Any reason not to fit a tee dee venturi/needle assy on the back of a Black Widow? (Other than having to make a custom backplate that will take a reed in front and threaded for the venturi at the back?).

I notice that most rear-inlet engines have the carb mounted at an upward slant (eg the Queen Bee). Is there any reason for this?

What would the downside of such a conversion be? (I have a couple of BWs without fuel tank/backplate, and would like to use oversize conventional fuel tanks with them).

Buy this and screw it on:
http://coxengines.ca/product.php?pro...&cat=25&page=3

I think you are trying to reinvent the Thermal Hopper.

Thanks for the suggestion .... however I don't want to do a standard backplate, I want to see if the "trumpet" air intake of the Tee Dee and the multiple spray holes improve the performance of the engine, over the standard backplate design.

The angled design of the QB is mostly likely from a practical point of view. Easier to prime (there is no access to the exhaust with the muffler on) and easier positioning of the tank.

Again, take a look at the Space Hopper and similar designs;

Tarno used to have a motor mount for reedy Cox to accept their carb(same tread as Cox .049/.051 venturi/1/4"-32). They do work(I have two of them)well with venturi!

Uhh OK.. I want to see a 21 year old Vixen in my room.
Ain't gonna happen either

You will want the Tarno AR carb. Details can be found here:

http://coxengineforum.forumakers.com...-2-ar-details?

As Bare mentioned above they are hard to find if not almost impossible. Usually command a hefty price on ebay when they do show up.

I did manage to find and buy a Tarno AR but that 21 year old Vixen remains elusive.

Well I happen to have a Tarno A carb here (I thought I would try one BW with the Tarno and the other with the Tee Dee venturi/needle) and tonight I turned a backplate to fit on the BW, with a reed (e clip) on one end and threaded for the carb on the other end. By sheer coincidence, it looks a lot like the AR except the round flange is bigger and drilled for the 4 screws that go into the crank case, and I left my carb boss a full .500". However, I wasn't paying close attention (winging it) so I bored the hole .250" which is the major thread diameter on the Tarno carb, not the minor diameter. So I quit in disgust and had a beer. Start over tomorrow. In any case, I like the sheet metal mount and will try to develop my own version of that as well, it calls for a smaller and thinner flange. I hadn't really thought through how I was going to mount my back breathing Black Widow ... I figured I would use the larger flange somehow and a firewall with a hole in it.

Actually, you are making at Thermal Hopper, rather than a Space Hopper. Incidentally the carb fittings on the Space Hopper are the same as on the TD. I have checked. I imagine the Thermal Hopper was the same, but don't have one to check. There was a cage mount for the Thermal Hopper.

Solved the turning mistake by boring out the hole and CA'ing in a bushing with the right .250 OD 32 pitch internal thread. Inserted a steel reed and an E clip. Bolted the backplate the the BW, affixed the carb, and made a mount from a piece of brass sheet and bolted the engine to the mount (I should have copied Tarno's solution as I later found out.). Got the assembly screwed to my vise (I use wood shims to hold stuff) and hooked up fuel and electric. Well to my surprise she roared to life and blew the gas tank and the clip off! Some more Heath Robinsoning and I got it to run, but then the engine mounting bolts vibrated loose. After some crawling around, I found the little nuts and bolts on the floor and this time I CA'd the nuts. Got it to run again but the whole head/cylinder assy started rotating off the motor while running! That's where the carb came in handy, I was able to shut down and fix that. Final run was successful, see video. I need to fine tune the air bleed a bit I think. Next step is to put this on a Baby Flite Streak RC and annoy the neighbors. Love that raspy exhaust note - don't know RPM but seemed pretty decent even on 15% nitro

Custom Conversion RC Cox 049 Black Widow (1 min 43 sec)

that thing sounds good. kind of makes me wanna do the same thing lol

Another way to do it is to cut a slot in the tank and just get the needle assembly over the inside venturi stub. I have seen this done and it worked very well with a 1/8" hole in the venturi and a bladder and 80% nitro. Sometimes the fit is just right. It is 1/4". A hole must be drilled to match up with the needle assembly, or if you want to try the three holes, it could be done with a groove as on the original Tee Dee venturi if you have a lathe. The original venturi is not needed at all. You will ruin the tank for anything else, and the plastic backplate is easier to play with.