Petrol engines aircooled performance enhancement
Improving Performance - Aircooleds
Depends on how much money you want to spend, don't expect big gains without spending some money, but they are very tunable, a good set of twin 36 webbers and a free flowing exhaust will gain you some useful bhp, or the next step balance the bottom end and increase the capacity (cc) and compression. Forged 96's pistons will give you a 2056cc without major changes to the rest of the bottom end. It's probably best to have a read up, here are some links.
There's some American sites and the type 4 engine is what you have (CU code)
Dyno (rolling road) tuning
IMHO If you don't want to break the bank then stick to making sure it runs spot on and never lean if it runs lean then that means HOT head and shortened engine life its what kills most a/c engines, for about 80 quid you definately should have it set up correctly on a rolling road. Probably the best £80 you will spend - the Raceshack guys know the type 4 well and have tuned mine and loads of other buses.
Then fit yourself a cylnder head temp (CHT) gauge and oil pressure and temp if you can CHT gauges and pyrometers here (fits under number 3 spark plug) Airworld UK, CHT thermocouples & gauges
CHT Gauges: (CHT guages!)
Do a search on the 80-90 Tech forum for a list of bits you need for CHT gauges. Here is a Tech Archive link from 2005 Measuring oil or cylinder head temps
Fitting CHT gauge is the best way to keep an eye on engine temps. And as you probably know the a/c engine benefits from being driven with plenty of revs and I mean plenty!
You could always go the whole hog and spend a stupid amount and end up with something like mine by adding Nitrous!