I'm still pretty new at all this so just want to get some feedback:

I'm running an OS 120AX with a Perry Pump and fuel tank at the CG. I installed a 'T' near the carb and a return line for excess fuel as per Connely/Perry instructions. I just switched to a fuel with full synthetic lubricant (what was available from a trusted brand), and installed a new OS A5 (cold) plug as per the OS manual. I set my needles as I always do, low end based on a perfect transition and high end a slight pitch richer than max rpm and flew. It was a bit windy and there was not much of a smoke trail. I landed my plane and noticed the exhaust was a mix of orangy brown and smoky grey. I tried making everything more rich (to the point of a dead stick on a touch-and-go) but could not get a heavy smoke trail. The good things I noticed were a very wet exhaust stream on the ground, and lots of oily goo on the bottom of my plane. I ran my plane at WOT on the ground and had another guy hold it nose up with no change in RPM.

Let me back up a little here: I ran this same setup the other day with a different brand fuel, with a small mix of castor in the lubricant. I did the same routine and noticed some castor varnish on the engine, and opening up the engine found dark brown varnish on the piston and in the exhaust. I found I had an OS8 plug in the engine which I think is too hot for the 120AX. I also found that this fuel has much less power and transition performance from the newer (Wildcat, 10% nitro, 18% full synthetic) fuel. It also had the same lack of smoke. I took this as a sign of running the engine lean.

I just want to be sure I'm not leaning out in the air. The fuel is a different color (and smell!) then I'm used to, and I'm not sure why I couldn't get more smoke from the engine. Like I said I am pretty new and feel like there are a lot of variables here, and no one in my club has experience with a Perry pump, nor is there really anyone I fully trust with tuning an engine. Right now I am nervous about performing vertical maneuvers without seeing the excess smoke trail.

My intuition is telling me that the synthetic oil might leave less of a smoke trail, the greyish color in the exhaust is probably OK and normal, just different from my 46AX with castor oil, and that the pump seems to be working well on the ground so I should trust that it is working well in the air.

Well thanks for reading and for your thoughts.

Lots of people are taught that you need a smoke trail or its running lean. To me it means my plug is too hot. Switch back to a hotter plug and you will see that you will have to richen the mixture, but the RPM may be down. Your smoke trail will return. But I assume you put in the colder plug because of high nitro, so you may as well get used to no or little smoke trail. Some synthetic oils also leave less smoke.

G'day Mate,
The heat range of the glow plug is dependant on the nitro content of the fuel, not the engine. I run 10% nitro in my OS 160FX, & use an OS #8 glow plug, you won't need a colder plug until you use at least 20% nitro.
The vapour trail that you may or may not see, is dependant on tuning & atmospheric conditions, you may see the trail when it is cooler, & not see it at all, if it is a bit warmer, it is not smoke, if it does actually smoke, on the ground, then you are running way too lean, & burning the oil in your fuel.
Do you see oil vapour coming from your exhaust, when you run up your engine on the ground, it is not smoke, to check this put you finger in the stream of vapour, & notice how oily your finger is. If it was smoke, it would not be as oily, because it would be burnt & black.

I have no idea which plug is right for this engine. To test the right heat range, run it on both plugs. The one that gives the highest RPM is the right plug. The speed at peak is a combination of the fuel mixture and ignition timing. If a plug is too hot then you will have to richen the mixture to quench the plug and a smokey exhaust results, the synthetic oil will not burn but will vaporize to a finer smaller droplets. If the plug is about right then there is not as much quenching of the plug and peak will occur with a leaner mixture and at a higher RPM.

OS claims the appropriate plug for larger engines like the 120AX is the A5/cold plug (now called #10), and it is what came with the engine.

Anyway it sounds like I should pay less attention to the smoke trail. Like I said I'm still pretty new and have a lot going on at one time with this plane with the pump, return line, etc.

Thanks for the help.