I assume she has stored this bike all winter with ZERO fuel in the tank and carbs. This since the 1) perils of ethanol fuels and 2) the topic of modern fuels going stale (yes, even ethanol-free) within weeks has been beaten to death on every motorcycle, chain saw, and lawn mower bulletin board for the last 20+ years. Bad fuel IS the number one issue.
If that knowledge is our common baseline, then...
Her problem is symptomatic of the gas cap breather system being clogged. This is a common issue for bikes in multi-month storage. There are multiple fuel caps in use, but suffice to say she should be able to find a small hole(s) on the underside of the cap. She should be able to easily push-pull air through that hole at lung pressures. (That's right, stick the gas cap in your mouth and breathe !) In the 70's this was a simple hole with a labyrinth to keep fuel from sloshing out. In the 80's the EPA made them implement a more complicated system to prevent fuel spills in case the bike fell over. If the cap can be disassembled, then sometimes you can remove the fancy rubber flappers to return the cap to the pre-1980's simpler (and never-have-this-issue-again) labyrinth system. If dis-assembly is not possible, hit the breather hole with high pressure compressed air to "wake it up" !!
OR, if she has recently replaced the original screw-on fuel cap with one from a car. A car fuel cap will not work on a bike because cars have their own fuel tank breather system which is NOT implemented in the cap.
The issue revels itself as you ride down the road far enough to pull a vacuum on the tank. Due to the vacuum, fuel refuses to flow to the carbs and the bike stalls out. The rider thinking he/she has run out of fuel opens the cap to check. The vacuum is removed and fuel flows. Mysteriously, the bike will then crank right up and runs for another 4-5 miles. Then the cycle repeats.
• The simpler screw-in flush mounted caps of the late 70's can simply be un-screwed a half turn to break the seal, but fuel may start to slosh out onto the rider if the tank is full.
• Be aware that a rubber-bottom tank bag can also be sealing the tank fuel cap from breathing ! BMW supplies an over-fill drain on most tanks with the flush-fitting fuel cap. When a tank bag is present on top of the tank, the fuel cap "breathes" through this over-fill system . If that over-fill drain is clogged then it has the same impact as a clogged fuel cap breather.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
My daughter in Anchorage Ak has a 1978 R80/7 and could use some help in diagnosing and repairing her bike. Runs fine for awhile then cuts out and dies. Lots of new parts. I'm in Nh and can't fly up this year to help. Says she is going to bring it to the Bmw dealer and I fear that is going to be frightfully expensive. So if you can help, send your contact info to me and I will forward it on to her.
Thank you very much for your detailed reply. But it is not a breather system problem. She was having this problem last August, before putting the bike away. Has new points, plugs, condenser, valves adjusted, timing good, New carb diaphragm s, very low mileage rebuilt engine, with new nickasil cylinders, New valves, guides,springs, etc. I sent up some good plug wires and coils and good handlebar switches, but I don't think they have been installed yet. She needs a knowledgeable airhead to help her out. Or a shop specializing in airheads, which we haven't been able to locate.
You're correct, those intermittent issues can be hard to locate. And with a dealership mechanic that's never seen one before you can quickly exceed the value of the entire bike at the $60+ per hour labor rate.
My money is on battery, battery connections, and electrical connections dealing with an electronic ignition (if fitted).
Best advice is to find the closest Tech Weekend and trailer it there. If Washington state seems like a hike, then remind her that we're always open in Cornelia, GA !!
All the best.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Advertisement
