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[Thumper]

The basics of the basics:

There are 4 circuits that correspond to throttle positions:

0-1/8: idle circuit: affected by the idle jet  (aka pilot jet, low speed jet or slow jet), the idle mixture screw, and the idle speed screw.

1/8-1/4: The cut-away portion of the throttle valve (the cylinder that moves up and down inside the throat of the carb).

1/4-3/4: The needle as it rises and falls inside the jet

3/4-Full Open: The size of the main jet

Additional information on the circuits:
1) idle - this runs your bike at idle. The idle jet regulates the amount of fuel at idle. The idle mixture screw regulates the idle air/fuel mixture. Typically, if this screw is on the motor side of the carb then it is a fuel mixture screw (screw in = leaner, screw out = richer). If the screw is on the airbox side then it is an air mixture screw (screw in = less air = richer, screw out = more air = leaner). The idle speed screw regulates the idle speed - typically by lifting the valve-slide.

2) 1/4-3/4 and 3/4- full open: - The main jet regulates the amount of fuel once you've opened the throttle enough. A tapered needle goes in and out of the main jet to regulate the amount of fuel that will go into the air/fuel mixture. The needle sits in the slide valve. The slide valve is raised via vacuum and the rubber diaphragm; or, on older models, directly by the throttle cable.

3) The cutaway part of the slide valve is engineered to transition from the idle circuit to the main circuit at throttle openings of 1/8-1/4.

4) The jets are simply brass tubes through which fuel passes. The bigger the hole the more fuel (the richer the mixture).


Tuning
It might help to determine at which throttle position you are having problems.
Carbs are designed to address 4 general positions: 0-1/8, 1/8-1/4, 1/4-3/4 and 3/4 to full open.

The carb's mechanical design has four circuits that correspond to each of these throttle positions - and each can be adjusted. Identify the throttle position that has the problem and you've likely identified the circuit that needs adjusting.

0-1/8 - idle circuit: affected by both the idle (aka pilot) jet and the idle mixture screw.

1/8-1/4 - The cut-away portion of the throttle valve (the cylinder that moves up and down inside the throat of the carb).

1/4-3/4 - The needle as it rises and falls inside the jet

3/4-Full Open - The size of the main jet

Amal provides a good sequence for tuning a carb:

With a thoroughly warmed engine, go out and first test the main jet.
Run your bike fast, getting the throttle to 3/4 - full open.
Determine if there are any rich or lean problems there.

Next, adjust the idle: Adjust the idle *speed* screw to slow the engine to the point it begins to falter. Quickly adjust the idle *mixture* screw left or right to make the engine idle as fast as it will go. Go back to the idle *speed* screw and slow it back down to faltering just like before. Again, adjust the idle *mixture* screw to bring the idle back to as fast as it will go. Usually, two cycles (as just described) will do it.

Third, verify that the throttle valve cutaway is correct by driving at 1/8 to 1/4 throttle.

Fourth, verify that the needle is sitting at the correct position by driving at 1/4-3/4 throttle.

Finally, go back and readjust the idle (2nd step) as needed.

General trouble shooting:
(This is very general information. Manuals like Pete Snidal's and the link to Amal below provide step-by-step troubleshooting procedures).

When I tackle a carb problem the first thing I do is to try to isolate it and identify it. This isn't as hard as you might think given the level of complexity of carbs. As an example, your right cylinder is running hot which is usually indicative of a lean condition and yet the plug is darker than the left side. This could easily be due one of the carb circuits being rich (like the idle circuit) while another is lean (like the main circuit).

Break out your carb into it's different circuits (as described above) and the components that make up each one individually and eliminate them one at a time as possible causes.

I also eliminate simple things like: putting in a new spark plug; examining rubber manifold fittings and spark plug wires and boots; make certain that the air filter is clean and sealed properly; is the exhaust system thoroughly sealed and firmly seated in the exhaust port?

For many carbs:

Idle Circuit:
Idle jet and passages - Are passages blocked? Is it possbile that the idle jet should be increased to enrichen or decreased to lean it out?

Idle mixture screw: First determine: does it regulate air into a gas mixture (usually called the air mixture screw); or does it regulate gas into an air mixture? This will dictate whether you screw in or screw out to lean/enrichen the mixture (usually the two types of mixture screws turn opposite).

Idle speed screw: often adjusts by mechanically raising or lowering the slide

Main Circuits:
Main jet: might have to be increased in size (enrichen) or decreased in size (lean)
Needle height/position: might have to be raised (enrichen) or lowered (lean). You can also raise/lower by 1/2 position by using a small, thin washer.

