Carb Circuits & Throttle Positions

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Carb Circuits & Throttle Positions

Post by 4Strokes » Wed Jun 22, 2016 8:54 am

Carburetor Circuits and Throttle Positions by White Brothers
Original supplied with White Brothers E-series Disc type exhaust system but can be very helpful with tuning most any carburetor.

While tuning and/or diagnosing any glitches in your powerband that may arise, the following will provide you with a basic understanding of which circuits are affected during a particular throttle position. In most cases, adjusting the number of ESeries Discs will improve and/or fix most circuit problems. In case additional adjustments are required, this section will help you locate and solve the problem. It is important to remember that adjusting carburetion should only be attempted on an engine in good running condition that is thoroughly up to operating temperature. Do not make adjustments all at once, and most importantly, do not make adjustments until first trying the exhaust system with the existing jetting. Then trying to adjust by varying the number of E-Series Discs. Find the affected throttle position (circuit) and work on that area only. Doing one step at a time will ensure optimum performance and smooth efficient operation.

1a. Pilot Jet
Up to 1/4 throttle - Controls fuel flow through the idle circuit - Mixture is adjusted by means of the mixture screw - There are two types of mixture screws, Fuel Screws and Air Screws:
  • A Fuel Screw, as the name implies, meters fuel - A Fuel Screw will be located on the engine side of the throttle valve, usually on the bottom of the carb - Turn in to lean mixture - Turn out to richen mixture - The engine should have a smooth, steady idle with the screw between 1/4 and 3-1/2 turns out from fully bottomed (gently!) - If the engine requires more than three turns out, the pilot jet may be too lean - If it requires less than 1/4 turn, it may be too rich.
  • An Air Screw meters air - An Air Screw will be located on the air cleaner side of the throttle valve, usually on the right or left side - Turn in to richen mixture - Turn out to lean mixture - The engine should have a smooth, steady idle with the screw between 1/2 and 2-1/2 turns out from fully bottomed - If the engine requires more than 2-1/2 turns out, the pilot jet may be too rich - If it requires less than 1/2 turn, it may be too lean.
1b. Pilot Jet Symptoms and/or adjustment
With the idle at approximately 1000 rpm, turn the air screw mixture in slowly until the idle slows or becomes erratic, then begin turning the screw back out until it again becomes erratic. Count the number of turns in between the two positions. Set the position of the air screw between these two conditions, which should also be your smoothest idle . Finally, adjust the idle screw if necessary to approximately 1000 rpm again.

2a. Jet Needle
From 1/8th to 3/4” throttle - Controls fuel flow through needle jet - Raising or lowering the clip location on the grooves, richens or leans fuel flow, as well as determines at what throttle position the tapered part of the needle is to be raised out of the needle jet - Raising the needle (lowering the clip) richens the mixture - Lowering the needle (raising the clip) leans the mixture.

2b. Jet Needle Symptoms and/or adjustment
Accelerate to 1/2 to 3/4 throttle, preferably in top gear. If acceleration seems weak, soft or flat, or the engine responds slow when the throttle is quickly opened, the mixture is too lean. Lower the needle clip one position to raise the needle. Conversely, if acceleration is crisp and strong, but hesitates or shutters as the throttle is closed from 3/4 to 1/2, the mixture is too rich. Raise the clip one position to lower the needle. The optimum setting will be crisp acceleration, with no load up (rich) as the throttle is backed off.

3a. Main Jet
From 3/4 to full throttle - Controls fuel flow as the jet needle is lifted far enough out of the needle jet. Until this point (throttle position), the main jet has no effect on mixture strength. A larger # jet richens the mixture - A smaller # jet leans the mixture.

3b. Main Jet Symptoms and/or adjustment
The main jet is the last jet you deal with and by far the easiest to diagnose. You will only change the main jet if a problem exists at 3/4 to full throttle. Any other problems in the carburetion are not affected by the main jet. The two easiest ways to find the optimum setting are:
  1. Set-up an area approximately 50 to 100 yards long. Have someone time you through this area. As you pass the entry, open the throttle fully (on the main jet circuit) and motor through this area. Essentially, the jet size that provides the least amount of time to run through this, is the correct main jet.
  2. Starting with a fresh set of spark plugs that have been run for at least a few miles to gain some color to them, run the engine (under a load) for approximately a minute or so at 3/4 to full throttle. Then, in a safe area, pull in the clutch, hit the kill switch, and leave the throttle in its wide open position; all at the same time and coast to a stop. Then remove the spark plug and inspect its condition/color. The porcelain should be a reasonably clean color/condition, light gray to brown is good. If the plug is black and/or sooty, the main jet is too rich (big) and you should go down in size to a smaller number. If the plug is white, with no apparent change in color from brand new, and the engine appears to be running hotter than normal, it may be lean (small). You need to go up in size to a larger number.

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