I have a real strange engine problem and I was hoping someone could possibly shed some light on it for me. I have a VW2180 with a Zenith carb. The induction system for the carb has a round hole in the cowl that is about an inch and a half diameter. Behind that is a rectangular airbox that has a K&N filter element that is about 4 X 7". That goes into a piece of 2 1/2" SCAT tubing into the carb heat box which is mounted to the front of the carb. There are some pictures at www.engalt.com/kr2.htm
My engine has been running pretty smooth except for a fraction of a second stumble about every thirty second to a minute. During a cross country last week at 8,500' it would give me the little stumble about every fifteen seconds after I was flying for a few hours. It isn't a stumble that would make you land, but one that you pay attention to. I have a cockpit mixture adjustment and had it out about an inch during cruise. The engine would run rough and loose power at two inches out, and it would still give the stumble and decrease RPM if it was pushed in any more so the best I could tell without an EGT is that I had the mixture adjusted correctly. The oil temp is fine and the CHT was only about 280 degrees. Just for grins I tried pulling the carb heat in case there was some icing even though there was no visible moisture and I was in Florida on an 85 degree day. I noticed that when it was all the way on I got the normal decrease in RPM, but when it was about half way on the engine smoothed out, picked up 100 RPM, and ran perfectly. My first thought is that maybee I had the mixture lean because the mixture richens a bit when carb heat is on, but with the carb heat off I got a definite RPM decrease if I pushed the mixture in more and it didn't make the engine run any smoother. Another thought is that I was getting a lot of ram air pressure which was leaning the mixture, but again, that doesn't explain the fact that the engine runs best at altitude with the mixture pulled back some. I thought that maybe I had to much resistance through the filter and opening the carb heat some gave me more air, but that would have made it richer and leaning it more would have made the engine run smooth, which it didn't. Mark had a post a while ago about someone who had a ram air system that didn't port the float bowl air vent to the same pressure that is at the front of the carb and I possibly have the same problem, but I don't think that I am getting that much ram pressure with the filter and I have the same symptoms on the ground or in very slow flight when I have almost no ram pressure. Any one have any clue what this could be?
