Irregular idle and backfiring at highway speed

Is this probably due to gummed up carbs?

Other info: Bike is a stock 82 XJ650 Seca and has been out of commission for a while. I recently got it back on the road with a new battery and tire. Nothing on the engine has been replaced. Carb boot are looking cracked but the bike runs pretty well once started and around town. Starting usualy takes a few tries.

 

Have you checked the basics? Plugs, air filter and valve adjustment, after that I would look for exhaust leaks. Then a carb clean and overhaul.

 

FIRST ITEM: Check rear brake shoes for delamination: http://xjbikes.com/Forums/viewtopic/t=15874.html

Now then. Yes, it's "gummed up" carbs. And possibly way overdue for a valve adjustment.

You can't just take an old bike that's been sitting, throw in a battery and change a tire, and hit the road. There is a process involved, the Antique Bike guys call it "recommissioning" and I like it. You said yourself it has been out of commission for a while. It needs to be properly recommissioned.

Read this rant: http://xjbikes.com/Forums/viewtopic/t=24010.html

And this: http://xjbikes.com/Forums/viewtopic/t=14581.html

And start shopping for a service manual.

Carb intake rubbers (manifolds) commonly crack all over on the surface. As long as the cracks don't go all the way through, they do not need to be replaced. As long as they aren't leaking, it's better not to mess with them. It's VERY EASY to break a manifold bolt off in the head. You can clean them up, and pack the cracks with black RTV; paint with liquid electrical tape; or even "skin" with hunks of bicycle inner tube.

However, if you have a vacuum leak between the manifold(s) and head, then they will need attending to.

Backfiring is generally an indication of fuel starvation or an extreme vacuum leak; do not keep riding the bike if it's backfiring you will hurt it. And don't ride it at all until you check those rear brake shoes.

 

Turns out it was just bad gas (new gas). I had similar symptoms to this guy:

http://www.triumphrat.net/twins-technic ... as-or.html

So I added a small amount of Heet to the gas and it cleared up within a couple of miles.

Thanks for the additional info. I do have the service manual, although I haven't done a thorough overhaul of the bike. I did take a look at the rear brakes when I removed the tire to be replaced and there didn't appear to be any issues with them coming apart.