Good power under load, a little 4 stroking otherwise

[robimagu]

First this is my first build with a 66cc motor. I'm in the Denver area, so altitude is an issue. The motor now has about 7 hours and 70 miles on it. It runs great under a little load, ie accelerating or a slight climb at both half throttle and WOT (only short burst just recently), but four strokes on flat surface or when decelerating. The carb slide is in the top grove at this point (5 grove pin). Im using a NGK B7HS and regular unleaded with 16:1 Castrol 2T.

The plug is black and a little on the wet side. I know I'm a little rich but don't want to go to far the other way.

Will a smaller main jet fix both half and WOT?

Will going to a higher octane fuel and/or the change to 24:1 effect this?

Brian Robinson

 

[Skyliner70cc]

Solder and redrill jet one or two sizes should do the trick. If you get a tuned pipe, you'll be ok with the mixture and may have to drop the needle a click or two.

 

[AussieSteve]

robimagu, if the plug is black and wet it's more than a little rich. A smaller main jet is the answer. To avoid drilling, you can buy a range of main jets at Sick Bike Parts.

The main jet mainly affects WOT mixture but does make a difference below that.
I'm in the same boat - to get a good WOT mixture, I fitted an 0.72mm main jet, but it's still slightly rich at 0-3/4 throttle, even with the needle at it's lowest setting. (5 notch, like your's.)
Goes pretty well all 'round, though and especially at WOT.

Why a B7HS? That's a little cold and might be exacerbating the problem. You'd do better with a B6HS, or a B5HS even.
When you reduce the oil/fuel ratio, your engine will run even richer. (Not much, though.)
Regular fuel is best, high octane would be a waste of money on a stock low-compression engine. (6:1 CR)