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Cox Engine of The Month
September-2021
crankbndr's

"Cherry Bomb" .051 engine

More info on this engine!




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Post  Gordinir8 Sun Sep 12, 2021 12:32 pm

Had today my first flight of my tee dee with carburetor mod. First of all i got to say that this is a way faster engine from a black widow but i guess you already know that. Now i didn't manage to hold idle speed and as soon i throttle back engine stopped. Needle was adjusted at top speed with 20% nitro fuel. Not sure if one of two small screws on top of carb are for idle adjustment.
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Post  davidll1984 Sun Sep 12, 2021 7:43 pm

Yes it is the slowdown screw I can already see by the photo which is too close it is that of the small holes one in the Front side its for barel and idle
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Post  ffkiwi Mon Sep 13, 2021 1:05 am

Assuming the TD carb has typical internal geometry, the idle setting is governed by the rear screw in the top of the carb-this actually bears in the throttle barrel-and normally impinges on a flat or slot machined into the throttle barrel-and so how far it is screwed in, limits the closing of the throttle-how far you can physically rotate the barrel towards the fully closed position-so in practice you adjust the 'crescent moon' opening that remains when the throttle is closed by screwing this rear screw in or out till you obtain a satisfactory idle. The front screw (which is often on the side) in bigger throttles, intersects a hole which allows air into the throttle when it is closed-the screw allows you to block or partially block this small hole-and this in turn allows you to adjust the idle mixture-but it works in the opposite sense to a needle valve-you screw the screw in to richen the idle and screw it out to lean the idle mixture. This is typical for any simple airbleed RC carb-you adjust the throttle opening to get a suitable idle, and the airbleed adjustment to get the correct idle mixture. [A very few airbleed carbs-usually those fitted to diesels-have a fixed airbleed which is not adjustable-eg some PAW engines and some MP Jets.]You will find you need to do a bit of fiddling with each adjustment to get a stable idle and pickup again. Fuel and prop size will effect things slightly. Equally important to be aware of-you'll not get a 2000rpm tickover like you get on a big engine-the small ones simple don't like running that slow....even the Queen Bee 074 idles at about 5-6000rpm...at which speed it's producing very little thrust...

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Post  balogh Mon Sep 13, 2021 3:08 am

Among the Instruction Sheets here you will find the description of the TD RC050 engine with instructions on idle setting of the carb.

https://9824b54b-a-62cb3a1a-s-sites.googlegroups.com/site/coxenginecollection/instructionpapers/TD.050Instructions.pdf?attachauth=ANoY7crYhyWnnMekfonq1_RY4vf2bFlX5cGuG7n9ztTqls5MujWLBoTm2sS2l9CaxtoyfE088ea-h3fi8jEVFehoDEpPehEl74EDFvB6QOnzY2H84ea-s_7udGMrAPoliBywKIPxgzrADt4eeGvJXQRzyBlnkY37HezUstD7zy8vseKmbAT4yR-BwFQL2_OogHO6iP8VFcAbFkR-XTyO4dGYkSzBSPFzwDPQmo8Duc4VM4GLVfKzXOwTTPxXrWGiTXfmvQGSxQlx&attredirects=0

You may also want to use a Kamtechnik turbo head with a hot turbo plug that will not flame-out at a sudden opening of the throttle barrel when a shock of fuel droplets instantly hits the glow filament...

Otherwise, if the engine stops when the throttle barrel is closed towards idle, then you need to unscrew a bit - like 1/8 turn counter-clockwise or more - the air-bleed bolt which is in the front of the carb, to lean the fuel-air mixture for idle run..sort of experimental setting..

Even though I am a very forgiving COX fan, I must admit that the aluminum material of the TeeDee R/C carb is rather soft and sooner or later the setting screws will loosen in their threaded bores due to engine vibration, so, before they are lost mid-air,  it is not a bad idea to unscrew the bolts and expand the retaining springs a bit so that they will pull more on the bolts when re-installed, and better withstand vibration. Tiny washers put under the spring may have the same result, but with compromised screw setting range......I would hesitate using non-permanent locking agents or similar, that may dilute in nitro, or, eventually prove permanent, and then you are done with the ajustability of your R/C carb for good..
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Post  Gordinir8 Mon Sep 13, 2021 10:31 am

Thanks, i don't remember having this manual, already stretched the springs and added washers on their ends. I don't much care about the idle stop screw since i can adjust barrel limits with my radio. Ill give it a try to see what happens, neighbors going to love me for one more time.
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Post  Scotland421 Mon Sep 13, 2021 5:49 pm

