I’M READY FOR MY SCREEN TEST, MR. WEINSTEIN

I’M READY FOR MY SCREEN TEST, MR. WEINSTEIN

6 October 2020

When things go wrong with your CO2 engine’s operation, there is a host of malfunctions or complications that could be at fault. In some cases, the same symptoms may be caused by one of two separate but distinct malfunctions. Let’s face it, troubleshooting a CO2 engine can be a frustrating task.

Generally, these can be broken down to two basic conditions: CO2 [liquid or gas] is going somewhere where it shouldn’t, when it shouldn’t: or CO2 is not going somewhere when it should. It’s this second case I’ll be addressing here ~ the unseen neglected charger screen.

Let’s describe a scenario ~ when you put a fresh CO2 cartridge in your charger for your first flight of the summer.* You poke that charger on the filler for about 60 seconds for a good charge. Removing the charger, you get a good pop, flick the prop and nothing happens – or maybe a little popcorn fart of a run. Now, the most important thing to do is nothing. Light a smoke and think. I can think of a few system problems that may be manifested here. I’ll address some of the others later, but now we’ll focus on the screens.

In the MODELA, GMOT and Brown Jr. chargers there is a brass screen in the plumbing system. Its purpose is to protect the integrity of the engine’s plumbing and to stop metal, corrosion and paint from nicking, scoring and otherwise wearing of the plastic and chromium plated surfaces of the check balls. In one flying season, a scratch on a chrome-plated ball, during all the wide temperature swings from sub-zero to hot day, expanding and contracting that scratch will soon cause a void in the plating and flakes will soon evolve from the void, and be ingested further in the engine, abrading other parts, metal and plastic alike. The screen usually does a suitable job stopping these particles.

The Brown screen is found in the bell portion of the brass filler.
It’s hard to see, especially if it is older and covered with crud. If it’s bright and clean, reassemble the filler. Make sure the filler’s stopper stick or ball can move and not stuck. If you can, unscrew the tank fully from the system and with the charger, shoot a brief blast through the filler to ensure the gas flow is not impeded between the filler and the tank. Why? While not strictly scientific method, CO2 motor troubleshooting is most efficiently accomplished by moving from the known to the unknown.

The MODELA/GMOT style charger on the other hand, has its crap-trap in
the head of the charger. To access it, unscrew fully the head from the charger body. There is a slot across the head, use the GMOT charger wrench, if you have one, to unscrew the retainer nut. Pull out the rubber neck seal and piercing needle and voila! there it is. Unlike the Brown screen which is a single
circle of brass screen, the GMOT style is a screen sandwich with the “buns” being two 9mm circles of brass screen with a similar diameter section of some fibrous material.

Add caption

I have seen many Modela and GMOT spares kits, but never one including this piece. (If I was to try to replace this material, I would start with some sort of synthetic material used for air filtration. Cotton or wool I believe would be more prone to retain moisture.) Maybe, and this is just speculation, it might help diffuse any liquid CO2 before it heads down to the cylinder. Maybe only Dr. Modela knows for sure. Like the Brown, clean or verify it’s clean, reassemble and make some sort of gas flow check.

So here we are. We don’t know why our motor is misbehaving, but we at least know it’s on the engine side of the screen

*Usually you neglect to clean, dry and oil and wrap your engines in paper for the winter, as most of us, at the time, rarely think the last flying session as the last one of the summer. Time and moisture around copper and brass in an enclosed system is inviting corrosion. Take care of those screens, will ya?