Switching to Mikuni carbs has rarely helped anyone that I've known. Don't get me wrong, they are excellent carbs, but no one in the BMW world is going to know anything about them... and thus when the first signs of trouble show up no one in the BMW world is going to be able to offer ANY help. And when you take it to the Yamaha shop, where they do know about Mikuni carbs, no one will know anything about BMWs and they will refuse to help.

Mikuni carbs are great for the carb expert who can do all the work himself/herself. Which I take it you are not, since a very simple Bing is giving you big issues. In your case then, you're much better off to stay with the well documented, well known, and much better understood devil you already have.

Not to put off on your father, but if the idle mixture (really the "idle quality") screw has no effect, then your problem is going to be in the SMALLEST jets and passages. Places where he may not be able to see. Or, it could simply be an issue with having allowed ethanol fuel to stand in the float bowl for several weeks at some point. Try these tips on for size.....

• Float level is CRITICAL for good idle. Floats and float needles need to be replaced about every ten years or anyway... so just go ahead and buy new ones. Then take extreme care to set both fluid levels the same.

• Sealing of the intake is critical, so I would also order new, supple intake hoses that will insure a tight seal of the carb to the cyl head.

• The idle jets are the smallest jets, and therefore 1) much more likely to get plugged with trash or varnish from ethanol fuels, and 2) much more likely to be destroyed if wires and other clearing techniques are used to unstop them. REPLACE THEM !

• When the idle screw is removed and the idle jet is unscrewed, carb cleaning sprays and compressed air can be used to blow through the connecting passages to check for cleanliness.

• As a matter of course, since the carb needle wiggles (hammers) back and forth during running, the carb needles and needle jets need to be REPLACED every 10 years. That's simply normal maintenance. When this step is taken, care must be used to get the needles back into the stock (and matching) clip positions.

► On a twin cyl bike with dual carbs, all the above has to be undertaken with absolute precision. The reason is very easy to understand and state ("If you want the 2 cylinders to run alike, then both carbs absolutely must BE alike"), but often times the level of workmanship is not up to the task. I've found most people want to treat carb work with the same level of care as slicing carrots, when it needs to be up to the level of a high school chemistry class.

► When all this work is completed, the carb work is NOT considered finished until the carbs are syncronized at 1000 RPM (idle) using the Idle Speed Screw AND at 1800 RPM using the cable adjusters.

► Your plugs, plug caps, and plug wires are also highly suspect if they are over 10 years old.

Try those suggestions and let us know.

Thank you for the response, and great points around the devil you know vs the devil you dont. I'll work through these items with my pops and report back.

Please re-read. I've added more ideas to my last post.

Thanks.

Update so far... I ordered and replaced the following on both carbs:
float, float needle, idling jet, jet, and main jet.

I also adjusted the throttle cable tension as they were looser than they should have been. After the work, I am now able to better adjust the left carb and the throttle response is much better(assuming due to the throttle cable adjustments).

I have yet to be able to properly balance the carbs, and still find that when i short the left side, the right side seems to never "almost stall". It keep son trucking. When i short the right side, the left side stumbles almost to a stall within three seconds. I have some more work to be done here. I have recently came across a local independent BMW mechanic and will be getting with him to try and properly balance these. I cannot rule out user error here.

Other than the idle imbalance, the bike runs good, and at speed doesn't miss step.

I'll update once I am able to get a proper balance.

Thanks for the help,
Travis

I have yet to be able to properly balance the carbs, and still find that when i short the left side, the right side seems to never "almost stall". It keep son trucking. When i short the right side, the left side stumbles almost to a stall within three seconds. [snip] I cannot rule out user error here.

Various carb tuning techniques that include "shorting" the plug wires, un-plugging one of the plug caps, laying the plug atop the cylinder and then running the engine on a single cylinder, etc, etc, can be harmful to your ignition system and cause permanent ignition system damage. That is, damage to the ignition system that can only complicate tuning the carbs. (In other words, you're only digging deeper, when the stated goal was to "fill in the hole".) Therefore, these type "tuning techniques" rank right there with "internet rumors" and should be avoided.

The correct way to tune carbs is with a manometer that measures the difference in the level of vacuum between 2 or more intake tracks, and by listening to the beat of the engine. If you can't make an adjustment by listening and measuring, then IMHO you have business getting near the carbs with a screwdriver to begin with. Here is a link to an effective manometer you can build for under $5. Click Here And please, don't laugh at the price until you try it !

Any carb cleaning and/or jet replacement on your part was for a reason. The reason was blockage, but because it's not humanly possible to detect and remove every speck of crud from the ENTIRE fuel system, once the engine is running you need to follow that up with "cleansing type" fuels and possibly even an additive so that the entire system continues to be cleaned. Fuel system crud is more akin to an infection, in that a single dose may improve performance dramatically, but is not going to fully clear the problem. The best fuel additive I know of is StarTron, the details of which I will leave to you to research. http://www.starbrite.com/startron

When you work on carbs, process and technique are everything! Carb tuning is by its very nature a "dynamic" adjustment and is therefore the last tuning done AFTER completing a long list of "static" adjustments (like valve adjustment). A whole host of ignition system inspections and other settings need to be brought into spec before the carbs can be effectively tinkered with. So any person that walks up to a bike and start twisting carb screws is makes the HUGE assumption that every thing else is in perfect condition and adjustment. Which in the case of a vintage motorcycle a very dangerous assumption.

In short, I don't think the mechanic can do you as much good as YOU CAN DO yourself at a lower cost. Set the timing with a strobe lamp, adjust the valves, replace leaking rubber intake parts, adjust the carbs with a manometer, and start using a high-test fuel with cleansers for a month or so... and I think your bike will come around on its own. I really do !

Hope this helps.

I'm telling you all this because as a professional vintage motorcycle mechanic, (when I know the history of the machine) I routinely replace the fuel and go ride the bike for 2 hours. Then I charge the customer $150 for "working" on their bike for 2 hours and send them home. And customers are ecstatic. Just working the cleanser type fuels through the fuel system has that much of a positive effect !!

If I do not know the history, then all I can do is start back at the beginning with a full tune up and a 15 minute test ride, which all total takes 3-4 hours, depending upon what I find.

All I'm saying is this: You know the maintenance history of your vintage bike. You won't need to repeat your completed work. The new mechanic you found does not know any of the history. He'll need to repeat everything.