Interesting engine stumble at speed

[Snakeoil]

Solved the problem and learned something in the process.

I pulled the 135 Mains out and reinstalled the 130's that came with the bike. Both with and without the baffles the bike runs great! Back to it's old self again.

My theory for what was going on is as follows.

With the bike at high revs and WOT, when you start to close the throttle there is a very brief moment where the revs stay at or very near where they were at WOT. That means that the air flow thru the carb also remains the same. But for that same instant, the slide closes and this means for the same volume to flow, the velocity must increase thru the venturi. When that happens, it draws more gas thru the needle/main jet. If you are running rich, it dumps an over rich mixture into the engine and in essence floods it for that brief moment. At the same time the revs decrease, the flow decreases which means the velocity thru the carb decreases and the mixture returns normal. This is why I could not get the stumble to be anything other than momentary. This would be the same as you closing the choke and reopening it quickly. Engine would flood and then clear.

What I think this demonstrates is another symptom of an engine that is running too large a main jet. It's the first time I've ever run into this. But that's what makes this fun. Learn something new every day.

regards,
Rob

[brewsky]

Glad you got it sorted out.

I sometimes think there is some psychology at play when tuning. You already have a suspicion you are rich (or lean) somewhere, then go out an road test for feel, and confirm the flat spot, stumble or hesitation is because of your initial beliefs. So you adjust accordingly, only to find out you made it worse.

I'm going to try and mount a video camera so my A/F meter and throttle position can be captured together on the road to really see what's going on under different conditions.