I just put a rotax 503 single carb into a minimax and I have an ultraprop II 3 blade 59in fan on it. According to ultraprop, this with a 10 degree pitch block should be about right. But Im only getting 4200 rpm out of the engine. WAYYYY too much prop for this engine. I have a 2.25:1 belt drive on it. Im guessing if I take 3in off each blade to trim it down from 59 to 53in diameter, it should get me closer. Or I could just buy a two blade hub and go that route instead. Any suggestions? I don't mind trimming down the 3 blade unless its gonna be bad performance vs a longer 2 blade. More ground clearance after trimming the 3 blade would be nice but not really necessary.

I'm running a 60 x 42 2 blade (wood) on my 'Max. With a 447, it's a bit coarse but gives 6100 static and 6800 in climb. I'm fitting a 503 twin carb but keeping the same prop. Here in the UK, the 60 x 42 is suitable for both engines apparently. I'm yet to find out! With the 447 I was getting 60kts at 5100rpm and around 9lph.

No. Don't cut it down. 53" even for a three blader sounds very small for a 503. I don't think that your engine is healthy. It should be pulling more than 4200. Check with a good optical tacho first and have somebody check your pitch block is right. You are overpropped but only a bit, not that much IMO.

The engine is fine. Excellent compression and revs to 6500 and way beyond if I let it with the ultra prop I original 3 blade prop which has ALOT less pitch built into it. The orginal ultra prop 1 is flat poor quality blades. The ultraprop 2 however is a really well designed quality prop. This is just waaay too much prop for this engine. But their customer service is excellent. They just offered to exchange my 3 blade hub for a 2 blade hub to try that. So I will try that before any cutting.

And i checked prop rpm vs engine rpm with a laser tachometer and the laser and tiny tachs both agreed perfectly with each other.

With a 2.25 reduction you definitely don't want a 3 blade prop.  

I used the original Ultra Prop I many years ago but found them inefficient.  Glad to see they have improved the product.

Hard  to beat a ground-adjustable 2-blade Warp Drive prop..............

Yeha I have that warp on my weedhopper. It is the best prop bar none.