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Carburetors Under Pressure

Testing carb modifications in five inches of water
RELATED TOPICS: engines | mods | maintenance
large 44mm Mikuni round slide carburetors tuning snowmobile
Large 44 Mikuni round slide carbs are still popular on large-engine drag sleds due to their consistent tuning properties.
Vintage racing is now the fastest growing segment of oval racing. With as many as 800 entrants at Eagle River’s Vintage World Championship, there is some serious tuning going on. The nice thing about vintage racing is the good old sleds, but some aren’t all that old to me!

The older sleds (25+ years, going backwards from 1992) are lightweight and easy to work on – few with electronic fuel injection and no big 4-strokes. This is 2-stroke heaven and carbs only! If you are racing on your own budget, then this is the place to be, and the competition is very serious. In many cases, you are limited to just modifying stock carbs, like in the improved stock classes. But it is not just racers doing carb mods. What if you are stuck with a 38mm Mikuni round slide? How do you maximize performance?
In many vintage classes, the choice of carbs includes free and flat slides, or larger 44 round slides, or even larger aftermarket flat slides. Particularly popular are the larger open mod classes on the drag racing circuit. In order to put some statistical numbers on the flow properties of the various carbs available to racers and normal consumers, we went back to our Superflow 110 flow bench and ran the numbers on both 38 round slides and TM and TMX 38 flat slides. We then modified the 38 round slides by mounting UFO inserts and also boring out the 38 round slides to 39.5mm.

DIY mechanics may wonder why we use only five inches of water pressure to test the carbs, when automotive carbs were often tested at as much as 28 inches. Well, the automotive companies are in constant competition, and high numbers impress customers, so somehow the test pressures (or vacuums) kept creeping up. I asked the question to a Holley engineer some years ago, and he agreed that five inches was probably more realistic for a 2-stroke with a crank case pump.

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