Posts
3
Joined
12/11/2013
Location
Murrieta, CA
US
Edited Date/Time
12/15/2013 7:28pm
I am new to the site and was hoping someone could help me out. I am working on my first 4 stroke rebuild, so here is the deal.
I bought this 07 yz450f. with a new bottom end already installed. The whole top end was shot so I bought a new cylinder piston and rings and everything that involves the valves (all OEM). the only things aftermarket engine wise is stage 2 hot cams and a full pro circuit exhaust and I also re-jetted the bike according to the PC website.
got this bad boy put together turn the gas on pull the choke give it a few kicks and it backfires dang near every kick, shooting flames out of the exhaust. every few kicks feels like it has good compression then a few more kicks feels like no compression and I get some kick back. with the choke off it does not backfire while kicking. with the choke on or off it took forever to start the timing is on point, I check the valves what seems like a million times. am I looking for a slight drag when measuring the valves? I don't have to force the thickness guage in but it also does not slide right in. maybe my math is off? got it running and the pipe glows red within 20 seconds and it will on idle with the choke off. when I hit the kill switch I get a nice little burp from the exhaust. I am at a loss here so any help would really be appreciated. thank you in advance for any responses.
I bought this 07 yz450f. with a new bottom end already installed. The whole top end was shot so I bought a new cylinder piston and rings and everything that involves the valves (all OEM). the only things aftermarket engine wise is stage 2 hot cams and a full pro circuit exhaust and I also re-jetted the bike according to the PC website.
got this bad boy put together turn the gas on pull the choke give it a few kicks and it backfires dang near every kick, shooting flames out of the exhaust. every few kicks feels like it has good compression then a few more kicks feels like no compression and I get some kick back. with the choke off it does not backfire while kicking. with the choke on or off it took forever to start the timing is on point, I check the valves what seems like a million times. am I looking for a slight drag when measuring the valves? I don't have to force the thickness guage in but it also does not slide right in. maybe my math is off? got it running and the pipe glows red within 20 seconds and it will on idle with the choke off. when I hit the kill switch I get a nice little burp from the exhaust. I am at a loss here so any help would really be appreciated. thank you in advance for any responses.
That would be the first step to take. The header glowing is indication of lean on bottom (pilot jet).
Better yet, completely go through the carb and clean the heak out of it.
The Shop
Also double check the valve clearances. It's ok for them to be a bit loose, but not any tighter than the manual tells you.
Did you check the valves for sealing when you rebuilt the top end? With the age of the bike, they could be leaking just from wear.
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