Baffled with carb problem ... (long, but not winded)

[StephenB]

I seem to not be able to get of my left carb to work as it should:



For comparison, here's the right (good side):




Here's where I am in my investigation which was so far mainly comparing left and right "settings":

Compression: equal on both, measure with carbs off (9.5 or so)
diaphragms: ok, no rips or holes
floarbowl level: equal, float bowl parallel to carb body method
Needles: 1st from top, tried 2nd (stock) with same result
Needle yets: 2.68, "goodplug" O-ring was slightly ruptured
Confirmed position of needle in the jet - same height (measured from below with main jet removed)
Main jet: 160 (have not confirmed whether they are what they say they are)
idle mixture screw: new with new orings and springs (adjusted, about 1 and 1/8 rev out for both), bore is clean and free, o-rings seal as they should
idle jet: 45
Gas mileage: about 37-40mpg mixed
Plugs: new BP7ES (have not confirmed spark quality yet)
Coils: Dyna dual (green), about 4-5yrs old (have not confirmed spark quality yet)
Ignition: points with Dyna booster ( (have not confirmed spark quality yet)
Valve play: 0.06/0.08
Rockeram axial play: close the zero, but freely moving

Other observations:
downpipe temps: the "bad" side seems to run hotter (I always thought rich runs cold?), measured about 30F higher with wireless IR thermometer
when removing the idle mxture screws they are both dripping wet with gas (that is maybe normal)

I am scratching my head:
All "enrichening" factor have been looked at except for reducing the jet needle or trying a 2.64 and play wirth needle settings again. Both carbs were always different, but it seemed to have deteriorated.
I can also swap the carbs and see if it is indeed carb related. I was hpoing I would not have to do that 'cause somebody comes up with a magic trick!
I can also clean the carbs again although they seem to be squeaky clean and I run them with (VW) in-line paper filters from day 1.

Ideas anybody?

Thanks, Stephen

[Kurt in S.A.]

What is age of floats? Just setting them to where they're parallel is really not good enough. You need to measure the amount of gas in the bowl after allowing to fill and then turning the petcock off. I forget the numbers, and it depends on the 32/40mm carbs, but it's something like 22-24mm in depth measured in the center of the bowl.

That's where I'd look to be sure one side is not really running richer than the other.

Kurt in S.A.

[StephenB]

Kurt, I never understood why many believe that high float bowl lever richens the mixture? What's the rationale? How could it if the Venduri effect is sucking the gas in, through a series of variable resistances (jets, needles etc) (just like resistances in an electrical circuit) from the bottom center of the carb bowl. We all know the effect of bogging when the float levels are too low, but too high -> there is an overflow directly to the outside. The float bowls are designed to allow working with different float bowel levels, that is the whole idea, is it not?

BTW, Bing's type 94 instructions don't mention amount of liquid, just parallelness of the "float top edges to the top edge of the float chamber".

Also, with regards to the overflow function, Bing states:

The space above the fluel level is connected to atmosphere by two ducts. When these ducts are blocked, an air cushions forms above the fuel level. The fuel will not lift the float sufficiently to close the needle valve and the carb overflows.

I know one duct is the brass pipe sticking out the top of the float chamber and a bore on the bottom of the float bowl. What's the other one? The one in the middle sticking out the center of the bowl?

[Kurt in S.A.]

Stephen -

Snowbum mentions height of fuel here:

http://bmwmotorcycletech.info/bingcv-2.htm

My understanding is that the higher the fuel, the easier it is to suck the gas into the carb, thus it tends to be rich.

Kurt in S.A.

[Jean]

which bike?
I'd switch SP wires first and see if there's a problem with the ignition. Is it possible to turn the coil around and fire the opposite plugs?

I had a similar situation when my coils started going bad. One was weak and misfired...Yes, the pipe discolored and got hot since there was 'afterburning' IN the pipe!!.

[George Ryals]

Maybe the right side has an air leak or gummed up atomizer and is running lean instead of the left side running rich.

You gave a compression of 9.5 instead of something like 145 psig in each cylinder. Did you calculate compression ratio from the pressure readings from your compression gaugeor something?

You could have a worn needle and needle jet in the left carb...or the wrong needle to go with the correct needle jet.

[George Ryals]

The hole at the corner of the float is the other vent to the atmosphere.
The distance from the top of the fuel level in the bowl and the top of the atomizer nozzle (or bottom of the carb throat at the idle ports) is the static pressure that has to be overcome by carb venturi vacuum to lift fuel air mixture into the carb throat. If one carb's fuel level is 20 mm below the atomizer nozzle and the other is 25 mm, the one at 20 mm will lift more fuel at the same venturi vacuum than the one at 25. It has 20% less static pressure to overcome.

[StephenB]

http://bmwmotorcycletech.info/bingcv-2.htm

I am sure, snowbum has a lot to say but I have yet to manage to read one of his articles in its entirety.

The engine is the 1980 100CS engine that lives in my G/S.

Sorry about the bars: 9.5bar translates to 138PSI which is what I measured with a standard compression tester.

Now, I made one more vital test: went out today in beautiful sunshine on one of those long straight gravel roads. Warmed the bike nicley up (30min) and then ran one of these straight sections at 4000+rpm. Cut the engine after 5min and checked plugs -> they were both equally PERFECT. Absolutely magnificent. Nearly white isolator, dry black rim and two color electrode with a color change at the right spot -> textbook type appearance. Both were looking just like the "goodplug" above!

So, I must have looked at the results of the idle circuit as the picture were done after coming back from a previous ride with the last 2-3min riding at low speed through the subdivision where my home is.

So, it was only my left idle circuit that was f***ed up. I didn't bother to look that up again after the ride because I don't have the bike idle very often or long. and the off-idle performance leaves nothing to desire.

So, contrary to my intial thought there is nothing really wrong after all, as soon as I am off idle, I'm good. What a relief.

Thanks for your comments though, especially the one on the venthole.

[Duane Ausherman]

The only way to do a plug test is just what you described, run it and kill it. Otherwise you have no idea how it was running in the murky range below full main jet.

It might be worth chasing, but at least now you know where to look.

[Major Softie]

Kurt, I never understood why many believe that high float bowl lever richens the mixture? What's the rationale? How could it if the Venduri effect is sucking the gas in, through a series of variable resistances (jets, needles etc) (just like resistances in an electrical circuit) from the bottom center of the carb bowl.

Stephen, vacuum is pulling the fuel up. Gravity is pulling against it. Regardless of restrictions, higher pressure will result in more fuel being delivered. Raise the fuel level in the bowl, and you have reduced the gravity side while retaining the same vacuum side - thus, more fuel pressure against the restriction (jets).