75/6 Carburetor problem (?)

[Major Softie]

Ken, you should host one of the rallies in Boise. That way some of us PNWerners and some of the left coasties could make it.

Of course you must find some Idahoian Wood Nymphs first.

SOME people have managed to make it to OK even from the left coast . . . and from Canada.

[davidaid]

So the dealer was able to order a replacement 140 jet. My carburetors are 64/32/09 & /10. Interesting to note the Bing manual says those should be for the 75/5 with a jet of 135 and Clymer says those are for the /6 or /5 buy with a 135 jet. I guess the good news is with the new o-ring for the idle jet it is running much better. I don't have the "choke" reaction when accelerating. Not sure it is quite right yet, but at least it is now rideable. Thanks all for your help!

[Jean]

Uh-huh. MY BMW workshop manual (really OLD!) also says those carbs are R75/5.
R75/6 and /7 shows 64/32/13 &14 with a 145 main jet. A 140 might be a little bit lean...
That "choke" reaction could be the diaphragms...Have they been replaced lately?

[Jean]

Found some additional info...64/32/3 &4 which may have come on some earlier R75/5s DID use a 140 main jet. (This from an OWNERS Manual spec list for R75/5.) Later /5s had the 9 & 10 series carbs with the 145 jet.
My R75/5 came to me with 64/32/9 & 10 carbs which were a bit of a pain in the choke and acceleration department, so I've got 64/32/13 & 14 fitted. I assume they have 145 jets, but haven't looked.
I sold the 9 & 10 to someone who wanted "..the CORRECT carbs" to install on their bike. I wanted the ones that worked.
Another thing I uncovered: the diaphragm that fits a Stromberg CD 150 carb will also fit a 32mm Bing...MG Midget, Sprite had this carb.

[davidaid]

Since there is nothing like resurrecting an old thread...

The bike still never ran quite right. I narrowed the problem down to an idle jet - I would change it between left and right and fiddle with the carburetor in which that particular idle jet was installed. New o-ring, made sure it wasn't clogged, etc. Rode the bike off and on for a while and still wasn't quite right. I can pull on the throttle cable for that particular side and it will choke out and it doesn't run "right" when cold. Best I can describe it is doesn't really like to warm up and top end power is less definitely down.

Took the idle jet out and looked at it again and noticed a really small crack in the "up" end of of the jet. I assume that is the problem, but it seemed like such a tiny crack - you really have to squint to see it. The guy at the dealer couldn't even see it, but he said he could feel it with his finger nail.

Anyhoo, my question is - Would this small of a crack make that big of a difference?

I tried to take a picture, but I couldn't get a good one.

[Duane Ausherman]

I like to use a loop to inspect that jet. A really tiny clog can cause trouble. Never have seen a crack, but am not surprised that it could cause a problem. If it changes sides with the jet, then you know for sure. Glad that you reported your finding.

[Major Softie]

Took the idle jet out and looked at it again and noticed a really small crack in the "up" end of of the jet. I assume that is the problem, but it seemed like such a tiny crack - you really have to squint to see it. The guy at the dealer couldn't even see it, but he said he could feel it with his finger nail.

Anyhoo, my question is - Would this small of a crack make that big of a difference?

Very interesting.

I wouldn't have guessed that such a crack would make the difference, but your experimentation has proved that it does.

[davidaid]

Second to last up date. I got the new jet today and installed it. Seems to be much better. It doesn't seem to choke any more on the side with the new jet when using the "pull on the cable" method of testing.

I haven't had a chance to go ride and properly adjust the carbs, so that will hopefully be the last update. Still a little amazed that such a small crack could make such big difference. Glad it was an easy fix - after a long diagnosis.