051 Tee Dee Cloning to a .050 Tee Dee R/C - RC Groups
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Aug 22, 2011, 10:41 PM
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051 Tee Dee Cloning to a .050 Tee Dee R/C

Background: Im trying to turn (clone) a .051 TD into a .050 RC
using a new carb. from Bernie. The carb. seems to work fine but I cant get it to
idle as slow as Id like. I am using an old standard COX muffler.
The prop is a COX 5x3. Engine runs well but the idle is a bit high 5-6K.
I live at 5000 above sea level. I would like to get it to idle at 3-4K if possible.

I have several questions which are as follows:

1. Did the .050 TD RC have SPI ? My .051 TD has .004
of SPI and screams with the normal intake setup.
2. Is this SPI the reason for not being able to get a slow idle?
3. How should the air bleed be setup and is it critical how it
is set for low idling?
4. Could the size of prop. influence idle speed?
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Aug 22, 2011, 10:51 PM
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earlwb's Avatar
SPI does affect idle speed. If you use a muffler so you can get some muffler pressure, the SPI causes the engine to starve at low speed and stall out then.
So if you are using an RC carb you can't idle below a certain point well as the SPI will choke the engine on exhaust gasses and without muffler pressure, you have a lack of fuel draw at low speed, so the engine stalls out on you. Now without SPI, you can run a muffler which helps idle as it helps to retain some heat as well as provide a little muffler pressure so the engine can still draw some fuel at low speed.
A larger prop helps to provide a little more inertia at low speed. But with the little 1/2a engines there isn't a lot of weight to the little props anyway. Ao it probably doesn't help all that much.

A idle speed of 5 to 6 thousand RPMs is pretty good actually. I think you are in the correct idle speed range for it. The little props are not all that efficient. So one should have no problem doing touch and go's for landing and takeoff practice. Some of the RC Cox engines I ran using exhaust sleeves were only slowing down to about 6,500 rpms with 7,000 being about normal for them.
Last edited by earlwb; Aug 22, 2011 at 11:15 PM.
Aug 23, 2011, 06:47 AM
Registered User
Curious if yoou tried it without the muffler and if so, what the RPM range is?
Aug 23, 2011, 06:58 AM
Tiny diesels.... I love 'em
what fuel are you running?
Aug 23, 2011, 01:25 PM
Registered User
What? are you trying to accomplish?
The max rpms of a TD OR the slower idle of a throttle ringed reedie?
IF you must have a lower rpms idle.. Unecessary IMO, with a 5 x 3 prop fitted.
Then fit a reedie cyl / piston ?
Or.. gasp a reedie engine with the venerable throttle ring Those were surprisingly good.
Aug 23, 2011, 03:29 PM
Tiny diesels.... I love 'em
or buy eewww norvel
Aug 23, 2011, 03:30 PM
Tiny diesels.... I love 'em
At 5000' above sea level props are less efficient anyway so even at 6000k you will not ROG with a 5 by 3
Aug 23, 2011, 05:27 PM
Registered User
A couple of other things to try- instead of replacing the piston/cylinder with a non-SPI just retime your cylinder by putting timing shims under the cylinder before you screw it onto the crank case. Also, re-shim your head and/or use a lower compression head.

Good luck,

Aug 26, 2011, 05:55 PM
Registered User
You might want to hunt up AndyW's threads in RCUniverse a few years back on throttling 1/2A's. (Search their 1/2A forum)
Lotsa carefull reading in there:-). Up shot was IF I read it right, SPI has little or Nothing to do with throttling and idling.. His claim :-)
Anyways he had many good suggestions.
Best one was using a VA 049 engine which, he claimed, throttled superbly.. oem.
Norvel is IMO a likely good bet as well.. those Are of better design than the venerable Cox's.
Aug 26, 2011, 07:43 PM
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surfer_kris's Avatar
I'm getting an idle around 4000-4500rpm on the cox 05RC, but you don't need that low idle, especially on a 5x3 prop. Set it up for max high end and then live with what ever low end you get, 5000-6000 will certainly be fine for flying. On 10% nitro the TD head is a little too high compression. You might get a better idle on the regular head and a 6x3 prop. Or you simply shim the TD head. The best runs that I've had are with a turbo head and a hot plug, gives better low end and gain about 200rpm on the high end as well.

I had to look up "venerable", I think you guys have too high thoughts about these engines. A Norvel (with added adjustable airbleed) would be a saint by the same measures:
Aug 26, 2011, 10:08 PM
Registered User
Too literal a translation :-)
Venerable was a tongue in cheek reference to what is held as 'better' in retrospect than it actually ever was.
Also a term associated with Nostalgia which IMO is nothing more than "Selective memory " :-)
Aug 27, 2011, 04:26 AM
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surfer_kris's Avatar
Oh, I got that all wrong then...

They are not that bad thought, I don't care much for the sure-starts, but the TDs are good engines. The 05RC will spin a 6x3 prop (graupner) at about 4000-17000rpm on 10%nitro. This is not bad for such an old design. I have a short video of a bench run (see below). It runs a little better now with a Turbo head and a hot plug, and I have it in a "Yellow Jacket". The only real issue I've had is the muffler. It vibrates a lot at certain revs, and rapidly wears through the aluminium and the plastic lip in the case etc. Had cox not ended the way they did, they could surely have improved on that too.


Direkt link
Aug 27, 2011, 11:55 AM
Registered User
No they are not bad at all !
I spent years of Summers as a Kid fooling/flying with these.. Wonderful memories. Hardly use mine these days but I will not part with them.
Cox's were great.. then, don't forget these were the ..only.. decent motors available.
There were.. No.. other real choices :-)

Entirely Different world today.
Silly little unpretentious 8$ HK 2730 electric Motors can equal and often outperform most Any 049 .. absolutely true.
Try to get your TD to swing a 4.7 x 4.7 Cam prop to 24k rpms :-)
None of Mine will.
Aug 27, 2011, 12:38 PM
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surfer_kris's Avatar
Electric motors have been around for a very long time too, I just find them really boring. I can't understand why anyone would want to remove the fun part...
Aug 27, 2011, 08:06 PM
Tiny diesels.... I love 'em
Me too I don't like the electric whiny sound i like the raspy sound of cox engines as a guy inknownonce said "the cox 010 a mosquito on steroids"

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