Hi I just discovered that I could interact with this forum without being a member.
This post is a follow up to the earlier discussion on running rich at max throttle/rpm.
I recently aquired a pair of 1978 Motobecane Model 70's with about 500 miles each.
When I got the first one running, it would 4 cycle at about 23 mph. Removing the airbox and cleaner raised this to about 27 mph.
When I got the second one running, it would run 27 mph with the air cleaner and air box in place.
For some unknown reason, the first one had a 58 main jet and the second had a 54 main. Swapping jets swapped the symptoms. I used a clock bushing tool to reduce the 58 jet to slightly smaller than a 54 and the number 2 now runs a little better than the number 1.
When the weather warms up a bit I will be doing more testing as I have the advantage of two identical bikes to compare. I will post the results.
On the rich/lean thread, I agree that reducing the oil/gas ratio does not affect the mixture but it does reduce plug fouling caused by the rich mixture and I suppose if you are running that rich, less oil in the gas is ok.
Does anyone know if this rich at high rpm problem is the manufacturer's method to restrict speed to < 25 mph? It seems like such a crude way to do this, but evrything on my mopeds is esentially like new and stock, so I can't see a better reason for this problem. I just have a hard time believing that a solution resulting in bad gas mileage and fouled plugs would be the best they could come up with.
For those who are considering a smaller main jet to help with this problem, keep in mind that the tendency for the mixture to get richer, increases as the throttle is opened wider. This is because the air box and filter create a greater pressure drop as the air flow increases. I think that this may cause a too lean condition at lower rpm. I will try to test this by doing plug chops under various conditions. In any case, when installing smaller mains, check the plugs immediately for good color. Remember, 2 cycles run best just before they sieze up.