I have a 1.60, with the stock muffler and upright installation in a 4*120 it ran perfect ( unbelievably perfect ). When installed on its side with a pitts style muffler it was rich in the mid range. I ended up using a remote servo controlled needle valve ( used a throttle curve function on my radio) and onboard glow to tame the problem ( but never made it perfect ). Eventually plugging one of the exhaust stacks to get it correct, but it hurt performance.

But as I understand, a sliding barrel carb is designed for a specific fuel pressure. If you change the muffler or any thing else the fuel pressure is no longer optimal. Hence requiring a pump and regulator set to the proper pressure to perform properly.

This is a common problem on Super Tigre engines, especially the big ones.

If this problem occurs, the idle needle valve can typically adjusted to get good mid range and throttle response from mid range to WOT. But, the idle will most likly be far to lean.

I have machine the slot in the barrel to change the low end mixture rich while keeping the mid range just right. Effectively I changed the carb to perform optimally with a different fuel pressure.

Search the ST threads for more info. Also search for "pe reivers" ( he is a 2-stroke expert ), he has documented modification I described ( it wasn't my idea ). check out his site: www.mvvs.nl

The carb modification will make the engine run well, but you need to have the resource to modify the slot. Some have done it with a dremel tool, I am a perfectionist so I set it up in the mill and machined it that way.