Cox Diesel

Godsey3.0 Sat Mar 03, 2012 12:08 pm

Hello. I have a Babe Bee with 1 Bypass port and SPI that I converted to diesel. It has a nice pylinder fit. The issue I am having it that the Teflon Discs keep rupturing. I have broke two so far in about 10 runs total. I tried an aluminum one made out of a pop can. I could not get it to go like that. Whats going on? I am using regular Davis Diesel Fuel. I may get some of the 1/2a stuff.

Surfer_kris Sat Mar 03, 2012 1:36 pm

I have mainly used the peewee on diesel and I could never get the Teflon discs to work. The head would either come loose or the teflon would be pinched off by tightening the head. So there was always a little leak. A metal disc works much better, but you have to be careful with the compression setting. The metal seals much better and I had to rework the counter piston a little to be able to lower the compression enough (shaved of about 0.15mm). One could possibly use a few head gaskets to achieve the same effect though.

The best solution thus far has been a thin sheet of brass, the performance is the same a with an aluminium sheet from a soda-can but the brass lasts much longer. I have not yet broken one in +10 flights.

Cox International Sat Mar 03, 2012 1:49 pm

We supply our diesel head conversions and diesel engines with the Teflon discs (and not a metal barrier) because they are intended as pre-set breaking points before there is any engine damage.

Metal will work but one must be more careful to avoid engine damage.

With the Teflon discs:

Use at least a 6" prop (7 or 8" is better) as high RPM's will rupture the disc.
Do not over-compress the engine (that is the most common cause of disc failure).

Godsey3.0 Sat Mar 03, 2012 2:20 pm

I have been using a 6x3. I have a Texaco Prop in the Cart at your site Bernie.

Godsey3.0 Sat Mar 03, 2012 3:36 pm

I got it to run with the aluminum disc. The contra has to be screwed all the way in though. When it is like this it is really poppy and slow. I leaned it out in hopes that it was just rich but that did not help. The compression is not as high with this disc compared to the teflon. What is a way I can increase the compression?

Godsey3.0 Sat Mar 03, 2012 4:00 pm

Ok. I got put in a gasket to help it seal. It ran awesome for a while. It peaked and held it. The contra still had to be screwed in. It ran 1 minute and 33 seconds then died. I flipped it over and the aluminum disc had failed. The only thing I can come up with is fuel. I got it at the beginning of December and used up 3/4 of it then let it sit till now. I will get some Davis 1/2a and try that.

Surfer_kris Sat Mar 03, 2012 4:04 pm

Check how far up the piston travels in the cylinder with the head of. It should go all the way up to the level of the head gasket. If it doesn't do that you may have a piston that is really worn in the conrod ball-joint.

A 6x3 will be too small for the .049, that's what I run on the peewee engine. A 7x4 should be a better match, aim for about 10000rpm when fully leaned out.

If you don't have enough ether left in the fuel the compression setting will go up too.

**RknRusty** Sat Mar 03, 2012 4:32 pm

Surfer_kris wrote:If you don't have enough ether left in the fuel the compression setting will go up too.

A small amount of fuel in a large container will take on a lot of water and lose the volatiles fairly quickly.

Godsey3.0 Sat Mar 03, 2012 4:33 pm

The piston is good. It has a nice fit. It just is not getting enough compression with the metal disc. With the Teflon it got to a point where it could not be turned over. With the aluminum it does not work like that.

Surfer_kris Sat Mar 03, 2012 4:37 pm

RknRusty wrote:A small amount of fuel in a large container will take on a lot of water and lose the volatiles fairly quickly.

I'm not a chemist but it sounds like you are referring to methanol or other alcohols?
In diesel fuel it is the ether level than is important and it is easily lost due to its high vapor pressure.

**RknRusty** Sat Mar 03, 2012 4:44 pm

Surfer_kris wrote:
RknRusty wrote:A small amount of fuel in a large container will take on a lot of water and lose the volatiles fairly quickly.

I'm not a chemist but it sounds like you are referring to methanol or other alcohols?
In diesel fuel it is the ether level than is important and it is easily lost due to its high vapor pressure.

Ahhh, right you are. That's why brake fluid has ether in it. Like you say though, it off gasses pretty quickly, depending on the container.

Surfer_kris Sat Mar 03, 2012 4:44 pm

Godsey3.0 wrote:The piston is good. It has a nice fit. It just is not getting enough compression with the metal disc. With the Teflon it got to a point where it could not be turned over. With the aluminum it does not work like that.

If you cannot turn the engine over you have way too much compression. With a good fuel you only need a little higher compression than what you have on a glow engine. Sounds like you may have lost too much ether in the fuel.

Godsey3.0 Sat Mar 03, 2012 4:53 pm

I can only really crank up the comp with the teflon. The aluminum will not let me do that. I can screw it all the way in and can turn it over with ease. With the teflon I only had to screw it in a 1/4 of a turn to get it running good. I got several runs like that before the disc failed. I think it is a combo of bad fuel, small prop, and badly made contra substitutes.

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