TUNING THE SLIDE-TYPE MOTORCYCLE CARBURETOR

TUNING THE SLIDE-TYPE MOTORCYCLE CARBURETOR

Whether it's an Amal, - Standard, Monolbloc, Concentric, TT, or GP - or a Del'Orto, Mikuni, or Micarb, there's a sameness about slide-type carburetors as found on motorcycles built during the past 60 years and more. Air is admitted into the intake tract by means of raising a slide in the body of the carburetor, and consequently raising a fuel control needle attached to, and usually inside, the slide.

Fuel is maintained at a constant level in an adjacent float chamber, usually part of the carburetor body, by means of a float and valve arrangement, and is admitted into the airflow beneath the slide through a series of jets, the most important of these being the needle jet - the one the tapered needle slides in when the slide is moved. The size of this jet, and the taper and relative position of the needle are the determining factors for mixture (air/fuel ratio) in the range of 1/4 to 3/4 throttle, give or take. Once the throttle is above 3/4, the needle jet passes pretty well all the fuel that comes its way. It is regulated only by the size of the restriction below the needle jet, which is the main jet. Changing the size of this jet thus controls mixture at the "top end" of the throttle, and to an extent, the rpm range. (Since mixture is dependent to a degree on airflow, the upper end of the throttle range wants to be tuned at the upper end of the operating rpm range.

To set the jetting for either of these ranges, we do a "plug chop." The tip of the spark plug insulator changes colour with mixture - a white tip shows too lean a mixture, a black one, too rich. The ideal is just the sandy side of chocolate brown.

To do a plug reading, you want to run at 1/2 throttle for a minute or so, pulling upwind or up a hill, say, and then suddenly hit the neutral finder, if so equipped, or pull in the clutch, and turn of the key, coasting to a stop at the side of the road. Where you take the plug wrench out of your pocket, and pull out the plug and have a look. White plug, clip down, black plug, clip up.

For Main Jet tuning, repeat as above, but at full throttle. White plug, bigger jet, black plug, smaller jet. If it won't get up onto full throttle because of bucking and missing, you have to look at your pipe and if you see black smoke, you know it's 'way too big, so you'll have to try a smaller one. If it's too small, you'll get a white plug tip on full throttle, if not a lot of starving, bucking, missing, spitting back from the carb, (classic weak mixture) etc. Your main jet will have to be an awful lot too big before its size will affect the mid-range of the needle adjustment.

The needle position should already be set by this time. In extreme cases, you may not have been able to get a rich enough needle setting, because your mainjet may have been too small.

Variations in main jet size shouldn't affect needle setting, although if I've made big main jet changes, I always check the needle setting again.

The first 1/8 of throttle opening is affected by the pilot, or idle jet setting, and throttle cutaway affects the "transition" from idle circuit to needle. Adjust pilot jet at idle, alternating between the idle stop screw to get the lowest possible idle, and the pilot jet screw, to bring the idle up as much as you can. Throttle cutaway can only be changed by changing your slide, but fortunately, this is an adjustment that is seldom necessary. The indication that you need to will be hesitation (or worse) as you increase throttle from just above 1/8 to the needle range.

That's about all there is to getting your carb tuning "in the ballpark." Do remember that too lean a mixture will bring about overheating, often detonation and subsequent holing of the piston(s).