Stuka Stunt Main Forum · 9 of 9 known posts recovered
Jay
treat each day as if it were your last, because one day, it will be.
1) Drill the backplate and tank (together!) to .090 - .093"
2) Use a fresh Tee Dee cyl. & piston
3) "Roll" the mylar reed between your finger & thumb to get a concave shape, install it with the concave side facing the venturi. It should "squeek" like a clarinet reed when you blow against it after it's installed. Yeah, yeah, the experts say that the reed MUST absolutely seal perfectly but what we're after here is mixture flow at high rpm's. Try it, you'll like it.
4) Use Babe Bee head with a couple extra gaskets and 25 - 35% fuel.
5) Use 5x3 black prop
6) Lap the crank to the case (if you're using brand new parts) with Simichrome polish.
7) Flatten all mating surfaces, head included, on piece of glass with 600 wet/dry and light oil.
8) Measure the SPI with a freshly reset rod/piston and if it's not .016 - .018" (if my creaky memory serves) surface the cyl. seating area on the top of the case until it is.
9) Make a needle "sealing device" as pictured from half of a nylon control horn back plate drilled to allow needle to tightly thread into it. File 2 notches so it will fit down between the backplate nipples to keep it from turning, fit small O-ring between nylon piece and backplate. This device prevents air leaks and the needle will STAY PUT. If your engine is happy, you shouldn't messing with the needle much!
"He who dies with the most good Cox engines wins!"
Also, I use the old metal backplates with the 2 nipples plugged with JB weld and a "stunt" tank but if you don't have one of those, just make your nylon piece long enough so that the flat butts against the firewall to keep it from rotating as you adjust the needle.
"He who dies with the most good Cox engines wins!"
I like that needle retainer. Good idea.
One note is that the high Nitro and drilled venturi will both drasticly increase fuel consumption. You pays for what you gets somehow. I think the other modifications will improve power, and especially reliability with only minimal impact on fuel consumption.
One other item I might quibble about is lapping the crank to the case. The anodizing inside the case is pretty thin, and you don't want to cut through it while lapping. It is also somewhat porous, so you might retain some of that ultra fine abrasive. I prefer to polish the crank to fit by chucking a prop stud into my drillpress, screwing on the crankshaft, and using that as a "lathe" at fairly low speed. Then I take 600 grit paper soaked in oil and wrap it half way around the shaft, pinching it tight up against the shaft. The paper has sort of a keyhole configuration. This puts perfectly even tension on the paper and tends to make the shaft rounder, smoother and, of course slightly smaller.
Keep testing the clean, dry shaft and case until the shaft will spin freely several revolutions when you snap it over with your fingers. Give the shaft one more shot, but with 1500 or 2000 grit paper for a mirror polish.
Larry Renger
think S.M.A.L.L.
After re-reading the original post my response may have been "barking up the wrong tree." I just assumed the gentleman was wanting more power & speed. Most of my reedie finagling was performed on u/c sport racing Deweybirds and Skyrays. We were having a ball with 'em for a few years and then it died suddenly. It was great fun with 50 lap, 1 pit heats and 100 lap, 3 pit finals.
You're right about the increased fuel consumption, although it didn't seem to be a drastic change on a "happy" engine that was well broken in and had a great needle setting. We were also grinding all excess material away from the backplate, tank walls and venturi tube to increase the tank capacity. Every little bit helps!
Duhhh, dunno why I didn't think about polishing the crank to relieve a tight front end and leaving that precious anodized layer alone. Just plain common sense escaped me on that one! I was lightly lapping the thrust face of the crank to the case to try to reduce friction there as well.
Happy New Year everybody!
"He who dies with the most good Cox engines wins!"
Jay
treat each day as if it were your last, because one day, it will be.
If you're just looking for a solid run, use the needle mod and make sure all part mating surfaces are flat, use new gaskets and tank seal, run 15% nitro with 20% castor, screw in a nice shiny new glow head, set the needle a hair rich and go have fun! 
"He who dies with the most good Cox engines wins!"
The last version of the Black Widow had about 0.082" from the factory for both the plastic and metal backplates. However, if your engines have ever been apart near any other Cox parts, you might have a venturi as small as 0.040" which was used with the old Baby Bee engines. I have heard folks say the orginal Black Widow venturi was in the 0.060" range.
In Mouse Racing you will see engines with 0.098" venturis and using 40% - 60% nitro. But then they normally don't stunt and they only need to get 34 laps on 42 feet lines. (2:15 - 2:40 and 1 pit stop in a 50 lap heat, 5:20 - 5:40 with 2 pit stops in a 100 lap main event) (BTW, the low times are for the folks that are good at it. Something I'm not)
Dave
Bill