1975 ATC 90 Info

[fourustircom]

Hi, Folks!

The little beast runs after sitting in the weeds for who knows how long! Brake is unlocked, chain oiled, carb cleaned, points set, but haven't checked the timing or valve lash yet. Idle is great, but has BAD stumble when throttle is nailed. Probably would die if I didn't let off the gas. Backfires too. Float level is set to 20mm. New P7 plug is sooty. Choking has little affect.

Any ideas?

Thanks!

[Doctorturbo]

Fatten up the main jet.

Because it not a pumper carb you have to go over center on the jetting and make it more fat to cover up the air hole when you nail the throttle.

[fourustircom]

So, the main is probably plugged? I pulled the jet, but didn't pull the guts under it. I sprayed a bunch of carb cleaner through it. They didn't act this way stock did they?

Thanks!

[Doctorturbo]

So, the main is probably plugged? I pulled the jet, but didn't pull the guts under it. I sprayed a bunch of carb cleaner through it. They didn't act this way stock did they?

Thanks!

Maybe, maybe not.

When you don't have a pumper carb and rack the throttle you get a big slug of air going into the engine with not much gas. The air also comes to a standstill. A pumper carb overcomes this by shooting a shot of gasoline down the throat of the carb so you don't have any throttle lag.

To overcome this problem with a non-pumper carb you need to richen up the main on the carb. This overrich condition helps the transition of the lag. It's sort of like a band-aid way of doing thing but it's the only choice you have without a pumper carb.