1975 R90/6. Surging.

[chasbmw]

I have just enjoyed the first day of the this very wet UK summer on my r90/6, A nice 200 miles on A and B roads, still on the same tank of petrol, not yet hit reserve and the new Michelin Pilot tyres are really nice and stable.

However there is a problem with the carbs at closed or low throttle settings between 2500 and 3000 revs as the bike surges and cannot find a happy setting, this is really irritating in town.

The bike is dual plugged, runs a highish compression ratio, carbs rebuilt last year, I did fit springs to the slides, (springs were not fitted to this model). Is surging a sign of running too rich or too lean in the transition from idle to needle?

Thanks for your help

Charles

[vanzen]

I have experienced "surging" as a result of both lean or rich conditions.

My questions to ask will be:
1. Did you recently tune the carbs before the "surge" was evident ?
2. What carb changes / adjustments have you made relative to the "pre-surge"and "now-surging" condition ?

[moosehead]

Had same problem on my 75/6...turned out when I rebuilt the carbs I didn't tighten up enricher cover bolts very well and one side was loose. Once snugged everything down and added a little blue locktight everything back to normal. Guess with enricher cover sucking air the carb woould be running lean?

I got the idea from snowbum's article on surging...I know lot of people don't like all the colours but it works for me. Also article has other ideas on surging as well. Check it out....wear sunglasses

http://bmwmotorcycletech.info/bingcv-2.htm

Point #2 Surging 30-50mph...don't have to read any further

[chasbmw]

Surging cured".............

I took the carbs off, checked the enrichener the enrichener screws which needed a quarter turn to be fully tight and gave them a dab of loctite as well. Whilst I was in there I checked to see that all the internal passageways were clear and replaced one of the idle jet O rings.

I also moved the needles from the leanest position to No 2 and put it all back again. I have taken a brief test ride around town and 90% of the surging has gone and what is left has dropped down the rev range.

So subject to another longer road test, I think that I'm done.

Charles

[vanzen]

Take that long road test, then while the bike is still HOT –
retune your idle.