My 1985 K100 runs great for about 20 minutes, then starts to cut out, and within a few minutes cuts out completely. I have to wait anywhere from an hour to overnight for it to start, then it starts nicely again.

Today it stopped about 6 or 7 hours ago, I just started it and it ran rough, some smell of gas (maybe just firing on 2 cylinders) then won't start at all.

Does this sound like the 
- Hall Sensor going bad
- Bad coil?
- Something else?

I've read I can test the Hall Sensor with a hair dryer - but are there 2 Hall sensors? I guess if I ascertain it is the Hall Sensor(s) then I can investigate further

Or could it be a coil has gone bad? If so is there a way to test for that?

(Or am I on the wrong track entirely?)


FWIW I just replaced the air intakes, cleaned and tuned the throttle bodies with a manometer and when it runs it runs well.

It certainly sounds like the hall sensors and a good place to start looking. You can get the sensors quite cheaply but they need to be fitted to the plate which isn't a difficult job. If you go to the Forum page and use the google search I am sure that many posts about where to get them and fitting will come up.
I would not discount the possibility of it being a coil but the hall sensoes are the likely culprit.

Thanks much! I'll go ahead and head down that path.

timz10000 wrote:My 1985 K100 runs great for about 20 minutes, then starts to cut out, and within a few minutes cuts out completely. I have to wait anywhere from an hour to overnight for it to start, then it starts nicely again.

It's like you have copied and pasted the first paragraph of numerous threads about bad Hall Effect Sensors(HES). 

This is how they start to fail, and finally stop working altogether.   The hair dryer will make them cut out sooner, while a shot of freeze spray will get them working temporarily. 

Replacements are available.  There have been s couple sources posted here, and a search on Hall Effect Sensors should turn up threads where they have been posted. 

Replacement requires drilling out the rivets, riveting the new sensors, and some soldering.  Takes a couple hours, but not impossible.  Should be doable for about $50 or less.

An alternative would be to search eBay.  There should be a complete plate
assembly ready to install there.  Price should be $100 or less.

Replacement requires drilling out the rivets, riveting the new sensors, and some soldering.  Takes a couple hours, but not impossible.  Should be doable for about $50 or less.

An alternative would be to search eBay.  There should be a complete plate
assembly ready to install there.  Price should be $100 or less.

Great, thanks. Looks like the plate assembly is more like $200 on eBay and Amazon right now but I'll watch it for a few days and see if anything cheaper pops up, or maybe just dive into the replacement routine.

Euromoto Electrics (EME) have both Bosch and Endurolast HES the later being the cheapest option if you are after a complete unit. I'm pretty sure the Bosch unit is plug'n'Play but the Endurolast has a different plug but they give you the makings to replace the socket upstream. Endurolast HES

If you want to fit them yourself then you need to read this especially the link to the fitting guide.