My 1986 FXSTC Evo with a TM40 carburetor is misfiring. What I mean by that is, when I turn the throttle, say, to go from 5 or 10 mph to -> 25 or 35 mph, it starts to stumble or misfire. There's a noticeable loss of power as well. Now, I cleaned the carburetor multiple times while setting the floats with a gauge, and also installed another known good tm40 with the same result. So, I don't think it's the tm40 carburetor.

The other thing that's happening is the plugs get very sooty, almost instantaneously after one short ride. Not oily, but a dried black soot covers the entire end. The spark plugs tested good using an ohmmeter, around 4 and change, and 6 and change for both at 20k. But when checking for spark against the heads, at least to my novice eyes, it looks kind of weak. There is a spark for each plug, however. I checked the gaps and ordered 2 more online. Do you think there may be a problem with the ignition system or something else? If you have any ideas on how to proceed from here, I'd appreciate it. Thanks.

 

It sounds like the coil is dying. A weak spark will make the engine miss under a load.

 

i would suspect the ignition by the description of the plugs, plugs are cold sooty. if you have a service manual there is probably a procedure to test the coil.

 

If you can run todays **** gas and get clean plugs...LMK where yer gettin yer gas... If that was My Bike I would be checking/replacing the VOES...First
You do Have the Accelerator Pump turned all the way Off for testing/tuning...Right..as per Instructions....

 

I use ethanol mainly. Here's where you can find it:

https://www.pure-gas.org/

Nowhere in the Mikuni instructions does it say to turn off/disable the acc pump to tune the carb.

 

Thanks. I do have the service manual and will test the coil.

 

Leave it to the Japs to lead you HD asshats down a Bad Road....

 

Genius take. But as already mentioned, two known good carbs were tried.

 

And...A carb that works on one Bike ...should work as well on Any Bike....

 

Same bike, dummy.