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44 is the Best Number
The “good old” 44 round slide carbs are coming back into popularity again. Over the years, a number of larger flat slide carbs have been available, but they require a higher level of expertise to make them function properly over the full range of RPM and throttle position. With its three distinct tuning circuits (pilot, needle jet and main jet), the round slide is very predictable and tunable with a large Mikuni inventory of tuning parts readily available.
It was interesting to watch the Goede Brothers machine at the Eagle River World Championship this last winter. While others experimented with reed valve inlets and large flat slide carbs, the Goede sleds were clean, simple and effective, winning the championship with piston-ported cylinders and 44 round slide carbs. There is a lot to be said for simplicity, especially if it means the sled can be tuned to a higher level of perfection. The larger 44 round slide carbs have been around as far back as the early 1970s, and they were always easy to dial in with their large selection of tuning parts.
When I was an Evinrude development engineer, we had a 440 prototype piston-powered engine on the dyno. We invited Mikuni to come in and tune some 44 round slides for the engine. Two Japanese engineers showed up at 10 a.m. and had us make inserts to hold the slide at ¼ and ½ throttle. We then made several dyno runs, and the Mikuni engineers read the fuel flow gauges. Several adjustments to needle jets, needles and main jets were done, and they had the carbs dialed in before lunch. Very impressive, as another carb manufacturer had been attempting to calibrate some carbs for our consumer sleds for three months and was still not done with the production calibration.
The simplicity, consistency and parts availability of the round slides have many racers going back to them because they are so easy to tune. We put some 44mm round slides on the flow bench to see how they compared to the smaller models. A stock 44mm round slide flows 134 CFM. Install a UFO, and the flow increases to 138 CFM. This is a 3% improvement at full throttle, but we were most interested in better throttle response in this case. The 44s can be bored out to 46 in the throat and tapered to 48mm at the flange. When combined with the UFO, this increased the flow to 141 CFM, a decent increase over the 134 CFM of the standard 44, although not as large a relative improvement as we saw with the mods on the 38 round slides. With the larger carbs, the main improvement we look for is in the throttle response, especially on the drag sleds, and the UFOs are a benefit in this area.
Our tests led to more questions. What about the flow in rack carbs or fuel injection bodies? What about the butterfly action compared to slides? Since we fired up the flow bench again, we are also going to test reed valves, so look out for some future comparisons on those and other items of interest.