My 49er usually run at 40lbs at 50mph. Lately, it's been fluctuating. Runs at 20lbs then goes up to 40lbs when I let off the gas. Sometimes near 0lbs at idle. Is the pump getting tired? Is the gauge going crazy? Does oil viscosity have a role in the pressure? I'm confused? Thanks for any advice.
Mechanical gauge or electric?
It sounds vacuum/crankcase pressure related. Not knowing how those motors vented the crankcase, (this is way before PCV) I'd make sure any breather tubes, crankcase vents are clear.
The conventional wisdom says 10 psi for each 1000 rpm. I would first take out the oil relief valve and make sure the spring in it is not broken. Do you have a manual? Pretty simple job.