Sportster 717 - Power loss + surging when warm

[SuperMod]

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Sportster - Single engine 717
Works fine until warm then power loss and surging.
Engine temp seems Ok, not too hot?
Fresh plugs and main fuel filter cleaning didn't change anything?
Pulled the intake off and went for a rip w/ open carbs = same result.
Haven't tried turning the fuel flow to Reserve trick yet.
Opened the elec' box lid and all dry also seems Ok w/ visual inspection.
Tried swapping plug wire leads and works the same both ways...is there a right/wrong way?
Haven't done any comp test yet being it works so good until warm.
Thinking of going for a rip at night w/ buddy driving and watching for spark jumping under load.
If nothing else appears, I'll be looking towards the carbs next.
What problems usually cause these same symptoms?

Thanks in advance for any help!


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[Dr Honda]

With your description... sounds like it's overheating. BUT... if the carbs are running rich... that would give the same effect. (Since it needs more fuel when cold to run properly) But, you say the temp is OK. How do you know? How did you check? When having issues, you can't trust the temp sender and buzzer.

Try changing the fuel valve... but I'm guessing the issue is in the carbs, and they need to come apart.


As far as the ignition... it doesn't cause surging. It would cut on/off like someone was throwing a switch.

 

[SuperMod]

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Thanks for your input and good suggestions!
How best can I check engine temp?
When the problem occurs, it acts like it may be cutting back to one cyl. so why I was wondering about an ignition problem.
What of ignition lead orientation, do these engines work the same regardless of which way they're hooked up?
Reading other threads here looks like I should look for original grey fuel lines which may be collapsing internally?
As to changing the fuel valve, are you suggesting I rotate to Reserve position and try it there?

Thanks!
~

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[Dr Honda]

It fires both pugs together. SO... you can actually swap the plug wires, and it will run just fine. BUT... if it feels more electrical to you (like one plug shuts off) then there is a simple test. Get a couple inline spark testers, and plug them on. Then go for a ride (with a helper) and watch them.

The other way is to carefully touch the base of the spark plug. If one plug isn't firing... it will be cool. BUT... if they are firing... they can be very hot. I check by touching all the time. Lick your fingers, and give it a quick touch the first try. You will know if they are hot or not.

Temp is easy. Grab an IR temp gun, and just shoot it at the cyl head. 190 or less is what you want. Most of the time, they are 130~140 when water is flowing.