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Rebuilt bottom end, now she won't start!

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Hey guys,

First post. Just wanted to see about getting some tips on what I am doing wrong. Long story short,

Bought a 03yz250f off someone for 500 bucks. Needed a new bottom end. Friend and I installed all new bearings/seals and put it all back together this morning. Go to kick her over and it won't start. After about 15 kicks, she finally starts for maybe a half a second and then dies. Smoke/back firing out of the pipe during every kick.

The motor spins when you kick her over but she doesn't start. Checked for spark, no issues there, etc... We triple checked our work on the cams and the timing looks correct.

I've been told I may need to bump start to get her running since all the bearings are new and a little stiff. We made the mistake and gave it gas a few too many times and flooded it I'm sure as there was fuel coming out of one of the fuel overflows. I then learned that you shouldn't touch the throttle when starting it!

So you guys think I should bump start? Any help would be appreciated!

Ps: double spacing may show up since I am posting this from my phone at work going crazy while the bike sits at home!!!!

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Hey guys,

First post. Just wanted to see about getting some tips on what I am doing wrong. Long story short,

Bought a 03yz250f off someone for 500 bucks. Needed a new bottom end. Friend and I installed all new bearings/seals and put it all back together this morning. Go to kick her over and it won't start. After about 15 kicks, she finally starts for maybe a half a second and then dies. Smoke/back firing out of the pipe during every kick.

The motor spins when you kick her over but she doesn't start. Checked for spark, no issues there, etc... We triple checked our work on the cams and the timing looks correct.

I've been told I may need to bump start to get her running since all the bearings are new and a little stiff. We made the mistake and gave it gas a few too many times and flooded it I'm sure as there was fuel coming out of one of the fuel overflows. I then learned that you shouldn't touch the throttle when starting it!

So you guys think I should bump start? Any help would be appreciated!

Ps: double spacing may show up since I am posting this from my phone at work going crazy while the bike sits at home!!!!

If everything was done properly and cleaned properly it should start first or second kick. I have done complete engine rebuilds i.e. crank, crank bearings, piston, rings, cylinder, valves, and never had an issue with starting it up easy. It is a little finicky when it is first ran but still shouldnt take but a couple kicks to start. Did you check the valve clearance when you had it apart? Did you disassemble and clean the carb good?

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No. Didn't check valve or clean the carbs. Mainly because it ran when we bought it. Just needed new bearings.

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Check the valves. I did the same thing on my last bike, which was an 03 YZF. I put in a crank, and had to have a friend adjust the valves. After that it ran like a champ...until the transmission grenaded.

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What most people tend to forget to check is the timing chain, if there is the slightest kink the bike will not run properly(or at all for that manner).

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Also make sure your flywheel is seated properly on the shaft. I have seen guys put these back on and mash up the woodruff keep even though it appears to be on correctly its not. If its not aligned properly on the shaft it can spin off center and change the timing....causing the bike to run poorly and or backfire. Prob not your issue but worth looking into.

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