2008 yz450f engine problem or paranoia

Dear thumper talk,

        I now own a 2009 yz450f and a 2008 yz450f. I have also owned another 2008 yz450f in the past. Anyways my new 08 has a problem. Now when im riding the bike unlike my other two bikes it has a weird slight hesitation to its throttle curve plus extremely loud backfiring on decel, and when the bikes at idle a very slight throttle chop can kill the engine.(not right) When I got the bike it had a DRD exhaust on it. I swapped it to the stock pipe. When I switched it to the stock pipe I put in a new header gasket and swapped the carb perfectly to stock jets(yes all of them, perfect condition and stock) and settings with the exception of the o ring mod and the pilot air screw at 2 and 3/8 turns, (that worked perfectly on my other two bikes.) However this bike started very easily like first kick, but I had realized that the valves had not been shimmed by the previous owner in many many rides. So I did them, right off the bat i noticed that the cam chain tensioner was upside down. Anyways I shimmed them, they were a bit out of spec so I went down in shim sizes accordingly. They are now perfectly in spec, I checked them and double checked them more than twice. Now when I re installed everything I put the tensioner in the right way and went to start the bike now, I noticed that the bike on average takes 5 kicks to start cold (super abnormal) and those same negative problems with the bog shut off and super loud backfire crack still happen. (could the tensioner being upside down cause a timing issue?) (Could i have a blown gasket somewhere?) (could the cam chain have become defective due to the uside down tensioner?) (does anyone know what this could be caused by?)

Edited by vic2340


You see the funny this is i cleaned the entire carb and it looks brand new I cleaned out all the jets and passages, i had to switch a few jets to go back to stock and I made sure everything was clear. Theres no harm in checking again though. I will do that now. Im selling the bike and I want it to be perfect but if the next owner doesnt care about the problem then neither do I. I will keep you updated on anything that i find out.

Pilot jets are so small that having a dried film on them only .001" thick has the effect of going two or three sizes smaller.  Mine will barely run when that happens.

Sounds like a plugged pilot to me.R

Pilot jets are so small that having a dried film on them only .001" thick has the effect of going two or three sizes smaller. Mine will barely run when that happens.

you guys were fricken right it something with my pilot jet. I really cant believe it. I had to adjust my pilot fuel screw to 3 and 3/4 turns out to get the bike to run correctly ( way to many turns out) I think the stock 45 pilot jet has a film over it of some sort. I have a 48 jet and will try it. If that doesnt work then i know i need a new 45 that isnt defective. I didnt want to believe it since my other two bikes never had a problem with a 45 pilot.

I new 45 was the fix. The 48 ran to rich.

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