1978 YZ250E fork preload

Sofiedog
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Sofiedog
Edited Date/Time 6/13/2017 8:46am
I believe these are the stock 38mm KYBs. I've been trying some things to get them tuned up. The fork spring preload seemed way to high in stock form with very little static fork sag and a stiff feel. I'm going to try to see how they feel with the 2" preload spacer not installed. As you can see from the pics, there is still preload for the spring provided by the fork cap. Anyone else try this? I hope to get out and ride it next week to try. It has about 1 - 2 inches of fork sag now, with me on it.

with spacer


without spacer, tube all the way up


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450exc115
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450exc115 6/7/2017 8:05pm
6/7/2017 8:05pm
Don't confuse 1978 suspension theory with modern day practices. Those springs are buttery soft and required the preload and even a few psi of air to make them better than a wet noodle. Smile

What you want to do is get the right springs for your weight add some race tech emulators and enjoy a more modern fork feel.

Sofiedog
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Sofiedog 6/13/2017 7:05am
6/13/2017 7:05am
I had a chance to get it out yesterday with the lower fork preload. It felt pretty good, for a '78. Forks are stock. Not much adjustment capability. You can change the fork oil viscocity and vary the air pressure. I'm also trying to vary pre-load to determine effects.
It definitely needs something close to the recommended 14psi in the forks. It has an effect on fork sag and damping.
I tried lighter weight fork oil, 5W, but it didn't provide adequate damping. The front end chattered at high speed on rough ground. The 10w feels good. I may also try something heavier like 15W to determine the effect.
After I get it dialed in as best I can stock, I may have the forks modified by a professional. I want to see the limitations and understand and feel the problems of the stock fork before I ask to have them improved. Sounds like RaceTech emulators are a common mod. Has anyone else tried to tune these forks before adding emulators?

wfoyz250
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wfoyz250 6/13/2017 8:44am
6/13/2017 8:44am Edited Date/Time 6/13/2017 8:46am
I have a couple of suggestions. Nothing wrong with understanding what you have to work with but in all honesty it's best to consult racetech before you get too much involved in the forks.
They can get you dialed-in from the get go with proper spring rates, emulators and other tricks to make your suspension work. They've been doing it for a long time so give them a call.
Another item to note is introducing air into the forks.......there's no reason to do so and only complicates things. Air in a hydraulic system is a no-no and should be bled from the forks for more consistent fork action.
I have a system on my forks that plumbs the two chambers together to an adjustable proportioning valve with "on the fly" bleed-tee which is plumbed into the center of the handlebar.
The handlebar is closed on both ends and acts as a reservoir for increased oil capacity and air accumulation can be bled from the bar.
This setup is not offered by racetech and is a system from a third party.

Again, consult with Mathew Wiley at racetech for technical assistance.

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