Idles poorly, surges and dies after 40 minutes of riding

Melon

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Hello,

I have an issue that I cannot find a solution to. My 1980 Special starts first kick, has a steady idle and rides fine for ~40 minutes. During the first 25 minutes the idle rises about 2k, which I adjust for with the idle screw while i'm at a stop light. Consistently, around the 40 minute mark, it starts behaving differently. The first signs are a slight jitter in my at-idle RPM seen through the tach, which progresses into noticeable surging as I ride the bike. This happens both at steady throttle and during acceleration. At this point I have 2 minutes of riding until the bike shuts off at the slightest release of throttle. If I let the bike idle in this state, it will stumble intermittently until it finally dies. When I try to kick the bike over again, it turns over about 5 times and makes an aggressive sputtering sound, which almost sounds like its through the carb. I usually let the bike cool off and it starts up first kick, but the previously described symptoms return, proportionately to how long I let it cool off. I have repeated this one three separate days and the same thing happens every time.

Engine:
-Jugs bored and honed with #6 oversized pistons, valves re-seated by a shop
-new cam chain
-Carbs are BS34's with #45 pilots and #140 mains, float level set at 27mm, fuel screws at 3 turns out, new boots and gaskets, Uni pod filters, boot barbs capped with rubber stoppers
-New gas cap, and replaced the vacuum petcock with a old style on/off ball valve petcock
-Compression after a few kicks (cold) is ~150psi on both cylinders

Electrical:
-new wiring harness from Mikesxs
-PMA kit https://www.mikesxs.net/yamaha-xs650-xscharge-permanent-magnet-alternator-kit-pma-200-watt.html
-Battery removed and replaced with capacitor
-TCI replaced with e-advanced https://www.mikesxs.net/blog/how-to...-xs650-gen-ii-e-advancer-electronic-ignition/
-new ignition coil https://www.mikesxs.net/yamaha-xs650-ultimate-high-output-coil.html

I tested the coil after the bike died and it reads 2.4ohms//16.8kohms. When cold it reads 2.5//13kohms. The webpage gives the specs as "Primary resistance 2.45 ohms. Secondary resistance 15k ohms" so these values seem in the ballpark.

As you can see ive invested a lot into this bike, and have done a decent amount of research. but I am totally stumped on this one. My first thought was the coil, but it still seems to have spark when kicked after dying, as it sputters for a bit.

Any ideas?
 

bosco659

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Gas tank not venting properly? Try riding with the cap not latched tightly at 40 minutes.

May also be the ignition components failing with heat. I’d try the fuel first because it’s easy.
 

Melon

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Gas tank not venting properly? Try riding with the cap not latched tightly at 40 minutes.

May also be the ignition components failing with heat. I’d try the fuel first because it’s easy.
The cap is new, and I've opened it intermittently while testing this. The symptoms appear to be a function of heat
 

Wingedwheel

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One thing that stands out to me is all the Mikes parts. Did you own this bike before the parts were installed and if so did it run fine then?
 

Halftwist

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I would highly suspect your e advance. When they get hot they go to hell. Those xs charge units are famously unreliable.

Check out the gonzo thread and get in touch with Jim.

I’ve been running a gonzo box for 2k miles without a hiccup. Slight mods to the wiring harness on your 80 tci bike and it’s a 13 dollar part, cheap enough to run a spare.
 
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