Ae silencer would aid in ae better idle.
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Post  Gordinir8 Tue Sep 14, 2021 2:48 am

[quote="balogh"]
You may also want to use a Kamtechnik turbo head with a hot turbo plug that will not flame-out at a sudden opening of the throttle barrel when a shock of fuel droplets instantly hits the glow filament...[quote]  

tee dee carburator 31yt9a10

I have this head already, is it ok to use on tee dee and which exactly is the turbo plug you mention?
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Post  balogh Tue Sep 14, 2021 3:15 am

I am afraid what you have here is not the turbo head but a normal COX adaptor for normal glow plugs. The problem with these plug adaptors in small engines is, that the tiny volume of air captured between the plug and adaptor threads adds to the total compression volume in the cylinder, and thus the stock compression ratio drops, as drops the engine performance in consequence.

Turbo heads are specially shaped with the tapered tip of the turbo plug bottoming out and sealing in the tapered plug hole bottom,  thus detaching the volume between the threads from the compression volume. It results in higher compression ratio than e.g. the stock COX high compression heads.

There are 2 types of turbo plugs available on the market, for which different  Kamtechnik turbo heads are needed.:

1. the Nelson turbo plug, which has imperial thread and is a bit larger in diameter. For this plug, the Kamtechnik head is here:
https://kamtechnik.com/product/ktk-21/  Buy this one if you think you have an uninterrupted supply of Nelson plugs. In Europe these plugs are rather rare, so I prefer the other one:

2. normal turbo plugs with metric thread, that are much more widely available than Nelson heads, for which Kamtechnik heads are, however, currently out of stock:
https://kamtechnik.com/product/ktk-11/

I highly recommend the use of turbo heads and plugs for not only Tee Dee but all COX engines, because it really boosts the engine performance. The stock COX production engine 290 - the red postage stamp backplate reedie - as an example, spins a 4.5 x 3.5 prop at above 28k in my Quickie100 RC plane when unloads in the air...

https://www.coxengineforum.com/t13833-quickie-100-rc-flight-video?highlight=quickie+100
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Post  Gordinir8 Tue Sep 14, 2021 6:44 am

So i adjust my engine as per instructions and managed to have a pretty good idle, don't ask me how many revs cause my stupid meter refuse to read. I think that i can have even better results with fresh fuel and definitely will buy this turbo head but unfortunately is out of stock as you mention. Besides i got some pretty good speeds with my q tee that makes me want to push it a little more. Thanks for info.
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Post  MauricioB Tue Sep 14, 2021 8:43 am

Look, I read your concern. What I share is no longer news, many know it here. What I show you is the idea of a friend present here in the forum, Mr. András Balogh.
It's all about combining a throttle control ring with the carburetor and it really works perfectly.
This is my video doing my operating experience:
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Post  balogh Tue Sep 14, 2021 12:49 pm

Thank you, Mauricio, for referring to my idea. Below is the concept that proved quite good.:

tee dee carburator Dttd_d10


It really worked well, here is a video:




I have applied it to a Black Lynx reedie too:
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Post  Gordinir8 Wed Sep 15, 2021 12:30 pm

You know this came through my mind to combine carburetor and ring and seems to work fine but i see there is also a turbo plug. I think i will do all this mods because after all this is the fun part. You are doing great work guys, thanks for sharing your ideas.
Thomas
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Post  balogh Wed Sep 15, 2021 12:39 pm

The turbo plug is mainly used for ramping engine performance up. Plus turbo plugs come in "medium", or even "hot" version, that are not susceptible to flame-out in idle run even if there is no muffler to capture heat in the cylinder.

Mufflers and throttle rings add backpressure and help keep the engine heat during idle, preventing flame out.

So with muffler or exhaust throttle also used with a RC carb, a hot turbo plug is not a must and even stock COX heads will sustain low idle run...see the videos Mauricio and I shot where stock high compression COX heads (Mauricio: a Merlin clamp and plug, not a turbo) were sufficient for maintaining the idle  because the simultaneous closure of throttle ring kept the engine heat inside..this is why the stock COX TD050 RC engine included a muffler with the RC carb..

So in - my -  conclusion , a safe idle with TD RC carb

1. needs a turbo medium or hot plug if no muffler or exhaust throttle is used
2. does not necessarily require turbo plug if muffler or exhaust throttle is also used.
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