Need some input here
Could be an intake leak as the seals get old after a while. Several different methods of testing, but most require the spraying of another combustible aerosol around the seal area on the intake and noting the change in idle, etc.
Griz
A vacuum leak in the petcock or vacuum line could account for the motor being starved for fuel. This would be a good time to go over the fuel system and clean the fuel strainer at the petcock (or replace, as mentioned). Also remove and clean the float bowl and carb jets. A new fuel line (and crossover line) wouldn't hurt if you don't know when they were replaced. Cleaning out the fuel tanks would be advisable if there's a lot of junk in the petcock strainer and float bowl.
(edit), I see Sir Graham has posted simultaneously with me.
(a) shut off the petcock
(b) disconnect the fuel line from the carby (be careful...from the factory the inlet nipple on the carb is plastic, and they tend to break easily. If your's does break, no biggie. It can be replaced.)
(c) pull the line back through to the left hand side of the bike
(d) turn the petcock on and drain the tank into a suitable size gas can. Not a lot of people have five gallon cans sitting around. Holler at your neighbors and have them bring their gas cans over for a free fill-up. Be prepared to wait a while. It takes a fair amount of time to drain a full tank.
If you have the vacuum petcock on there, I'm not sure how to get it to flow without putting vacuum on the control hose. Hopefully someone chimes in with that info.
I agree with the above advice regarding getting the new petcock installed and seeing how things stand from there.
If you find that your problems are still there after the new petcock is on, don't fret. Cleaning out the carb is a bit of a fiddle, but nothing you can't handle!
Oh, and while you are at this, you may as well head down to the hardware store and get a new piece of fuel hose. Can't hurt. Get a couple of small hose clamps while you are there, in case your bike has the crimp kind now that you have to cut off.
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Dual fire you can swap one end of the wire only, or swap the complete wire front to rear to help isolate. If it's single fire, you have to swap both ends at once (complete wire from cylinder to cylinder). A single fire system can also drop a cylinder via the ECM, so check those leads with a VOM as well.
It sounds worse than it is. You'll track it down in 15 minutes.








