MX Z Adjustments

December 27, 1996

Dear Ralph:

Eric Enk

Eric:
Yes and no. (What a politician!) The lower radius rod mounting hole on the trailing arms is primarily used by oval racers when they lower the front suspension, and should not be used for trail riding. Using this position is not necessary to get the camber to 0 degrees. This is easily accomplished by adjusting the ball joints on the end of the radius rods. True, adding negative camber will increase the “bite” of the wear bar during cornering because the weight shift in a corner is always to the outside, and the negative camber presents the wear bar at a more aggressive position for this weight shift. (Negative camber is when the top of the spindle is closer to the machine than the bottom of the spindle).

Trail riders usually set the camber at 0 degrees, while cross country riders may set it slightly negative (1-3 degrees) for better “bite”. You are correct in that 0 degrees camber is better for straight line tracking than negative camber, but ski alignment is the first place to look (set at 1/8″-1/4″ toe-out). As for keeping the skis on the ground, since you have already widened the stance I would try shortening the limiter strap, decrease the center shock preload, verify that the stabilizer bar moves freely, and possibly increase the diameter of the stabilizer bar.

We experimented with removing the stabilizer and increasing the front shock preload. This helped the skis to stay on the ground, but there was more body roll when cornering and the straight line ride quality was noticeably rougher.

Backfire

December 27, 1996

Dear Ralph:

Neil Youngren

Neil:
Under no circumstances (other than in an emergency to make it home) should you render the throttle safety system inoperable. You should check the throttle cable tension and free play before messing with the safety system. If proper cable tension does not fix the problem, hold the throttle lever stationary and press the throttle. If the system is working properly the engine will die. Another possibility is the kill switch which can cause erratic low rpm running. The connector for both of these systems is under the handlebar pad (brown and black wires). If everything still works after you unplug this coupling, there definitely is a problem. If the problem is in fact the safety switch and not the cable (highly unlikely), replace it.

A Waste of Money ?

December 27, 1996

Dear Ralph:

Lee in Spokane

Lee:
This is not as easy of a subject as it may first appear. Spring fatigue and replacement should be done on a case by case basis, and there is no good rule of thumb to go by. Most springs will take a set after a few hundred miles, and then their fatigue should be pretty gradual from that point on with fairly consistent performance. As long as the clutch tuning is done after the springs take a set, then there should be no reason to replace them each season. (If you want a primary spring to take a “quick set”, compress it in a bench vise overnight.) I would replace springs in the clutching system based on performance, not as a matter of precaution.

Dependent on the quality of the wire material that the spring is made of, and how close to the “physical limits” the spring is being stressed to, you will have some springs that have a poor “memory” and do not always return to the desired tension. As for the secondary, you could measure the pre-tension (with a spring scale or fish scale) and maintain it to the same pressure and be OK. Yes, measuring the free length is a good indicator of primary spring condition, but this spring is one that will fatigue and the shift characteristics will change. Usually your operating rpm and engagement rpm will drop when the free length becomes shorter. This one will drive the unsuspecting tuner nuts. Know your free length and measure it when troubleshooting.

Suspension springs are another story. After consulting with a major shock supplier to the snowmobile industry, they informed us that whenever a shock is replaced the coil spring should be replaced also. The suspension springs are placed under extreme loads (very close to their design limits) and will exhibit fatigue and, especially with matched sets, the variation between the springs can be quite great. This was news to me, as I have never replaced a suspension spring unless it was broken or clearly shot. Since the shock supplier doesn’t sell springs to go with their shocks, I would have to tend to believe them.

Indy 500 Carb Question

December 27, 1996

Dear Ralph:
I have a 1992 Indy 500 and I recently bought a PSI pipe. When I had the stock pipe in the needles were dropped one setting in the carbs to get rid of a nasty hesitation at half throttle. If I leave it where it is now will I burn it up with the PSI pipe on it? Thanks!

David “500″ Meier

David:
The best advice in this situation is to start with the carb calibration recommendations of the pipe supplier. Their recommendations are based on the assumption that the rest of your engine is stock – and their specs are going to be a good starting point. You may very well end up with the needles where they are right now, but I would raise them back up a notch or two just to be safe, and work your way back down if needed. If you have made any additional mods to the engine it would also be wise to increase the size of the main jets and work your way back down. Better safe than sorry.

Share Your Secrets

December 27, 1996

Dear Ralph:

Tom Scheideman

Tom:
There aren’t really any instructions, books, or sources that will give you exact porting specs for specific engines. One of the best sources of porting information is “Tom Turner’s Two Stroke Software”, which is a software package that will give you the ability to calculate port specs for any state of tune you may wish. This is pretty much for those with experience in the field, definitely not for beginners.

The art of porting has long been regarded as a “black art”, and those who have spent the years learning the art are not readily willing to give up their secrets. Kind of like a “magician’s silence”.

Many of today’s engines are so close to the edge in terms of “maximum safe” porting that it is very easy to go too far. But, you can never go wrong with matching the flow areas and blueprinting the engine. Matching the ports is all we’re doing here. You want to take a look at what the factory was trying to do on a mass production basis, and “clean it all up” with some time consuming “hand massaging”.

Do not attempt to do an “eyeball” porting job. You will have to have the capability and tools for measuring and cutting accurately. Take a digital dial caliper and measure all of the port dimensions – you’ll find that some dimensions are bigger than others by as much as 0.5 to 0.75 of a mm. Since you can’t add metal to the ports, all you can do is take metal away from the “low” or “small” dimensions so that they match the “high” or “big” dimensions. The port contours are very important, and you do not want to be altering them without good knowledge of why and what you’re doing.

