My ST 51 on gas uses the stock carb and one click either way makes a noticable difference. The 28 degree advance seems good for my engine. The idle is rough so I have the CH module to retard the spark at idle. I made an adapter for my 61 Evolution for a Walbro and later removed it to try the stock carb both worked well. I put the ST 51 carb on a weedeater with only a 1/2" aluminum adapter and that worked very well. The Walbro seems to be a little big to get between the prop and cylinder on less than a 61. I had to use a 1/4" or better washer on the prop drive to clear the prop/carb. I use gas and 18% synthetic but have not run the 51 enough for a positive evaluation of the con rod. Prof G.