Puzzling me, my old Yamaha XT500

[duckling]

I've had this particular XT500, a four stroke single and my fourth, for about 18 months. It had on a megaphone style race exhaust for the past riding season. Just as it started to get cold here in Idaho, I ditched the pipe for a factory muffler and installed it after I refinished the metal. It was sandblasted and painted and I bolted it on and started it right up. It ran for about 10 seconds, then as I revved it a little, it died and hasn't started for anything again. My question is, though it has fuel, spark, compression (a recent rings and head job), could the mixture have changed so much by the different flow characteristics that it wouldn't run without another jetting change?

Thanks for your thoughts, fellas.

[reelrazor]

I'd crack the pipe loose at the head and see if it runs.

I am thinking that the stock pipe you put on had been sitting for a while-maybe years?

With it's tiny passages thru the muffler, it may well not flow enough air to be able to run.....nothing to do with mixture/jetting....air can't get out-so fuel/air mix can't get in.

Those exact symptoms will be seen on any engine with a spark arrestor when the spark arrestor is carbon clogged(or chainsaw engines when the mud daubers plug off the muffler can....


Cracking the pipe loose at the head is a fast and easy way to see if excessive backpresure is the culprit