Perhaps the easiest thing to do (that will benefit the long term durability of your engine) is the chamfering of the ports. Factory engines are pretty “raw” in this respect, but are getting better. Take a ball hone and make 2-3 quick passes from both the top and bottom of the cylinder to get rid of all the “sharp edges” on the vertical portion of the ports. The top and bottom edges of the ports are best champhered with a small Dremmel type tool. You want to be able to run your finger along the edges of all the ports and not feel any sharpens. If you’ve never tried any of this before, don’t try it on your new engine. It typically takes several hours of experience and a few failures along the way to gain the ability to do this right. Doing this type of work is where the differences are in having an experienced two-cycle shop do your work as opposed to having an auto-parts store bore your cylinder, or the local “hack” mechanic.

Freeplay Removal

December 27, 1996

Dear Ralph:

Lewis Brazil
Sioux Falls, SD

Lewis:

Normally the “freeplay” in the handlebars is caused by the combination of loose bushings, hyme joints, and wide tolerances all the way from the handlebar pivot under the engine to the tie rod ends and down to where the skis bolt to the spindles. Yes, sometimes a single bushing will be the culprit, especially the ones in the bellcrank (sometimes called steering frog). You should check each hyme joint, bushing, and pivot point individually and replace components as needed. Higher quality rod ends and bushings are available from specialy suppliers (sold in most any aftermarket catalog in this magazine) that manufacture the pieces from better materials to tighter tolerances.

Roller Clutch Blues

December 27, 1996

Dear Ralph:

Michael Albrecht
Fargo, ND

Michael:
We have been using roller secondaries for three years now, and will admit that there is a steep learning curve when it comes to getting the calibration of the entire clutch system dialed in. It is all a matter of matching the shift curve of your clutches to the powerband of your engine. Easier said than done, but once you get it dialed in you will be happy.

You have to change your thinking when installing a roller secondary. Our experience has been that the roller unit upshifts so quickly that if you make no other changes the engine will be loaded too hard too fast, similar to installing too steep of a helix or having primary weights that are too heavy. If you bought the kit and were told that it would work with no other changes, then you’ve been had. By virtue of what the roller does (reduce the friction) you must make changes to the helix angle and spring rate to compensate for the change to the shift rate. The typical recommendation is to install a helix with a more gradual angle to slow the upshift (typically 5 to 7 degrees), and even after that you’ll likely have to increase the spring tension. On the smaller sleds (under 800 cc in this case) increasing the spring tension two or three holes will do the trick, or install a higher rate spring or even one that is longer for more side pressure.

The most common mistake is that after installing the roller kit the tuner is not willing to go tight enough on the secondary spring, as this goes against everything they’ve ever been taught. And how many tuners have a bunch of helixes that are shallower than what comes stock ? Most of them have steeper, more aggressive helixes which are the exact opposite of what they need. For example, if the sled comes with a stock 36 degree helix, installing the roller makes it shift as if there was a far steeper (like a 50 degree) helix in there. If the engine doesn’t have enough torque to pull this load, it will bog. Going to a shallower angle slows the upshift, allowing the engine rpms to build before the load is applied.

If you can not compensate enough by changing the helix angle and spring tension you could use the flyweights in the primary to get the calibration right. Lighter flyweights will not load the engine as hard, but will also raise the operating rpm. What you need is less weight on the initial shift area on the flyweight (up by the pivot hole) as the shift occurs (allowing the power to build), then throw more weight at the engine (more weight in the middle of the flyweight) as the power and load increases, and have enough weight at the tip of the flyweight to keep the fully shifted rpm where you want it. What you need, in comparison to a normal flyweight, is one with less mass at the pivot area, more mass through the middle, and the tip mass will depend on the engine.

If where you purchased the roller kit from does not have specific set up calibration parts or specs for your sled, you either need to have a box of helixes to play with (at varying shallower angles), have some higher rate springs to work with, be able to grind some weights, or be willing to experiment with some of the “adjustable weights” now on the market. Adjustable weights are available from Thunder Products, Hooper Racing, and Tri-City Polaris. They all use a different method of adjusting the weight, but they all allow you to vary the placement of weight along the arm (in varying degrees). Remember, it is a matter of matching your shift curve to the powerband of your engine. The roller secondary accelerates the initial portion of the shift pattern. A shallower helix slows it down, as does a lighter flyweight. Having less weight at the pivot area allows you to let the engine power build as the roller secondary quickly shifts. Finding the right combination throughout the entire powerband is the trick.

On our Arctic Cats we’ve been able to just go to a shallower helix angle (48 degrees) and increase the spring tension to take care of most of the bogging. On the Yamahas we also used a 48 degree helix but had to install a stiffer spring. With our Polaris engines, the low end torque was not great enough to pull the load, so a “reverse angle helix” like a 34-36 can be used with a blue Polaris spring. We also increased the engagement rpm to have more power available and used a set of adjustable flyweights to get an Ultra to work. Engines with lots of low end torque seem to respond far better to the rollers than do engines weak on the bottom.

Once you get the shift pattern matched to your engine (which is what the factory tried so hard to accomplish) you will be happy with your roller unit. Until then, you will be fighting bogging and over revving conditions, and I can understand your dissapointment.