Tighten exhaust, end stumble?

[buggrin]

I posted earlier about a bottom end stumble that suddenly started one day after running the bike hard --WOT thru the gears to near redline RPM. Cleaned carbs twice, replaced jets & plug caps, etc., but never did get rid of the stumble. Plug color test shows nice tan color so I hesitated to fatten up the bottom even tho it seemed lean the way it was stumbling.

Now the strange part. Noticed a slight leak in the header gasket on the far right cylinder. Tightened it and the stumble immediately went away. Now it pulls all the way up from 2K. Not real strong at the bottom but steady (even at big throttle opening where it used to bog and stumble) and really starts pulling over about 4K. Does this make sense? Why would a very small exhaust leak cause stumble?

The guy that cleaned the carbs said the stumble might go away after a few hundred miles as the air flow becomes more laminar due to slightly smoother venturi surface because of small particles filling some small pits. Sounded like hocus pocus to me, but something sure made the stumble go away. Spooky, but I'll take it.

[DiverRay]

These bikes are VERY touchy when it comes to the exhaust. A leak is less back pressure on just one of four, and you WILL notice it. I had a leak on the #2 header to muffler clamp, and put up with a lot of noise and little pull. It was $26 for a replacement seal, but it has worked! Bike is now running like it used to, and pulls strong all the way to redline!