Vintage BMW Motorcycle Owners

Lately, when I rev the engine a bit, it takes a while to get back to idle rpm after the throttle is released. Or, for example, when I come to a stop and pull in the clutch, the engine very slowly comes down to idle speed.

Anyone have any idea what might be causing this??

Thanks.

My first thought is that the mechanical advance is handing up either from being dirty or the springs are not pulling the advance weights in.

Did it just start doing this? Any maintenance event that precedes this?

Kurt makes a good point in regards to the advance mech as well as any maintenance done recently such as a tune up or valve adjust.

It would help to give the mileage, and last time the valves were adjusted and any kind of tune up was done. Do you still have points or is the bike converted to a pointless system?

The reason I ask and make a point about these things is that carbs should be the last thing to mess with when trouble shooting and airhead. Okay, not helpful but as Kurt mentioned a sticking advance unit can cause the problem. Points wear out and timing changes and that can cause idle RPM changes. Loose or tight valves can cause idle problems. Surprisingly carbs are pretty bullet proof if they get good gas. One of the weak links for them is the cables controlling them. So, on top of the advance system there is a possibility of a cable or cable hanging up.

One of the common problems with carbs is holes or worn diaphragms, your symptoms don't point to this being your problem.

Hope this helps, good luck. St.

Thanks for responses.

Mileage is 72,000. Valves adjusted about 3000 miles ago. Points are practically new, maybe 1000 miles on them. Recently I adjusted the carbs, but just messed around with the adjustment screws to get them in sync.

Still the advance unit could be suspect. However, if you just recently made some carb adjustments, that's the first place I'd look. Did you get the engine/carbs warm before making the adjustments? If not, then the bike is reacting differently when it's warm to the cold adjustments. You should probably go back to the starting point and redo the synch with a warm engine and do all three parts of the synch - idle mix, idle speed, and cable tension.

Again, Kurt makes a good point.

Problem with making carb adjustments other than the cable adjustments is the bike has to be warmed up. Now some bikes warmed up means five minutes or less of running in the garage, for others like My R80RT, it means a two hundred mile ride. I went nuts the first few years of ownership with the bike fiddling with the carbs trying to get things right.

After awhile, I found a lot of the problems I attributed to the carbs were in fact caused by the valve adjustment. Since on my RT, I have the electronic ignition, the ignition system was not a problem. On my old R75 with points, the points, and advance mech could cause problems.

Sorry, I am off track, The trick is, if the advance system is working fine, not binding and the points and timing are fine and adjusted properly, the valves are adjusted properly, then, you have to get the bike warmed up to the proper level to do the carb mixture adjustments. The tricky part is know just how warmed up and just how long the bike can sit at idle while the carbs are adjusted.

I will go back to may original statement, the carbs are pretty bullet proof ONCE they are adjusted for mixture properly, there is little to change in them. Cables and moving parts do change. A sticking cable can cause the problem you are having just as well as a mis adjusted mixture. This sticking cable may not just be on the throttle part of the system but it would be a good idea to check the choke cables and system as well.

At the mileage you have on the bike, have you ever checked the grease in the twist grip? On my bikes, there is an adjustable throttle lock to be used as a crude cruise control, I have inadvertently used mine and forgot to twist them back to loose. Causing the throttle to react slowly when released. Poor grease in the twist grip can do the same thing.

Good luck St.

I have a question related to the mechanical advance. I would like to define what “fine“ means when describing its performance. When I pull mine off the two wings that are pulled toward the center by springs have a fair amount of play before they contact the force of the springs. In other words there seems to be slop in them. Should they be pulled in tight to the center at all times? I mean while the unit is in rest?

I would think it should be somewhat tight to the body of the advance. But there's a couple of ways to see if it's working. If the bike is adjusted with the S-mark in the timing window at idle, you should be able to use a timing light and see that. Then increase throttle and find the F-mark in the window...and no father...around 3000 RPM. If that happens, they you're advance unit is working well on the ends of the timing function...it doesn't say anything about how fast the timing is advance...a weak spring will let the weights come out early which provides for more advance than might be required.

The second thing you could do is to remove the front cover...remove the battery negative connection first...then start the bike. Use the timing light to visualize what the advance weights are doing at idle and all the way up to full advance.

As a follow up, I ordered the two little springs that hold in the wings on the spark advance. I installed them and the bike does not wander in relation to idle.

In other words it no longer races and then dies down and races and then dies down while it's idling.