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Bike doesnt want to start 05 YZ250F

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I have just bought a 2005 yz250f from a guy who said its timing was off and thats why it didnt run. It was off, so i retimed it. I know i did it correctly, as i spent a lot of time reading up on the procedure. But now it still doesnt want to start, i kick and kick and kick, but nothing. So i thought maybe the plug was bad, it was not. I have cleaned the carb in and out, but still nothing. So i got to thinking I'll try to bump start it, it started to fire, but sounded like it was running dry. Wouldent respond to the throttle, and would only fire as long as you pushed it. I put gas directly into the carb, and it fired for a few seconds then quit, any ideas?

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Difficult to start 4T MX engines can usually be traced to 2 common problems:

-dirty / plugged up carburetor from improper storage with ethanol blended gasoline  

-compression loss due to valve clearances out of spec and/or worn valves & valve seats 

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I fully cleaned my carb, and the compression is good. it could possibly be the clearance, but the bike would still start if they were wrong just backfire and sputter a lot. 

i had another thought, could it be a fuel filter or fuel pump?

i'm not really familiar with this bike, is there a fuel pump?

if there is and its electric could it be my stator?

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Gravity fed from tank to carburetor, no fuel pump.

Start by checking the valve clearances before suspecting anything else.

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Your compression,

how many PSI, or what leakdown percentage ?

 

Otherwise it's just a guess (say like going by kick start lever resistance...)

 

People here really want to help but you have to do your part as well,

once you publish the valve clearance numbers and actual compression reading you'll be able to either rule out or, pinpoint the problem easier.

All part of owning a high performance MX race engine, otherwise there are TTR's for those wanting minimal maintenance.

Edited by mlatour
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it was not the valve clearance. i checked and they were gapped the right height for my bike. i'm almost positive it has something to do with the carb or maybe a clogged fuel filter.?

 

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Okay guys, i hvabe went through the bike more now. I took off the top end, and here what i replaced. 

The piston rings

The valve shims

The cam chain 

The tensioner (i put in a manual one)

The guards for the chain

Retimed it 

After all of that, the damn thing still wont start. So now ive got the whole carb off the bike, any ideas about what might be wrong? Its my first time with a mx carb. What shpuld i be looking for?

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If the cam timing was off when you got the bike, it's possible that the cam sprocket has somehow slipped on the cam itself. They're a press fit and I've had this happen to my YZ450 after it exploded. Was there any sign of piston to valve contact? Also, make sure that there's the correct amount of oil in it - too much and they are also hard to start.

 

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Did you inspect the valve faces and seats?

Despite proper shims/valve clearance, if the valvetrain is worn it won't build enough compression to start.

(compression leaks past the valves as they're not seating evenly on the seats)

 

A leakdown test before dis-assembly could have pinpointed if the valves needed replacement but,

tearing that far into an engine they should have been replaced, same as the piston (not only the rings).

 

It's 12 year old MX race engine with no past maintenance history,

by only replacing a few parts (worn valves and piston are the most risky of sudden failure)

you're unfortunately likely to tear back into it sooner than anticipated.

Edited by mlatour

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I did also replace the gaskets, sorry i forgot to mention. I'm going to revuild the carb myself, but i think ill have my local highschool shop do a leak down test. 

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The basics - fuel delivery, compression, spark.  What measurements did you shim the valves to?  Do you see spark on the plug if you pull it and test?

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