Off-Idle Stumble - Causes?

[WingMan71]

Troubleshooting an off-idle stumble on my '76 GL1000 LTD problem. (Now owned by a friend of mine.)

After about 1,000 miles of riding it has developed an off-idle stumble upon cracking the throttle open fairly quickly, not when opening the throttle just gradually.

Carbs were removed by me, completely disassembled, sonically cleaned, and re-assembled with a Randakk's Cycle Shakk carb kit about a year before I sold it. I know what I'm doing and have rebuilt dozens of carbs on vintage bikes. So, they were done right. Pilot screws set to 3 turns out, as recommended by Mike Nixon's book. Carbs properly synchronized. Bike was running great when I sold it.

My buddy says he keeps Stabil in the (ethanol) gas, but the bike gets ridden only weekly. So, there's that issue. I'm thinking partially plugged idle jets.

Would appreciate any other ideas regarding the off-idle stumble.

[redglbx]

Replace the low speed #115 air jets in the brass elbows with #110’s that should fix it.. some go with #105 or 107’s but I’ve had good luck with the #110’s . This is a noted problem with the early wings but doesn’t affect them all. Easy to do on the bike. I get mine from PJ Motorsports.

[WingMan71]

Thanks much for that tip and the source for the jets.

[OhioBill]

Check the resistance of spark plug caps. I chased the same problem with carb overhaul, off and on multiple times. Turned out to be spark plug cap.

[gltriker]

How many years ago did you last time have these carburetors apart ?
Leave jet sizes be and go through the entire system, again.

Drain the stabilized gasoline and start the engine afterwards with new fresh gasoline.
Actually, simply a new fill of non-treated gasoline into the system might be the best way to approach the "mystery" ?

[redglbx]

Ohio Bills spark plug cap suggestion is a good one,,, measuring across just the caps should read less than 5k ohms.

[WingMan71]

Check the resistance of spark plug caps. I chased the same problem with carb overhaul, off and on multiple times. Turned out to be spark plug cap.

We'll do that.

Thanks for the suggestion!

[Whiskerfish]

Check the header temps at idle and make sure it is firing on all 4. Also do the idle drop out. Easy checks. Most times I have found a stumble is due to it not hitting on all 4.

[WingMan71]

Check the header temps at idle and make sure it is firing on all 4. Also do the idle drop out. Easy checks. Most times I have found a stumble is due to it not hitting on all 4.

Thanks. We'll check that.