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Carburetors Under Pressure

Testing carb modifications in five inches of water
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?
38mm Mikuni round slide carburetor snowmobile
This 38 Mikuni round slide carb is used to check modifications to the slide and bore.
stock 38mm round slide UFO insert carburetor snowmobile
The stock 38mm slide on the left has a large wall on the back to restrict flow. The slide on the right has a UFO insert with smoother flow.
Why We Do What We Do
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.

A 2-stroke has only a fifth of the crank case compression ratio of a 4-stroke, which sucks directly into the top of the cylinder. On a 4-stroke engine, he felt that maybe a test pressure in the 15- to 20-inch area would be more realistic. So when you see test claims of 650 CFM (cubic feet per minute) and wonder how they can get such large numbers through the small throats of some of the 4-barrels on display, you might want to check at what pressure the carb was tested. If you calculate the CFM required for a 250cc cylinder running at 10,000 RPM, you get close to 90 CFM. If you calculate the CFM required for a 400cc cylinder running at 9000 RPM, you get close to 130 CFM. The fact that the 38 carbs test in the 90-110 CFM range and the 44s in the 130-140 CFM range justifies the five inches of water as a realistic test pressure when comparing 2-stroke carbs.

We flow-tested our carbs in four throttle positions: ¼ throttle, ½ throttle, ¾ throttle and full throttle. The full throttle numbers are always accurate, as the slide is all the way out of the bore, while the part throttle numbers can vary slightly as it’s difficult to place the slide accurately in every carb, and there may be slight variables from test to test. Still, the overall trends were clear. The test results are seen in the chart above, and the trends are interesting.
stock 44mm round slide UFO insert carburetor snowmobile
On the left is a stock 44mm round slide. On the right is the same slide with a UFO insert in place. In front of the two slides is the UFO insert itself.
vintage carbureted snowmobile
Vintage sled racing is growing fast, in part because old carbureted sleds are simple to work on and can be found affordably.
Carb Test Results & Products
When comparing the three different 38 carbs, there is clearly a difference in full open flow. The trusty round slide carb flows 96 CFM, while the early square slide TM carbs jumped the flow up to 100 CFM. The square slide carbs were originally 4-stroke units, but they were also popular on snowmobiles mounted on a twin or triple rack (aka “rack carbs”). The 38 TMX is a 2-stroke carb originally developed for a Suzuki motocross bike. It has a shorter body, smoother bore and shorter, D-shaped slide. The TMX 38 carb takes max flow up to 108 CFM, a 12% increase over the round slide carb.

The TMX carb has several advantages. Overall length is a full inch shorter than both the 38 and 44 round slides, which is beneficial on high-revving engines that need short intakes for good power at higher RPM. The shorter slide also has less cushion underneath it. When lifted, it increases vacuum around the needle jet quicker than a full round slide. This improves throttle response and acceleration, and makes it a favorite on snocross machines. Because of the quick vacuum buildup, TMX carbs usually only run 25 pilot jets, where round slides may require 45 pilot jets.

If you are required to use stock round slides, it is possible to modify them to match the performance of the newer TMX carbs and still stay within racing rules. Thunder Products offers a product called a UFO (ultimate flow optimizer). This is a streamlined insert that reduces the “cushion” under the slide and also improves the airflow. We installed the UFOs on our 38 round slide test carb, and top end flow increased from 96 to 104 CFM. This is a healthy 8% increase at full throttle, and 5% more at half throttle.

This is still a little short of the TMX’s 108 number, but we had one more trick in our bag. We bored out the 38 round slides to 39.5mm, and this brought the flow number up to 106 CFM, a 10% increase over a straight 38. After adding the UFO slide to the 39.5 round slide carb, the full throttle flow numbers went up to 110 CFM, actually 2 CFM more than the TMX flat slide. This represents an almost 15% increase in flow over a standard 38mm round slide. With the UFO insert, we also had throttle response and acceleration comparable to the 38 TMX.

It’s interesting that you can match TMX flat slide performance by modifying the carb by boring it out and installing UFO inserts. When you install UFOs, Thunder Products recommends that you reduce the pilot jet size to half of normal, which makes it close to the 25 pilot jets used in the TMX. This proves that the quick vacuum buildups from the UFOs match the quick response of the TMX flat slide.
Mikuni Carburetors Airflow Cubic Feet per Minute CFM 5” Water Pressure chart
Click chart image to enlarge
Olav Aaen's Carb Tuning Handbook Aaen Performance
We’ve heard this referred to as the “Carburetor Bible” on more than one occasion.
Superflow 110 flow bench carburetor modifications
The superflow 110 This flow bench is a useful tool when checking carb flow modifications.
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.
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