April 3, 2003
Addressing 2002 Summit Calibration Issues
Is your 2002 Summit 800 not up to snuff? That model year of Summit 800s suffers from well documented calibration issues, with the two most noticeable being poor running quality and throttle response in the mid-range and aggressive deceleration when the throttle is released. The carburetion issue exists at all altitudes, but seems to get extremely obnoxious around the 8,000-9,000-foot mark. At 10,000 feet and up, where Colorado’s deep snow exists and Xtreme tests extensively, the sled is particularly unpleasant to ride.
Carburetion: After a brief examination of Polaris and Ski-Doo carbs—both use essentially the same carburetor produced by Mikuni—we noted an interesting difference. Polaris uses a needle jet with the traditional “hood” on it, while Ski-Doo elected not to. Since the Polaris sleds run as good as they ever have in the mid-range and the Summits do not, we wondered if that missing hood could be part of the issue.
In order to test the theory, we began working on our 2003 Summits, which also happen to come with a hooded jet. In our water tank, the 2003 calibration seemed vastly improved, with only a slight rich gurgle in the mid-range and almost no hesitation under snap throttle acceleration. We wondered, then, how bad a 2003 would calibrate with 2002 carbs. To find out, we promptly installed a set of 2002 carbs on the 2003. In the water, with the sled under load and with the 2002 carbs installed on the 2003, the 2003 had difficulty even picking up rpm in a snap throttle situation. More often than not, the sled would simply die altogether if you kept the throttle mashed. That indicated to us the 2002 carbs were incapable of delivering enough fuel for acceleration, while the 2003 carbs did so easily, without being overly rich at steady state throttle.
With that knowledge in hand, we began to look for a reasonable way to change the needle jets, so that replacing the carbs would not be necessary. Luckily, with the help of Joe Dispirito at Cyber-Dyne Research in New York, we arrived at a solution for replacing the needle jets. The solution remains fairly complex and time-consuming, as the carburetors must be completely disassembled in order to replace needle jets that were never designed to be removed. Fortunately, it’s still more economical than replacing the entire rack.
With the process of modifying the carbs done, we could begin calibrating the new carbs, both in stock and RMX form. We attempted to make the new carbs work with DPM, but the programming is so rich in the mid-range to try and cover up the lean bog, that we couldn’t mechanically lean the jetting far enough to make the sleds run correctly. Thus, our solution involves removing the DPM, venting the carbs to the airbox, replacing main jets, pilot jets, needle jets, needles and resetting the fuel screw. With all of that changed, the sled now gurgles significantly less and responds when you smack the throttle. In stock form, you can still hunt for the lean spot and find it occasionally, but in all our test riding, it never bogged while riding normally.
In order to calibrate the main jets, we ran the sled for about two miles at wide open throttle down a hard packed road repeatedly. We’re not sure where you would ride for longer than that at full throttle, but if you do, it’d be a good idea to burp the throttle occasionally. In order to make the sled run right in three feet of powder, the jetting has to be fairly lean, meaning a little caution when trail riding is in order.
Why does it work? It’s helpful to consider how carburetors function to get a better understanding of the problem with the stock carbs. As the piston starts back up its stroke, having gone through bottom dead center, effective crank case volume grows, thus causing a vacuum in the crankcase and intake tract. It is this differential between the low pressure in the throat of the carb and the constant pressure being applied to the float bowls that causes gasoline to rush up through the needle jet and mix with incoming air on its way to the crankcase. Needle jets are normally built with a “hood” on the top of them so that air coming through the carb must go around and over the hood, thus causing a venturi effect much like the wing of an airplane. It is because of this venturi that the area directly above the float bowls inside the hood sees an even greater pressure differential and increases the “pull” on the fuel in the float bowl. Since the 2002 Summit carbs lack the hood, that means the pressure differential between the float bowl and the throat of the carb is limited. This condition is referred to as lack of signal. Essentially, it makes the carbs very difficult to tune because the pressure differential is so low that even whopping jetting changes result in very small changes in fuel flow. When you combine low signal carburetors with our high altitude air that has the molecules spread out, you have made a bad situation even worse and the signal is further degraded.
Furthermore, two-stroke engines are far less efficient at lower rpm because they lack the pumping action of the tuned pipe, which makes the carburetion problems more pronounced in the mid-range between 4500 and 5500 rpm. The final complication for mid-range carburetion is that there is a real possibility that you are likely to abruptly pull the throttle open, thus giving the engine a huge shot of air. In a properly tuned combination, you would have enough signal that as the slide is lifted and air volume increases, fuel would be picked up and carried into the motor as well. In this case, all you get is a lot of air with not enough fuel, which results in a huge bog and the rider inspecting the windshield from the wrong side of the sled.
