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Maybe Trey Jorski will chime in. He goes "kinda" fast on a 83 CR480.
First try adding preload to the rear shock. Then I would actually move the rear axle back in the swingarm to give the bike more trail. I'm having a similar problem with my 79 yz400 but I know it's my dual rate spring setup so I've gone back to a factory Yamaha progressive spring and will test it out this weekend.
If you can't dial it in with fork height, tune the shock spring.
The Shop
Wheel base is different for different types of competition. Trials and Short track prefer some thing shorter to allow them to turn. They tend to be shorter wheel based bikes. Desert racing is looking for high stability with minimal turning. They prefer the longer wheel base. Each time you adjust the chain you are either making the wheel base longer (tightening) or shorter (loosening). I am not aware if your 480 is that sensitive to the change, butit sound like it.
As far as Jorski goes, I think he typically shortens his throttle cable about 3" to insure the slide is safely out of the way in the carb.
I'm only a clubman rider, but through experience do know this.
You should be setting your sag every time you adjust your chain, or have a big breakfast / take a King Kong sized crap.
Having the correct spring rates and setting the race sag are always your first port of call as all your front end geometry starts from the angle of the bike as dictated by the rear end.
If he really is super tall, he's going to need much stiffer springs and more rebound damping if he hasn't already done so. Note that the shocks from this era have very little rebound damping so stiffer rebound is a must. I usually use 7wt shock oil in lieu of shuffling shims. Pro suspension guys will scoff at this approach, but it works for great for vintage racing, where the motos are short.
Taller riders put more load on the suspension, so correct springs & damping rates are critical, especially the shock.
The suspension/spring rates are good and the sag is well within the ball park but I'll fool with it a bit and see how it effects the handling.
So many variables, so little knowledge.........
Since I'm taller than the person the bike was designed for I've found that I need less sag. I suspect you are in the same boat. On my modern bikes I'd swapped the springs out for my weight and then would go test to determine baseline sag settings. I wpuld start with what the book says to run and then ride it. Then i would add two turns of preload and go test again. I would repeat this till the bike was unstable or the front end knifed, then back it off to the previous setting.
At our size no bike is setup for you and I.
Sag was 110 mm and I've screwed it down to 100mm ( one full turn) and will report back because I know you're all breathless to know the results.
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