Bing CV Carb Cap Plug

[Gibson]

I was having somewhat sloppy response just off idle and at small throttle openings slight surging. The spark plugs also did not read the same on both sides. I was having also having difficulty synchronizing the carbs.
I bit the bullet over the weekend and tore down and re-built both carbs. I went one size larger on the main jet and the next size up on the needle jet, (for E10) replaced both needles in the original clip position. I also replaced both diaphragms and floats and thoroughly cleaned up carbs. Polished up some galling on the slides also.
The reason I suspected a torn diaphragm was I was measuring different vacuum reading on each side, when I adjusted the carbs for the same reading on both sides, it would run terrible. The problem turned out to be a leaky plug cap on the top of the right carb. diaphragm housing. Once the leak was fixed, I set it up by ear and it now runs smoother and responds more crisply with no more surging. I suspect will sync right now with gauges.

[Kurt in S.A.]

You could do something like this to help seal the top:

https://www.motorworks.co.uk/vlive/Shop ... A_50_50_BA

Kurt in S.A.

[Gibson]

Thanks Kurt, those are great!

[SteveD]

Some silicone underneath...a Deutschmark would be better but they're a bit hard to come by these days!

[Seth]

I had a similar problem with my Mystic.
I used some silicone on the top edge of the cap and crimped it back down some with a socket.
I had noticed the cap dancing round in a circle before any problems with performance.

I know you have a Mystic as well. Another problem I had was the pin that locates the front main bearing had come out.
Heard it knocking around inside the motor and it came out in an oil change (A magnetic drain plug would be helpful).
The bearing didn't rotate, so I just had to get a new pin and peen it in place. Keep an eye out, as it is something that has affected motors of those years.

https://forums.bmwmoa.org/showthread.ph ... l-pan-find
http://www.ukgser.com/forums/archive/in ... 09646.html

[Gibson]

Hi Seth, many thanks for the tip in the locating pin! If I drop the oil pan is it visible? If it is still where it should be, I will peen it some more to make sure it stays.
I have another question for you about your Mystic, are the fork tube tops flush with the top tree? I looked at some pics of other mystics and they are all even. On mine, the forks protrude about 1/2". Someone may have dropped mine 1/2" to lower the bike perhaps?

[Seth]

My fork tubes are flush. If you move them up, it should quicken the steering some.

No, you can’t see the pin. It’s in the front main bearing holder which bolts into the front of the engine case. The bearing is pressed into the “bearing cover” in the fiche. Then it’s drilled to provide oil to the rocker arms and the pin is to prevent it from spinning in the cover. If it does, oil doesn’t reach the rockers and the rocker bearings fail and no oil is provided to the top end.

[Gibson]

Thanks Seth, I am going to move the fork tubes back to stock position to slow down steering on highway a hair.