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main jet test
Way back when, in the 1920's there were two main manufacturers of motorcycle carburetors in England, Binks and Amac. The Amac was a variable venturi carb, like the keihin and the mikuni, except it required two lever control. One control lever (on the handle bars) lifted the air controlling piston, and the other raised and lowered the needle. The Binks had a better idling circuit and used emulsion tube control of low end mixture. Amac was the larger company and bought out Binks so as to be able to use their patents. They changed the company name to Amalagated Carburettors, Limited. Amal for short. within a year or two, they were able to utilise the emulsion tube idea in order to have a one lever carburetor. Then three or four years later, someone came up with twist-grip control. So endeth the lesson for today.
So, I was going through some old notebooks I was going to throw away, when I came across my old tuning notebook for the Amal GP carb fitted to my BSA Gold Star. I was going through it when I had an "Oh yeah" moment. Some other tuner had told me, and I wrote it down, on a simple main jet test. He called it the Roll-Off Test. This simple little test will tell you where you are at, main jet wise. First, get the engine well warmed up on your way to your favorite low travel, but high gear road. Now get well up into the power band in high gear. Apply full throttle. Keep it going like this for 3-4 seconds so that the carburetor is settled in and the engine pulling hard. Now rotate the throttle back to 7/8ths of it's wide open position. The pecularity of these style carburetors is they will richen up for about a second when you do this. So, if you get a tiny power surge, your main jet is too lean, or small. If the engine staggers slightly, or has a hard hesitation ( a hiccup), it's too rich. Simple, huh?
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