September 27, 1995
Dear Ralph:
Please help me! I have a 92 RXL. The problem is that I can’t trail ride it. Running it hard on the lakes is just fine, but when you are on the trail it starts to “pop”, even when the plugs are new. Everything else is set to factory specs. Nobody seems to have an answer.
Robin Woolner
Schreiber, ONT
Popping and backfiring can be caused by different things. The first place to check would have been the plugs, but you’ve taken care of that. If there was water in your fuel or if you had a sticky injector the problem would be there all of the time. The key here is that it “pops” only when trail riding. Because of this, the most likely cause is the throttle safety switch system. Its basic function is to cut the ignition if the switches detect that the carbs (or throttle bodies) are open when the throttle is closed. When ignition is cut and turned back on, backfiring and popping is common. On the RXL, one switch detects if the throttle lever is all the way against the block (closed). The other switch is up at the pivot pin. This switch is normally open, and closes when pressure is applied to the throttle lever (at the same time the block switch opens.) If the block switch opens and the pivot switch is still open, the ignition will be cut. Verify the proper operation of this system and recheck the safety switch adjustments. The other scenario I’ve seen is with the way riders hang on to the handlebars. Some people “push” on the handlebars (and the throttle lever) as they hang on, applying pressure to the pivot pin. This pressure can keep the pin from moving and closing the pivot switch, ala the “pop”.
September 27, 1995
Dear Ralph:
Much has been made of the multi-angle helix in the driven clutch. I’ve heard and read that it can substantially improve acceleration without hurting backshifting in most models. Snowmobile shows are rife with machinists that have “their own” version of this little item. They all claim that their version is best. What do you think ?
Howard McCollister
Crosby, MN
When it comes to clutching, you will find plenty of people who claim that they understand how it works. Many will tell you that all the others (their competition) who think they understand clutching really don’t, but they do. It is actually a very complex subject that could cover volumes of information. Here’s my two cents worth.
You have two sides of clutching, the theory side (how it is supposed to work) and the practical application (what really does work). You will find disagreements on the theory of how clutching is supposed to work, but nobody can argue with what does work. And, it is a proven fact that multi-angle do helixes work.
However, I think the (very effective) multi-angle helixes are nothing more than a “band-aid” for the inability of clutch tuners to control over-rev conditions. A steeper initial angle loads the motor quicker, resulting in improved acceleration (if the motor isn’t loaded to the point that it bogs).
True CVT clutching theory indicates that the main purpose of the secondary (driven) clutch is to provide just enough belt tension to prevent slippage, and good backshifting characteristics (the torque sensing aspect). Spring rates and helix angles should be selected to satisfy these criteria first. All RPM control (ideally) should be done by the primary (drive) clutch.
Therefore, the main purpose of the primary clutch is to control engine RPM and upshift characteristics. You want to apply the load to the engine as fast as possible without bogging it. Due to the inability (and sometimes lack of understanding) of tuners to do this (control the engine rpms with the drive clutch), they have used the “back-door” method (multi-angle helixes) to help perform the engine loading function. Still, it is a very effective way to get better performance out of a machine.
A steeper initial angle (at the top) on the helix upshifts the clutches faster, placing more load on the engine quicker – but on the flip side it slows backshift response. A lower angle gives a more responsive backshift, but a slower upshift. Typical multi-angle helixes use a combination of angles with “shift points” rather arbitrarily selected to help load the engine based somewhat on it’s power curve. If a sled has a 36 degree angle as stock, the tuner may select, say, a 42-36 degree helix for a stronger initial upshift, then come back to the standard angle for backshifting characteristics. The problem comes in selecting the initial angle, the point (on the helix) at which the angle decreases, and the final angle. Was the stock angle selected as a compromise, or was it picked as the best selection for backshifting ?
In my opinion, the IDEAL way to approach clutching is to start with a straight angle helix, with the angle selected for backshifting characteristics, and the secondary spring tension as loose as possible while still preventing belt slippage. Then all engine RPM tuning should be done with the spring and weights in the primary. The stock spring is usually pretty good, the problem is with the weights. Usually there is not enough mass up by the pivot to control the over rev (especially on the higher horsepower machines) – and this is where the multi-angle helix comes in.
Typical RPM tuning has been done by installing heavier or lighter weights, usually with the same curvature and weight distribution ratio from pivot to tip. The mass of the weight is selected more for peak shift RPM, with the low end and mid-range shift characteristics pretty much being “settled for” as is.
Grinding some of the material off of a heavier weight has been one way to change the effect of “off-the-shelf” weights. But, up until recently the art of manipulating the distribution of the mass (from pivot to tip) in a weight has been a “secret”, or confined to those who grind sets for their own custom application.
