So you want to ride a Free-Air

    The interest in vintage snowmobiling is growing every year with clubs and vintage websites popping up all the time. More and more people are looking for vintage sleds and undertaking restoration products to get these old  beauties back on the snow.

    First a couple of words of frank and honest advice for those interested in running a sled described in this website.  By the late 70's liquid cooling had replaced free-air cooling in the sport performance market, and for good reason.  Even though the liquid cooled sled was much heavier, the ability to provide high horsepower with reliability doomed the free-air.  The same holds true to this day.  I get a lot of mail from folks restoring an old T'NT or RV and I always give them the same caution.  Always ride with a buddy, or if you're out alone, don't travel anywhere you can't walk out of.  These old sleds are basically museum pieces, to be ridden with care and respect and not pushed too hard.  If you thrash one, it's probably going to break.

    If you're planning to get an old free-air running and terrorize the trails or rule the lake, you'll be disappointed.  The most powerful consumer sled of the ones listed in this site, the '78 RV, pumped out around 62 horsepower, about the same as a 500cc fan today.  If you're expecting to keep up with your neighbors new MX-Z 600, you won't.  Suspension wise the story is the same, the '74 F/A that I ride has about 3 inches of travel front and back, even on a groomed trail you're going to take a beating.

    If after all that you still want a free-air, and I hope you do, there are a couple of things that might help you out.
 

Polar-Fire Ignition Woes!

    I've often thought that the Bosch Polar-Fire ignition system was Germany's way of revenge for World War II!  I get a lot of letters from folks that have partial or no spark in their old T'NT or R/V.

Let's assume you're T'NT F/A has good spark on one side, and none on the other.

1.  Try unplugging the connector that hooks the engine to the rest of the sled, it's the large plug on the flywheel housing. If you get spark on both sides now, you've got a wiring problem on the sled, (ignition switch or kill switch) if you get no spark, plug the connector back in.

2.  Try swapping spark plugs, if the spark switches side, you have a bad spark plug. You should be doing all this testing with new plugs anyway.

3.  Next swap spark plug leads, they unscrew from the coils, if the spark switches sides, you have a bad lead or cap.

4. Next in the plug going into the bottom of the CD box, swap the positions of the purple and purple+yellow wire. (the plug should have the original positions printed on the side)  Use a very small screwdriver to bend the locking tabs on the spade connectors so you don't damage the
connector.  If the spark switches sides, you have a problem with the trigger coil on the stator plate, or the wire leading from this coil. The flywheel needs to come off (puller available from Ski-Doo dealer) and the wiring behind the stator plate needs to be checked for pinches.  Shorted wires is VERY common with these ignition systems.  The trigger coils can also become damaged, in that event, you'll be looking for a stator plate, your only option is a used parts dealer.

5. If the spark stays on the same side, you have a problem with the CD Box, It is possible that it's the high-tension coil on the side that isn't working, you should switch the high voltage coils on the side of the box (involves cutting wires and resoldering).  If the spark moves, it is inside the  box, if it stays, then the coil that you moved is bad.  Either way, the coils and CD box are available from Steen-Hansen in Edmonton 1-800-661-9468 or in the US: LLP Manufacturing1-800-222-3375.
 

Piston Problems

    The most common ways a free-air engine failed were either detonation meltdown, heat seizure or piston skirt collapse.  Piston skirt collapse is discussed in the 1974 page.  Piston seizures are caused by incorrect carb jet setting, high ambient temperatures, too slow speeds, air leak in the carb flanges or faulty crank seal, improper ignition timing or too low octane gasoline.  Which ever the cause, you'll need a piston or two.  There still are new pistons available from after market suppliers for 340 and 440 T'NT F/A's and 340 R/V's.  Your local sled dealer can probably still order them. Gasket sets are still available through the after market.   All other models will require you finding them from someone's existing stock.  400 F/A and 245 R/V pistons are very scarce, old Blizzard pistons are nearly impossible to find.  The web is handy, I always direct people to the VSCA site (link on first page) they list a bunch of vintage parts dealers.

    If you do find a set, and get your engine back up and running, you want to keep it together. Make sure you have new seals and gaskets in the engine, rebuild the carb(s), make sure the ignition timing is correct.  If you ride the sled and it bogs down off the line, then takes off, you're likely too lean, reset the carbs.  If it runs too hot to touch at all, you're likely too lean.  If you hear a metallic rattling sound and think it's a clutch noise, it's probably detonation and you only have a few seconds to back out of the throttle and save the engine.  You should ALWAYS use premium grade gasoline.  You may also want to consider running two head gaskets per cylinder to reduce the compression ratio a bit.  The goal is to build in some reliability, not win races.

    As I said, I hope you'll take up the challenge of restoring one of the machines described in these pages, and that you send me pictures when you're done!