View Single Post
Old 01-05-2009, 11:32 PM   #68
JR Greenhorn
Gnarly Adventurer
 
JR Greenhorn's Avatar
 
Joined: Aug 2008
Location: Litchfield, MN
Oddometer: 336
Quote:
Originally Posted by Dirtsampler
Several of the tracks here are short and often muddy. This being a rather
large critter, any hints on fork position in the clamps for different conditions
would also help.
I don't expect you'll have as much trouble as you think. The '83 YZ chassis hugely favored stability, and I'd expect the IT chassis to be at least equal in that regard. When I first started riding my buddy's 490, I hadn't a lot of dirt-bike experience, but at some point I realized that on the 490 I almost never had the urge to put my foot down. You know, the classic rookie move to "catch" the bike if it slips out on a turn. Eventually, I realized it was likely that stable geometry giving me a sense of security and plantedness. Obvioulsy that comes at the expense of quick steering, but that's what it is.


These big engines are very versatile and have a lot of inertia. You won't struggle with stalling in the mud as much as you might on a smaller bike. Especially with the IT state of tune. It should be an absolute tractor.

I was surprised at how the YZ490 didn't feel that held back by its 4-speed transmission. My KTM has a 5-speed, but 1st feels pretty pointless a lot of the time and I rarely get a chance to wind out 5th. My bike would probably be fine with a 4-speed too, except when 1st gear is needed in the tough stuff to prop up the lacking low-end power the "little" 440 makes.

Compared to the YZ transmission, the IT adds a lower 1st gear, then 2nd-4th are about the same as 1st-3rd on the YZ, and finally the IT has a slightly taller top gear than the YZ. Bottom line, the bike has a wide spread of gears, and should be able to pull that tall gearing without issue.



If you're new to open classers, experiment with setting your idle speed lower than you might be used to, and learn to use the clutch to uncouple that engine inertia to keep it from driving you ahead even when you're off the power. Pulling in the clutch on a big bike will also help you make tighter turns.
On a little bike, you use the clutch to prevent keep the engine from being overpowered by the bike (bogging or stalling). On a big bike, you use the clutch to keep the engine inertia from overpowering the chassis.



For tires for the mud, pair a Michelin S12 front with an IRC M5B rear. The M5B comes in wide sizes for open class bikes. The M5B is really amazing in the mud, but rocks and hard terrain will chew it up fast. This is the combo I'm running on my KTM, and I love it. However, my next rear will be a Michelin S12. It also comes in wide sizes (although I've heard their wide sizes run a bit narrower), and I've heard they hold up a bit better on varied terrain, considering the S12 is a soft terrain tire. Basically, I've been told the S12 is a "general use" soft terrain tire, while the M5B is a mud/sand specialist.



If you're not used to drum brakes, you'll want to run grooved shoes, especially in the mud. Vesrah makes them:



You can also just cut similar grooves in the shoes you have with die grinder cutoff wheel or similar. Wet braking will be dramatically improved with grooved brake shoes.


Does your IT have the dual-leading-shoe front brake? The YZs got them in '81 (but I don't know with the ITs). If yours is a single-leading-shoe, the DLS setup will really help you get the most that drum brakes have to offer, while staying period-correct.




In case you're interested, here are the power differences between the '83 IT and YZ 490s, from period magazine tests:
(sorry, I don't have the '82 numbers)

..............IT........YZ..
RPM......RWHP....RWHP
3000.....15.24...........
3500.....18.09.....11.5
4000.....20.21.....16.0
4500.....24.31.....17.7
5000.....26.70.....18.9
5500.....32.09.....22.8
6000.....34.65.....27.3
6500.....34.65.....31.6
7000.....33.80.....36.8
7500..................40.7
8000..................43.9
8500..................44.7
9000..................41.5
JR Greenhorn is offline   Reply With Quote