Tuning issue post bbk install

[rabidnz]

Hi scooter experts (if you are just guessing please refrain :zap2: )
I have a small question. I have recently installed an autotech cast iron 70cc bbk on a keeway matrix scooter. Before the bbk install it was equipped with a stage6 medium contra spring,
red clutch springs(could be factory, unsure),
and an s-drive variator.
im guessing the weights have been changed but i didnt have a scale on hand to weigh them, their colour has all worn off, but there are no flat spots ,and the belt appears in new condition. clutch shoes have a good 3-4mm of meat left on them. 82 mj and latte br7hs plug. 110psi compression throttle open and stock expansion chamber. Stock oil pump.

The performance is very lacking compared to what I expected, as I am a manual transmission sort of person i thought i would ask the experts.
Clutch engages at 4k revs, a bit too high for me but no big issue, take of to 20mph is ok, kinda zippy but then the revs hit a ceiling at around 7200 and the bike will pull those revs all day at about 35mph, though when on the stand i can rev into the powerband about 8500. I feel if i could get the revs to hold up in the 8ks then i would gain the 10-15mph that im missing .

Any suggestions? is the contra spring too strong for the rollers and not letting the front sheave close all the way?

[90GTVert]

If you haven't done it already, you will likely need to re-jet the carburetor.

110psi is a bit low. Check your squish clearance.

[2strokd]

I agree with Brent and would like to add that either the weights are too light or the torque spring is too heavy. Combine that with steep final drive gears and you will get what you explain.

[rabidnz]

cheers mate, have done numerous plug chops and now am left with a 1.5mm sandy band , so if the jetting is that far out perhaps im going to have to make a leakdown tester. Have not checked the squish yet, im guessing its over 1.0, but still should be able to pull 8.5krpm. The rollers feel very light in my hand i would guess 4g , i have a stock shortcase 10mm engine awaiting rebuild (how did you go with the usps claims website Robert hehe ? ) so I will see if the stock rollers out of there help at all, otherwise will try returning to the factory spring. I doubt that the final drive gears have been altered from factory due to the extreme amount of noise and revs it makes to get itself and hold speed at 35mph!

Thanks heaps for your advice fellas, will let you know how I get on.

[speedy1125]

contra spring is too heavy and rolles too light IMO too.

[rabidnz]

redid head gasket, compression up slightly to 120 but i agree still too low for a brand new cylinder, perhaps it will seat better as it runs in. Squish is a giant 1.48 thats nothing to worry about but definitely not performance oriented - will hopefully get the head milled accordingly to get .8 or so squish. Did another plug chop and its showing a little on the rich side now, though without it being able to rev out its hard to plug chop for max revs. Rollers are 4.5g so im hoping thats my problem, the one local scooter shop/servicer out of 10 which actually has ANY rollers in stock only has 5.5s or 8.5s, so I will go with the 8.5s and try 3 of them and 3 4.5s first, moving to all 8.5s if there isnt much difference. Thanks heaps for confirming my suspicions folks, if it doesnt work ill be back for more troubleshooting

[90GTVert]

If it's a metal gasket you could probably remove it and use copper spray to help it seal. Most of the metal gaskets are .3-.5mm thick.

[reveeen]

With, well, limited compression, one wonders if you have a stroker cylinder there (a cylinder meant to match up with a stroker crank to give 80-90cc's) The timing numbers would tell the tale (or simply the limited port uncover @ BDC).

[1quikvento]

I am no expert, but I had similar issues after I installed my cylinder kit. After I got rid of my factory exhaust and put on a lv tt, my scooter was a different animal. I know the keeway pipes look like an expansion chamber, but its prolly got a catalytic converter stuffed in it.

Try an exhaust that isnt restricted, maybe? I was stuck around 40-42 and after I put the pipe on I then tuned my trans and Im getting speeds of 49-52 now, still not fully tuned how Id like it.

I will gladly trade you a stock contra spring for the medium stage 6 one. I even have a soft stage 6 contra Ill trade..

[1quikvento]

I would measure the belts width as well to be sure its not getting thin. It might look okay, but with a few mm's missing, you wont be reaching as tall a gear.