I have an XYZ 50 engine in my small Ziroli P47. The engine has good power but it is giving me a problem that I just cant seem to solve. Engine runs fine on ground, with good transition. Engine runs fine during take off roll. Once the plane gets in the air, the engine sags for about 1-2 seconds then comes back up to power. I thought it might be a pressure problem, so I soldered a tube to the diafram plate and extended it into the fuselage. This had no effect. This situation is giving me heart problems and it is only a matter of time before it will cause a serious problem.

It is strange that it is only exibiting this behavior when the plane leaves the ground it occurs about 4-5 seconds after the plane takes off. After the power comes back up it seems to run just fine. During todays flight it seemed to sputter a little during the first victory roll. I have replaced all the fuel lines and the tank is tight. Any help would be appreciated.

Do you think it's possible there is inadequate cooling of your engine?
Being in a scale plane, airflow is often an issue. Overheating may well cause the symptoms you describe.
Just my 2c worth

No, it is not a cooling issue

I once have worked a day long on an XYZ engine solving multiple problems all concerning carb tuning, carb pulse and engine manifold isues. In total about 10 of them IIRC.
After ironing out all errors, the engine ran like a champ.
So NO, I would not like to comment on possible issues for this problem. Give the engine a thorough overhaul with much attention to detail, taking nothing for granted.

Donald,

It seems to be happening when the torque demand is high (rpm low), the throttle is full open (Venturi effect poor), but the low pressure in the crankcase is marginal and the nose is pointing up; which are the worst working conditions for the pump in the carb.

Replace the flexible parts of the carb if they may be aged.

Best luck with that!

I'll give that a whirl, thanks

Donald - I was having problems with my XYZ50 - was getting harder to start and then would not start. Took the carb apart and I found lots of garbage in the two internal screens. (I was new to gassers and forgot to install a fuel filter). Also one of the the diaphragm's stop-valve flaps was a little raised. I ordered and installed a new diaphragm, no difference. But I had also had plugged the choke plate hole with a small bolt and nut, and used lock-tight on that, and I think that lock-tight may have flowed into some places that it shouldn't. (In hindsight I should have removed the plate before putting the bolt in). I soaked the carb in Acetone, but still had problems.

So I ended up getting a replacement carb from Hobby City (only $30), and the engine runs like a top now.

So I've got the XYZ 50 running nicely now. Steady idle and very good high end (will tac it tomorrow). Was a combination of opening the low speed needle and also adding liquid gasket to the surfaces and gaskets around the carb manifold. Seems like there might have been a slight air leak that was increasing with the higher RPMs. Now with the liquid gasket she's running very nicely. I also saw that the Halls Sensor had loosened slightly and shifted a touch, so I re-centered that. That didn't seem to make a difference.

One more recent issue with this plane - now when the engine is running the elevators shake and move intermittently on their own - my guess is radio interference. This intermittent movement is independent of whatever controls I do. I've had this plane for over a year and this is a new development. Can radio interference pop up out of nowhere? From what I see I can't find any obvious wear on any cables. Am wondering if Ch 2 on the receiver might be going bad (though the elevators work fine when the engine's not running). Any other thoughts? Guess I can swap in another EI to see if that's the cause. Any ideas on what I should look for that might be generating radio interference?

I had an XYZ 50 that had the blue plastic cased ignition. Even on 2.4 it had a lot of RFI. The surfaces jumped erratically at high throttle. A cheap fix for troubleshooting is to wrap the ignition with a formed tin can(not the aluminum type, but the magnetic type.) and ground it to the motor case somewhere. Swapping a good CDI in is probably an easier troubleshooting attempt, but I just wanted to see if it would work. Of course, the more costly fix is to buy a new ignition.

I had an XYZ 50 that had the blue plastic cased ignition. Even on 2.4 it had a lot of RFI. The surfaces jumped erratically at high throttle. A cheap fix for troubleshooting is to wrap the ignition with a formed tin can(not the aluminum type, but the magnetic type.) and ground it to the motor case somewhere. Swapping a good CDI in is probably an easier troubleshooting attempt, but I just wanted to see if it would work. Of course, the more costly fix is to buy a new ignition.

Funny - as part of my troubleshooting I actually re-installed the original blue-plastic cased EI that came with the engine. And the engine runs great! As for the suspected radio interference, the elevators still had that intermittant movement - so either it wasn't the Rcexl EI, or both EI's were having problems. Then I swapped out both of my elevator servos - Hitec 5645MG's - with two Futaba 3305's. And the intermittant movement is completely gone. So looks like the Hitec's were worn (even though I could feel no gear slippage and they worked fine without the engine on). Or could it be that, because the Hitec are digital they were more suseptible to radio or electrical interference?