The introduction of new “adjustable” weights (Thunder Shift Kit) open up the potential to tune the drive clutch the way it should be done (to a finer degree). The adjustable feature of these weights allow the tuner to vary engine RPM in distinct phases of the upshift. Adding more mass to the pivot area of the weight has the same effect as the steeper initial angle on a multi-angle helix. Instead of being stuck with choosing a weight and it’s total mass suited for the top-end shift rpm, you can manipulate the low end and mid range and still end up with your desired shift rpm.
Granted, there will be some gains to be made with additional grinding and further tweaking with helix angles and spring rates, but for the most part we now have a greater ability to keep the RPM control up front where it belongs.
September 27, 1995
Dear Ralph:
Why are the OEM pipes so heavy ? If the aftermarket pipes are so much lighter and provide more horsepower, why can’t the sled manufacturers make their pipes that good right off the bat ? Are there some type of restrictions that limit their ability ?
The manufacturing techniques are different from OEM to aftermarket. The OEM pipes are made of relatively thick metal and mass produced with electric welding. This technique doesn’t work on the thinner (lighter) sheet metals that the aftermarket shops gas weld.
The OEM pipes are “over-engineered” somewhat for strength and long-term durability. Thinner metals are more apt to crack with the cold snow spray coming in and hitting the hot pipes. Since their sleds are used by a wider range of buyers, the OEM pipe is usually tuned for a broader powerband (so it is easier to clutch) at a lower operating RPM (for durability).
Contrary to popular belief, the manufacturers do not have a ton of people that can spend countless hours developing the “ultimate” pipe. They are faced with production deadlines, and need to have their dimensions figured out well in advance so that the (expensive) tooling and stampings can be made in time for sled production. Once they come up with their pipe specs (and spend over a hundred thousand dollars for the tooling to stamp the pipes) they’re less likely to make any changes (expensive) until the die wears out.
The aftermarket shops (since they are smaller) can make changes quicker, and actually get to spend far more time “field” testing their pipes. Instead of being stuck with the same pipe for years, they can continuously make it better – based on field testing and feedback. And, their pipes can be made to perform better in a specific application. For example, you can get better cylinder filling at a higher RPM for more horsepower, but the power will be concentrated in a narrower range (harder to clutch). Excellent for lake racing, but terrible coming out of corners on the trail. Or, they can make a “nasty” trail pipe with less hp at the top. It’s a give and take – they can even make them with a similar power curve to the OEM, but quite a bit lighter. It’s your choice.
September 27, 1995
Dear Ralph:
What is the difference between “case-reed” induction and “piston-reed” induction ? Are there advantages or disadvantages to either one of the methods ?
On a case reed motor the carbs and reed block is mounted down on the crankcase instead of the typical location up on the cylinder. This provides a straighter shot into the cylinder since there is no piston running up and down blocking the passage. Another advantage is in lubricating the rod end and crankshaft a little better. Since the carbs sit so much lower on the motor, tilting it forward is often necessary to get everything to fit in the chassis.
With the piston reed the carbs are mounted higher which makes it easier to package the carbs and the engine in the chassis. The reed block is mounted between the carb and the piston. Performance comparisons show they’re usually pretty close.
September 27, 1995
Dear Ralph:
My dealer told me that when I buy a set of aftermarket pipes to make sure that they have rebuildable silencers. Why is this an advantage over the non-rebuildable types ?
If the silencer has a “fiber-pack” it will burn out after about 2000 miles (or in a few years). When this happens the pipe will get a bit louder and you could loose some of the back-pressure that might result in a decrease in performance. Think of it like shocks. If what’s inside goes bad the rebuildable unit allows you to fix it instead of buying a new one. If you go for the non-rebuildable silencer, find out how long it is expected to last.
September 27, 1995
Dear Ralph:
With the never ending emphasis on increased performance, why are we still seeing mostly round-slide carbs (Mikuni VM) on the new sleds? Especially since the flat slide TM and TMX carbs can provide an improvement in throttle response, tunability, and power. The flat-slide technology was developed over ten years ago and applied to motocross bikes, so why have the snowmobile manufacturers been so slow to respond ?
Dean Pepperdine
Tipp City, OH
Yamaha does have quite a few flat slides on their machines. For the most part it is a matter of cost – the round slides are cheaper. The majority of “touring” snowmobilers may not even notice the difference, or even care. For them, the extra cost would not be justified.
But for the “other” riders (like you) it seems odd that the new “Super 600′s” don’t come with them. On these machines they could make of a few hp difference on the top end, and you would get much better throttle response. The throttle range where most of the riding is done is where the most noticeable improvement would be. Don’t forget that fuel economy is better too.
The advantages of flat slides are undeniable – but it all comes back to cost. A 38mm round slide runs about $125, where the flat slide runs about $175. On a three cylinder machine, that’s $150-$200 more. Multiply that by a several thousand sleds and the manufacturers save hundreds of thousands of dollars. The question is whether the majority of the buyers (of a certain model) would be willing to pay the difference.
