Carb jet drill
Some will cringe, some will rejoice, but whatever the case, tonight I took the leap and drilled the main jet in the carburetor. How did I get here? The CHTs sure rise fast at takeoff and in high power climb. I only see about 100 degrees EGT rise (8000′ DA, 75% power). I think I could use some more fuel flow on takeoff. So, drilling the main jet is how I’m going to attempt to achieve that.
Disclaimer time: I don’t have a certified engine so no danger of jeopardizing paperwork, I’m well aware that I could destroy the jet and have to replace it for a few hundred dollars and much down time, I built this thing to tinker with, not just fly, this ain’t my first rodeo when it comes to carbs. I was very comfortable tearing down and rebuilding auto carbs by the time I was about 12 years old. I’m no expert, but it isn’t rocket science in there.
So with all that accepted and out of the way, I started turning wrenches and had the carb all pulled apart on the bench in about an hour. All you really have to do is separate the bowl from the top half. I had previously ordered a gasket kit and new retainer washers from ACS. I learned at a very early age not to start such a disassembly until you either have new gaskets or enough material and time to make new ones. The gasket needs to come away with the top half of the carb since it is under the float. I did have a bit of a hard time getting the two halves apart after the bolts were out. They were just well stuck together. I used every trick in the book before finally getting out the putty knife. Note that I didn’t have to remove the airbox from the bottom half of the carb. I just left it all assembled. I also found that it is much easier to remove the whole carb heat muff from the exhaust crossover tube than trying to mess with the SCAT tubing that is very tight fitting.
Back at the bench with the carb in two halves, yep, looks just like the good-ol-days. I removed the main jet from the bowl body. A #42 drill bit (.0935″) fit in the hole just right. I drilled that out to #41, then to #40 .098″ (very carefully). I understand that Precision Airmotive has said in the past that each # size drill bit will open it up by about 1gph. That is as far as I was willing to go for now. I think this is a pretty conservative modification. I reinstalled the main jet and its retainer washer. Then removed a cotter pin holding the float pivot pin in and removed the float. This is necessary to install a new bowl gasket. It all went back together the same as it came apart. I got it bolted back on the airplane but still need to finish hooking up the controls. Oh, yeah, while I was at it, I completely removed the carb temp sensor. I’m tired of swapping it out just to watch it bounce all around on the instrument. I’m done with that little piece of crap.
