I put a fuel flow meter on the FireFly to try and figure out how to get better fuel economy on cross countries. One day as I was taxiing out to the runway, I noticed that the fuel flow was higher with the throttle almost closed as compared to the 3000 to 4000 rpm range. I thought about it some, and I realized that if the fuel enrichment valve spring was too weak, the high vacuum in the intake manifold caused by a closed throttle could lift the valve off the seat.

Since I am getting a Victor 1 ready to mount on the FireFly and the Victor 1 uses the same carburetor, I removed the enrichment valve spring and compared it to the spring on the 447. Sure enough, the spring on the 447 was a much lighter spring. The new spring, on the right, has a free length of 2-1/8 inches, and the old one had a free length of 1-1/2 inches. I checked the Rotax site for updates, and I found nothing that mentioned this spring had been changed.

So if you have an older 447, mine is four years old, and if your engine is loading up or will not run steadily at idle or low speeds, you may want to check your enrichment spring length and if necessary purchase a new spring. It could make for a safer short or soft field take off.

Since the change in springs, my engine idles in a more uniform manner than it ever has before. At the lower idle speeds it still bangs the gears. I can set the throttle at any speed from 2300 rpm down and it will run at a constant speed. But the high fuel consumption at low idle speed (1600 rpm) did not go away. This indicates the pilot jet/air screw is much too rich at these low idle speeds.

I did change the air screw to lean out the engine and I found out why I have it so rich. It is to reduce the banging of the gears in the gear box. Also, if one leans it out too much the engine does not want to spool up from an idle. At this point I believe the only way to get around these problems is to put a delay in ignition pickup circuit to further retard the spark. This would make it idle smoother and not hesitate to spool up too. I will not be trying this because the Victor 1 is just about ready.

I have found that at engine speeds below 2,000 rpm, the Bing throttle valve spring is too weak and lets the throttle valve and needle valve vibrate up and down pumping fuel into the venturi. I believe what happens is that as the needle is bounced up additional fuel flows into the float bowl to make for the lost volume. Then the throttle and the needle come back down against the stop. This causes a local increase in fuel level in the main jet percolation area which greatly enriches the air/fuel mixture that is passing up between the needle and the jet. Basically, the vibrating needle and throttle slide in combination with the float valve act as a pump, and pumps fuel into the venturi.