R90/6 carb setup

[estabon1533]

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. I obtained them from Bob's BMW.

[melville]

Look for a spot where the cable housing ends aren't seated. Either at the handlebar or at the adjusting barrels on the carbs.

[Rob Frankham]

1) Are the throttles staying open because the cables are pulling them or because they are closed as much as they can and are not getting to the stop screws? - Disconnect the cables and see if they close more.

2) Is the handlebar throttle control set up correctly? There should be indexing marks on the gears. If the gears are meshed wrong then the cables may not be able to fully retract.

3) Idle speed is affected by ignition timing and idle mixture. Go back to basics, check the ignition timing and set the carbs to the initial setting in the book. Once the engine is running, check the timing with a strobe and adjust the idle mixture.

4) If you still can't get a decent idle, then you probably have an air leak either in the carb or in the rubber connections to the head (or in the head itself ).

Note: when carrying out the final air mixture and idle screw adjustment, the engine needs to be thoroughly warmed through... preferably after a decent ride (10 miles at least) only then will a stable idle be achieved. It's not unusual or a fault for the engine to idle low when cold or even stall. It will need a litle throttle to keep it running until it warms up.

Rob

[SteveD]

Rob has it covered.

Re #4...air leak. Before you did anything, I think I'd just check a couple of things re a leak.
1. whilst idling spray something all around the carbs and connections, whilst listening for a change in the idle. You might find a leak.
2. does the /6 have a removable spigot at the head where the carb manifold attaches? If so, ensure both are tight. I had a similar problem on my 82 RS years ago. Correction of the loose spigot solved it.
#4...


I'd be pissed if I did all the other stuff to find that it was a simple spigot leak.

[estabon1533]

Thanks to all of you for your replies! Rob, your #2 suggestion turned out to be the source of the problem. When I reassembled the throttle control at the handgrip after placing the new cables, I didn't take close enough note of the gear alignment. Once I rechecked and corrected that, I now have plenty of adjustment available on the threaded section on the carb, and the idle is down to where it should be. I love this forum!

[Rob Frankham]

Glad you've got it sorted... thanks for letting us know the outcome.
Rob