After getting my Powered Parachute (PPC) back home I wanted to start it up to see how it runs. The seller had sent me a video of the engine running but I wanted to see it for myself. It started up okay but ran pretty rough and would NOT idle correctly. If I brought the throttles back anywhere near their fully back position the engine would die. After talking to a friend who is pretty knowledgeable on PPCs and doing some online research I decided to remove the intake silencer. Most sources said they don’t really work all that well on quieting engine noise and don’t do anything to make the engine run better. So my first modification was to remove it.
Next I covered the carburetor intakes while I ordered the parts I would have to replace as a result of removing the intake silencer. First, I would need two air filters, one for each carburetor. Next I would need two new main jets as Rotax has a main jet recommendation for use with and without intake silencers and the jets are two different sizes. So while I was ordering the correct parts I covered the air intakes.
While the parts were on order I also decided to check the other components of the carburetor to make sure they were clean and correct. I checked the main jet, idler jet, needle jet, and jet needle (yes those are two different things) to make sure the correct parts were installed.
So after verifying and cleaning these jets and the needle I reassembled the carburetor and ordered two air filters and two 158 main jets.
After receiving the main jet installation is very simple. Remove the carburetor bowl (slide one clip out of the way and drop the bowl) and unscrew the main jet at the bottom of the carb. Screw the new jet in and replace the bowl. Next I put on the two new air filters.
Now after having verified the right parts in the carburetor it was time to adjust the idle. I went with Rotax’s standard recommendations to start with. Setting the idle speed on this carb is kind of like hitting a moving target. There are so many adjustments to make and every adjustment affects every other adjustment. Rotax recommended screwing in the air regulating screw all the way in and then back out 1/2 turn. They recommended screwing in the adjustment screw all the way in and then out 3 full turns. This is were I started and the only difference from these defaults was that I ended up screwing the adjustment screw out 2 1/2 turns. This gave me an idle speed of 2200 rpm with the throttle at the rearward stops. Now, here is the translation. The air regulation screw controls the idle mixture, so the closer you are to sea level, the richer it needs to be. The higher up you are, the leaner. What they call the adjustment screw actually just limits how far the throttle slide inside the carburetor will travel to the closed postion. So once you get the idle RPM running nicely with the air regulation screw, you set the minimum idle speed with the adjustment screw. If I were to give them names I would call them the idle mixture screw and the throttle stop screw.
Next I have to take the PPC out to the airport and do some full throttle tests. I don’t want to do that in my neighborhood because it’s way too noisy. More to come!