R80/100 carb frustrations... (cont.d)

[Brickboy]

Hi gspd, yes, I've been having similar thoughts. I've only ever seen brass ones before but they are indeed identical - or at least they look it...

I have the idle at 1000rpm (on the tacho) and it is smooth and balanced according to the Carbtune gauges. No stutter on pick up - very smooth in fact

I did try gentle cable pulling. No obvious stutter or bogging down I could spot.


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I've tried swapping the screws over. Interestingly, the non-brass (SS, I think) screw will not seat as deeply in the Rh carb as the left. The brass one is fine both sides and so the bike runs really badly if they are swapped over. I do wonder about the thread quality on that ss one...

Returned to their original carbs the Rh brass screw still cuts off that cylinder when screwed in fully. I wasn't convinced it cut off the lh cylinder when in that carb but it all ran so roughly it was hard to tell. Perhaps it's time to buy replacement idle screws so a pair are on their way as we speak. Nice and brassy!!

[gspd]

I've only ever seen brass ones before but they are indeed identical.

If they don't seat the same, they obviously can't be 100% identical.

I've tried swapping the screws over. Interestingly, the non-brass (SS, I think) screw will not seat as deeply in the Rh carb as the left.

Maybe the hole in the L carb body was enlarged or damaged by someone over-tightening the (improper?) SS screw into it?
Those screws should only be lightly bottomed when turning them in.
I've repaired a few carbs over the years where the soft brass tip broke off in the carb without causing any damage.
The stainless steel tip would not break off as easily if bottomed too tightly and could possibly enlarge the hole in the actual carb body's soft aluminum if forced in.
I have a suspicion that the seat/hole in the left carb is fucked up.

...it is smooth and balanced according to the Carbtune gauges. No stutter on pick up - very smooth in fact

Your bike might run really well as it is, but once the problem is fixed you'll probably realize how smoooooth it's really can to be.
As it is now you're balancing the carbs before properly adjusting the mixture, it should be the other way around.
Post back the outcome after installing the 2 new screws.
Hope that works.

[Brickboy]

Well, who knows. By screwing in the new, correct idle screw I can get a beautiful tickover... but only with the new screw screwed as tightly home as I dare. You can clearly hear that cylinder fire up as the screw starts to hit the stop - it ticks over beautifully on both cylinders and picks up smoothly and without any stutter, pop or bogging down. I'm inclined to agree that something is awry with the left carb - what it is I cannot picture but it does run very nicely indeed.

[Kurt in S.A.]

Well, it suggests that the idle circuit is running rich. Right at the point where you cut off the fuel, the mixture becomes perfect and it fires. What this is, not sure.

Kurt

[Brickboy]

Yep, I agree Kurt. After some faffing around I connected up the Carbtune again and reduced the throttle stop setting on the lh carb until the lh pot cut out. Then I backed out the idle screw and it fired up again - the best setting is about a third turn out rather than the book recommended 3/4 turn but I've balanced the idle and the pick up. I think that's the best I can do for now.

[gspd]

Plausible explanation: Maybe the hole that the mixture screw tip goes into has been enlarged a bit by the old SS screw so now 1/3 turn out is giving the same amount of fuel that 3/4 turns out should. As long as the cylinder stops when screwing it in all the way, and starts firing smoothly when backing it out a bit, you're right in the ballpark for the mixture, possibly even dead on.

[Seth]

Looking at the picture of the screws, someone really cranked hard on the stainless one. Look at the damage to the screwdriver slot.

[Brickboy]

Yes, Seth, I think that's what may have happened which fits in with gspd's suggestion that it has enlarged the passage the tip engages with.

Still, at least things are almost 'normal' now on the adjustment front. Doubtless there will be more fiddling around but for now I'm just going to enjoy riding it.