killer bee venturi diameter

A 3/16 .187 bit will drop about half way in from the reed side, where the reed sits is slightly larger and the other end is slightly smaller.

You sure you are not just hitting the spray bar? I was under the impression that the passage was more or less conical.

Phil

OK, I measured what I have on hand, I do not have a red killer bee backplate available without taking apart a plane and engine to measure the parts.

The black back that Bernie sells is a tight 11/64 after the nva is removed. Two turns of clear Scotch tape on the twist of the drill bit pass the reed seat, but 3/16 does not.

The reed seat on the Sure Start is the same size as the black back but there is a large plastic support block inside the air passage at the NVA. The black back has support webs on the outside of the air passage that is not on the sure start.

It would appear that running a 3/16 in from the reed side to the NVA will provide the effect that I am looking for.

I had it backwards in my mind which way the cone shape was for the air passage. Thank you crankbndr for the measurement.

Getback is referring to porting a fuel tank as that is very close to 5/64 I port fuel tanks to. I port to a convenient drill size not maximum numbered drill size as for sport flying the few thou smaller is still quite a hop up on a baby bee.

I'll try to get this engine running soon and see if I got it right.

Phil

I drilled out the support block to 11/64 and drilled the reed seat side to the hole for the NVA at 3/16.

I too have used the 11/64" drill bit, a #16 works as well. I think my success has been using the 128 TPI spraybar and needle. It doesn't unwind or vibrate like the stock needle. The friction nipple also tends to work better on them. The finer threads allow for a better adjustment and a slightly leaner setting while the stock unit does just the opposite.

I hope to find 4 screws and run this thing today. I have the gasket and the sealant. I am not sure what reed I found but it is an opaque white plastic material. It is either Mylar or Teflon. I am inclined to think Mylar because the edges are very smooth and Teflon often has a little fuzziness on the very edge.

The trials of an unsorted box of parts.

I want to use the choke tube. I did remove the filter screen and do not plan on replacing it. I need to find a washer and some medium fuel tube for making a replacement spring and added seal. I hope it all fits.

I am pretty sure I don't have another fine NVA to use. I'll look while looking for screws.

Phil

My KB back-plate was accessible.. so I took some measurements with a vernier-caliper. First.. it's important to note that the intake side is in fact "oval".





and gradually tapers larger moving forward.. ending-up symmetrically-round at the reed-seat.




This shape/taper is probably "key" to getting fuel to draw un-assisted.. with such a large venturi? (it's shaped that way for a reason..) To get my engine to run past a prime using a hard-tank.. I was successful switching to a larger fuel-line. Prior to doing that; I'd see bubbles forming in the smaller fuel-line as the prime burnt-off. Attaching a larger ID fuel-line seemed to be less-resistant to flow. I was however using a very small 6.15cc uniflow-vented tank. This was on my Rare-Bear Speed model.. as you probably remember.

Modifying a stock horseshoe backplate would be dicey. I measured one of my "black" ones.. to find that there "is" a .001"  taper forward of the NVA. I measured .174" near the spray-bar, and .175" at the reed-seat. It's less of a taper (by a good .004") as compared to the KB.. but a taper nonetheless. I didn't measure the intake side.. because I didn't want to remove the filter-screen.

I bought an inexpensive new digital caliper which seems to be spot-on accurate. But I still love my old dial caliper. You can take it out to 5 digits easily. See, 0.16925". And I learned from Ian's pictures to get the most accurate outside measurement by squeezing the forks together rather than forcing the wheel tight. If the material isn't too flexible.

Thank you! That is pretty helpful, but I can't replicate the taper. I am sticking with my 11/64 to 3/16 as it is already assembled. I need to find party balloons, tiny rubber bands, and a block of wood to screw it down to to test run. I made a hole punched tube for a bladder pickup.

If weather and children cooperate... Hah!

11/64 prior to the NVA is .171875 but that is too many sig figs, so lets call it .172

3/16 after the NVA to the reed seat is .1875 ... again too many sig figs, so call it .187

This gives an annular shoulder of .015 diametrically, .0075 by radius. There is also the area of the spray bar removed from the cross section. I hope it will draw fine.

I'll try to get a picture attached in a minute. Not exactly impressive looking since it is otherwise stock.

Phil

Oh, I didn't warm things prior to work. The spray bar just pops right out with the push of a small Phillips screwdriver.

Phil

Getting closer. Hopefully after dinner.



Phil

I've had trouble getting those spraybars to stay in place and not turn with the needle. JB Weld didn't last long either.

