First of all, I know this isn't everyone's favorite engine, but I was starting out and wanted something inexpensive in case my H9 Arrow ended up like my PT-40 before it (can you say "matchsticks"?).

Anyway, it seems every time I go to the field and get it started for the first run of the session, it runs fine. I know how to lean it (using pinch test), and it seems to run smoothly without much tweaking. But from the second flight on, I can't get it to run at full throttle without getting surges. Up to 3/4 throttle it still runs smoothly, but seems down on power compared to the same throttle settings when the engine is running perfectly. It keeps the plane in the air, but sounds positively sickly. What am I doing wrong?

Clarifiers: Have run 5% fuel through on the first gallon assuming that was better for breaking it in, but tonight switched to 10% Cool Power. The same surging problem occurs with either fuel and I saw no difference in performance even on the first (usually best) run of the night. Have also changed glow plugs....again, first run is good, subsequent runs get gradually worse. I've never tach'd the thing since I don't have a tach (but I plan on getting a cheap one soon). Have run both 10x6 and 11x6 props, with the only noticeable difference being that the bigger prop acts like a nice air brake when coming downhill throttled back.

Theory #1: Does it have something to do with the heat of the engine itself affecting the mixture (i.e. - as it gets hotter it vaporizes the fuel differently causing it to run too lean)?

Theory #2: I've never ever touched the low idle setting on the motor. Could never keep it straight which way to adjust it and for what. Could the vibration inherent in these 2c engines have caused this adjustment to change in an unhealthy way?

Theory #3: There is no......Theory #3.

Theory #4: I have noticed that it runs more consistently on cooler, less humid days and runs worse if humidity is high.....still, I always assumed that the high speed needle would allow me to adjust for that.

Thanks in advance for any guidance you can offer.

David

Generally, on here, The surging problem is considered to be one of balance or that of starting ot with just the right fuel mixture. That when getting in the air, the engine unloads some whereby it runs faster. This produces more muffler pressure which enrichens the mixture and slows the engine down thereby reducing pressure whereby the engine goes leaner and so on.

However, being the time of year it is, You may not be setting your engine rich enough before take-off and the thing is getting hot on you? I'm in the Deep South right now and most everyone at the flying field is having heat related engine problems and surging that they didn't have in the winter months.

I'd stay with the 10-6 and make certain that I had the engine enrichened from the peak lean point determined by the pinch test. Better yet, do the pinch test with the plane vertical and richen it a few clicks from that point?

Enjoy,

jim

Thanks for the input. I'll try that next time. Hey, at least I'm not wrecking the thing (not even when it quit on me yesterday in a relatively low altitude vertical maneuver....pushed full forward just before the stall and managed to DS it in after it passed over the hill bordering the flying site...landed perfectly sight unseen!).

Any engine that doesn't quit on me must be running pretty good? But, then again, I don't fly 3D.

Enjoy,

Jim

It may be a mixture problem, as previously described, but it could also be the head bolts or back-plate bolts loosening. LAs are noted for that little trick, & it can also produce surging.

That's news to me...I'll definitely check that out when I get home tonight...thanks!