I ride an '86 Evo Sportster 883/1200.
What is causing my engine to surge? It's worse from 35-45mph in 3rd & 4th gears (4 speed tranny) while cruising.
Could it be the ECM??  Or out of time, causing a wrong signal to be sent to the ECM??
I put a newly tuned CV carb on it a while back.. Runs great except for surging....
I noticed the surging after replacing the clutches...

Is it only while riding?

Does it idle evenly?

If you hold the throttle at around 3,000rpm in neutral does it hold that rpm, or vary?

Had that problem once with a sportster.I had taken the carb off.Found the rubber boot between the carb and the intake was out of place.

The rubber boot is fine.
I will test it when the sun comes up Lucky.

besides the rubber seal on the back of the carby there are also rubber seals where the intake mounts to the inlet...check the seal that they are intact and not broken...oh and by the way they are a royal pain in the rear end to replace...get a good night sleep and take your patience with you when you start...you'll need it!

It could possibly be the new clutch if the engine revs slightly when applying power especially in the higher gears it could be a little slippage in the clutch. Have you re-adjusted the clutch since install?

That was kind of my though when he said only while moving and he put a new clutch in.