solex basic queston(s)

c.routhier · 06:34 AM

can anyone clear something up for me? I've been reading the manual/repair guide and i'm confused about two adjustments on my carbs (solex). Idle Mixture and Idle Speed.

I assume that the idle mixture (black knob) adjusts the gas/air mixture (and not just for idling?), but it affects the idle speed? One side seems to be running richer than the other and i'd like to make a small adjustment, and i can't find anywhere which way to turn the knob to lean the mixture.

and then i think the idle speed seems clear, as it just physically pushes the throttle open?

thank you,
chuck

Jim Chambers · 03-15-2004, 07:17 AM

You have it right. However, the mixture adjustment is for the idle circuit only. If you are doing it by ear (far better to buy and use a UniSyn) , set the speed at about 1000 rpm with the speed screws both the same (with engine off, set each until it just contacts the arm, then screw each in the same amount, maybe 2 turns), then start engine and turn each equally to obtain 1000 rpm. Then adjust each mixture screw, one at a time, in until rpm drops slightly, then out until it no longer increases, then in about a quarter turn. At that point the idle speed will have changed. Adjust each speed screw equally until back to about 900-1000 rpm

c.routhier · 03-15-2004, 07:23 AM

Thanks Jim, I'm unclear on one thing "However, the mixture adjustment is for the idle circuit only." what does this mean exactly? is there an adjustment for non-idle circuit, or does adjusting the 'black knobs' (idle mixture) actually adjust the overall gas/air mixture for entire rpm range?
I have a UniSyn; I suppose i would use this to set the "idle speed" screws. then adjust the idle mix... or am i still confused?

thank you,
chuck

Jim Chambers · 08:14 AM

Not sure of exact RPM where main circuit takes over from idle circuit but probably 2500-3000. Mixture screws "in" to lean the mixture. Main jets/air jets determine mixture and are not adjustable. If you are too rich at high rpm you may have a plugged air jet or passage that is impeding volume of air mixed with fuel. Also could have a stuck float allowing excess gas on that side. It has been years since I worked on a 912 Solex. Take a look at the manual see how accessible main jet and air jet are. Maybe you can remove and clean/blow out.

With regard to idle adjusment, if you have unisyn, first use it to balance carbs equally with the speed screws. Then use it to obtain highest reading on the guage with mixture screws of one carb. Then do the same with mixture on other carb. Then reset balance/idle speed to desired rpm. Guage should read equally on both carbs.

c.routhier · 03-15-2004, 09:28 AM

thanks Jim, this was very helpful. The carbs were just 'cleaned, blown out' and when i got the car back it was fouling plugs on one side, the mechanic leaned out that carb and now it's fine, but the other carb seems a bit rich, according to the plugs - on one side = toasty brown, on other = blackish, carbon-y.
regards, chuck