Got a timing light and a tachometer? Basically you want to make sure that when you're actually driving your heavy bus, you don't over advance your ignition. In a normal car you can hear when your advance is set too far but in a bus the engine is a long way back and theres a lot of noise
That can cause your engine to run hot, and will burn up your valves. Equally, having your timing too retarded can also give you a lot of heat.
The distributor may be DVDA (dual vacuum dual advance) which was an emissions thing - basically it slowed the engine quickly by retarding the ignition. Often the retard side doesn't work and to be honest that's not a problem - the advance side MUST function though.
Setting the timing at idle is a good starting point but you don't drive at idle and you also don't know whether your distributor is working as it should. The VW type 4 engine expects between 28 and 32 degrees of mechanical advance at 3500 rpm regardless of the distributor in it. I set mine to 30. To set it up a tachometer is useful but not essential because at some point all of the mechanical advance will be "in" (the exact RPM depends on the distributor you have). You should get hold of a timing light as part of your basic tookit though and do the following:
* Disconnect all hoses to the distributor. That will stop the vacuum from interfering with the reading.
* Increase revs to 3500. Noisy but necessary.
* Adjust your distributor so the timing light falls between 28 and 30 degrees on the timing scale. That's your maximium mechanical advance.
* Let it fall back to idle - note where the timing falls - it should ideally be around 7.5 degrees but don't adjust it if not, the bus will still run fine. Just make a note. That way you can check it quickly with your static timing light in future.
* Tighten the distributor down and reconnect the hoses.
* Rev it to 3500rpm again. You should see the vacuum advance kick in and advance it up to around 40 degrees. If it doesn't, either your carb isn't sending any vacuum (check by feeling for suction on the end of the hose) or your distributor isn't picking it up (often because the vacuum can on the side of it is broken, check by sucking the hose connected to the distributor and seeing if the plate inside it moves). Either way your ignition will be retarded when you're driving and your engine will run hot (and poorly).
You can get a cheap Gunsons timing light for less than £20 that will do the job.
Hope that helps.
