75/6 Carburetor problem (?)

[davidaid]

Hi all,

I have what I think is a tuning problem with my right hand side carburetor. When giving a little throttle, it seems to choke out (only way I can think to describe it) on that side and adjustments to the throttle adjust screw end in the same result. When I try adjusting the idle mixture screw, any change in idle in minimal. These adjustments are while I'm trying to balance the carbs.

I have adjusted the valves, cable play and compression is good. I ordered the Bing guide, but haven't received it yet, so I thought I would ask you all.

Thanks!

[Deleted User 62]

Did you check the diaphragms for holes? You can check them for function by removing the airbox to carb tube and watch for the slide moving upward when you twist the throttle, bike running.

[Major Softie]

A carb not reacting properly to idle mixture adjustments is a pretty classic symptom of a vacuum leak. The diaphragm is one place that could be happening, but there are others: like the intake manifold.

[SteveD]

When I try adjusting the idle mixture screw, any change in idle in minimal.

I'd remove it and clean it and the passages. Is the o-ring kaput?
Are the jets clear?

[Ken in Oklahoma]

Hi all,

I have what I think is a tuning problem with my right hand side carburetor. When giving a little throttle, it seems to choke out (only way I can think to describe it) on that side and adjustments to the throttle adjust screw end in the same result. When I try adjusting the idle mixture screw, any change in idle in minimal. These adjustments are while I'm trying to balance the carbs.

I have adjusted the valves, cable play and compression is good. I ordered the Bing guide, but haven't received it yet, so I thought I would ask you all.

You didn't talk about the recent history of your carb. Particularly if it has been sitting up for an extended time you could have some clogged internal passageways. A thorough cleaning might be in order. Dipping or "boiling" the carb is usually next to useless. There are some very small passageways associated with the idle that must usually be mechanically cleaned. A spray can of carb cleaner and a strand of copper wire are my tools of choice. Actually I use a guitar (little) E string or B string but those bits of music wire can easily damage the passageways, since zinc is so much softer than steel. I sort of round off the end of the wire with a fine grinding wheel or even a whetstone.

It helps if you know where the passageways start and end. You can tell a lot by looking at the carb body. You will see some lumps which have been drilled and plugged in places to create the passageways.

Here is an old Butler and Smith article with some cutaway diagrams which should help you to understand what is going on inside the carb body:

http://www.omnilex.com/public/bmw78/cvcarb.pdf

Your prime area of focus will be in front of and in back of the throttle butterfly on the bottom. Those holes are associated with idle and off idle performance, and of course the idle jet. If I remember correctly there are three of those very small holes, one in front and two in back of the butterfly. You blow carb cleaner through the passageways to make sure that they are open, using the thin wire to clean them out if needs be. I don't quit until I'm convinced that all passageways are open.

While working on the carb you will want to disassemble it, inspecting and cleaning the rest of the carb. I generally leave the enricher alone if the carb "choke" has been working OK. There is a paper gasket on the enricher body which can be problematic. If you do take the enricher apart be careful to note the position of the block you remove. It can be put on wrong.

While working on the carb you may find other issues that need attention, particularly the various O rings used. Since you've ordered the Bing manual I presume you've also ordered a couple carb kits. Some people will buy O rings of the right size from an automotive store or even from a Harbor Freight O ring kit. I haven't done so, but in a pinch I would.

While you're at it you might want to measure and record the amount of needle projection from the slide, and/or note which needle groove the clip is on. This information might come in handy some day.

Oh, I forgot to mention that there could be an issue with seals on the butterfly shaft ends. Replacing those seals involves disassembly and installing new screws that hold the butterfly to the shaft, along with peening the ends of the screws so they won't come out. I would leave the seals alone, at least on this go-round, unless you have reason to think they need attention. Spraying some ether or WD-40 (I've heard) will show up any air leaks past the butterfly shaft ends.


Ken

[davidaid]

Thanks all for the responses.

The carburetors were "gone through" by the dealer about four years ago and they worked fine. However, the bike for the most part, has been sitting for 2.5 to 3 years due to its owner acquiring a newer model.

Anyhoo, I did look at the diaphragm and there were no holes - in fact they looked really good on both sides. I did try to clean some internal parts as best as I could, but all the little parts made me nervous until I can get the Bing manual (ordered, but not yet received). Pardon as I cant' remember the names of the parts, but I did get to the jet from the bottom side of the carb between the floats and all was clear. I also took off the side where the choke cable attaches and looked for gunk and clogging there - all was clear - and made note to reassemble in the same way. The o-ring looked good.

I'll see what the manual says and go from there and work on the idle mixture screw per SteveD and the intake manifold per Major.

[Duane Ausherman]

You have been given a range of advice................ from people who have never done this for a living on those exact carbs. Think about that.

Only Steve hit on it.

Testing is easy, instead of using the throttle, grab the cable at the top of the carb and gently lift up. You will instantly see if there is a problem on that side just above idle. If you aren't comfortable doing it that way, then use a screwdriver to increase the idle. As you turn the tool, the idle should gradually and smoothly increase. If it Fails to respond, or actually drops in rpm, then you have found that you have a problem on that side. it could be on both side, but worse on one side.

By far the most likely fault is a plugged up idle jet. Remove it and positively clean it out. Take a good look and you may see light, but the hole must appear round. Poor eyes may require a loop to see it.

Don't use compressed air and don't use some liquid cleaner. I use a "hair" that sticks out from my wire brush to clean it out.

This process will fix well over 50% of bikes with this symptom. If both sides are clear, then proceed with the advice shown above.

[ME 109]

I use a "hair" that sticks out from my wire brush to clean it out.

That's gotta hurt man!

[davidaid]

Testing is easy, instead of using the throttle, grab the cable at the top of the carb and gently lift up. You will instantly see if there is a problem on that side just above idle. If you aren't comfortable doing it that way, then use a screwdriver to increase the idle. As you turn the tool, the idle should gradually and smoothly increase. If it Fails to respond, or actually drops in rpm, then you have found that you have a problem on that side. it could be on both side, but worse on one side.

That's exactly what it was doing. After getting a little more bold, I took out the idle jet which I had not yet done previously. The o-ring was all torn up on that side, so I will need a new one (obviously) and a new main jet because of a mess up on my part. Hopefully, I should be good to go with those little repairs.

thanks all!

[chasbmw]

To Check to see whether or not the internal passages are clear, spray carb cleaner 'Up' the idle jet hole, if the passages are clear you should be able to see carb cleaner fountaining out of the 3 tiny orifices in the bore of the carb, adjacent to the throttle valve.

Make sure you protect your eyes when doing this.