Carb idle mixture

I read on some Norton-related site recently - wish I could recall just where - that the optimum setting for the idle mixture on the Commando/Amal is not the point at which you get the fastest/smoothest idle but with the mixture screws out beyond that (leaner). I had never heard of setting the idle mixture on any carb on any kind of vehicle at anything other than at the point at which you get the highest RPM for the particular idle throttle opening.

I don't understand how other than the fastest/smoothest setting could be called "optimum" but I'm willing to learn. And how could you find this "optimum" setting anyway? Is there anything to this? (I'll continue to see if I can find where I read that)

Found it:

"This is where a bit of skill & experience comes into play. By turning one screw a little at a time with the engine running (on both cylinders) you can gauge any difference in running and find the sweet spot. When you hear the engine revs race, you want to turn it back a 1/8 to ¼ turn - the sweet spot is not at the extreme."

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

I was wrong about going leaner - he says to turn it "back" which I would take to mean richer. So his adjustment makes it even richer than the best running at warmed-up idle. Makes NO sense to me.

"Best running" is not the same spot as fastest idle. The fastest idle will be a slightly lean mixture. Witness the effect of not turning on the fuel tap when you start the engine - the idle will increase just before stalling out with a low float level.

The factory procedure is to adjust for "most stable" idle. In practical terms you need to compromise sometimes between hot / cold temperatures and faltering when blipping the throttle. Not all bikes behave the same way. The Amal gurus say that the ideal spot is 1-1/2 turns out, as that's the design nominal point to match the fuel pilot jet.

I go slightly richer than optimim to avoid occasional pops on the overrun. The symptom is 1 pop when shifting up thru each gear if idle is adjusted for max RPM. Going a little richer may cure that. MM you are idling too low at 500-700 rpm - just my opinion. 900-1000 is normal for my bike and most Nortons I've ridden.

Russ

Russ, I agree, My hot idle is 900 (thereabouts, based on the "wobble" in the tach) but the bike will idle at 500 just after starting, slowly increasing to 900 on it's own as it warms up.

"The fastest idle will be a slightly lean mixture. Witness the effect of not turning on the fuel tap when you start the engine - the idle will increase just before stalling out with a low float level."

Good point. Usually when I shut the bike down for the day I turn off the fuel and let it run the carb dry. The idle speeds up just prior to shut down.

Mike,
The just slightly richer than the fastest setting also helps the transition point between idle and just off idle. If you left them at the fastest setting often you will find a flat (lean) spot when turning on the throttle.

SU's & Strombergs are tuned like that, fastest running then in about 1/4 or 1/2 a turn.
According to the Norton manual, you are meant to disconnect one cylinder & do the running one.
Must have another bash at mine, as the tickover is erratic.

Well if you insist on doing it that way use a grounding cable on the plug you wish to kill. Just pulling off the plug wire will kill your coil. When the secondary fires off it must have a pathway to ground, this should not be the side of the coil to the frame. The manual is wrong.

Hi all, I was always taught by my old dad, who's biking days started in the 1930s, no matter what type of carb, to lean it out until it just starts to falter, then richen it up by about half a turn. I have always done my bikes this way & it seems to work.
Terry

My 2 cents. I think you should do the adjustments while test riding the bike.
I start with the mixture screws 1 1/4 turns out (or start at 1 1/2).
But FIRST make sure the idle circuit is free and float level correct, or you will be waisting your time.

- Start with a good test ride to you make sure the engine is warmed up properly.
- Then start the adjustements of the mixture screws. As mentioned by NORBSA you can get an off-idle flat spot if the mixture is too lean. This will make it difficult to get away from a stand still. It might idle nicely but has no low rpm torque when releasing the clutch.
You should enrich the mixture until it pulls "like a train" from stand still. Adjust the mixture back and forth until you have the leanest mixture that still allows a good start pull. This will take a bit of tweaking to achive.

Using a Carbtune (or similar equipment) to get the sync perfect also helps to get the low rpm torque.

Correctly tuned you should have easy starts from stand still and an 850 engine (with 21 tooth sprocket) should pull from 30 mph on top gear.

Cheers,
Per