Today for some reason my beta was playing up.Before riding carb and filter were cleaned but whenever I opened throttle from tickover to wide the engine dies for about half a second then picks up.this sounds a very short lag but it makes a hell of a difference in the sections. Checked plug and mixture appears spot on. Wondered if it was some electricl problem when on heavy load. On higher revs bike runs fine.

Any ideas greatfully accepted

My personal opinion is that the mixture is too lean. Try adjusting the air screw by rotating it clock-wise, and see if it solves the problem.

Either **** in the pilot circuit or dial it out with the air screw as suggested above

Whacking the throttle open quickly tends to cause the bike to run lean because it drops the air speed through the carb. This is the reason for accelerator pumps. Any engine will do it under load if the throttle is opened fast enough but your bike may be jetted a bit lean so the situation is a bit worse. One factor to consider is temperature. If the air temperature is cold and your bike is jetted for summer then it helps to go up one size on the pilot and main jets. Just for starters you can try raising the needle one notch to see if that helps. Another thing that might cause this is a broken reed valve petal. At low throttle settings the carb is closed down so air velocity through the carb is fairly high and constant. An engine designed for a reed valve has aggressive intake timing. When a reed is broken mixture can blow back through the carb loading up the engine as it gets multiple passes through the carb until the engine can get up enough RPMs to keep the column of air in the intake moving forward even when the intake manifold is open on the piston down stroke. Reading the plug isn't going to tell you much for this since it is a transient condition and really doesn't affect plug temperature much.

It's highly unlikely this has anything to do with ignition.

Thanks to all but especially Dan who mentioned the reeds. Pulled them out today and one set were not seating correctly and were leaving a visible gap.Stripped them and refitted and bike is now running spot on.Yipee roll on playtime.

mine did that yesterday i adjusted the air screw working fine now (fingers crossed).

Thanks but I can't take credit. A slightly misfit reed probably won't have a big effect. More likely you loosened up an obstruction in your jetting by taking the carb off and jiggling it around or sealed up an air leak in the carb/reed/engine interface.

In any case I'm glad it's running good now. It's happy time. Go forth and play and I'll be envious as the woods here are still deep in snow.