Hi, I just bought an old 250 Yz and the fork is bottomed out. As i understand there are no springs in them? How much oil and how much air should be in them?
I had a '76 YZ125X which had the same forks as the 250 and 400.
Never could get them to work right!
You are correct, there are no springs. Each fork leg has two chambers. If I remember correctly, the top one on the center is for higher pressure. The other one is lower pressure. I really don't remember what the pressure should be. Maybe try 10 - 15 psi in the lower pressure and 30 - 40 psi in the high pressure? Sorry, don't know the metric conversion.
It is hard to get the left and right side pressure to match equally.
For oil, I also can't remember, but I would drain all the oil, compress the fork and start with 125mm from the top with the forks compressed. Just a guess - that usually works on non-air forks of that era!
Good luck. My '76 125X was a good bike for the day, except for the forks. The '76 air fork is interesting to look at, but it is not so good on the track.
Yes, you will need to do some testing to find a setting you will be happy with. The pressure is pretty easy to experiment with. Back in the day, we brought a tank with compressed air with us. Now you can get an electric tire pump where you pre-set the pressure, and it will fill to that amount. That would make it easy to experiment.
The air forks are an interesting piece of Yamaha motocross history, so they might be worth keeping.
Suspension was improving very rapidly during the late 1970's. The '77 frames, front and rear suspension are totally different than the '76, and supposed to be much better. I know the rear shock will not interchange, and the '77 and '78 had leading axle forks. So that would change the geometry (maybe for the better) if you did a fork swap. Not sure about fork tube diameter on the later bikes. It might be larger.
Yeah, the forks are for sure crazylocking! And i heard that it was the same guy that made citroens gas/hydraulic suspension who designed the shock on the old Yamaha.
Have to test them first before doning something drastic..
Got to rebuild the engine first also, hehe.
The bike is in pretty bad shape, but i got it for almost nothing.. 100 dollars and i got an PW50 too.
Wow, I had no idea those forks had no springs.:-o
I saw many of those back in the day, with those big cans on top of the fork legs . . . crazy looking yes!
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