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I have an 05 crf250 that has an of idle bog. I found gas blowing back out of carb. Enough to make my hand wet after a couple revs. I think the bog is either from air blowing back [blowing gas away from cylinder] from cylinder due to a timing problem. How do i find out my timing. I have swapped cdi box and set throttle position sensor to spec. Internal engine components good including valve shimming. Camshaft timing dead on. went throug carb and replaced needle and seat, slide needle and holder [had wear marks on mid section]. Ap pump spraying correctly and duration is good. its blowing the fuel away from combustion chamber and back out carb. bike idles great runs sweet of idle. Just has that bog. any help would be great. Im already tying my noose. :bonk:

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you must have an intake valve at 0 clearance. Thats why its pumping out the intake on the upstroke.

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Those specs are wrong to begin with. The intake is .04-.06 and the exhaust is .10-.12. Try shimming them to the correct clearences and the see what happens.

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I have them shimmed to spec .12 intake and .28 exhaust

If the numbers you state are in mm, then the adjustment is fine. When the intake slams shut, it will cause a reversal of flow and spitting fuel backwards is fairly normal at low engine speeds. See all the threads on the AP mod thing and see if it applies to your bike. At least see where yours is at anyway. Is the jetting stock? Ever played with it or is the bike used (just picked it up)? You might want to go through the carb and write down all the particulars of your jetting and compare them to stock to baseline where you are at. Then you can go back to stock and start over or at least have known, unplugged jets before the next round of diagnosis. Good luck.

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Thanks for info. Had it since new. Ran great till i dumped a valve. Left bike sitting there all winter. Had the valve done. They have a good seat. Pulled the carb down three times and cleaned with compressed air and carb cleaner. didnt get any on rubber. all components are installed correctly. The bike ran mint untill this. Replaced float seat, ap pump, jet needle and seat. All stock setting. Never had a low end bog or any bogging problems till this. As I sit here i just thought about my engine locking up with the valve got dumped. Maybe my cam gear spun on camshaft. Its only pressed on and if the valve stops then so does the cam but not before the bottom half has a chance to smash your valve a couple times. I am a ASE master certified tech[shoe salesman]. Im confident in my carb repair and settings. Im pretty sure its timming prob. Give me an opion. please?!

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Did you reset the float height? the retutn spring has has to be without preassure on it and if you hold the carb upside down the weight of the float will press the spring giving you about 2-3mm difference in float height leaner.

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