I put new slip on mufflers on my 1986 250. I am getting fouled plugs and itís running rough with backfires. Guessing I need to rejet carb to match the much more open pipes. Any ideas how to know which carb jet and where to get it? Much thanks.
jrk1086 - fouled plugs would seem to insinuate too much fuel. Logically swapping to slip on mufflers would open it up somewhat and make it run more lean.
An actual backfire, fires into the intake ("back"ward). More than likely you're getting popping in the exhaust. Which also implies excess fuel in the exhaust. Either way, follow flitecontrol's link. He knows what he's talking about.
Now that I think about it, I've went down one main jet size just for research. And when it is cool out, and the engine is not quite fully warmed, I can get a pop out of the stock mufflers. It's weird, it's not what I would expect. It is when I have been running for about 30 seconds at about 55 mph, then I let off and let the speed coast down in 5th, slowing to make a left turn. When I clutch and blip the throttle (while pushing down to 4th gear), that's when I get a pop sometimes.
I'm not surprised that it does it. I'm just surprised it does it with my smaller main jet. However, I never rode my 2016 (new to me this summer) in the cool weather with the stock main jet.
Lots of main jet tuning coming in the spring.