Carb diaphragms -symptoms of failure

Roger Chatterton

Roger Chatterton

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Hi All, Having real problems hunting down a problem on my 100 GS PD, with 40 mm Bing Carb and twin plugged.
Done just about everything to carbs, checked valve clearances,( LH side exhaust valve was too tight, so reset that), swapped out Beancan, re-set the timing. I checked carbs and cleaned out the choke chambers as I thought they might have been sticking. I gave the diaphragms a thorough check under magnifying light,and in bright sunlight, but NO signs of pin holes ore splits, so didn't swap for spare ones, as they are pretty pricey (genuine BMW ones - the thinner floppy ones!).

But my problem of severe stuttering on a closing throttle remains - bloody bike tries to throw me off! Runs fine at high revs (over the 3000 rpm mark),very smooth, and I have checked on my vacuum gauge, and carbs balance fairly evenly, apart from when decelerating around 2500 rpm when the vacuum increases on the RH carb, and decreases on the LH carb.
I am beginning to question the diaphragms as culprits, but they look perfect, so maybe I should replace anyhow, but - what are the normal symptoms of a punctured or faulty diaphragm please? :mad: :rob
 
With a CV carb, it's the pressure on the diaphragm that lifts the slide when you open the throttle. With a punctured diaphragm, the slide won't lift and the bike won't pull more than roughly 3000 RPM.
Easy to check, remove airtubes and look to see if slides lift.

So it's not the diaphragms


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As per Chas's comments avove. When the diaphragm on my PD failed it it ran fine up to 60mph but after that it felt like one of the throttle cables had snapped.
 
Yes - thanks Fellas, - so I guess it's not the diaphragms then, as I'm getting the problem below 3000 rpm approx - which is the point at which carbs change from idle jet to main yet is it not? Idle jets are clean and the O-rings are good, as are the O-rings on the piston slide. Pistons lift and fall easily, and throttle cables are not frayed, and the little jet that sits in the corner of the float chaber is also clean as is the hole at the bottom of its chamber.
Plenty of fuel coming through, and float needles look good around the rubber tips.

I'm really flummoxed here ......
 
Put a strobe on it and check that the timing is advancing and retarding. A dual plugged bike should be at about 28 degrees fully advanced compared to 32 degrees stock.


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Yes, as mentioned- changed the beancan and adjusted timing on the strobe. Timing advanced and retarded OK, but can't quite specific degrees of advance etc.
 
28 degrees should be when the top of the Z is at the bottom of the window.


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There are two holes on the end of the carb, below the intake.
If you spray carb cleaner into the large one it should spray freely out from round the needle, the small one should exit from one of the pin pricks downstream of the butterfly.
If you remove the float bowl and spray into the pilot jet it should come out the other one.
There is also a small hole in the slide which vents the centre if the guide tube , so check that too!
 
Thanks for advice everyone! I've spent time cleaning the little vents by the butterfly, and pulled the mixture screws and replaced one slightly suspect O-ring and running has certainly improved slightly.
But still getting the hiccups around 2000-2500 rpm, but quick blip on throttle seems to clear it for a while.
Bike is running well under acceleration- smoother than before, but hiccups and stutters unpredictably. Tickover is also pretty even.
Looks pretty evenly balanced on my Vacuum Gauge -
Any further suggestions? .....:eek:
 
Have you checked carb to inlet tract rubbers? it is not unknown for these to collapse/leak air on high vacuum load ie overrun, early stages of WOT etc... and since you have done a large mount of carb related work without succes/much change it is possible the fault lies elswhere - coil/leads/ignition ecu breaking down perhaps......test by substitution.
 
Indeed - will be checking coils etc on Friday, but it's tricky to test the ICU. I don't have a spare, but may have to buy one. Anybody know where I can get the Siemens or Telefunken replacement- cheaper than Motorworks new BMW one I suspect!
What would a faulty ICU symptom/s be?
Beancan has been substituted with no real change.
 
Indeed - will be checking coils etc on Friday, but it's tricky to test the ICU. I don't have a spare, but may have to buy one. Anybody know where I can get the Siemens or Telefunken replacement- cheaper than Motorworks new BMW one I suspect!
What would a faulty ICU symptom/s be?
Beancan has been substituted with no real change.
 
Having spent time checking things, and having read an excellent section in the Bing Handbook (the American pamphlet that exactly described my symptoms!), I think I can say that the problem is fixed.
The solution was .....(and I hate to say this as I thought I had cleaned them thoroughly), - blocked idle jets. Apparently these operate pretty much exactly in the rev range where I was having most trouble, namely around 2000 - 2500 rpm. Applied lavish amounts of carb cleaner, allowed to soak, blasted with compressed air on both carbs , checked the O-rings and also soaked those tiny orifices near the butterfly, and again a blast of air through.
Bike now seems to be sorted- but worth noting that the L H side carb seemed worse, probably due to oil dripping in from the oil breather in the air intake, whilst on the side stand.
So my thanks for help and suggestions, and especially to the Bing Handbook, which is very clear and straightforward with its help and diagnosis. Well worth the money ...mind you I didn't pay for it , "Oldenoughtoknowbetter" did ! But not wildly expensive , even from the States. Paid for itself now , it has !!!
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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