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idle mixture screw has no effect - what could cause this? '80 R100RS

ezwicky

Member
My newly-restored '80 Euro-spec R100RS has an issue with the right-hand carb. The idle mixture screw has no effect. In the past when I've seen this, it's always been the o-ring messed up and a replacement fixed it, but not so with this one.

Bike was a non-running crusted barn find 3 years ago, now everything is new or rebuilt. I did the carbs, and have done about 6 pairs previously, so I am confident in my abilities but ready to admit I might have made a mistake somewhere.

Before I remove the carb and tear it down for inspection, do you all have any suggestions on what might cause the idle mixture screw to not have any effect?

The bike runs great btw, so whatever "default" state the mixture is in apparently works, but I have no ability to adjust the idle mix on that side for fine tuning.

Vacuum screws and rubber washers are new and tight.
Starter fluid sprayed around do not cause any effect.
Enrichener cable is correctly adjusted.
Throttle cable is firmly in the ferrule.

Thanks in advance,

Eric Z
Richmond VA
US
 
Seems like air is getting thru somewhere else. Leak at the carb head rubber? Carb butterfly oriented properly?
 
Seems like air is getting thru somewhere else. Leak at the carb head rubber? Carb butterfly oriented properly?
I'll check the butterfly. Possibly I could have installed it wrong way round, but I'm usually pretty careful about that. Starter fluid sprayed all over the carb and instake rubbers (on both ends) has no effect so I don't think it's an air leak there.
 
Agreed, air leak at the rubber that mounts the carb to the head. Could also be the head gasket allowing air into the combustion chamber.
 
Agreed, air leak at the rubber that mounts the carb to the head. Could also be the head gasket allowing air into the combustion chamber.
Air tubes are tight. Starter fluid sprayed all around the seams caused no rpm fluctuations.
I just put the carb back together. Had the top and bowl off, shot air through all the passages, visibly inspected all the passages I could see through. No debris, no obstructions. Getting ready to put it back on the bike.
 
"Throttle cable is firmly in the ferrule"... Do you have a little freeplay before the cable's core starts pulling up? If you're actually "above" idle on that side, the screw won't have any effect.
Are the choke cable, and the little choke valve inside, installed correctly...?
Idle jet installed correctly? #45 typical, 40 or 50 for some variations.
 
"Throttle cable is firmly in the ferrule"... Do you have a little freeplay before the cable's core starts pulling up? If you're actually "above" idle on that side, the screw won't have any effect.
Are the choke cable, and the little choke valve inside, installed correctly...?
Idle jet installed correctly? #45 typical, 40 or 50 for some variations.
Yes on all the above. I tried another idle mixture screw off one of my spare carbs and it's doing the same thing. I already had the carb disassembled on the bench this morning, blew out all the passages, carb cleaner sprayed throughout, same problem when remounted the carb.

All intake tubes tight and airtight.

At this point I am thinking it's some anomaly in the carb body..... this bike was a non-running barn find three years ago so I don't have a baseline. It could have had this issue when it was last running for all I know.

It runs great, except for the idle mixture adjustment. One thing that I discovered: it's not actually the case that there is no adjustment. What happens is that there's no adjustment (rather, no *effect*) when turniong the screw, until almost seated fully. About a 1/4-turn before that happens, it starts to stumble and dies. Backing out a 1/4 to 1/2 turn effects the necessary alteration and it runs well.

I'm going to look for a used 40mm carb body. I love rebuilding carbs so it will be an interesting project to see what happens.
 
What is the geometry of the tip of the idle mixture screw? Is it the same as the other side?

The latest symptom you’ve posted suggests potentially an all-or-nearly-nothing fuel flow past the tip, possibly from the tip being deformed or damaged, or significant crud around the seat. Just a guess, but thought I’d mention it.
 
What is the geometry of the tip of the idle mixture screw? Is it the same as the other side?

The latest symptom you’ve posted suggests potentially an all-or-nearly-nothing fuel flow past the tip, possibly from the tip being deformed or damaged, or significant crud around the seat. Just a guess, but thought I’d mention it.
Yes they look the same. Same length, same degree of taper at the tip, no discernable difference.
 
Have you pulled the carb and bore-sighted through it at a light, with the idle stop screw backed off, to see if that butterfly plate is closing properly all the way around? If you see light around the edge, loosen the screws and adjust until it seals properly.

Best,
DeVern
 
Mark has a good point... Have you cleared the air passageway? If I recall correctly, "un"-screwing the mixture screw lets in more air.
The mixture screw's O-rings may be damaged; also note that some used two O-rings instead of one.
 
If I recall correctly, "un"-screwing the mixture screw lets in more air.

On Airheads, the CV carbs mixture screw controls a gas circuit, so opening results in more gas. The slide carb for the R60/5 etc. are different and that is controls an air circuit. The slide carbs before 1970 were also air circuits for the mixture screw.
 
This morning I dropped the float bowl, removed the idle jet and the idle mixture screw. O-rings looked fine. I blew out the idle jet just for good measure, and before I re-installed the jet and screw, I blew the passages, blth ways.... IOW first into the idle jet hole and out the screw hole, then the other way round.

Blew Berryman's up in there too.

Same symptoms... idle mixture screw has very little effect when adjusted. Max idle speed achieved when screw is almost entirely screwed in.

I'm almost convinced at this time that there is something flicklified in the carb body. My mechanic says he wants to look at it before I buy a spare carb. I'm taking it to him tomorrow for first post-rebuild oil/filter change. He probably won't get to it til next week some time but I'll report back on what he finds.
 
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