Have you ever noticed that your sled seems to run a lot better when it is colder outside? It's not just your mind playing tricks on you, for most internal combustion engines it's a fact. It's a very complex topic, but I'll boil it into a quick overview.

Gas powered internal combustion engines require a specific mixture of gasoline to air. As air density changes (mostly due to temperature or elevation changes), the requirements of your engine changes as well. If the system that meters the gas/air mix cannot compensate for the changes in air density then your overall efficiency will go down. This is one of the factors that results in the changes in performance that you see as the temperatures (and elevations) change throughout your ride.

Internal Combustion engines need a specific gas/air mixture to run properly. Most modern made cars are fuel injected which can compensate for the differences in air density as you drive. This works to give you consistent performance and mileage from your car.

Some snowmobiles can compensate for changes in air density as well. Battery less EFI from Arctic Cat is one of the more popular systems and SKI-DOO has DPM that has been around for a few years as well. Arctic Cat EFI works very similarly to the EFI in your car. By using sensors it can determine air density and change the mixture of air/gas as you are driving. DPM uses a different method to accomplish a similar effect. DPM has a small air pump mounted on the snowmobile that connects to a group of hoses, which connect to the float bowls on your carburetors. The DPM brain then uses the air pump to change the pressure in the float bowls to change the calibration of the carburetors, which changes the gas/air mixture.

Both of these systems have been around for a few years as a reliable solution. Although they are known to work well, they are not installed on all the snowmobiles made by Arctic Cat and SKI-DOO, and Polaris and Yamaha do not have a solution available currently.

In short, the majority of snowmobiles running today don't have a way to compensate for temperature changes when they ship from the factory.

Controling the weather

The complete system installed and ready to run click for larger image

I found with my 2000 Polaris XC 7 thatthe performance of the snowmobile would vary drastically with temperature changes. Since I wanted to have consistent performance from the strong 700 motor, I started looking for a way to keep the sled dialed in.

The good news is that there are a few different ways to do this. You have all sorts of options, some of them more cost effective at the expense of convenience and some are more automatic. I tend to ride quite a bit, and I didn't want something that was going to add more maintenance factor to the sled. I would much rather find a way to invest in some initial setup and then to have a reliable solution for the rest of the winter.

For this, I turned to the Holtzman Tempa Flow. It is a small unit that is quick to install, requires a small amount of time invested for setup and I found it to be very reliable for over 1500 miles.

The basic idea of the Tempa Flow is that it changes the float bowl pressure in relation to the velocity of the air running through the carb while factoring in air density. To do this the Tempa Flow requires a vacuum source (which is installed in your carburetor), a metering device (the Tempa Flow body itself) and connections to your carb ventlines to help change the air/gas mixture.

The instructions are different for some sleds, so if you would like to investigate installation procedure for other snowmobiles you can check out the detailed installation instructions for all types of snowmobiles at http://www.holtzmaneng.com.

A typical install

The Tempa Flow is not a difficult thing to install. If you are fairly confident in doing your own basic service and have had your carbs apart for cleaning and adjusting then you should be able to handle the installation. As always, though, if you are unsure or uncomfortable with doing the work yourself you can take your machine to your favorite dealer to have the Tempa Flow installed.

The testbed for this project is a 2000 Polaris XC 700 equipped with Keihin D slide carbs. The installation has common steps regardless of the type of carbs on your snowmobile.

The carb before the vacuum fitting has been mounted click for larger image

The carb after the vacuum fitting has been mounted click for larger image

Once the fitting was mounted in the carb with the epoxy dried, I took a small file and ran it up and down inside the slide groove to make sure that there would be no interference with the slide movement. This is a step worth spending some time on as a slide being stuck in the open location could very easily result in big problems. After this was complete, I cleaned the carb thoroughly, attached the hose that came with the kit, the retainer clip for it and put the carb back into the snowmobile.

The hose that you connected to the fitting in the carb will be the vacuum source for the Tempa Flow body. All the ventlines from all the carbs are then connected together and connected to the brass fitting on the Tempa Flow body.

Airbox

Now that the fitting has been mounted in the carb you will have to mount the Tempa Flow body. Following the Holtzman instructions, the newer Polaris sleds require the Tempa Flow body to be mounted in the airbox. As I already had the airbox out of the snowmobile (it's easier to take the carbs out if the airbox is not in the sled), I removed the lid from the airbox and found a suitable mounting location as per the instructions.

Note: Some snowmobiles require the Tempa Flow body to be mounted in other locations. Follow the instructions that accompany your unit to see where yours would need to be mounted.

Tempa Flow mounted in the XC air box. Box is laying upside down. click for larger image

Head of the Tempa Flow, notice the slot screw for adjusting click for larger image

I verified the Tempa Flow was set to 1/8th turn clockwise position and headed out on the trails.

I made a first run down a river at 30 mph for ½ mile and then stopped to check my plugs. They were still a very dark brown/almost black. The lighter the plug color, the leaner the mixture. Given that I knew I was still on the safe side. I made a few higher speed runs and again checked my plugs. They were always a very dark color. My goal was to lean out the snowmobile so that I had a good performer, but I also didn't want to run on the edge of a meltdown. I knew I was going to stay in the safety zone, so I was looking for a tan color on my spark plugs. I started rotating the Tempa Flow screw to 1/16th turn counter clockwise from the initial setting. I then repeated the runs up and down the river, checking my plugs as I had done before. Now the spark plugs were starting to become a very defined brown instead of black. As the color of the plugs was starting to lighten I knew we were making progress. I then set the Tempa Flow so the screw head aligned directly with the starting marker and repeated the runs. For all the runs, the plug color was showing as light brown. Still in the safe zone, and the sled was performing amazingly very well for a 25 degree (F) day.

Over the next day of riding I would stop periodically to check over the system to make sure that nothing had come loose and that the plug color was close to the safe brown color. It was working perfectly. Not only was I getting excellent performance, my XC7 was getting over 12 MPG.

I proceeded to ride the XC7 with the Tempa Flow installed for the rest of the season. I have 1500 trouble free miles on it and I have to admit it was one of the best purchases I have ever made for my snowmobile.

Notes from the field

• As the Tempa Flow needs the vacuum source to operate, it may not work in lower throttle positions. Don't expect it to do much as your snowmobile is idling or if you are driving at 10mph.
• On the Keihin carb, you can grind off a corner of the slide to expose the Tempa Flow fitting at lower throttle positions. I did not try this last year but I do plan on trying it this year to see if it makes a difference at lower trail speeds (20-30mph).
• I found that the Tempa Flow needed time to cool down after stopping at a rest stop. If you come to a stop and pull off the trail for a while, you might be on the trail for upwards of 10 seconds for the Tempa Flow to start working properly. I found that if I held the throttle position constant when starting out I could actually feel the engine come into its own as the Tempa Flow started to cool down.

How safe is it?

In a nutshell, very. The main issue all tuners need to keep an eye out for is if the engine starts to lean out (not enough gas in the gas/air mixture). If this should occur all sorts of bad things can start to happen to the engine. The Tempa Flow requires the use of a vacuum source in order for it to operate. Without the vacuum source, or if the hoses should come loose while riding, the snowmobile will run in a more rich condition (getting more fuel). This means that you won't be getting top of the line performance, but you will have a reliable snowmobile. Even if you didn't secure the fitting into the carb very well and it came out on the trail, the system will not cause problems that will result in expensive repairs, something that is key for a guy who does 3000+ miles a year or for someone who only has a limitted number of riding days in the season.