There was a good explanation with pictures on the old Spitfire site . There seems to be two schools of thought regarding the side adjustment screw . I went by the instructions where the side adjustment screw was for the air/fuel mixture at idle and the slots on the needle were for the high speed air/fuel mixture adjustments . Of course there will be some overlap and one adjustment might affect the other . The other belief is that the side adjustment screw is for the air/fuel mixture period and the inner needle slot adjustment is not mentioned . Larry (TRW) could one of the original "angel wings" clear this up . ..............................D.J.
D.J., you always have some pretty sharp answers for folks
in my case, i'm sure the screw is for mixture, and i'm always tweaking it (even on-the-fly) for various conditions.
i simply ignored the needle adjustment, figuring that the "longest" setting was best for a minimum idle speed. instead, i opted for making sure i properly adjust the throttle for a full-travel of the slide from closed to open, then i use the carb. top adjustment for idle.
Most carbs have a slow circuit and a fast circuit. There is a Jet for both circuits. The Carb for these engines have only one circuit. And only one Jet. This jet can be changed. But not needed. The side screw is for idle. That is all. If you look close at the needle, you will see that it is tapered. It has 5 slots. Typical factory setting is in the middle. You can lean or fatten up the run by raising or lowering the needle. If you raise the needle, it will run fatter "richer", cooler, safer. If you lower the needle, it will lean it out, making the engine run leaner, meaner, hotter , more power, and will not last as long. Take your pick. Either way, enjoy the ride.....
Mechanical slide and constant velocity carburetors used on motorcycles use a needle valve to control fuel flow through the main jet. The tapered needle is fastened in the slider portion of the upper carburetor and it slides down into the main fuel tube. The taper on the needle determines how much fuel can be drawn into the intake stream. The needle position corresponds to throttle setting, all the way into the tube for idle, and sliding most of the way out of the tube for wide-open throttle. Carburetor needles may be adjusted to lean out the main fuel circuit, or make it more rich. Raising the needle in the slider will make the air/fuel mixture more rich, and lowering the needle will lean it out
My carb has 4 positions on the choke lever; Open.....1/3....2/3.....full choke.
I thought the engine needed more air so I took the filter cover off the carb and put 2 zip ties around the case to hold the filter in. The engine seem to be breathing better at first, but later I noticed the infamous whine of a lean engine. I have to add that there's a long hill with a considerble grade to it that I use to go to the market. I've gone up it with; 1. The cover off the air filter. 2. The cover off A/F ,the choke set @ 1/3. 3.The cover on. 4. The cover on, and the opening on the bottom of the air filter widened with a drill bit.
The best seemed to be #2.
It had the same amount of power as #1, but sounded like a tuned engine that wasn't stressing, and loss most of the lean whine.
I have also noticed that these engines are very sensitive to air temperature outside.
This mourning was good, but in the afternoon when the temperature went up it seemed to be leaning out.
Now off to buy a feeler gauge.