I haven't begun looking at this problem yet, but maybe I'll start by gathering suggestions for what the problem could be.

485cc Yamaha, 1986, 6000 mi. *When the engine gets warm, one cylinder goes dead on idle. *Rev it up, and it comes to life. *Why might this occur? *Plugs are fine, new NGKs.

Twin carb motor??? If it is, I would bet that it is in the carbs. If the motor or cylinder is getting too much fuel, the plug will not fire it at idle. Once you rev the motor up you clear the fuel and it will begin to fire.

Just a thought.

Only one of 3 things cause combustion.

1. *Compression - Easy to check
2. *fuel and A/F ratio - simple enough to check
3. *Spark problem - sometimes difficult to check

Just a thought, I once had a motocross that had similar problems, coil was weak. Once engine came up to temp the CDI would malfunction.

Could be anything - just my 2 cents

Stator might be giving up on you. They can be tested for resistance.

thanks so far. *I did check the carbs/jets this winter upon discovering the problem--all clean, so I don't think that's it.

TORS system still hooked up?

I`m thinkin we can switch the carbs and the plug wires from cylinder to cylinder to verify if the dying clyinder is due to one of those systems and then if its not you will have to leak the motor down. Good luck. I`m not familer with your sled, what the ell is TORS?

T.O.R.S. (throttle override system) is designed to interrupt the ignition (miss) to keep the RPM below 2800 if the carb fails to return to idle when the lever is released.
But I don't know if it would be on a 86. I think my wifes XLV had one.

O, cats use something like that too.

Ya they had the T O R S in 85 , You can disconect the 2 wires on the carbs '' black and orange '' And put them together to bypass it . Another thing you say that when it warms up it starts missing on one side , could be the coil heating up and not sparking . And last of all Crank seals do a leak down test . good luck