flames!

[markryam]

hello - just had my head done and pro-circuit cams in my 08 yz450f.

sound great apart from when i let of i get a back fire and a quick flame from the exhaust. is this due to the cams or is something a miss?

i did pull the cover off to check is was timed right and it appears so.

mark.

[grayracer513]

This happens on decel?

[markryam]

just decel. when the bikes on the stand, warming up. you back off the revs and it flames say every other time. and pops once quite loud.

do you think its possible the valves are leaking?

i have made sure the exhaust is leak free. and it never did it before , and havnt touched the idle circuit.

just to give you some extra info - before it had the cams in it had siezed on the inlet cam, jumped a couple of teeth, this has been repaired and the pro circuit cams installed at the same time , valves re-shimmed etc.

really barks on throttle, which makes me think it may be the duration and overlap of the cams maybe.

id value your input gray, as you seem to be a bit of a guru on the yz450 engine.

mark.

[grayracer513]

Decel backfire is almost always either an exhaust leaking air, or a lean idle circuit. A little bit is normal for a YZF.

Read:

https://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

[Ga426owner]

What did you get done to the head ... curious?

sounds like it may be lean...where is the airscrew at?

Timing is perfect correct?

Until you run it on a track - fully warmed up, then you can tell what is up, Stands do not diagnose well ?

[todds924]

How can you check your jetting on a stand????? In neutral??????

[grayracer513]

Only works for the pilot.

https://www.thumpertalk.com/forum/showthread.php?t=327405

[markryam]

yes - i was refering to the idle/pilot jetting.

well i rode today and there appears no advantage than stock. you would expect a decent gain with the pc cams and high comp piston.

i have this spec in my yz250f with a leo vince fastbox and cairolli pipe and its awesome.

so i think i need to check the timing, but with a dti and degree wheel.

does anyone know the timing specs ie. valve opens 274degrees after top dead centre. so i can check that everything is right disregarding the dots on the gears.

thanks guys.?

[markryam]

picked up a timing wheel today, and i have a dti so all i need are the specs.

ta.

[djtroy]

I was in the pits and both RC's and Bubba's bikes had fire coming out of the PC exhaust when the mechanics were warming them up.

[todds924]

picked up a timing wheel today, and i have a dti so all i need are the specs.

ta.

Stock specs??????????? If they are PC cams, call them and ask what there degreed at...If your looking for stock specs, i believe they are in the service manual. Do the new cams have adjustable sprockets?

[grayracer513]

...If your looking for stock specs, i believe they are in the service manual. Do the new cams have adjustable sprockets?

Not that I've ever seen. They only list the lift, and then only if you do the math yourself. But the cam grinder should have that info, too.

[markryam]

yeah grays right - there not in the book - im trying to get the info from pc were the cams should run. - i'll post a reply if i have some luck and if indeed somethings a miss!

mark.

[jayfugate]

Decel backfire is almost always either an exhaust leaking air, or a lean idle circuit. A little bit is normal for a YZF.

Read:

https://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

When you say exhaust leaking air, do you mean exhaust pipe, or exhaust valve?

[grayracer513]

Pipe, usually at the mid pipe/header joint, or at the head.

How would an exhaust valve leak air?

[todds924]

yeah grays right - there not in the book - im trying to get the info from pc were the cams should run. - i'll post a reply if i have some luck and if indeed somethings a miss!

mark.

Do those cams have adjustable sprockets??????

[markryam]

the gears are pressed on, i just want to check - that way i know nothing else has moved and the cams are right. somethings slightly out and i need to check this first.

pro circuit wont give the specs out so im running out of options.

[todds924]

the gears are pressed on, i just want to check - that way i know nothing else has moved and the cams are right. somethings slightly out and i need to check this first.

pro circuit wont give the specs out so im running out of options.

Thats what i thought..The only adjustable cam sprockets i have seen are on sportbike cams. If they are new cams just throw them in and check what your working with. Or have you tried that already and things don't seem to be lining up proper?

[markryam]

well to update everyone - the inlet cam was out!

i took the top off, put it on the marks, went round the other side to look at the lobes, you would expect pretty much a mirror image between the in and ex cams, but the inlet was set into the head, so i moved it a tooth so the lobes mirrored each other ... no more flames and what a beast!!!

so were sorted.?

had to fit a gytr 9oz flywheel now to calm it down!!!