Re: What is the difference between new cox parts?

EXModelEngines Fri Nov 09, 2012 10:04 am

iskandar taib wrote:
EXModelEngines wrote:Yes, all the stunt backplates are modified from the Babe Bee backplates that we have available. Some of the raw backplates had the smaller venturi (Babe Bee) and some larger (Black Widow).

How interesting!! Back when I got into models (early 70s) the Black Widows were advertised as having more power.. an additional "xxx RPM with a given prop" (my friends and I agued for a long time over what "a given prop" was...). I always assumed the difference was due to the twin bypass cylinder, but maybe that wasn't the whole story! I don't recall mine being more powerful than Babe Bees or Golden Bees my friends owned - there probably was a lot of variation.

Iskandar

The cylinder makes a world of difference without a doubt, but opening up that venturi on the backplate as well as the fuel tank bowl will allow the engine to perform better (and I would argue more consistent).

Usually you will see RPM figures advertised when using a 5x3 prop. That is how we advertise ours anyway; "5x3 prop & 25% nitro"

Matt

**RknRusty** Fri Nov 09, 2012 10:40 am

pkrankow wrote:Huh. That is an obvious blue ring inside that intake...Is the air passage supposed to be that much smaller than the hole in the back plate (venturi?)...

...I have also run home blended methanol/castor sans nitro. Yellow HEET fuel additive is methanol, and castor from the pharmacy. The Cox 049 engines run OK on it (not great), and my bigger engines act satisfied but mellow.

Phil

The venturi inlet is no doubt .082". As long as the round washer(frequently called an o-ring) seals between the backplate and around the venturi tip, it shouldn't matter if the backplate hole is wider. What you see in the picture should run just fine.

I use drugstore castor as an additive, but if you use it as your main component I understand the de-gummed racing castor like Klotz Benol is much better. You want some varnish in your engine but not an excessive buildup.

http://www.klotzlube.com/proddetail.asp?prod=BC-175_Pint

pkrankow Fri Nov 09, 2012 1:11 pm

I ran the home blended without nitro because I was stuck without fuel (but not for long!). I will look into Klotz degummed castor for my next gallon of fuel, probably in about a year at the rate I am using it.

Phil

pkrankow Sat Dec 15, 2012 8:52 pm

Is there a tolerance difference between the plastic backs and the metal backs?

On the stunt/black widow tank I could not get the metal back to seal, and put the plastic back on. Sealed right up and the engine runs like a demon!

Now on an extended run bee I am having a similar problem. I over drilled the venturi in the plastic, it measures .090 with my inside/outside calipers. I am unsure of how it grew this big as I used the 5/64 drill bit as recommended. I think it just snagged and peeled out a little too much all around. I can't keep the engine running.

I tried putting both my spare metal backs on and they won't seat (the black widow back too, I did the same thing before switching to the plastic back). They won't seat with no pickup and just assembling the tank to the back with my fingers. I have 2 extra metal bee backs and the metal black widow back. I tried laying thread in as a gasket and did not have results that were meaningful. Either the thread would peel out as I assembled, or I couldn't get it to stay in place. I was using petroleum jelly as a tack agent. I was using the heaviest thread in my wife's sewing box. Should I have tried using a piece of Dacron control line?

Tomorrow I get a tube of aluminum safe anaerobic sealant.

Phil

**RknRusty** Sun Dec 16, 2012 4:07 pm

Dacron line makes a pretty good seal, so I've been told. The metal backs frequently have hairline cracks that you can't see, but they do leak. If you can't get the venturi to seal against the air inlet, the only suggestion I have is to try some different thicknesses of fuel tube slices as a gasket. Also make sure your tank screws are not bottoming out in the crankcase. I've had to shorten them. Or even better, use a 2-56 bottoming tap to extend the threads deeper into the holes. The holes are always plenty deep, but not always threaded deep enough. The tanked Bees can be truly maddening.

pkrankow Sun Dec 16, 2012 6:17 pm

The rim is not sealing, the venturi seems too long on the tank.

I filled the groove in the rim with anaerobic sealant, cleaned the inside of the tank with isopropyl alcohol on a paper towel, then assembled and cleaned the outside of the tank. After about 30 minutes I attached a clean fuel line section to one vent/fill then covered the other with a finger.

With the needle closed it resisted blow, and with suck it held the fuel line to my tongue. I let it stay for about a 10 count and decided it was good. I may run it tomorrow.

Phil

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