09 High Comp Piston - Possible Jetting Problem

Hi,

Had a search but cant find anything along these lines.

I just put a high comp piston in my bike, raced it for the first time at the weekend (after a bit of a break in during practice). Bike is stock with exception of a DEP powerbomb header and 13.5:1 piston. 97 Octane fuel. It pulls like a train when the wheels are on the floor and sounds good.

The problem I have is when the engine has no load (in neutral or in the air), it doesn't rev out cleanly against the rev limiter, it kind of rev-bog-rev-bog cycles (kind of hunting). Basically I'm happy with it until I need to panic rev in the air, and then it doesn't inspire confidence at all!

Any ideas? I was thinking this may be a jetting issue (main jet too rich?). I'm yet to pull the plug and check the colour, will do that when I go out to the bike next.

Any ideas anyone?

Did you rejet it yet, or did you leave it as it was before you changed out the piston? What octane fuel were you using before high compression. The best pump gas I can get here is 91 octane, and I used some VP U4.4 just to see what it would do and my jetting was off by a bit even with just a fuel change. Gave me a bog and a hesitation through the rough stuff.

Did you rejet it yet, or did you leave it as it was before you changed out the piston? What octane fuel were you using before high compression. The best pump gas I can get here is 91 octane, and I used some VP U4.4 just to see what it would do and my jetting was off by a bit even with just a fuel change. Gave me a bog and a hesitation through the rough stuff.

Still on standard jets. I ran 97 Octane Fuel before and after the change.

octane is not a problem if it were your bike would blow up slowly inside detonation basically try changing oil cleaning the filter getting a new iridium plug and go give it a good bashing until there is no more bog left

OK, I checked plug, it is pretty black (rich). It only starts to go rough half to full throttle with no load. Oil and filter are not the issue - I change oil every 2 races and filter every race.

Do I need to go down on the main jet or the leak jet or both?

Cheers gray, will give it a try tomorrow.

Only other thing that has crossed my mind is cam timing. The flywheel has 3 marks, the centre of which is top dead centre. With the cam chain tight, the dots on the cam sprockets are sitting about 5mm above the cylinder head surface. Would the bike start easily and run so well under load if i was a tooth out?

The flywheel has 3 marks, the centre of which is top dead centre. With the cam chain tight, the dots on the cam sprockets are sitting about 5mm above the cylinder head surface. Would the bike start easily and run so well under load if i was a tooth out?

The bold is incorrect. Moving clockwise on the flywheel, or watching the marks roll past the window, or starting at the left, the first two marks are joined by a horizontal bar to form an "H". These are for strobe checking ignition timing, and are both BTDC. The third mark stands alone, and is referred to as the "I" mark. That one is TDC.

You'd have issues if you were a tooth off. If you moved the exhaust off the mark one tooth either way, you'd either have not enough compression to start, or so much that you could barely crank it. If you got it running with the X cam one tooth advanced, it would be timed like a WR. The intake is another matter. It would barely run one tooth retarded, and would be scary close to bending valves if it was advanced at all.

Cheers gray, feel an idiot now! :thumbsup: Wish the manual said the 3rd one was TDC!!

From your description, if I put the crank on the right mark, I'll be spot on on the exhaust and a tooth advanced on the intake. I've been a lucky boy - I raced it like that last week!

I'll time it up properly tomorrow, and let you know if thats the issue (I am fairly confident that will be the issue - it's that only thing that changed significantly enough to cause the issue the bike has at the moment.

To be honest, given that the crank turns twice the speed of the cams, it's very unlikely it makes that much difference to the cam timing.

Thanks for your help on this gray. It was as I thought - 1 tooth advanced on the inlet cam. Did it and fired it up tonight, revs out properly now. :thumbsup:

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now