450F YZ450F 2005 roll starts but won’t kick start

subserver

subserver

Hi everyone! I am new here so apologies if I have not formatted this correctly.

I have a YZ450f 2005 model that my friend blew the head on valve bouncing. It was pretty ugly inside to say the least I have rebuilt it (new piston, barrel, and entire head), but sadly I can’t get the thing to kickstart anymore. It used to start first pop pretty much every time. I can bump start it then it runs fine, just can’t kick start it.


I’ve checked compression, sitting at 150PSI. The cam chain appears to be set correctly, lines up the I on the crank then the dots On the cam with the case. I’ve put in a new spark plug, didn’t change anything. Auto decompression seems to be working fine too. I’m not really sure what else now, was going to take to the mechanics in town to have a go but thought I’d try my luck here first ☺️

Any ideas what might be up/need to check? Any help would be greatly appreciated!
Cheers,
Ryan
 
P

PawPaw

150 psi on the compression test is actually high for an auto decompressed engine. When you said head work....What was done to the head? Have you replaced the cam chain? What do you have the valve clearances set at?

Paw Paw
 
subserver

subserver

Hey guys, thanks for the advice. The valves were pretty worn out so I have gotten a new set as the old ones were pretty hard to get correct clearances for, I had to file down the shims and still couldn’t get the gap required. Wasn’t far off, but I guess this motor is pretty picky huh.

Back to work for now so will be continuing the repair another day, but thanks for the advice. I’ll get a new cam chain and replace the valves , will pay special attention to get good clearances this time and see how that goes

also for note the compression was 150 when spinning over fast enough for auto decompression to not be enabled. When auto decompression was working (aka kick starting) it was only 50psi
 
P

PawPaw

You do understand that you will need to also have the valve seats either cut or replaced when you replace the valves, right?

Paw Paw
 
2Wheels

2Wheels

From what I read it said new piston new head with Old valves? Filing shims? Terrible idea you should be able to get where you need to be with standard increments. Quite the contrary, that motor is not extremely picky, it just needs to be rebuilt properly.
 
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M

Malik

subserver:
Hey guys, thanks for the advice. The valves were pretty worn out so I have gotten a new set as the old ones were pretty hard to get correct clearances for, I had to file down the shims and still couldn’t get the gap required. Wasn’t far off, but I guess this motor is pretty picky huh.

Back to work for now so will be continuing the repair another day, but thanks for the advice. I’ll get a new cam chain and replace the valves , will pay special attention to get good clearances this time and see how that goes

also for note the compression was 150 when spinning over fast enough for auto decompression to not be enabled. When auto decompression was working (aka kick starting) it was only 50psi
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Good morning

Sorry to ask after this long time

What was the problem? My yz have the same condition and it would really help.

Thanks
 
subserver

subserver

Hi, the issue ended up being the hotstart plunger in the carb. It was not fully closing causing the bike to run lean. Replaced the plunger and oiled it up, runs the best it ever has now.


Cheers,
Ryan.
 
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