Motorcycle idle speed?

With the back firing, I would also guess lean air/fuel ratio. Does it have after market exhaust? Even adding just a slip-on style pipe it will lean out the engine because the engine is able to pump more air out the exhaust and it will need jetted.

If everything is stock, then you might just have a clogged jet in the carb. Due to the age, check for intake leaks in front of the carbs. Those rubber boots take a lot of heat and dry out over the years. They get brittle and crack allowing air to sneak in ahead of the carb. That extra air doesn't have any fuel in it, there by causing a lean issue.

You can check this by lightly dusting the rubber boots with carb cleaner or contact bake cleaner while the bike is running. If the idle increases you have a leak. The flammable cleaner is getting into the intake and burning in the combustion cambers. ONLY lightly dust the boots.

You can adjust the idle up, but this is just a temp fix. It's not going to hurt the bike-it's just not fixing the root of the problem.

**note*** the first answer only works on SINGLE cylinder motorcycles. If you start messing with the air mixture screws, your going to have trouble. Mainly because your going to throw the carbs out of sync on a multiple cylinder engine.

Well then you'll never get it fixed........if your having an idle problem the only way to fix it is with the carb if the air/fuel mixture screw doesn't work. Here's how to tell.......turn the air/fuel screw all the way in and mark it with a pencil on the screw and to part of the carb in a straight line then turn the screw out 2 1/2 times that should be close to stock setting for all motorcycles.....if it's still too fast or too slow turn it 1/4 of a turn in if too fast and out if too slow......at each 1/4 turn wait 15-30 seconds to see if it makes a difference. Hope this helps.

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If your bike has a carb, the float level may be set a bit high. At an idle, the too high level richens the idle mixture and eventually chokes it down. Or, the idle circuit air or fuel screws may need to be adjusted to lean out the mixture a bit. On some carbs the screws control air and others they control fuel. Make a mark where the screws are set at and then try turning them (one on each carb) in say a quarter turn and see how it runs. If it hesitates or runs worse, turn the screws a quarter turn from the starting point in the opposite direction and see how it runs. If things improve but still aren't quite right adjust a bit more. If the carb is stock, I wouldn't go more than about a half turn from the original setting. If your bike has fuel injection, the throttle position sensor may be on its way out.

First thing - turn up the idle.

As an engine wares out from normal usage - the idle has too be adjusted to compensate.

Adjust each mixture screw out 1/4 turn at a time - that should clean up the back firing - but first change the spark plugs.

My best guess is that the carbs need to be synchronised. Backfiring is a classic sign of one of the carbs being out of synch with the other. You can do this yourself with a set of vacuum gauges. I had the same problem with a BMW R80ST.

Like Mike says some body better want to work on the carb they dont repair them selves