1976 R90S Misfiring/hesitation

[DLC3]

Duane...
My static timing looks good. I then put my strobe light on the advance unit, but was not sure what I am looking for. What I saw was the advance cross piece was showing consistently at about 10 o'clock and 4 o'clock under the light. Was that what I should see?

[Deleted User 62]

With the timing light on the advance unit, slowly twist the throttle. You should see the weights move smoothly out to full advance at around 3000 rpm. They shouldn't hang at a lower rpm and then jump out at higher rpm. This would indicate the advance unit is sticking where it rides on the camshaft. To fix or clean it you have to remove the advance unit and clean/grease the camshaft and the inside of the advance unit tube as well.

[Duane Ausherman]

Follow Tim's advice. Sorry, I assumed that one should use the strobe to observe the flyweights through the rpm range. I should have spelled it out, but I probably have on my website.

To fix a sticky advance unit can be far more than just cleaning and lube. I have info and photos on my website.

As a test, I put "bmw motorcycle sticky advance unit" into Google and got my website as #1. I went there and read it. The info is there.

I will never understand why people ask here first. Nearly every question that one could ask about these old bikes is already answered on one of several good sites.

Learn to use Google first and save time.

[DLC3]

Thanks Duane...
Will go to your website now. I did use the strobe light on the advance as I increased the throttle. What I saw was a blinking strobe light as I heard the engine doing its misfire/hesitation thing. The advance arm that opens as the rpms increase seemed to be opening smoothly.

[Deleted User 62]

Check that the plug wires are tight and fully inserted into the coils. They should be long enough to go though the rubber thing under the tank. If not, wind and vibration can make the wires pull out of the coils. Another thought, how old are your plug wires, caps etc?

[DLC3]

Gentlemen...
Thanks so much for your insight. After reading Duane's site about advance unit problems, watched the weights on mine as I increased the rpms. I could see some blips in the weights repositioning rather than spreading evenly as rpms increased. I suspect I should get a new advance unit and try it, which I will do.

[Deleted User 62]

Advance units are very expensive! I had a similar problem on my R75/6, the advance unit grease had deteriorated to the consistency of dry jam. Capital Cycle sells the correct grease for lubricating them.

[DLC3]

Tim...
I removed the advance unit. It is properly greased, but one of the arms rubs a rough area as it opens and closes. I just ordered a new one ($250... gold plate?) and am looking forward to trying it out. I have a long list of what this motorcycle problem is NOT. Soon, I will have replaced every part on it... and then I will know it is my twitchy right hand on the trottle that causes this hesitation! Then I will have to ride two-up and have my wife work the throttle, but she likes to go slow. I do think you guys solved this one and will let you know when I get the part.

[Major Softie]

good luck - hope you have it right. Personally, even if I was planning on ordering the new one, I would try smoothing the trouble spot on this one and make sure it improved the situation before I spent the money on the new one.

[ME 109]

This motorcycle runs perfect at full throttle, pulling strong and fast all the way up through the RPMs.

What more could a bloke want.


The difference between running perfectly, and shitty, is the throttle position.

If it runs perfectly at 4000 WOT, and then it runs shitty at 4000 with less throttle, how can that be electrical?