'05 YZ250F Backfires - Mixed Symptoms?

[tightwoods]

The bike is a 2005 YZ250F with 62 hours on the engine. It backfires at low rpm and when downshifting. The cam chain was replaced and valves reshimmed at about 30 hours. Valves, springs, piston and rings are original. Valve clearances now are within range but on the loose side. I would have expected them to be tight. A leakdown test shows slight leakage down into the bottom end. There doesn't seem to be any leakage out of the intake or exhaust valves. How am I getting backfiring if the valves aren't tight? I would think that the valves and springs should be replaced anyway but what is causing the backfiring?

tightwoods

 

[oldguy]

could be running to lean. We have the ziptye fuel screw in both our YZ250fs and often make just a 1/4 turn adjustment as the weather/temp changes to get rid of backfiring

 

[tightwoods]

Pilot Jet Clogged

Thanks for the reply. The pilot jet was clogged with gasoline varnish. I've seen this happen on several of the 4 strokes with FCR carbs, and this is on bikes that get ridden weekly.

tightwoods