astronomer1

Background: I’m 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 can’t get it to
idle as slow as I’d 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?

66Malibu

FYI...
The Cox TD.05RC used piston/rod/cylinder assembly part #20145. The cylinder is marked #5 in the exhaust port but it does not have SPI.
It is ground outside around the port for the special muffler assembly.
This cylinder assembly is very difficult find ,since so few complete engines were made, naturally only a small number of service replacement parts were produced also.
Good luck in your search for hens teeth.....

Mr Cox

1) the .05RC does not have any SPI
2) I don't think SPI is any good for the idle, but the Norvels do quite well with SPI
3) you the airbleed screw as usual, i.e. for a good throttle respons (this is a little richer that just right for constant rpm). On the 05RC the screw covers about half of the hole.
4) A larger prop, e.g. a 6x3, will idle lower and a lower compression helps too. The high compression TD head is a little too high compression, you'll need a few head shims depending on the nitro level. A lower nitro level can also bring down the idle.

I think you might have too high expectations, there are very few 1/2A engine that idle reliably below 4000rpm and other that possible awe-factor (?) you will actually not need anything below 6-7krpm when flying, especially on a 5x3 prop. I think you should set it up for maximum high-end performance and then simply live with what ever low-end you get.

I've had my 05RC idle down to about 4000 on the bench (but don't need it for flying):

[youtube]http://www.youtube.com/watch?v=xEGKeaqu8Go[/youtube]

Cox International

My Feedback: (10)

The reason your 051 does not idle al low as an OEM 050 is because 051 has SPI and 050 does not. Switching to our TD (non-SPI) cylinder/piston will put you on par with an 050.

However, your current setup should idle around 5,500 - 6,000 and should be good enough for RC flying.

Another method to reduce RPM is to use our "type A" head conversion with the hot insert.

Bernie
www.coxinternational.ca

fit90

Got a good one here with mainly after market stuff from Cox International/Bernie. Old Tee Dee .049 with a MECOA long reach glow plug adapter, an O.S. A5 plug, Cox International's "Tee Dee Power non-SPI" piston/cylinder assembly, Cox International's Tee Dee R/C carb, three head shims from Cox International, an aluminum carb mount body Ipurchased on ebay (ridenfligh I think-his parts are exceptional), an APC 5.7X3 running on Sig Champion 25% with one ounce of castor oil added to each quart of fuel.

top end 15,000 RPM
idle4,000 RPM
transition GREAT

High end can be raised substantioally, but at the expense of idle.

Bob

Mr Cox

Giving away 3000rpm at the high end is not worth the lower idle, anything around 5-6000rpm is fine for practical use.

fit90

Mr. Cox,

You are aabsolutely right. However, I occasionally fly out of my front yard and it is heavily wooded with thick and tall pines, palms and oaks. My landing area is narrow, short and closely surrounded by trees. I normally fly electrics at home just so I can kill the power completely to make it in but sometimes I just need to put up a plane with a heart and soul (wet powered) and it breaks my heart when the planes end up spending several nights out in the trees because of my dumb thumbs.

You are right about the large sacrafice on the top end RPM though. This same engine with a #4 cylinder and a Galbreath head and Nelson plug would run beautifully from about 6,000 RPM to about 18-19,000 RPM with this prop and fuel. This would be a great power range for most planes.

Besides, I also wanted to see how low I could make this baby idle just for kicks.

I wlll try to get a pic of my front yard up soon so you can see what I mean. Also, the sound of a whining Cox still draws lots of attention from the neighborhood kids.

Bob

fit90

First pic was taken about where I turn base to final, second was taken from about where I stand when flying-looking at the area where I turn base to final.