[Steve Holland Posts: 1608 Status: Offline]

Assuming this engine isn't known for dissimilar mixtures going to the cylinders I'd be checking out the cylinder compression and if it has reed or rotary valves I'd make sure these are in good condition and operating to specifications. Crankcase seals are hopefully in good condition also.

[Turbohoolik Posts: 8 Status: Offline]

I chopped spark plugs, in this test I was running 290 main. It has been always that second cylinder is getting more fuel than first one. I'm running single carb. I have checked air leaks from intake many time. This is freshly rebuild engine. Before rebuild second cylinder was getting more fuel to.

[Nick Flint Posts: 4957 Status: Offline]

I owned a Rotax 503 on a kestrel Racing craft. It never missed a beat and as the carb was simply kept clean using a water/debris filter, and clean filtered fuel.
Perhaps the carb is fine, but other reasons for poor running. Quote from Kip Mac the  Kestrel and Scat designer ref the 503-


"If there's a problem use NEW plugs
If there is STILL a problem   "   "
If there is STILL a problem   "    ".


Unless you know the carb has been doctored unsuccessfully in the past, I would suggest a thorough clean perhaps ultrasonic?
503 Rotax were renowned for-
1 Reliability
2 Gulping fuel greedily 
Can you edit your post as some of the words are difficult to understand?
Good luck

[Turbohoolik Posts: 8 Status: Offline]

Hello again!

On my Scat hovercraft I have Rotax 503 with single Mikuni 38mm carb.

I'm stuck on carb adjustments.

It had 360 main and needle clip on the middle. Engine run fine till 3/4 throttle but on wott engine rpm drops a little.
I put in 330 main and lowered needle as low as possible. Now rpm are not lowering on wott but engine doesn't sound happy yet...

some jetting advice?