1982 R100RS - Hit Pothole, Backfired and Died

[mtsfestivus]

The other day I hit a pothole doing about 15-20mph and my bike sputtered, backfired and died. When I tried starting back up it tried to turn over then backfired again. Nothing. Trying one more time with no result, I flicked the kill switch off and got a pop. Seemed like there was some unused fuel in the cylinders. I checked the fuel on the spot and it's flowing fine. Eventually I ended up having to trailer her back home. Currently, it's not wanting to fire at all.

Before this happened, the bike recently started to stumble around 3500rpms. I assumed this meant I needed to replace old, worn-out jets since I just had a complete tune up. However, now I'm thinking it was the first sign of an imminent ignition issue.

I grounded the plugs to the cylinder head, one at a time, and got spark when I hit the starter. I also get spark when flicking the kill switch off. I should note that initially the sparks were intermittent, though now they seem reliable. I've been doing some tests on ignition components and am getting good voltage and resistance readings. I pulled off the coils and all the resistance readings check out, although I did notice some corrosion inside one of the towers. I cleaned it as best I could and got a good resistance reading when I connected the plug wire. I took off the front cover, cleaned the connections between the ignition trigger unit/hall sensor and the ICU. I checked the kill switch and everything looks clean inside. It seems to function properly when switched from Run to Off. Even though all these tests check out, could I still have faulty coils? Or would a bad Hall sensor still produce spark?

I checked the throttle/choke cables at the carbs and everything is in place. I opened the headlight bucket and everything is securely connected. All lights are functioning properly. I removed the starter cover to check everything there—all looks good. I'm just trying to pinpoint the problem before I blindly spend $500 to replace coils/Hall sensor, etc.

Could the bump have thrown timing or valves off? Has anyone had a similar experience?

Also of note, the bike has a new starter, alternator, diode board, rectifier, plug wires, and plugs. The carbs were recently rebuilt and a proper tune up was completed the day before this happened.

[ME 109]

Try spraying some starter fluid or whatever in a cylinder, put the plug back in, hit the starter and see what you get.

But I think it's electrical.
Check the left hand coil mounting bracket. It can break, and take out several grounds with it.
Check ignition switch wires, make sure they're all connected and give them a wiggle as you try and start the bike.

Edit, you can hit potholes much faster than that! Actually, if you go fast enough they disappear.

[mtsfestivus]

Ha, I thought I'd be fine! There's construction in my neighborhood, it was dark and there was loose gravel around so I didn't want to swerve. I was totally shocked when the bike just died.

Thanks for the reply. The coil mounting bracket is solid. I just took them out to test and reassembled. I may try some wiggling tomorrow.

Along with stumbling around 3500, the tach would also occasionally jump and bounce between 5-8000 for a couple seconds, giving false readings. This is when I'd be much lower, around 2-3000. Could this be another sign of faulty coils?

[ME 109]

Check your battery connections, and ground at the gearbox. Undo the battery terminals, clean, and reattach.

Electric for sure with the tach involved.

Check the short black wire running between the two coils.

[Rob]

There is also a major ground wire coming out of the harness and attached to the frame somewhere, often on one of the coil mounts. Check that for cleanliness.

And after cleaning, be sure to put on some grease to displace moisture. Anything will work, dielectric, axle, even vaseline.

[mtsfestivus]

Try spraying some starter fluid or whatever in a cylinder, put the plug back in, hit the starter and see what you get.

But I think it's electrical.
Check the left hand coil mounting bracket. It can break, and take out several grounds with it.
Check ignition switch wires, make sure they're all connected and give them a wiggle as you try and start the bike.

Edit, you can hit potholes much faster than that! Actually, if you go fast enough they disappear.

Alright, I double checked all ground connections and cleaned them all. The bike is actually firing now but still not running. I have the headlamp removed and when I wiggle the ignition switch from the inside while trying to start it seems to help, but still nothing. Any tips on opening the switch and cleaning it out?

[Major Softie]

Just bypass it. If it fixes the problem THEN you can fight with cleaning it out. If not, keep looking.

[ME 109]

Try spraying some starter fluid or whatever in a cylinder, put the plug back in, hit the starter and see what you get.

But I think it's electrical.
Check the left hand coil mounting bracket. It can break, and take out several grounds with it.
Check ignition switch wires, make sure they're all connected and give them a wiggle as you try and start the bike.

Edit, you can hit potholes much faster than that! Actually, if you go fast enough they disappear.

Alright, I double checked all ground connections and cleaned them all. The bike is actually firing now but still not running. I have the headlamp removed and when I wiggle the ignition switch from the inside while trying to start it seems to help, but still nothing. Any tips on opening the switch and cleaning it out?

Jump the ignition wires at the switch, if you can. That will prove the switch.
Did you pull the battery cables off at the battery? They can be tight, and fail.
What is the tach doing now, when you try and start?
Did you find and check the short black wire connecting the coils?
Let us know everything you do, that has been suggested. That's the only way we can trace the problem.

[chasbmw]

90% of airhead electrical problems are down to poor connections at the starter relay.

[SteveD]

90% of airhead electrical problems are down to poor connections at the starter relay.

Nah, it's always (almost ) the battery...or the carburetors!