If the fundamental design problem in the carburetor can’t be fixed, your only choice to get around the lean bog is to give the engine more fuel so that when the throttle is opened rapidly, there’s already enough fuel in the engine to mix with the huge shot of air. Unfortunately, that means for those of us who actually ride around in the trees or down a tight trail, the engine gurgles and carries on because it doesn’t need all that fuel for smooth, steady-state throttle operation.
This is also the reason the fuel mileage is so bad. If you rode your sled pinned all day, the mileage wouldn’t be as bad. However, if you’re powder riding and boondocking, you spend a fair amount of time at the mid throttle position, which is too rich. Unfortunately, with the stock carbs, if you lean out the mid-range to make the engine run properly at part throttle, the lean bog gets even worse. If you jet so rich that you don’t get the bog, you can’t carry enough gas to ride all day. Plus, the amount of unburned gas coming out of the pipe is enough to make you nauseous.
The compromise, then, is to leave the jetting somewhere in the middle, where they gurgle incessantly in the trees and still hesitate if you mash the gas. Last year, we found that our favorite altitude fixes of more compression and a more efficient exhaust pipe helped, but they couldn’t overcome the underlying carburetor problem.
Clutching: With the carburetion fixed, we moved on to the clutching. Last year, we spent more than $2,000 on every conceivable aftermarket super-whiz-bang clutching component on the market, trying to make the sleds faster. The bottom line was and is that if you have two sleds that turn the right rpm, no matter how you get there in the clutching, they run the same. Magnesium arms, titanium springs, roller driven clutches—none of it made the sled any faster. Some of them did make the sled feel different, but never made enough difference that we could recommend that you spend money on them. We even ran with sleds that had custom ground ramps and different springs. Again, if the two sleds turned the same rpm, they ran the same. The challenge with Ski-Doo clutching is that it is not terribly consistent from one condition to another. If you take a sled down a hard packed trail and then dive off in the powder, you will lose about 200 rpm even at the same altitude. Our inclination is always to tune for powder riding, because we assume that’s why you own a Summit.
The problem is, though, if you click your sled up far enough to hold rpm in three feet of snow at 12,000 feet, you’ll chop off about 8 mph on the hard packed road at 9,000 feet, because the sled overrevs and does not operate at peak power rpm. By the same token, however, if you click down and set the engine right at 7850 rpm for trail riding, when you get up high and load the sled on a hill, you’ll be lucky to see 7500 rpm and it’ll be a dog. The bottom line is that if you want to keep the sled running right all the time, you will have to adjust the clickers for conditions—there is no one clicker setting that works everywhere.
Having been disavowed of the idea that there was some magical clutching fix, we moved on to addressing the biggest driveability complaint about the clutching. That issue is that the sled slows down unnaturally, almost like a four-stroke with engine braking. When Ski-Doo installed electronic reverse, the driven clutch had to be redesigned. Like all reverse-enabled clutches, they ended up with a rear spring that only works on compression and is not torsionally wound like driven springs have always been. In order to try and keep the sled turning rpm, they built a monster spring for the rear clutch. Unfortunately, the spring is so strong that it limits the sled’s ability to coast. For 2003, Ski-Doo replaced the driven spring with one that’s much lighter. Trouble is, they also installed an HO engine with significantly more power that could pull the lighter spring and still turn the proper rpm.
Our solution was to put a stiffer spring in the primary to get our rpm back and install the new spring in the rear so the sled would coast. Overall, it’s a much better solution. You can run click No. 3 for trail riding or in hard snow, No. 4 works well in moderate loads up to about 10,500 feet, and No. 5 still turned rpm even at 11,000 feet in deep powder for us. As was mentioned earlier, though, the difference in top speed between No. 3 and No. 5 is about 8 mph, so it’s important to assess your conditions and tune appropriately.
With all the changes made in the clutching and jetting, we observed gas mileage improvement from about 0.8 mpg when doing nothing but fast trail riding to almost 2 mpg boondocking. The more time you spend in the mid-range, the greater the improvement. Once again, how you ride will impact exactly how much improvement you see.
With the updates installed, you can actually begin to enjoy the Summit chassis because the engine isn’t constantly doing something annoying. All sleds originally sold by Xtreme will be updated for free. For everyone else, the price on the entire update is $329 installed or $299 to simply do the carbs and provide you with the clutch components.
If you would like to have yours done, check with Xtreme Performance Center (800) 269-0038.
Montana's Yellowstone Country