set of reeds. we decided that if we tried it again we would do it with a set of reeds that was going to be replaced and glue one side of the old ones, replace the other side and once the "learner" was up to it, replace the glued up set. The Occasion never presented itself. Since we are going to set one of my 80's up for Gene who is turning 85, I may just try it, if the retarded overbored cylinder isn't enough.
 
Top