I am in the process of rebuilding my yfz 450, useing a trinity 470 big bore kit w/ new cylinder,ported & polished head with a 12 to 1 piston. the bike won't run. it has fuel & spark but it doesn't fire up.. when you apply throttle while turning it over it seems to load up & then back fire hard out the exhaust . cam timing is set properly & the valves are shimmed to the correct clearances.none the valves are sticking, bent or leaking. but it has no compression.
seems like lots of suction. i even tried rotating the crank 360 degrees thinking i was out there but thats not the case. any help on this subject would be greatly appreciated, thanks.

 

When you mention everything is correct give the tolerances you are measuring! Maybe someone will recognize them as correct or not...

NO COMPRESSION?

1.) Rings installed correctly?
2.) Valves opening at the incorrect time? Are the valves completely sealed into the seat on the compression stroke?

Cam was not setup correctly when the piston was at top dead center?
Fool proof way to find the compression stroke when there is no compression to be had!

Remove the valve gap inspection covers.
Manually rotate the crank and watch the intake valve begin to close then and follow the rotation putting the crank maker on T for top dead center (this is the top of the compression stroke) and double check the gap on the intake and exhaust valves all 5. Sounds like the cams relative to the crank is 180 out of phase.

 

thanks for responding. i have my valve lash set at .20mm on the intake & .25mm on the exhaust as required by my hot cams card.today i removed my stg 1 hot cams & installed my stock cams, and i couldn't believe it my compression came back, it fired right up & runs exellent how can this be?manufacturers error? or my own ? i'd like 2 run the hot cams for performance reasons, should i reinstall them again or send them back, & get new ones?
when u compare them side by side the timing marks on the gears in relation to the cams look identical with a slight difference in lobe width. thanks for the help.

 

Did you compare the stock cam markings to the race cam markings?

Compared The lift of the cam ?

Did you use a micrometer to get a measurement?

.20mm to .25mm is not much of a difference between for a hot cut cam
0.00787 inch to 0.00984 inch
Sounds like the exhaust may be to tight I may be wrong.
What is the gap on the stock setup?

Get in touch with the manufacturer to make sure the card was correct in the gap Tol. for the lifters.

Tell them what had happend prove to them you you made no mistake by letting them know you
had placed the orig cam back and it runs fine.