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Need ideas, CRF450X won’t start

Joined
Apr 22, 2011
Messages
3,337
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395
Location
Princeton, Texas
First Name
Gary
Last Name
Waugh
Okay peeps, I need some ideas!! A friend of mine bought a non-running CRF450X about 3 weeks ago, we removed the engine, saw that the valves where burnt out and there was a lot of scoring on the barrel and piston. We purchased a new barrel, rings and a piston, we then purchased kibble white SS valves, lapped the valves in and assembled the head. Checked the play of the big and little end bearings (all was within spec) so we put the engine back together and put it back in the frame. We cranked it over on the electric start (with the auto decompressor disabled and we are seeing about 100psi, I completely cleaned the carburetor and confirmed the jets are clear and blew out all passages. The accelerator pump is Lso working correctly. So I know we have fuel and compression. I have removed the spark plug and cranked the engine over and we have a very healthy looking spark, so I figured the engine would start. But we crank it over and there is not the slightest signs of life!! I sprayed a load of easy start in the inlet tract and after a few seconds of cranking got a good bang in the exhaust system like a backfire) but still no signs of any life.. i am now struggling to think what could stop this engine from trying to fire up. The carb has all stock jets and setup, I reconfirmed the camshaft is timed correctly (it is!!) so any ideas what might be amiss? I can only think the spark is occurring at the wrong time, but even then I would have expected some signs of life (kick back, popping, something) but we still have nothing, the engine just cranked over smoothly and shows no attempt to start..

All ideas or suggestions greatfully received as I am starting to pull my hair out..

Gary
 
I am not sure you should have lapped the valves. I believe the modern valves and seats don't need it. I would ask obvious questions like is there fuel getting to the engine? Do you have a spark? Check the wiring to the coil, the coil, new plug? You eliminated compression, even though 100 psi sounds a little low for a modern engine. After that, you got me.
 
I had a similar issue with a yz450 one time. I "inherited" it because it wouldn't run after a new top end. It turns out they didn't replace the cam chain and cam timing was off because the cam chain was way beyond it's service life. The mark lined up an exact half tooth off. To be fair, it was a flattracker and lived at least a third of it's life on the rev limiter. It ran like a banshee with a new chain.
 
Cam timing is spot on according to the Honda service manual, the cam chain is well within the service limits, so that really doesn’t”t seem to be the cause, I have checked it 3 separate times as I thought it must be the timing, but it is spot on every time..
 
That compression seems low. Without disabling decompressor, it ought to make 50-60 psi. With decompressor disabled, it should make 140-170. Even at 100, it should fire.
Since you tried ether and it loudly fired out the exhaust, that tells me igntion timing is off or cam timing is off. Since compression is low, it would seem valve timing or exhaust valve seating would be the likely culprit.
I've hacked on motorcycles since I was 15 and piston/turbine aircraft all of my adult life. I know I have had more than one instance of just knowing full well that xxxx was right only to find out later...I was wrong.

I'm sure it is somn simple!

PS - Did you replace the valve seats when you put the kibblewhites in?
 
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Did you pull the flywheel? I ask because the key maybe sheared or missing. If the flywheel is not on the crank correctly your timing marks don’t mean much. I would remove the valve cover and pull the spark plug. I would then put the bike on a stand and in high gear slowly rolling the rear tire forward. Watch for intake valves to open and piston to go down then intake valves closed piston rising, go slow to TDC then check timing marks.
 
If you positively have fuel, air and spark, the engine is probably out of phase. That boom you got with the starting fluid is a red flag. If it seems like you timed it perfectly, doublecheck to make sure you're not perfectly timed to 180º off.
 
Thanks guys, I will go and check again, I did remove the flywheel so I guess it is possible the woodruff key dropped out, will check again and check the TDC markings. I did not fit new valve seats, I don’t have the tools to do it and the existing seats looked very good.i thought a 100psi was a little low, but as it’s new piston, rings, valves, etc nothing has yet bedded in so I thought it wasn’t bad!! With the decompressor working I only get 40psi!! It should still fire if everything else was correct!! It has to be something simple, I just can’t work out what!!

Gary
 
Well it was the cam timing, turns out there are 2 timing marks on the crankshaft and I was using the wrong one!! Glad I always rotate the engine by hand 2 full revolutions before starting a new engine, as I know I have not done any damage to the engine!! Timed it to the correct mark, rotated 2 turns by hand, all was good so hit the electric start and it immediately fired up into a nice steady idle!! What a great way to end the night!! I will have to remember in the future that there are 2 marks so I don’t make the same mistake again.. thanks for all the suggestions, I will sleep happy tonight!!

Gary
 
Well it was the cam timing, turns out there are 2 timing marks on the crankshaft and I was using the wrong one!! Glad I always rotate the engine by hand 2 full revolutions before starting a new engine, as I know I have not done any damage to the engine!! Timed it to the correct mark, rotated 2 turns by hand, all was good so hit the electric start and it immediately fired up into a nice steady idle!! What a great way to end the night!! I will have to remember in the future that there are 2 marks so I don’t make the same mistake again.. thanks for all the suggestions, I will sleep happy tonight!

:chug: :clap: :chug: :clap: :chug:
 
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