Join Date: Mar 2009
Location: Fort Worth, Texas
Front Cylinder Backfire
I got my carbs working and it runs down the road well. I'm still having issues with the front cylinder backfiring and wanted to bounce some ideas off everyone. Here is what I know:
The bike is coastered and I checked the seals on the coaster plates and they are sealed well. The exhaust is straight through with no goat belly.
The backfire only happens on decelleration with the throttle valves fully closed. I can hold them open a little and it won't backfire.
It seems to be worse between 1000 and 3000 RPM's on decelleration.
It's worse when I have the idle mixture screw 2 turns out, the backfire is not as bad with the idle mixture screw 4 turns out.
With the choke full on it doesn't backfire, partial choke still backfires.
I moved the petcock vacuum line to the rear cyl carb to eliminate that as a possibility.
It's looking to me like the front cylinder is lean on decelleration. Could this be a Low Cut Valve situation? Doesn't the Low Cut Valve kick in when there is high vacuum like during decelleration? Is there any way to test it? I suppose I could swap the valves and see if the rear carb starts backfiring. Does anyone have any other ideas?
Last edited by AceMcgyver; 08-10-2009 at 04:00 PM.