QuestionHi Bill,
I have a 74 cb450 with Keihin carbs recently rebuilt by Dime City Cycles. My question is how do I sync and set the air/fuel mix? I'm a novice and need all the help you can give me. I'm pretty sure I have the timing set right, although when I put my hand behind the exhaust while idling, one side seems to have more pressure than the other. thanks again, Don
AnswerDon, the last thing to do on a tune-up is the carburetor synch and adjustment.
MAKE SURE that the ignition timing is set properly and the spark advancer is working as it should. Ignition timing is critical in establishing the intake manifold vacuum signals that the carburetors need for proper metering. If the timing is even a few degrees of from side to side, then it will affect the engine idle speed for that cylinder.
If you look at the throttle arms, there are idle speed screws on each one that turn into the carb body to change the angle of the butterfly plates on the throttle shaft. Basically, look at each screw contact end, back off the screws until you see a gap, then turn the screws in until the end of the screw contacts the body, then go in about 1/4 turn. Do this equally, start the engine and then let it warm up, checking the back pressure and sound of the engine as you balance the carb idle screws. The idle mixture screws underneath can be turned in about 1/4 turn each way from the standard setting (approx 1 turn from all the way in) listening for the engine speed to rise/fall until you find the peak rpm setting.
Once you have established idle mixture settings and the idle speed, shut the engine down and check your throttle cable adjustments. You have to be able to look/feel both throttle arms at the same time, so you can use the cable adjusters to ensure that both throttle arms go up at exactly the same time. Tighten down the lock nuts on the cable adjusters and you are done.
Bill Silver
