Low RPM stumble

Hey gang. Would appreciate some input here...

Pulled the carbs soaked the bowls and cleaned the main jets. Air screws 1.5 turns. New Dyna coils, plug wires and caps. Takes about 4-5 minutes to warm up with half choke but I get the occasional backfire through one carb.

When I pull away the old girl stumbles in the lower rpm range but once it hits about 3500 rpm it seems to clear itself and the acceleration is impressive. Anyone have some direction on where I should be looking?

Carbs are from WG #17.5 pilot jets, 1.5 slide cut outs, 5DL31-3 jet needles and #122.5 main jets. Float levels are correct as are slides.

Any input would be appreciated.

Cheers-Colin Firth

What year and model of bike?

How's the mechanical ignition advancer working? If it's sticking, this might be the problem. Most are behind a CD sized right engine cover at the end of the crankshaft. With the engine off, give it a twist, it should snap back to original position.

If it's stuck, make note of how it's assembled, as it can be re-assembled 180 degrees out. Usually some light grease or 3-in-one oil on the weight pivots and the tube it moves on are about all it needs. I check my bikes advancer every time I do an oil change for smooth operation.

Pulled the points cover and this is what I have for the Dyna S plate. I will have to pull the plate to get to the mechanical advance behind the plate and lube the mechanical advance? Noticed that the four mounting screws for the magnets were slightly loose.

After cleaning/ lubing, get out your timing light and check both pick-up coils are set correctly as they are independently adjustable. The 4 screws you said were loose.
make sure you have no vacuum leaks before carb adjustments.
Then a Dynamic Carb Sync, with a Manometer. Get the carbs sync real good, and the ignition set up. then maybe set the Idle mixture up slightly richer on fuel. Each screw will not be exactly the same for optimum running. Turning them out to 1-1/2 turns is just the starting point for a Dynamic sync.