Originally Posted by
1QwkSport2.5r
Some manufacturers use o-rings on the needles and some don't. I assume the engines that don't were designed at point in time where most folks didn't use muffler pressure. Some of the mufflers that came with some of my engines were not tapped for pressure but had the location cast into the muffler for it to be added. The engines that I run without pressure need the needle opened 1-1.5 turns further open; fuel consumption is up slightly. My .60 4-stroke picks up several hundred rpm if pressure is used but I don't have a good fitting muffler for that engine.
Conversely, my RC car engines won't run (even idle!) without pressure. The carburetor on my .28 monster truck engine has a 9.2mm carb choke.
This also brings up the issue of old engines using crankcase pressure. You don't see that in new engines anymore. Is that because old engines didn't have mufflers, but sometimes needed a pressurized fuel system? My older Fox 74 Eagle IV has a bump cast into the backplate to allow for drilling a pressure nipple. Mine is pre-drilled almost all the way through. The newer Fox Eagles that I have seen just have the bump, but not pre-drilled since that mode of pressurization is not common anymore.