I see a lot of clutching questions on this forum, and I would like to put in my two cents. I have clutched a lot Polaris' in my day, but the JD/Comet setup is a little more complicated than the Polaris due to the fact that Comet uses different spring heights for their engagement & shift ratios. That, & they have a lot of different primary springs & weights.
A piston-port 2 stroke engine, such as our mighty Deere's, does not have the torque of the newer reed valved, exhaust valved engines, so we have to clutch for higher RPMs where our engines make the most power. Any engine can benefit from some clutch tuning.
A higher engagement primary spring can get the engine closer to its power-band and improve the take off. A more aggressive helix & stronger secondary spring will improve the up-shift of the clutching. A steeper helix (although we don't have much to choose from), will make the secondary open faster and improve acceleration. Different weight profiles can really help a piston port engine. Weights with a heavier tip will improve performance. This is why in the last 10 years we have seen so many different styles of adjustable weights. We can place weight on the weights at specific locations to help tune the clutches to the torque of the engine.
I ran the Heelclickers on my XC700, and that improved the clutching dramatically. So much that I couldn't get the sled to hook even with 216 studs. I ran the adjustable weight in the middle & tip of the weight. It was a non exhaust valve engine, so the power curve was up around 6000- 7800 RPMs. That & different gears, helix, springs, made that sled beat any XC 800 twins it ran against.
We also want to keep the ratio of the clutches in its lowest ratio upon engagement. With a brand new correct length & width belt, you will want the belt no more than .030" away from the primary sheaves. This might involve removing shims from behind the spider. You can use Polaris plastic shims that install behind the clutch bolt to "push" the sheave closer to the belt, but I prefer to use very little shimming here due to the fact that when the plastic shims push the sheave closer to the belt, it is putting the primary in a higher start ratio, the weights are moving away from the rollers. I like to keep the rollers in contact with the weights. You also want that belt as tight as possible without squeals & belt burning, this keeps the ratio in the lowest at engagement. You do this by removing shims behind the washer on the secondary clutch. I keep my belt about 1/8" above the top of the sheaves on the secondary- again the lowest starting ratio.
A track dyno is a must for clutching, but we don't have access to those at every shop, so we have to do most tuning by the seat of our pants. A accurate tachometer is also a must, which I didn't have on my Liquifire when I started this venture, but I now have one.
You always want the engine to rev up to what the engine is rated for at its highest hp. For example, a XC700 w/o exhaust valves has to be running at 7800-8000 RPMs for the ideal engine horsepower. Under that or over that & the power falls way off. A lot of people buy "clutch kits". An improvement over stock, yes, but can leave a lot on the table as no two sleds are identical. It is a good starting point, but a lot of additional power can be found with over-the-counter parts.
Manufacturers sell the sleds with clutching set up to a "happy medium", as they don't know what conditions the buyer will be operating it. Weight of operator, snow conditions, temperature, etc.
Tight clutches are a must too, a worn primary will act inconsistent, and a worn secondary will bind and upset the whole ratio. A tight secondary spring will get the belt to return to the top of the secondary (lower gear) faster (backshifting), but it will slow down the acceleration of the ratios to a point. However, the JD secondary springs are not stiff enough to really impact this. For trail riding, you want quick backshifting. How many times have you let off in a corner, than nail the throttle only to have to wait for the power to come back? You can eliminate this with some tighter secondary spring settings. This will get the clutches into the lowest ratio for a great holeshot once back into the throttle.
A lot of trial & error goes into clutching, but once you find your best combination, you will have a much more enjoyable ride.
Gearing can also benefit a sled. How often do you do 80+ MPH? I geared all my sleds down (smaller top gear, larger bottom gear) to get the clutches to operate in the middle to high ratios. It makes for better acceleration, and improves the overall efficiency of the clutches. Once you gear down to say, 80 MPH at 8500 RPM, you will have to add heavier weights to the primary as the clutches "think" the sled is lighter due to the gearing.
Overall, these are things to watch for when clutching a snowmobile:
-Keep the belt as tight as possible w/o burning or squealing at idle.
-Keep the engine as close to the power peak as possible (for example, the engine makes 68 h.p. @ 8200 RPM, make sure it revs no more or less than 8200).
-Try to keep less than .030" clearance between the primary clutch and a new belt, if more remove some spider shims. This involves a special spider tool. Most shops will do it cheap.
-Keep the secondary set so the belt sits an 1/8" above the top of the sheaves, this provides a better "holeshot", and keeps the overall ratio at its lowest start. This involves shimming or removing shims behind the washer on the secondary.
-Install a higher engagement spring. Get a spring chart with the engagement & shift spring rates, and try to keep the shift rate as close to stock, or you will have to use heavier weights.
-Try tighter secondary spring settings, this will give you better back-shifting, which makes trail riding more enjoyable. Tighter spring setting will raise the RPMs, so adjust the weights accordingly.
-Gearing the sled for lower top speeds will improve clutching responsiveness, but will also increase fuel consumption. You will also have to change weights as this upsets the clutching ratio.
You can see a selection of Comet weights & spring charts on these pages starting on page 42...http://www.recreationsupply.ca/2009/2009-09_Clutch.pdf
I am not by any means an expert on clutching, but I have worked on a lot of sleds, for myself, and friends, and all clutches work on the same principle. The goal is to get the horsepower and torque through the clutches, gears and into the track.
Clutching 101
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Clutching 101
Last edited by ralphy on Sat Jan 03, 2009 12:14 pm, edited 1 time in total.
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