Are you still annoyed by the erratic high-idle problems so common with larger Mikuni TM flatslide carbs? If so, here is a product that does an excellent job at stabilizing the idle speed, but also improves the atomization of the fuel at partial throttle settings. The V.I.P., or Velocity Intense Performance, is a combination venturi divider and venturi modifier unlike any other similar device.
Think of 33mm carb performance (Yamaha) and that of a 38 or 40mm carb. The smaller carbs have excellent throttle response and fuel atomization, but the bigger carbs flow more air on top end. How can we get more of the benefits of both?
One way is to make the carb “act” smaller during the lower throttle settings – this is accomplished by a venturi divider. There are a number of simple venturi dividers on the market, primarily used in dirt bike applications, where an additional plate (or plates) are installed into the venturi to increase venturi airspeed and velocity.
The VIP divides the engine side of the carb venturi in half (horizontally). At any throttle position below ½ throttle, this helps to maintain airspeed velocity by itself. The VIP then takes this approach one major step further by adding a second venturi (now patented) to the floor of the carburetor.
Several things are happening here. The second venturi, or venturi ramp, that is part of the VIP design, is creating a slight (V-shaped) narrowing of the venturi on the bottom of the carb floor to further increase airspeed (by over 35%) at partial throttle settings. Look closely at the bottom of the VIP and you’ll notice a “slot” that exposes an orifice on the carb floor – that’s your pilot jet fuel delivery. Anytime your engine is running, fuel is being delivered through this opening.
One of the problems larger bore carbs have is the airflow is slow and in “chunks” with the larger bore engines, pulsing back and forth, so the fuel climbing the jet needle and being delivered from the pilot circuit often remains as large droplets as it enters the engine, instead of being atomized (broken into smaller molecules and vaporizing for combustion) into the air stream like it does so well in a smaller bore carb. Liquid fuel does not burn; power and throttle response comes from fuel that is atomized.
With the big carbs, fuel from this path often runs along the floor of the carb, dribbling into the reed cages. With the VIP installed, we’ve not only increased the air speed that is pulling this fuel into the engine, which by itself will help atomization and throttle response, but the VIP’s profile forms a ramp that lifts the liquid fuel up off the carb floor. When the fuel leaves this ramp, it becomes airborne into the higher speed air stream, helping to further atomization.
All of the test sleds that were using the VIPs last season report elimination of erratic idle issues. Keith Berard from the Twin Cities owns two Polaris XC 600s, and performed side-by-side testing of VIPs and reports the erratic idle is gone, but more impressive was the improvement in throttle response.
The reports are very much the same from Fett Brothers Performance, indicating the VIPs are improving both airspeed and fuel delivery, reporting extremely quick acceleration and throttle response, especially in corner-to-corner situations. They, too, report elimination of the erratic idle problems, but tell us that is moot compared to how well the sleds pull in tight corners and anytime the throttle is being swept from closed through mid throttle. In situations where the throttle is whacked wide open the VIPs are not nearly as noticeable, but overall you’ll experience less plug fouling and loading up.
The VIPs are easy to install. Remove the carbs from the rubber boots, install the VIPs, pop the carbs back into the boots, tighten the clamps and re-install the airbox. Small tabs are held in place by the rubber boots, so the VIPs are secure and will not rotate or move once installed. Testing so far indicates setting the needle one notch richer and adjusting the pilot screw ½ to 1 turn richer is all that is needed for a well-tuned sled. Most sleds with stock jetting can drop them in and be really close to perfect.
VIPs have been under development for nearly five years, and (as stated) the second venturi ramp is patented, making them truly unique. Whether you’re wanting to get rid of the annoying erratic idle speed so common on sleds with TM flatslides, or simply want your carbs to do a better job at giving you 33mm carb performance out of the hole but big carb airflow on top, the VIPs are worthy of our attention.
VIPs are offered in two sizes to fit 38mm and 40mm Mikuni TM flatslide carbs, so they’ll fit most any late-model carbed Arctic Cat, Polaris or Ski-Doo. (Yamahas all used 33mm TMs so they didn’t have these problems to begin with!)