My 140 DZ starts to give me some problems. The glow plug seems fine but the engine looses RPMs while in an upline (flying advanced IMAC).

I checked the valves and they are OK at about 0.08mm. The engine is not too lean and I can see a godd smoke trail at any throttle setting except in midrange where I see lots of smoke but the engine runs smooth in the whole throttle range.

Fuel : powermaster 30% nitro 23% low viscosity oil
Plug : OS #F 16 gallons of fuel old
Prop : APC 18x8W at 8300 or 8400 RPMs

My question is, the plug seems OK but it is obviously old (16 gallons old) so I replaced it but have not had a chance to test it. Can the plug be the cause of my engine running inconsistently the last few flights ?

The plug can cause problems, but in warmer weather you might want to try running a little bit smaller prop.

What kind of RPM gains would a APC 18x6W give me. With the 18x8W, the engine sure is not very cold but seems OK.

Raise the rpm by 500 and let me know what happens.