Round Slide Carburetor Synchronization How To:
Building the carb sync tool:
- Start with a piece of thin wood, 4 inches by 14 inches.
- Mark it with horizontal lines, every ? inch. Seal it with polyurethane.
- Find a jar lid, (baby food or other of similar size). Using a screw, mount it centered left/right, with the jar cover edge up, one inch up from the bottom of the wood.
- Get a six foot piece of clear and flexible gas line from your local dealer.
- Center it around the jar lid and mount it on the piece of wood. You can use "GOOP" on the backside of the hose, or drill small holes and use wire to fasten it in several places. Make sure both ends of the hose are the exact same length.
- Fill the hose with antifreeze, (70/30 mixture), so that it is up half way on each side of the wood, (maybe an ounce or less).
Using the carb sync tool:
Note: These instructions apply to round slide carbs. Flatslides, EFI, butterflies and diaphragms are different.
- The purpose of synchronizing the carburetors on multi cylinder motors is to balance the intake on each cylinder. This gives better throttle response, a smoother idle and better mileage. On a twin, match the carburetors together. On a triple, do two, then match the third to one of the other two.
Note: If the carburetors are way out of adjustment, you will need to do a "rough" adjustment first. If the carbs are close, skip down to the "Fine Tuning Adjustment" section.
Rough Tuning Adjustment:
- Remove the airbox for free access to the carburetors.
- Unscrew the idle speed screws, (big knurled knobs on the side of the carburetors you can turn with your fingers), so they are no longer touching the slides. You will feel the resistance change as you are turning counter clockwise. The slides should bottom out in the carburetors. If not, the cables may be too tight. In that case, use a wrench to loosen the locknut on top of the carburetor, and spin in the adjuster so the slide comes all the way down, and the cable is slightly loose.
- Adjust the idle screw in on each carburetor until it just touches the slide. Now turn it in another four (4) turns. The carburetors are now roughly balanced at idle. (Another way is to lift the slide, install a pencil, turn the idle screw in till it just lifts off the pencil on each carburetor.) The actual idle speed will need to be adjusted when the motor is running.
- Now squeeze the throttle slightly, (just off idle is enough), and look/feel the slide lift. Adjust the throttle cable up on the "slower" carburetor, until they both lift at the same time. Now you are done with the rough "off idle" adjustment.
- Next test the "freeplay". Squeeze the throttle slightly until you are just lifting the slides. How much clearance is there on the throttle lever? The manufacturers specify this as it affects the throttle safety switches on many models. The width of a credit card is about right. You will need to loosen the nut on each carburetor, and adjust in/out the same amount to get the freeplay correct. Now go back and check the synchronization at "off idle".
- For the final test, squeeze the throttle full on. Feel inside the carburetors; are the slides all the way up? If not, check the throttle cable condition, routing, and your previous adjustments. Also check for interference with grips, thumb warmers and the like. Repeat for 3rd/4th cylinder, then move on to fine tuning.
Fine Tuning Adjustment:
- Dig out your synchronizing tool.
- Put the sled on a stand, (very important for safety).
- Start your sled and warm it up, shut it off.
- Ensure the airbox is installed. Hook up your synchronizing tool to two carburetors, using the port on the "engine side" of the carburetor. (The ports will be covered with some type of rubber plug.)
Note: Some carburetors may have a smaller fitting requiring adapting. In this case, use a couple of pieces of smaller diameter (primer type) tubing forced inside of the carb tools larger tubing. Be careful to keep the lengths the same and ensure an air tight fit. (See picture.)
- Start the sled but do not touch the throttle.
- Ensure the carb sync tool is level, and adjust the idle screws to balance the antifreeze from side to side. You will need to adjust the screw on the carburetor with the coolant at the lowest level, in a clockwise direction. Adjust both carburetors up or down evenly to get the idle you want. Shut the sled off, swap the synchronizing tool lines, re-start, and double check your readings; they should read the same. (If the lines are not the exact same length, you will get different readings, adjust your line length.) On sleds with more than 2 cylinders, match the remaining cylinders to one of the first two.
- Next start the sled again, and squeeze and hold the throttle to bring the RPMs just off idle. Turn the throttle cable adjuster counter clockwise on the carburetor with the coolant at the lowest level, to even the coolant. On sleds with more than 2 cylinders, match the remaining cylinders to one of the first 2.
Note: Don't let the sled idle to the point of overheating. If you adjust idle in the summer, you will most likely have to readjust in the winter.
- That's all there is too it. Now go amaze your family and friends!