Carburetor problem?

[boman]

I have a R100S 1979. It is in good working order and was recently serviced. Now I have cleaned the carburetors (Bing) and the bike runs nice and smooth.

But when the bike is really warm after 40 min drive the idle hangs (in about 2000 rpm). I can get the idle to normal by breaking with the clutch engaged and thus forcing down to idle. Then the bike stays on idle (arount 1000 rpm).

The "butterfly" arrangement works nice and smooth and goes directly to "stop" position but the rpm goes very slow down and (most of the time) stops at 2000 rpm.

Any ideas?
Maybe better to change to Mikuni carburetors (seems to be issues with the Bings)?

Thanks in advance

Kjell-Ake Boberg
Motala Sweden

[ME 109]

Hello Boman, did the problem surface after the 'service' or after you cleaned the carbs?
The carbs on your bike are more than adequate, in good repair.

[Major Softie]

First two guesses:

If it was caused by your carb work: if you removed the butterflies from their shaft, then perhaps they are not aligned properly, and thus not closing properly.

If it's unrelated to the work you did: sticky advance mechanism.

[boman]

First two guesses:

If it was caused by your carb work: if you removed the butterflies from their shaft, then perhaps they are not aligned properly, and thus not closing properly.

If it's unrelated to the work you did: sticky advance mechanism.

The problem was there before my carb work and also other problems (no idle, sometimes runnin on one cylinder, leaking...)
I did not remove the butterflies from the shaft when cleaning.

I don't know what "advance mechanism" means. If it is the "pistons" that are lifted by the "rubber" then they where sticky before cleaning but now they are very smooth. Maybe the springs are to weak??

Hello Boman, did the problem surface after the 'service' or after you cleaned the carbs?
The carbs on your bike are more than adequate, in good repair.

Well... I like the looks of the Bings and It's the original. But when struggling with problems I have to consider the alternatives.

My problem seems rather odd. The bike runs as silk (I really like the Airhead "feeling" even if I have newer Oilheads in my garage) and I wonder why the problem starts when the engine is really warm. The carbs are not that warm...

Thanks for all your help. Really appreciated.

Kjell-Ake

[SteveD]

The two usual reasons for a high idle are:
1. When you tuned the carbs, the bike wasn't warmed up enough. The carb tune should be done after a 20 mile/minute ride.
2. The advance mechanism is sticking. Front of the engine, down amongst the points. If it's the advance mechansim, new carbs won't help.

These are both easy enough to fix and common enough problems.

[ME 109]

Well... I like the looks of the Bings and It's the original. But when struggling with problems I have to consider the alternatives.



Kjell-Ake

Not much breaks in the bings, but there are critical components to Check/maintain like idle jets and main jets, idle mixture, fuel and air passageways etc.
They should pull your S to redline easily, if the ignition system is also in order.

[boman]

The two usual reasons for a high idle are:
1. When you tuned the carbs, the bike wasn't warmed up enough. The carb tune should be done after a 20 mile/minute ride.
2. The advance mechanism is sticking. Front of the engine, down amongst the points. If it's the advance mechansim, new carbs won't help.

These are both easy enough to fix and common enough problems.

From what you say it seems to be the advance mechanism. I will check that.

Thanks for all your help, this is really a great forum, happy to find it and I hope I will be able to help others.

[Duane Ausherman]

Bowman, do you know of my website, W6REC.com? Google is your friend. Had you just used Google, you would have had the answer. I just tested it with the search phrase, "bmw motorcycle high idle" and got my page on testing this issue. It is http://www.w6rec.com/duane/bmw/advance/index.htm and it only takes a few seconds to make the simple test to know where to start looking.

Let us know what you find. Good luck.

[Bamboo812]

You don't have to pull anything apart to check the advance units function if you have a strobe timing light. With it, you check that the S mark shows at idle, and the F mark at full advance, around 3000 rpm.

[George Ryals]

In a lot of cases of fast idle after warm up, the cause is too lean idle mixtur. Try 1/8 turn richer on the idle mixture screws.