BMW CV Carbs No Power Problem - Can You Help

 Hi,
My BMW R75/6 suddenly will not pull above 3500 RPM. It idles, runs, and
pulls, normally till that point. The bike has Bing constant vacuum carbs.

Here is what I've done:

0.) I visually confirmed that the choke and throttle cables were working
properly.

1.) I checked the plugs - they looked bad so I replaced them with new
Champion plugs with the gap set at 0.026. This did not effect the problem.

2.) I check the compression - 90 PSI on both sides after 5 compression
cycles.

3.) I checked the spark on each side, while revving the engine high and low,
with a spark indicator (an inline device that flashes with the spark). Both
sides looked the same.

4.) I checked the spark, at the plugs, with the plugs removed and grounded,
while cranking the starter. The sparks looked the same with normal
intensity.

5.) I individually operated the carburetors by pulling on the cables at the
carburetor. Pulling the right cable the engine revs up to 6000 RPM. Pulling
the left cable the engine revs only to 3500 RPM.

At this point I suspect the left carburetor or less likely the left coil
(maybe a weak high rev spark?).

I've got the carb off. I'm looking at the exploded view in my Clymer manual
and it looks scary - that's why I'm asking for help.

Have you heard of this problem before?

Can you offer any alternative causes of these symptoms or am I on the right
track?

If you suspect the carb - are there any particular things I should inspect?

Thanks in advance for your help.


Cheers,
Tim
http://www.racetec.cc
ò¿ó

 
"Tim"

When is the last time you rebuilt the carbs? There aren't too many parts,
it's a pretty simple carb. I'd suspect the diaphram(s) for a crack but if you
haven't done it I'd get the Bing rebuild kit, including diaphrams.

 Fletis Humplebacker <!> wrote:


My thoughts as well.

I've had split diaphragms: the bike runs fine up to x rpm (x = a low
number!) and then no further simply because the diaphragms can't lift.


--
Trophy 1200 750SS CB750F2 CB400F CD200 CB125S DT50MX
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BOF#30 WUSS#5  The bells, the bells.....

 
"The Older Gentleman"

Yep, I had trouble keeping up with a buddy on an RT, my power
flattened out at 5 grand. Turns out the left diaphram had a half
inch tear in it. I couldn't imagine how it worked at all. I think the
Bings are highly underrated.

 
Fletis Humplebacker wrote:


All CV carbs are underrated...

Tear the diaphragm badly enough, and the engine has to run off the idle
jets, plus whatever mixture can be sucked out of the needle jet...