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Topic-icon '74 R75/6: High idle with mixture screw in

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2 weeks 3 days ago - 2 weeks 3 days ago #5919 by eurotourer
'74 R75/6: High idle with mixture screw in was created by eurotourer
1974 R75/6, Bing 64/32/10.

When synchronizing my carbs I noticed that I get the highest idle on my right carb with the mixture screw all the way in. The bike runs perfect and the plug looks good. When using a crystal plug I can tell by the flame color that the mixture is OK. I replaced float, valve, pin, needle , 2.68 idle jet, all o-rings and butterfly is closed. The choke is on the correct side. Can anyone tell me what could cause this?
Last edit: 2 weeks 3 days ago by Wobbly.

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2 weeks 3 days ago #5921 by Wobbly
Replied by Wobbly on topic '74 R75/6: High idle with mixture screw in
How long has this bike been sitting ?

Good mixture or not, I'd first remove the float bowls and look for a brown lacquer color coating the bottom of the bowl. Then I'd pull the low speed jets on both sides and see if you can see light through them. Then you can pull the mixture screw out and blow carb cleaner from the jet to the screw (or screw to the jet).

If your fuel is over 6 weeks old, then dump it all into the car, and go get some new high grade fuel with built-in cleaners.

Don't hide 'em, Ride 'em !!
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2 weeks 2 days ago #5928 by 4099
Hi,

You said that you are using a Colortune? Glass Spark plug to check the mixture on the right carb after you did an overhaul on the carb (with all the parts you mentioned it looked like an overhaul). And I assume that you replaced the O rings on the butterfly shaft also?

First question: I assume you are doing your idle mixture screw setting when you are at say 1000 RPM or less? This would make sure that you are running on just the idle mixture range and not into the needle jet range yet.. right?

Second question: When you set the mixture on your LEFT cyliner (using the Colortune plug) were you able to adjust that mixture in and out so that you could turn it way out and go RICH and see the bright yellow/dirty yellow flashing of a overly rich mixture?... and then turn it in lean enough so that you moved from yellow to "bunsen burner blue" (the optimum mixture setting) then on through to the really "light blue" (too lean really) and then all the way to where the LEFT carb quit working? That would be the way a normal carb was correctly working.... and what you SHOULD expect to also see on the RIGHT carb you are having trouble with.

Third question: Are you SURE that when you re-assembled the butterfly disc on the RIGHT carb.. that you oriented it correctly? .... that disc has a "bevel" around it to help seal the edges of the disc to the wall of the carb circular outlet. If you don't have it sealed really good.. it can leak air/gas and let the main needle jet contribute to the fuel mixture and maybe mess up your idle jet setting you are trying to lean out. When I put in the butterfly disk... I do not tighten the screws on the disc until I have slid the disk around a bit in the shaft slot.. and completely closed the butterfly tight onto the carb wall.. wiggled it some more... looked at it with a light behind the carb inlet (and the slide piston raised up with my finger) and looked around the edge for minimal light showing around it and the wall.... AND then I snug down the screws. I then use a pair of diagonal cutters to "peen" the screw threads.. OR I use a weak mixture of Locktite blue to secure the screws. It is also possible the idle jet seat has been messed up or the needle itself by folks tightening down the needle too much and messing up the point.

Are you sure you left a 2mm gap on the throttle adjusting screw and the throttle cable end? Can't have any tension there while idling... I know these are obvious questions but I did find myself adjusting the choke cable instead of the throttle cable on my bike when I was balancing my carbs last night...so mistakes can be made....

By the way.. does your Colortune fit your BMW spark plug threads correctly?..... I bought my Colortune years ago when I had a Moto Guzzi....
It fit the 1971 Moto Guzzi spark plug threads just fine back then.... BUT when I try to use it on my 1981 and 1984 BMW bikes I can only get the stupid thing to go in a couple of threads and then it gets WAY too tight.. I think that maybe BMW threads are tighter somehow than the Moto Guzzis were.

Also.. Wobbly asked if you checked your idle mixture jets, and could see light through them..... REALLY take a close look at your jets..... At one time I had a jet that you could see light through just fine... BUT it still did not work.... It turns out (under a really good magnifying glass) that while I could see light through it... it was plugged up with stale dried gas! It looked like an amber colored bit of glass! I had to poke it out with a soft piece of copper wire strand. Don't use a steel wire! Something soft like monofilament line or a bit of wire strand from a small gage copper wire.

Let us know what you find.

Clark of the South
ABC #4099

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2 weeks 1 day ago #5933 by Wobbly
Replied by Wobbly on topic '74 R75/6: High idle with mixture screw in

4099 wrote: REALLY take a close look at your jets..... At one time I had a jet that you could see light through just fine... BUT it still did not work.... It turns out (under a really good magnifying glass) that while I could see light through it... it was plugged up with stale dried gas! It looked like an amber colored bit of glass! I had to poke it out with a soft piece of copper wire strand. Don't use a steel wire! Something soft like mono-filament line or a bit of wire strand from a small gauge copper wire.


Exactly, this is why I recommend the replacement of the smaller jets. People generally have no conception of how exquisitely fine the metering is within the "common" carburetor. The machining and over-all result is on par with the finest Swiss watch.

I've kidded here about banjo strings before, but I actually keep a short section of multi-strand lamp cord in my tool box for cleaning small jets. The fine copper wire does really well at poking out clogs. But again that's only for a short test run to see if that's the problem. The jet still gets replaced.

Don't hide 'em, Ride 'em !!
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1 week 6 days ago #5942 by eurotourer
Replied by eurotourer on topic '74 R75/6: High idle with mixture screw in
I had no idea my thread was posted because I got an error message and also no email that someone responded. I did everything right according to your tips. I replaced the idle jet with one from my /5 (Same carb) and the engine died when screwing the adjustment screw all the way in. The carb could be adjusted.
The o-ring on the /5 jet looked thicker. The OD of the o-ring was 7mm. The one on the /6 was 6.6mm. After replacing this o-ring the problem was solved. I guess fuel leaked past the o-ring. All four carbs were rebuilt with kits from different companies. Forgot where I got the /6 kit from.
My colortune kit is #G4074 and no problems with the threads.

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