New problem with 1986 R80RT

[PeoriaMac]

Well, this was a first! This afternoon, the R80 started cutting out...just slightly at first and then going completely dead. I coasted to a stop, checked the battery connections and checked the carbs for fuel.
All seemed to be good. After a minute, the motor fired...and I got about a block, and it died again. More checking and it fired up again and I was able to make it home.
Additional facts: fresh gas, with about a gallon down from a full tank. Mileage: ? unknown to me. Speedo was replaced by previous owner, but bike does not show extensive wear. I had a tankbag on it for the first time, and the possibility of low air pressure in the tank did occur to me. This happened on my 78 R100. Today I noticed a "whoosh" when I pulled the hose off the petcock to remove the tank at home.
Still, I'm learning toward an ignition problem. Tom Cutter said not to pull the plugs to check for spark with these mono-shock models, so what's my next step?

Mac

[wecm31]

I had a tankbag on it for the first time, and the possibility of low air pressure in the tank did occur to me. This happened on my 78 R100. Today I noticed a "whoosh" when I pulled the hose off the petcock to remove the tank at home.

You sure present a good case that you are vacuum locking with the tank bag closing off the vent in the cap....and I've never had a "whoosh" when pulling a fuel line off. That would sure suggest vacuum too..

Spark check...I take an extra plug, put the wire on it, and with insulated pliers hold it against the engine and crank. With the carb bowls drained and the gas off, you don't get a start, just live cranking to check for spark. You don't want the test plug coming off the engine, that is bad for the Hall sensor in the ignition can. Gotta keep that loose plug well grounded.

[PeoriaMac]

The low air pressure in the tank appeals to me as a solution....but....I'm not sure it wouldn't have sputtered more before dying as it did. Which brings me back to ignition. That's a black art to me. My old airheads was a dual plugged R100/7 with two coils. This thing doesn't even have a coil. I spent 30 years with the old bike so I knew its foibles. This is like going back to high school...

Mac

[Stan_R80/7]

Take off the tank bag and see if the problem persists. If so, the tank bag and/or its location is causing the problem.

[dduelin]

Under the tank is the ignition control unit which is mounted on or next to the frame. The underside of it requires a thin layer of heat sink paste between the unit and the frame mount. The paste keeps the ICU from overheating and cutting out. My 1981 R100 had similar symptoms. It would run fine for a while then cut out momentarily then cut off only to restart in 30 seconds to a couple of minutes. The cut out was just like I turned off the key - the tach needle would drop to zero, maybe come back on once or twice then cut off completely. Radio Shack or the like has the paste for $3 or $4 a tube. Check the tank vacuum first - just leave the gas cap loose and see what happens. If that isn't it look at the heat sink paste.

[ISAMEMON]

crack-o-matic coil