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Post  roddie Wed Apr 12, 2023 9:05 am

The Cox reed-valve engines use a tiny/round metal/mesh "screen" where the air enters the engine (at the rear). Its purpose is to prevent debris (sand, grit, tiny seeds) from entering the venturi and fouling the reed-valve.

It's not common for the screen to pop-out.. but it can be removed. One reason for removing it would be to use the "Ace-style" venturi-throttle.. but for now; let's just assume that you have an engine with a missing screen.  

You'll want to have some type of air-filter in place before mounting/running the engine. I use a small piece of 1/4" thickness polyurethane (replacement) air-filter media designed for "in-window" type air-conditioners. Polyurethane is resistant to glow-fuel.

The filter material is spongy... and will compress flat between the engine's backplate and whatever firewall-type mount that you plan to use.

Some photos...

Air filter for tank backplate 20230410

Air filter for tank backplate 20230411

Air filter for tank backplate 20230412

Air filter for tank backplate 20230413

Incidentally; the aforementioned "Ace-style" venturi-throttle conversion will work just fine with this filter in place.

I also use this filter material as a vibration-damper when using an external (wedge-type) fuel-tank on a C/L profile airplane.


Last edited by roddie on Wed Apr 12, 2023 9:13 am; edited 1 time in total (Reason for editing : Ace throttle note)
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Post  balogh Wed Apr 12, 2023 10:14 am

Roddie,

Very good reputposing idea. I am also a pro-filter  COX fan, and add a screen to even a Killer Bee filterless backplate. Not only will this screen protect the reed from clogging, but, more importantly, will keep the engine interior clean from debris that may destroy the piston-to-cylinder fit. I do not sense any notable performance deterioration with screens having a mesh size similar to stock COX screens. I have fixed this one with hooking a small O-ring to the NV on the top and to the fuel nipple on the bottom, with a screen pressed underneath.

Air filter for tank backplate Killer12
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Post  HalfaDave Wed Apr 12, 2023 12:40 pm

Hi roddie,
I am a fan of keeping fine particles out of Cox engines.  Nice!
Here is a story,
We were 12yrs old, running BB.049s CL in school yards.
An article in a magazine suggested 'nylon stockings' as a filter.
Buddy cuts 1X2"  squares from his sister's stockings. We install them between tank/firewalls.
The effect was amazing.
Girls showed up.
And joined in the fun. Cox Cheerleaders.
They could out flip start, out fly us any day.
And they like it.
This is why I like Cox engines,
And it is the future,
Take care,
Have fun,
Dave
P.S. The nylon stocking filters worked out well ! Cool
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Post  sosam117 Wed Apr 12, 2023 3:35 pm

I have a nice punch set and purchased a roll of screen material just slightly smaller than Cox screen (metric sizing -- not inch).
Anyway, I make my own replacement screens for my engines.

If you didn't see my previous posting?
Installing a screen in a Tee Dee venturi

I have also cut them slightly oversized to "dome" the screens.
Air filter for tank backplate 13_ven11


I also have done this on the backplates of the old metal and plastic Bee plates.
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Post  aspeed Thu Apr 13, 2023 7:30 pm

Roddie, seems like a good idea if the motor is mounted solid enough. I have to ask what the exhaust outlet setup and wheel collar? on the needle valve is. Is that heat shrink tubing?
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Post  Yabby Fri Apr 14, 2023 2:27 am

I also use nylon stocking filters on my Cox powered air cars to keep the sand and dirt on the road out of my engine. I also have made a nylon stocking filter that fits on my fuel tank breather as I was driving my RC homemade Cox powered air cars on a road next to the beach and I was having problems with fuel feed blockages. made the filters and the problem went away. cheers sunny
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Post  roddie Fri Apr 14, 2023 9:34 am

aspeed wrote:Roddie, seems like a good idea if the motor is mounted solid enough.  I have to ask what the exhaust outlet setup and wheel collar? on the needle valve is.  Is that heat shrink tubing?

Hi Alan, The filter media will compress flat when the engine is mounted.. except for the area (depression) on the rear of the backplate for air to enter. That entire depression becomes an air-filter when this media is used. I haven't found that it negatively affects performance.. but may require a slightly different needle-setting than you're used to.

Regarding the needle-valve; I have a short piece of med./lg. size silicone glow-fuel tubing and a steel flat-washer installed, which cures a common problem of air-leakage where the needle screws-in to the backplate. The OEM "spring" needs to be removed for this.. but if the tubing is the correct length; it will act as a spring when it's compressed. The operating-range of the needle is only a few turns.

Here's a better photo. Note that the flat-washer bears-on the shoulder just below the knob on the needle to act as a stop for the silicone tubing.

Air filter for tank backplate Needle13

Regarding the "exhaust outlet setup"; it's a muffler that I designed some time ago.. and am still experimenting with. The outer-part of the muffler is made from two disc-shaped pieces of inner-tube rubber (butyl rubber) stapled-together.. and does look a bit like heat shrink tube in that photo, because it's wet with oil. In short; there's an inner aluminum ring (pc. of 1" tubing) with a hole drilled to accept a spent .22 ca. (long type) shell-casing which serves as a header-pipe. I recently sourced some silicone tubing that fits tightly over the header-pipe to allow for plumbing the exhaust further away from the engine. (LOOK HERE)
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Post  aspeed Fri Apr 14, 2023 10:29 am

That silicone tubing pic. tricked my eye/brain. I guess I have seen both mods before, just can't see well enough. I was king of hoping that heat shrink would work on the exhaust to hold an exhaust stack. Just to add to the thoughts on the needle valve, I have had a motor where the needle was so loose that it pulled right out. I rubbed a bit of release agent wax over the needle threads, (oil might work too) and then put a SMALL drop of red Loctite over the needle with a toothpick, and screwed it almost in. It sealed well and now the needle is pretty stiff and holds the setting. The permanent red locktite is longer lasting than the blue. If you put too much Loctite on, it will plug the spray bar hole and the needle will be in there forever.
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Post  HalfaDave Fri Apr 14, 2023 3:45 pm

Hi All,
Landing/crashing vertical on a hard packed sand baseball diamond is the worst.
Cleaning Cox .049s get easier after that. Smile
It is all relative... Smile
Is there a scale of how much dirt/stuff can pass through a Cox.049?
I bet there are great stories here...
Thanks CEF forum, Cool
Dave


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Post  Yabby Sun Apr 16, 2023 4:23 am

probably slightly off of Roddie's thread topic but,
with a friend did some Cox 049 engine testing
Cox 049 surestart crankcase, grey horseshoe backplate with standard gauze filter in backplate, TD cylinder, killer bee crank, 1 head gasket, standard cox glow plug.
25 x nitro, 20 x Castor, 55 x methanol
Cox 5 x 3 safety propeller
tach with Turnigy fidget tacho
Engine held 18.7k till it emptied 1/2 ounce tank.  Peak at 19.2 K when first started and cold engine
Removed standard gauze filter from grey horseshoe backplate and ran engine again, still hot/warm from previous run
Engine held 18.7k till it emptied 1/2 ounce tank. Peak at 19K

It would seem the engine we were testing with ran just as well with the standard gauze filter as it did without it. that is not what we had expected  Huh... but we were happy with the engines output anyway. Thumbs Up

Be interesting to see tach numbers for other cox engines with and without various filter types Huh...  Thumbs Up
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