Misfiring help
Thread Starter
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Cruiser
Joined: Oct 2017
Posts: 173
Likes: 83
From: Pennsylvania
Misfiring help
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.
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.
Last edited by Dirty Dancer; Aug 7, 2024 at 04:17 PM.
Seasoned HDF Member
Joined: Jul 2014
Posts: 8,608
Likes: 3,375
From: Cental, MI
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....
You do Have the Accelerator Pump turned all the way Off for testing/tuning...Right..as per Instructions....
Last edited by Racepres; Aug 8, 2024 at 07:03 AM.
Thread Starter
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Cruiser
Joined: Oct 2017
Posts: 173
Likes: 83
From: Pennsylvania
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.
https://www.pure-gas.org/
Nowhere in the Mikuni instructions does it say to turn off/disable the acc pump to tune the carb.
Last edited by Dirty Dancer; Aug 8, 2024 at 04:39 PM.
Seasoned HDF Member
Joined: Jul 2014
Posts: 8,608
Likes: 3,375
From: Cental, MI
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.
https://www.pure-gas.org/
Nowhere in the Mikuni instructions does it say to turn off/disable the acc pump to tune the carb.
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Cruiser
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From: Pennsylvania
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