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Trying to set a 100mm Sid fork up for my 12yr old, its an older model but dual air. When I set the sag at 30% with the positive air it is such low pressure that almost as soon as I add any air to the negative chamber it starts to pull the fork down. Is it OK to run with zero air in the neg for a lightweight rider?
That doesn's sound too healthy.
The starting point would / should be positive = negative.
You can get some pull down when the pressures get near equal. Not when negative = zero.
Do you know the history of the forks?
Edit - what psi is in the postive to give the indicated sag?
The forks came on a 2nd hand bike so no history but feel fine when pumped up for my lardy weight, it's when I take them down for the kids I see the problem. It was the same to some degree on my revs when setting them up for my daughter but she is a little older so the extra air pressure allowed for more in the neg. My revs have been serviced and work fine for me.*edit* Postive is below 100PSI
With DualAir, you normally set the positive air first so that the forks perform ok mid stroke and don't bottom out. Don't worry about sag at that point, just get the positive pressure set so they don't blow right through the travel. They will be too harsh at the top of the stroke without any negative pressure. Once you've got the positive pressure sorted out, add negative air until they feel ok at the top of the stroke. Generally the negative pressure should be about the same as the positive.