Misfiring after bike is warmed up.

My bike is an 83 xj550 maxim. The carbs are clean and all that jazz...

So the bike has been running great but last night it stranded me. The engine had just warmed up after a couple miles and it seemed to be misfiring slightly. I press on and it gets worse and worse to the point of having very little power. I crack the thottle and I can hear it misfire and the bike is trying to lurch. I pull in the clutch and the bike shuts off. I leave the bike in a parking lot overnight....

So I go pick up the bike today and it starts right up and runs like a dream, good power/response. Then...it starts to stumble again, after a mile er two, slightly then progressively gets worse...no power etc etc and dies on me in a parking lot.

I've narrowed it down to something ignition related. Battery/charging system are healthy. The plugs look fine. The plug caps test ok (slightly off from each other but still ok). The coils test at 12k between the wires with no caps but I tested them after they cooled down. Would these original wires/coils only show problems after the engine reaches operating temp?

So any ideas/help would be appreciated. I'm frustrated

 

brushes or battery maybe. could be the coils I guess? Have you load tested the battery and checked the brushes?

 

Vacuum lock in the fuel tank? When you open the fuel cap immediately after one of these episodes occur, do you hear a "whoosh" sound? That would indicate a fuel cap issue, as the lack of air entry into the tank will cause the fuel in the tank to "lock up" and not flow into the carbs.

 

Edited original post....Batt/charging system are healthy.

 

I will check this but I highly doubt it. Gas cap still doesnt seal that well even with new seal installed...

 

If ... "It runs just fine for a short while; then starts acting up", ... you might be "Living off the top of a bad battery".

Put a Voltmeter on the Battery Terminals and measure the Charging Current.
( +14.2 ~ 14.5V @ 2000 rpm's and above )

The Battery might not be capable of getting DEEP Charged.

Test the Charging Output.
Bring a Charged-up Battery in to AutoZone for a AMPS Test.

 

Might be weak vacuum pull from your carb-to-petcock. (vac. leak in carb boot, crack in vac hose, weak/collapsing hose, or problem in petcock) You can try running on prime for a quick test.

 

did you ever check the valves?

 

Awww, man, that's MY question.

Gotta start there.