Choke Tube "stuff"...

I'll be mounting a choke-tube equipped Cox .049 reed-valve engine in a flat-bottomed boat hull. I want the engine to be as low in the hull as possible. The fuel line will take an abrupt 90 degree turn where it attaches to the NVA's nipple to allow the backplate to sit on the floor of the hull. A bulkhead for mounting the engine will be "notched" for the fuel-line to pass under.

To assure that the fuel-line does not kink where connected at the NVA; I placed an appropriately-sized "ball-bearing" for the line to curve-around.





yea.. I know.. It's HEAVY.. as well as OCD overkill.. but I'm going to find a plastic or wooden "bead" to replace it.

Good work roddie Perhaps you want to try a fuel pickup spring inside of fuel tubing instead of ball bearing.

Levent Suberk wrote:Good work roddie Perhaps you want to try a fuel pickup spring inside of fuel tubing instead of ball bearing.

I "do" have some springs Levent.. but I'm not convinced that one would stay in place. The fuel line's length/orientation will be such that an internal spring could migrate back away-from the elbow/nipple.. due to vibration. As I mentioned; I plan to source a wooden or plastic "bead" to replace the heavy ball-bearing.

Simply placing a small 1/8th in. wide "zip tie" at a location over the fuel line where the internal fuel line spring is (doesn't have to be made super tight) could provide enough frictional resistance to keep the spring from migrating by vibration.

That is lightweight, certainly lighter than the ball bearing. Another option is small diameter copper magnet wire, maybe 8 to 10 turns before twisting ends together.

One could also use the soft steel wire from a food item, like the papered wire to hold celery stalks together or seal the plastic bag securing bread or sopapilla shells.

Good thinking, Roddie

If you find a wooden or plastic bead with a hole for a string (I guess it’d have to have a hole to be called a bead ), you can lock it in its place by drilling holes through the sides of the wood blocks and inserting a pin so that it goes through the hole in the bead.

Does it make any sense  Otherwise, I think trying to hold the bead in place while installing the engine inside the hull might be a bit fiddly task to do.

Nice Roddie. I’ll keep that in mind. I’m sure I’ll need to do something like that in the near future

One could just cut the fuel line just beyond the end of the spring and put an internal joiner or barbed nipple to hold the spring in place, and give you a convenient place to disconnect the fuel line...

All GREAT IDEAS on how to keep an internal fuel line spring from moving. THANKS EVERYONE!

We do sell the correct spacers for the Surestart engines: https://coxengines.ca/building-supplies/cox--049-engine-firewall-helicopter-mount.html

The 'Choke tube" is a priming device; blocking the top with a finger and flipping the propeller until fuel reaches the backplate.

Cox International wrote:We do sell the correct spacers for the Surestart engines:  https://coxengines.ca/building-supplies/cox--049-engine-firewall-helicopter-mount.html

The 'Choke tube" is a priming device; blocking the top with a finger and flipping the propeller until fuel reaches the backplate.

Hi Bernie, Might I suggest listing that item under .049/.051/back-end parts.. rather than listing it under building-supplies? That's where I looked for it; because I remember it being offered a while back. I just figured it was no longer available.

Roddie, appears you are resorting to an empirical method of expiramentation to find if using a flared opening with slightly reduced cross section will work for improvement.

Nothing wrong with experimenting, how NACA developed wing cross sections and data on lift and drag. Read up on it in various model airplane handbooks. Over 70 years ago, seemed a lot more were interested in theory than now.

Difference In areas = d^2 of choke tube / d^2 horseshoe back. (PI and constant 4 cancel out.)

Didn't one of our modelling engine sages said to drill out a Surestart back slightly to increase horsepower to clone a Venom reedie? What was it? 3/16 inch drill bit?.

Say Surestart choke tube is 1/4 inch diameter.

Then after reduction of area formula,

0.1875^2 / 0.25^2 = 0.0352 / 0.0625 = 0.5625 or coincidently 9/16.

Entrance velocity of air in choke is a little less than half that in the venturi. Lower velocity air has less frictional losses than higher velocity air in venturi cross section.

Hence probably why choke tube has minimal effect on engine performance. Yet to see someone running tach numbers on choke tube versus no choke tube to verify.

This is all back of an envelope (snail mail type) calculations, since tapping away on a cell.

Coincidently, Mauricio milled venturi and added insert thus maintaining original venturi inner diameter.

In your case, I'd be interested to see if with a slight reduction of choke tube area by the insert got offset by the flared end. Or, did it work? Or not?

roddie wrote:

Cox International wrote:We do sell the correct spacers for the Surestart engines:  https://coxengines.ca/building-supplies/cox--049-engine-firewall-helicopter-mount.html

The 'Choke tube" is a priming device; blocking the top with a finger and flipping the propeller until fuel reaches the backplate.

Hi Bernie, Might I suggest listing that item under .049/.051/back-end parts.. rather than listing it under building-supplies? That's where I looked for it; because I remember it being offered a while back. I just figured it was no longer available.

Great suggestion Roddie, and it has now also been listed there

I get these urges.. when I know that I'll likely need more than "one" of something.. I like the idea of the choke-tube. It can help to confirm the drawing of fuel in a suction-induction set-up.

I had on-hand; some .350" hardwood square-stock.. which is the ultimate size for making a pair of blocks for running the choke-tube.. so I marked-out a few sets.  

Easier said than done; the horseshoe (product-engine) backplate's mounting hole pattern is wider at the top.. than it is at the bottom. It's necessary to fit the choke-tube to the backplate in order to mark the offset hole-pattern on the blocks. To mark the holes.. I used a mechanical-pencil with the lead extended out to about 1/4".. which helps to accurately mark the holes. I made the blocks 1.125" long.. which is the minimum length that you'd want them. My piece of square stock was 12" long. I drilled the holes first using my drill-press.. then I cut the blocks to length in my bench-vice using a hacksaw (32 tpi blade).

The holes are drilled to .125" (1/8") diameter so as to be an extension of the holes in the backplate.




These mounts will be kept in my "Cox Box".. which has other homemade engine-mounting pieces that I've made over time.