Engine not working properly. Please help!

[JuanRicci]

Hi, I'm from an Aerodesign team and we own 2 Magnums .61 XLS engines.
We used to attach them to APC 12,25x3,75 propellers because we need to obtain the most thrust available and it was the best propeller for this engine is these considerations.
The combination between engine and propeller used to rotate at about 13600 rpm and suddenly both engines are not working so well and are rotating at about 12900 rpm. We are using the same fuel, same glow plug and same bench test than before.
Does anyone know what might be happening? Did someone had the same issue before? What can be done to fix this?
Thank you very much!

 

[Revere]

Could have been dirty fuel - been a long time since I messed with glow engines so I don't know the setup of yours, but, clean the carb/venturi whichever is applicable to your motor with alcohol, be sure the needle valve is clean all the way through. and look at the glow plug to see if the element is tarnished and replace as necessary.
Another thing to consider - temperature. since it's summer now and you said your bench testing, did you turn the aircon on in your test area? for different temps you need to run different "strengths" of glow plug to keep the motor happy.

 

[PsyBorg]

From my experience with nitro engines on my cars and trucks these motors need to be tweaked every time you run them once initial setup and proper break in has been done. Many factors can drastically change how they run at any given moment. Temperature is the most common but also humidity, altitude, water vapor in the fuel if its aged or is some cases sat in a gallon jug that was half empty and the trapped air condensed and put moisture in the fuel.

They can be very finicky at times as well as stubborn to get started if any of those factors changes enough. The 700 rpm drop you see is pretty standard range for a motor that was not optimized for current conditions from last working conditions. Keep in mind the more hours on the motor will also have an effect on how often and how much retuning will be needed. Depending on that particular set of motors running hours they may be starting to show signs of needing to be re-sleeved to get the compression back up and reduce the blow by past the piston.