Needle taper: The different tapers of needle affects the transition period as you go to fuller throttle openings (I've rarely had to change needle tapers)

Slide valve cutaway: different slide valves have different cutaways to allow different amounts of air passage. This is a transitional regulator. If you have misfiring or 'spitting' just off idle when in 1st or 2nd gear (much like: in the dead of winter when a modern lean-running bike hasn't warmed up enough) you probably need a throttle slide valve with a cutaway one step richer.


Note that for most carbs: changes to the idle circuit will only have a minor impact on the main circuit (because the idle circuit is always functioning) - but the impact is not significant except in extreme cases. One example is the Amal Concentric MK1: it's idle circuit has a significant influence all the way up to 3/4 throttle. However, you usually don't have to worry about changes to the idle circuit having a noticeable impact on the other circuits. Similarly, changes to the other circuits will not have an affect on the idle circuit.

Continue to positively eliminate things that you are confident of - like cracks in the carb mainifolds, or the exhaust headers being firmly seated, or the spark plugs both being new and swapping them to ensure the same results. Once you have eliminated each thing, you don't need to second guess it.


Misc:
The simplist method for balancing twin carbs (and often the only method needed):

With carb throats open to the naked eye: ensure that both carbs begin to open at precisely the same time (simply by watching as you roll on the throttle). Then ensure that they both reach maximum opening (and hit the stops) at precisely the same time.


Links:
http://www.hitchcocksmotorcycles.com/amal/mk1_conc_ht_7.html

http://www.iwt.com.au/mikunicarb.htm

http://www.motorcycleblog.org/bulletblog/afterexhaust.html

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

https://www.johnsmotorcycleparts.com/Keihin_CV_tuning_tips.htm

Matt

[luoma]

Wonderful information. Thanks.

[Kevin Mahoney]

Good post Thumper...now if we can get Rhett to post the carb book we have in the office........

[luoma]

Good info, but I still feel that I lack the skills to dial in my carb perfectly. I have the carb from the CMW performance kit and while it runs great, the plug is still sooty. I moved the needle a notch leaner and it helped some. I'm thinking of getting my dealershio mechanic to put a sniffer in the tailpipe to see which of the circuits is out of sorts.

[Thumper]

Good info, but I still feel that I lack the skills to dial in my carb perfectly. I have the carb from the CMW performance kit and while it runs great, the plug is still sooty. I moved the needle a notch leaner and it helped some. I'm thinking of getting my dealershio mechanic to put a sniffer in the tailpipe to see which of the circuits is out of sorts.

As long as it's running great

...it's running great.

Soot on a plug should not be a concern - as long as the bike is running well.
Many bikes run rich with choke on - enough to leave soot on the plug - but it quickly burns off the electrode under normal operations and has no adverse effect.

If mine were running great I would not be concerned with the color of the plug.

Matt

[luoma]

I would not be concerned if the carb was running a bit rich accross the board, but what if it was too rich in one or more circiuts, and not rich enough in one or more others? I want a smooth broad power band, at least as much as possible.

[Thumper]

Any circuit will be running rich or lean at any given time - the question is if it is running too rich or too lean.

Too rich or too lean will result in performance degradation.

If the bike is running well throughout the throttle range, it isn't overheating, and isn't fouling the plug, then your carb set-up should be fine.

If it's not running well throughout the range, then it's best to identify which circuit (or circuits) have a problem. Sometimes it's easier to eliminate the circuits that don't have a problem first.

Matt

[luoma]

Thanks Thumper, that sets my mind at ease. The only thing I can think of that may be wrong is that the top end of the rpm range tends to run out of poop earlier than when it was in stock form.

[Thumper]

Thanks Thumper, that sets my mind at ease. The only thing I can think of that may be wrong is that the top end of the rpm range tends to run out of poop earlier than when it was in stock form.

This might be the rich condition that you attempted to address by lowering the needle. Your description, "the top end of the rpm range tends to run out of poop", sounds real close to how I would also describe a rich condition.

You could try raising the clip one more position (to lower the needle) and see if that cures both the soot on the plug and improves the top end. The other thing to try would be to go one size smaller on the main jet. I'd try lowering the needle first.

[luoma]

Thanks. I've been avoiding doing that for fear it may be too lean, or lose power or something, but it is an easy fix and only takes a few minutes.