When I installed an Oxygen Sensor to jet my carb, it taught me a lot.  I was surprised to learn that even when jetted to run at the proper mixtures when at a constant throttle setting- the fuel mixture goes lean when you are opening or closing the throttle.  When you add throttle the amount of air coming in increases, and the vacuum drops and it pulls just a bit less fuel out of the float bowl.....once you stop moving the throttle the flow stabilizes and becomes normal again.

When you close the throttle, the engine vacuum increases - but the fuel jets are being closed off and limiting how much fuel can flow (The TEV valve is supposed to help that situation - but does not eliminate it entirely).  When you roll off the throttle, the mixture will remain lean until the engine rpm drops to a speed that matches the available fuel/air flow......you will never be able to completely eliminate the lean condition when you are coasting.

You might want to take the TEV valve cover off your carb, and see if anything in there needs to be cleaned, or if the diaphragm is still flexible.

Jetting may help your bike run better - it made a big difference on my bike, as it had a lean surge just off idle when accelerating.  If I pulled away from a stop and slowly accelerated, the bike did not run smoothly and I could feel it surge as I rolled on a a small bit of throttle - and if I increased the amount of throttle to about half throttle, the engine smoothed out.  After jetting the best results for me was a #150 main, #50 pilot and 2 washers on the needle in place of the white spacer....I first tried the normal 3 washers recommended and the lean surge was still there - removing 1 washer made the lean surge go away.

I don't have any really long/steep hills around here that require a lot of coasting - but we did have one member from Colorado that had a steep downhill that lasted for miles.....and he reported that his bike banged and popped and surged the entire time.....I don't think there is much you can do in that situation - except to open the throttle a slight bit to help cut down on the amount of popping/banging out the muffler.

Dave wrote on 05/05/16 at 05:22:50:


I like that nehavior from the bike. It makes it sound mean!   I don't think anything can really be done about it.