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Old 12-31-2015, 06:31 PM
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sbest sbest is offline
Motorized Bicycle Elite Member
Join Date: Nov 2015
Location: Nova Scotia
Posts: 249
Default Re: Engine won't go past 30MPH

I am 210 lbs stripped naked and dried off. My mountain bike Grubee Skyhawk 66cc does 30-37mph on the flat with a 44 tooth sprocket before any mods. It did noticeably improve during break-in over the course of 3 tanks of fuel. I had detonation problems that were solved by correcting the squish band shape and gap.

When I ported the intake manifold to match the cylinder, I lost speed. This was verified by switching back to a stock intake. The mis-match has an anti-reversion effect I'd suggest.

Matching the exhaust flange to the cylinder increased the torque in a slightly lower range making the bike have stronger power peaks at 35kph and 50kph. This is with the stock Grubee chrome exhaust. The black exhaust was noisier and made less power so I did not use it any more. The chrome exhaust with the end cap (baffle) removed is much noisier and has less power and speed.

Jetting is important. Plug should be a tan colour, not black and oily. I'd recommend an oil mixture in the 32-40:1 range with quality oil as there is too much carbon fouling with heavier cheap oil mixes.

Last edited by sbest; 01-01-2016 at 11:59 AM. Reason: refinement
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