1975 R90/6 high idle after carburetor rebuild

[samsonwilliams]

I recently purchased a 1975 R90/6, in basically good running order. It was idling high (2000 or so), and I pulled off the Bing carbs, replaced floats, o-rings, gaskets, etc. and also replaced the throttle and choke cables, which were getting frayed. After reassembly, the bike starts easily and runs well. However, the idle is still higher than ideal (around 1400 when cold, near 2000 when warm). I've adjusted the threaded cable adjuster down to its maximum level of "looseness", but I can see that the butterfly on the carb body isn't quite down to its stop, i.e. the cable is still pulling on it a little. Is there another site of adjustment I'm not aware of? The new cables appear to be the same as the ones I removed.

[hal]

Have you checked that the enrichers are mounted the correct way?

[SteveD]

Welcommen.

Did you take the bike for a 15 min warm up before doing the carb tune?
The butterfly being a tad open will do it too.

Is there another site of adjustment I'm not aware of?

The idle mixture screws?

[Rob Frankham]

Does this model have seperate control cables to the twist grip or does it have only one? If the latter, there is a splitter box under the tank that has a single cable adjuster.

Are all of the cable ends properly seated?

Are the bevel gears in the twist grip meshed properly. There is a line on both gears and these need to be matched. One tooth out and the cables will be too tight or too loose.

Are you sure the new cables are the same length as the old? Not unknown for new ones to be the wrong length.

Rob

[LargeStache]

I'm no expert, but it sounds like the throttle stops need adjusted. I just rebuilt the carbs in my '72 R75/5 and had to fiddle with the throttle stop screws to get the idle set correctly and synched. The cable adjusters are for setting the synch above idle, where the throttle is opened with the twist grip.

[barryh]

The way I'm reading the OP's description of the problem it's the cables that are causing the high idle rather than the throttle stops. He can only do what Rob described and check the cables are correctly seated and adjusted from the throttle at the bars all the way down to the carbs. If he still can't get some slack in the adjustment then the cables are at fault and need the cable outers shortening.

[Blapper]

Hi Barry,

Could you explain 'from the top' the correct method of adjusting the throttle cables, and explain how they could affect tickover given that the op says he has slack at the twistgrip please. This is a genuine question, I'm not being silly.

Chz,

Andrew.

[barryh]

Hi Barry,

Could you explain 'from the top' the correct method of adjusting the throttle cables, and explain how they could affect tickover given that the op says he has slack at the twistgrip please. This is a genuine question, I'm not being silly.

Chz,

Andrew.

When the carbs have been set up and adjusted correctly there should always be some slack in the throttle cable but not too much or the throttle will feel sloppy with too much dead movement before the revs rise. BMW specify 0.5 - 1mm. O.5 mm is probably too ambitious and most people will aim for 1 - 2mm. I'm still not convinced that the OP has that slack after reading the original post again.

Generally most descriptions for adjusting the carbs will recommend more initial slack than 0.5 - 1mm to ensure that the cables cannot possibly have an impact on the idle settings. Once the idle has been set to the correct speed the amount of slack should then be reduced to the chosen amount and the carbs balanced to ensure both butterflies begin to open at the same time. Several ways of doing that either mechanically or by vacuum measurement. As it happens I've recently done mine after cleaning and greasing the throttle gears and chain pull which should be done every 5000 miles. The mechanical method I use is to push fit a short rubber sleeve (blue) onto the inside of the butterfly shaft and pierce that with a light weight knitting needle. With these indicators on both carbs amplifying the rotation of the butterfly shaft, I sit astride the bike and open the throttle very slowly to see which indicator twitches first then adjust the cables to get them both moving at the same time. It's a very sensitive method and quite good enough if you don't have a manometer. If you do have a manometer then a follow up check of the vacuum signals will usually shows the balance is near enough spot on.

If using only the vacuum method you just need to be careful the balance is not too far out initially or there is a risk of pulling the indicating fluid into one of the carbs. With the manometer showing a balance at idle, the throttle is opened slowly and the two columns on the manometer should stay balanced if the cables are correctly adjusted. Generally one column will rise slowly and the other fall. You then adjust only one of the cables to bring the vacuum into balance and finally check there is still some free play in both cables with the throttle closed.

balancer2.JPG (124.92 KiB) Viewed 878 times

[Kurt in S.A.]

I agree with the initial idea of the small gap in the cables. When adjusting the cable tension off idle, one might find that one side is running faster than the other. It is important to slow the fast cylinder down by adding more slack to that cable. If you decide to speed up the slow cylinder, you run the risk of eliminating the initial slack that was set at idle. That could result in a high idle for that side of the engine.

Kurt

[Blapper]

Hi Barry,

Could you explain 'from the top' the correct method of adjusting the throttle cables, and explain how they could affect tickover given that the op says he has slack at the twistgrip please. This is a genuine question, I'm not being silly.

Chz,

Andrew.

When the carbs have been set up and adjusted correctly there should always be some slack in the throttle cable but not too much or the throttle will feel sloppy with too much dead movement before the revs rise. BMW specify 0.5 - 1mm. O.5 mm is probably too ambitious and most people will aim for 1 - 2mm. I'm still not convinced that the OP has that slack after reading the original post again.

Generally most descriptions for adjusting the carbs will recommend more initial slack than 0.5 - 1mm to ensure that the cables cannot possibly have an impact on the idle settings. Once the idle has been set to the correct speed the amount of slack should then be reduced to the chosen amount and the carbs balanced to ensure both butterflies begin to open at the same time. Several ways of doing that either mechanically or by vacuum measurement. As it happens I've recently done mine after cleaning and greasing the throttle gears and chain pull which should be done every 5000 miles. The mechanical method I use is to push fit a short rubber sleeve (blue) onto the inside of the butterfly shaft and pierce that with a light weight knitting needle. With these indicators on both carbs amplifying the rotation of the butterfly shaft, I sit astride the bike and open the throttle very slowly to see which indicator twitches first then adjust the cables to get them both moving at the same time. It's a very sensitive method and quite good enough if you don't have a manometer. If you do have a manometer then a follow up check of the vacuum signals will usually shows the balance is near enough spot on.

If using only the vacuum method you just need to be careful the balance is not too far out initially or there is a risk of pulling the indicating fluid into one of the carbs. With the manometer showing a balance at idle, the throttle is opened slowly and the two columns on the manometer should stay balanced if the cables are correctly adjusted. Generally one column will rise slowly and the other fall. You then adjust only one of the cables to bring the vacuum into balance and finally check there is still some free play in both cables with the throttle closed.

balancer2.JPG

Thanks Barry. We both do the same thing but slightly different techniques. Here is a question - I have slack at my cables, but if I turn my twistgrip against the stop it feels slightly springy. There are two marks on the gear wheels which I did line up.