Not today. Sorry. Maybe tomorrow after swim lessons.

Phil

RknRusty wrote:I've had trouble getting those spraybars to stay in place and not turn with the needle. JB Weld didn't last long either.

I've never tried pressing-out and swapping a NVA for that very reason. I wonder if a piece (2-3" long) of Teflon-tape was threaded through the hole first.. would provide enough of a "pinch-fit" to hold the NVA in place? Cut the "tag-ends" off after seated.. remove the reed-assy and trim any material that might block the spray-hole using a #11 blade, prior to assembly to the engine? This wouldn't effect the needle-seal itself.. and eliminates the chance of getting any adhesive on places where you don't want it. There are different thicknesses/densities of PTFE.. so one could experiment to see if a thicker-grade held tighter.

Another thing to try.. would be to take your donor-NVA.. seal both ends with fuel-tubing (over the barb and the needle-threads or needle removed) tape-over the spray-hole just enough to cover it.. and apply thin CA to the exposed-areas and let dry. Sprinkling a little baking-soda on, as an accelerant might even help to provide for a friction-fit? Remove the spray-hole tape, the end-seal tubing-pieces and install.. taking note as to the spray-hole position. Maybe baking-soda would dissolve in fuel and the assy. would eventually loosen? Maybe some other powderous substance could be sprinkled on.. that would be non-destructive to engine-parts if it found its way into the combustion chamber? Talc maybe? We're talking "trace-amounts" of whatever is used.

Paint?? Maybe the most fuel-proof you can find would work? How about tinning with solder for that matter?

test run!
Cox 5x3 safety tip rubber ducky (as pictured above)
Sig Champion 25%, end of the bottle from last year.

Steady 15k RPM with quite a large needle adjustment, almost 1/4 turn with little change.

I hope this will pull a lil hacker on 35 ft lines just fine. It seems like a strong steady engine.

There is no SPI on this engine.

Sorry the video is like 20 minutes long. 19 of which are my pocket.

Phil

Hey.. congrats Phil! Are you gonna' keep the snorkel on there? How far back from the back-plate mount-surface does it extend? Maybe 1/4" to 5/16"? I like the idea of being able to choke the engine with one of those.. but it does pose an issue with mounting.. plus the horseshoe's mount-pattern is not symmetrical.

Using a pair of 3/8" square hardwood blocks; flush with the edges of the back-plate, gives you 5/16" in between. (the back-plate is 1.062" wide) This would place the engine itself the same distance forward the firewall, as running an 8cc integral tank.

http://coxengines.ca/cox-.049-engine-firewall-helicopter-mount.html

Don't get more easy than this.

Phil

pkrankow wrote:http://coxengines.ca/cox-.049-engine-firewall-helicopter-mount.html

Don't get more easy than this.

Phil

Yea.. that is a lot easier. Most buyers would assume it's a heli-only part. Bernie should edit the description and/or offer that part in "kit-form" to include the choke-tube style backplate, for other applications.

How did you R & R the NVA? Did it press back in tightly? Be careful running without the screen in place. That choke-tube facing "straight-up" is just waiting to swallow anything small enough that might fall into it.. which could foul the reed. I'd stick a cotton-swab or better yet; a foam-type ear-plug in the tube when the engine isn't actually running.

The NVA went in as tight as it came out. I use a #2 Phillips screwdriver and press the NVA from the threaded end. The tip of the screwdriver is not big enough to pass the hole. I then thread the needle back in and press the needle and NVA the rest of the way out with my fingers - not much force needed.

To reassemble I decide which scheme I am using - directly down the intake or turned just till the hole is not visible. I chose turned till not visible.

To reassemble I pass the needle through the hole and thread the NVA on a couple turns. I then turn the NVA to the desired position and pull it into the hole slightly. Then I use the #2 Phillips screwdriver from earlier and press the NVA home.

This is the 3rd engine I have used this method and the reassembly uses similar force to disassembly.

I did leave the screen off, and I did bevel the shoulders with a larger drill bit. If I had not then I could put the screen back on as the sure start screen is not melted in but just pressed in with a light press fit.

Phil

Anybody know off the top of their heads what the screw length is for the sure start and a regular horseshoe back?

I know it is 2-56 thread size. I bought some taps at one point.

I'm just being lazy and don't want to take a screw out to measure.

Phil

pkrankow wrote:Anybody know off the top of their heads what the screw length is for the sure start and a regular horseshoe back?

I know it is 2-56 thread size.  I bought some taps at one point.

I'm just being lazy and don't want to take a screw out to measure.

Phil

.500" under the head.