The Cessna 120-140 Association Forum

Had a rough running engine twice now and here is the scenario:
Another pilot friend flying my airplane had this
First event at level off@4200’(1200agl) at power reduction to cruise engine got rough and lost 2-300 rpm- corrected itself and did it again. Over airport so he landed uneventful. Troubleshot in the air carb heat-switched fuel tanks- then landed. Mechanic suggested a bad mag. Replaced mag with new. Run up fine.
Second event( next flight) with me flying.—cruisedat 4500’ for about 30 min with all normal- then same thing-rough and rpm loss. Troubleshot all the above plusmag check left off/on right off/on —-no change. This eliminated an ignition problem in my mind. Landed 15 minutes later and engine smoothed out in that timeframe.
I highly suspect an intermittent stuck valve???? I’m stuck in the field(AMA) and will have an A&P look at it tomorrow, but I don’t want to walk in there blind. Any ideas or suggestions?
Thanks Ted

It could be...

The first event was right after a power reduction: prime time for a stuck valve. The second one wasn't, but that doesn't prove anything. Unfortunately, unless you can catch it while it's stuck, it's tough to prove definitively. You can ream all 4 exhaust guides prophylactically, but there's not much else to do. Check the valve springs, too, while you're there.

You don't have a Marvel carb, do you? If so, check the venturi and, if you have the one piece venturi, see if the updated emulsion tube is installed.

I do have a Marvel Carb. I don’t have all the paperwork with me right now but I know the one piece Venturi was installed— don’t know about the emulsion tube?

Im a complete rookie to these gals but was reading similiar stories in "live to tell" at backcountry pilot forum... several fouund issues w either small paint flakes in carb and one guy found fiberous material in his gascalator and figures it had come from someone filtering fuel through a chamee the wrong way... again take that for what it cost but it wasnt dissimilar occurances... i beleive the thread was called the sputter club or something to that effect...