912 Fanbelt Slipping

[ibutler]

I have a 1968 912, which has an annoying slipping fanbelt. The crankcase pulley has two slots, one for fanbelt and one for smog pump (not fitted.)

During normal driving everything is fine, but as soon as the engine is underload the slipping starts. Engine temp climbs abit but is OK, but enough to keep an eye on it.

Tried different fanbelts, tensions, new top hub and shims.

Any ideas?

Best

Iain

[mein12]

Hi Iain, you probably know this already, but the only way to tighten the belt is to remove shims. The larger OD crank pulley on the 68 is the one that drives the generator.
If this does not work it is possible that the rear generator pulley is not shimmed away from the generator pulley hub far enough.
When this happens the generator pulley hub will extend too far out and the nut will tighten against the hub instead of the pulley. It is almost invisible when it is real close. Make sure that you are tightening against the pullies and not the hub. I have seen this happen to lots of people. Good luck...JMB

[Alan.UK]

The only other thing is to check you have the correct size fan belt. I saw a similar problem once where the belt had an incorrect thickness and the pulleys were wearing it down, the belt became slack and started slipping.

Just and idea

Alan