Jennings
Basically, the problem there was that every time I closed the throttle the crankcases filled with gasoline, and there was a long pause after reopening the throttle before anything happened. And then the engine would catch and make a mighty lunge—which always seemed bent on carrying the motorcycle right off the track.
I cured the carburettor flooding by following John Brooks’ suggestions and drilling a.037-inch hole to connect the metering pressure feed from the crankcase to the carburettor throat behind the throttle butterfly. This uses the vacuum behind the throttle to damp out crankcase pulses that would otherwise maintain pressure under the metering diaphragm and keeps fuel dribbling out of the main nozzle at idle. I also plugged the idle mixture hold behind the butterfly—which meant that the engine wouldn’t idle—but that isn’t of any great importance in racing. The similar mixture hole just in front of the butterfly I left open, as it is needed to feed the engine at low throttle openings.
A tuning session at our local drag strip showed that the revised carburettors were an improvement, having much better throttle response and no tendency to flood when the throttles were closed. We also found that the mixture curve was wrong and was going a bit lean above 9000 rpm. To counter it with the McCulloch carburettor, you enlarge the main vent that bleeds under the metering diaphragm. Once done, you must also open the adjustable main jet a tad, but it does lift the top end of the mixture curve. I can’t tell you how much larger you should drill the vent. I overdid it on the first try, and then had to open the pressure feed hole into the chamber to compensate-which I also overdid—and was forced to drill the vent even larger. Anyone else going through the same process should first obtain all available small-numbered and lettered drills and proceed with great caution. The difference in mixture, with a very small change in air-orifice diameter, is surprisingly large. By the time I had learned this lesson, all of the drillings in my carburettors were too large, and although they were working fairly well, a less drastic series of passes with a drill would have been better, I am sure. Sometimes there is more learned in failure than in success. With my Bridgestone 350GTR racer project (a racing failure if ever there was one) I learned several things: that a conventional battery/coil ignition system is inadequate to the needs of a racing two-stroke engine—Which should be fed oil in a proportion of about 1:12 with its gas; and that the McCulloch carburettor does offer significant advantages in this kind of application, but that it is necessary to do a great deal of work with its fuel delivery system, and even more work with the metering system’s air bleeds. These carburettors work exceedingly Well in the low and mid-speed ranges, but have an inherent tendency to go lean at maximum rev’s and this must be compensated. I have not yet succeeded in this; the modifications made have improved things, but the mixture is still slightly over-rich at 6000 rpm (and downright sooty below that speed) while a shade lean up at 9000 and above.
Originally Posted by wobbly
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Recommended Rotax timing is 138/88
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