Yesterday I was doing some routine pattern work. OAT was around 90 and density altitude was around 2,000 feet. Normal for this time of year. I normally pull carb heat at 1700 rpm midway through downwind. I have always gotten around 150 rpm drop with it applied. This time I pulled it and got a sudden 500 rpm drop. The engine started to run rough and vibrate. I assumed carb ice so I left it on and landed. Once on the ground I did a mag check, static run up ( 2150 rpm ) and then pulled on carb heat at 1800 rpm and the engine dropped to 1300 rpm and slowly fell to around 1000 while running rough. I attempted to give it throttle and at WOT I was turning 1400 rpm with the engine running rough. As soon as I turned off carb heat engine went back to normal.
I came out tonight and same thing. I taxied out on the runway and did a full throttle run with carb heat on. Usually it will still turn 2000 rpm and tonight again only around 1300 slowly dropping to around 1000. Pulled the cowl and checked the air box and butterfly and both are fine. The air box only has 100 hours on it. All hoses look normal and I removed each with no blockage. Carb heat control operates normally. The only issue I have seen is a slight exhaust leak at the right side header into the muffler. Tightened the clamp up on that.
The carb is a stromburg that has been trouble free since rebuild last year. Engine starts on first pull even when hot. It runs up normally mag check is fine 50 to 75 rpm drop, plugs are clean, but pull on carb heat and the engine starts to die.
What am I missing here. Not sure what would cause this problem sudden onset like this.
I came out tonight and same thing. I taxied out on the runway and did a full throttle run with carb heat on. Usually it will still turn 2000 rpm and tonight again only around 1300 slowly dropping to around 1000. Pulled the cowl and checked the air box and butterfly and both are fine. The air box only has 100 hours on it. All hoses look normal and I removed each with no blockage. Carb heat control operates normally. The only issue I have seen is a slight exhaust leak at the right side header into the muffler. Tightened the clamp up on that.
The carb is a stromburg that has been trouble free since rebuild last year. Engine starts on first pull even when hot. It runs up normally mag check is fine 50 to 75 rpm drop, plugs are clean, but pull on carb heat and the engine starts to die.
What am I missing here. Not sure what would cause this problem sudden onset like this.
