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2006 YZ250f POPPING AFTER REBUILD

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Hears the scoop. This weekend I rebuilt my yz250f. new piston and timing chain. Valves checked out everything went pretty smooth. I set the timing which Ive done before but it can be a bit frustrating. I noticed that the mark on the fly wheel was lined up with the case as well as the cams even with the head. 12 pins between the marks on both intake and exhaust cams. Just to be sure i had it right i tried a tooth more each way on both and clearly that wasnt the correct alignment. So I went back and aligned it again and I notice the line on flywheel lined up and the cams lined up only the intake cam was just a hair below the head. However it i tried to raise the mark at tooth again it was clearly even more off so I was confident that I had the timing right. Well I fire the bike and it fires right away. 2nd kick. However now I have a good amount of popping and it was a little low on idle. I played with the fuel screw and not much success. I would normally say check the carb but it ran fine 4 days ago before the rebuild and nothing with the carb was done other then removal. I did put in a new plug which could be a variable here that i over looked. Maybe faulty? IF i turned the idle up it would be ok and idle. Just a good rev or 2 on the stand though and i get a good amount of popping with some flames as well. Just looking for ideas before I return back to it guessing away with no real thought behind it.

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I would say it is one of two things

1. Air leak - check the exhaust and the boot before and after the carb.

Spraying carb cleaner around the book will tell you if there is a leak.

2. Your timing is off. I have had to pull the plug and use a rod to make sure the bike was at TDC and re time the bike.

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I hear what your saying but the fly wheel can only go on one way. And as far as the cams they line up with the head. So how could u not be a tdc buy going what the manual recomends? There is the possibility of of loose clamp on the airbox boot.

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Also would the bike even start if the timing was off. Im pretty confident i have it spot on the ohly doubt is bc its popping. But ive checked it several times and seems like its where it should be

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By your description, I think you have it assembled correctly. The timing marks will be off by that little amount.

It sure sounds like a plugged pilot. I had a bike which ran exactly like once- it shot fire out the pipe just like you describe, crazy backfiring. Mine turned out to be combination of things, I think- old fuel debris in fuel screw area, and a terminal plugging pilot jet. Although since you had it running just a few days ago, that really "shouldn't" be the case, maybe. Your description indicates what is typical of a lean condition, so investigate any possible sources of lean conditions.

Also, if you get crud up into the fuel screw passage, then the fuel screw is essentially plugged and you're into massive lean problems. Is there a possibility that your carb could have gotten some debris into the air jet (on the intake bell side) and then that debris was sucked into the fuel screw area? Look at your manual for a cut-away view of the carb to illustrate the problem. Note that this illustration doesn't have cross-hatching lines so you have to study to see the construction.

Or.... Did you run the bike very long after the rebuild? Maybe, enough float bowl gas evaporated to cause temporary issues? Drain the carb and try it again. If that doesn't help, then I would suggest to verify all the carb boots are on correctly and that the pilot jet is clear.

Hope this helps and good luck with it.

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All great thoughts. Its tough to get ligit trouble shooting ideas here sometimes lol. I really wanted to go to the carb first but my initial thought was like i said i didnt do anything with the carb outside of removal so. Im gonna recheck that the carb is seated correctly. It seems as though this should be rather simple. Also I thought some bad gas but again i didnt have any problems prior to the rebuild.

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Well I took a quick peak at the bike tonight and didnt visually see any issues with the intake boot carb or intake box inlets. All were fastened properly and alighned with the coordinating notches on the carb and cylinder head. Any other thoughts guys? Im going to put in the old plug just to remove that variable and i guess if I have to inspect the carb next. I guess dirt could have gotten in during removal but this seems highly unlikely to me.

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So for those of you if any still following this thread I think I found the culprit. When installing the fly wheel the woodruff key and the key way on the flywheel were misaligned. So the key way in the the fly wheel is all chewed up and was off buy a very small amount. Looks like ill be getting a new fly wheel. My assumption is that the flywheel being that small amount off is what was creating the backfire out the exhaust. This would make sense as to when the fuel screw had no affect in how the bike was running.

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it could be your problem and I am curious if it fixes it when you get the new flywheel on.

Food for thought though, if that small amount of timing makes it back fire, could the TPS also be part of the problem. Correct me if i'm wrong, but don't the TPS tell the cdi to advance and retard the timing depending on where the throttle is? Did you test the TPS to see if it is working correctly?

I had my 03 do the same thing after shimming the valves and changing the cam chain damper and only had to put a leaner main jet as it was too rich and burning off the excess fuel in the pipe. The weather warmed up about 40 degrees from the last time it got rode so that was a no brainer.

Good Luck:thumbsup:

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Well My bike ran fine no problems last week. I just tore it down to do the annual fresh top end check valves and replace timing chain. So i knew the issue had to start with something I did. Where as the carb was only removed and not modified in any way. Im not familiar with the TPS. My carb has always run fine so I dont do much with it other then change a jet here or there and adjust the fuel screw based on temprature.

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