Try a new glow head and Cox Fuel.. a 6x3 Wood TF or Grey Cox or Master Airscrew should work fine, also the APC 5.7x3.. also you might have a little dirt in it.. I just flush it with fuel or rubbing alcohol and then run it. Check the glow plug and make sure it is snug.. if this fails take it apart carefully and check the reed for dirt.. this engine likes Cox fuel.. borrow some if can before buying..

From Aeromodeller Aug 1974

The Black Widow is intended for operation on
fuels of a medium nitro rating — i.e. about 15 per
cent nitromethane. Using such a fuel, our test motor
recorded a maximum torque of just over 6 oz.in. at
around 9,000 r.p.m. and a peak output of nearly .08
b.h.p. at between 15,000 and 15,550 r.p.m. These
are quite good figures for a ‘sport’ type .049 and are
reflected in some useful speeds on various props..

The most useful prop sizes with the Black Widow
should be 6 x 4 for a control-line stunt or sport and a
6 x 3 or ‘fast’ 6 x 4 for free-flight although, should
the need arise, the engine is capable of turning 7 in.
diameter props of 3 to 4 in. pitch, such as one might
wish to use for a large ‘motor-glider’ type model for
example.

Prop speeds recorded on test included 10,600
r.p.m. on a 7 x 4 Taipan nylon-glassfibrc, 11,200 on
a 7 x 31/2 Bartels epoxy-glassfibre, 12,000 on a 7 x 3
TopFlite wood, 12,600 on a 6 x 4 Tornado nylon,
13,000 on a 6 x 4 TopFlite nylon, 13,800 on a 6 x 4
McCoy nylon, 13,900 on a 6 x 3 KeilKraft nylon.
14,000 on a 6 x 3 Tornado nylon, 14,600 on a 6 x 3
TopFlite nylon and 15,500 on a 6 x 4 (nominal) D.C
nylon. The Black Widow was also checked on 51/2 x
3 and 5 x 4 Tornado nylon props but these took it up
to over 16,000 r.p.m. static which probably means
rising to 17,500-18,000 in flight and, as the b.h.p.
curve indicates, there would be no advantage in
propping the engine for more than say, 16,000 in
flight. No problems were encountered during the
tests of this new Cox model and the original Cox
glow head survived the entire series of test runs.
One final point. As supplied, the Black Widow
has the fuel pick-up tube located to suit an anticlockwise
control-line circuit. Purchasers who wish
to use the engine for free-flight - or a clockwise
circuit - are therefore advised to re-position the
pick-up accordingly.
Power/Weight Ratio (as tested): 0.55 b.h.p./lb.
Specific Output (as tested): 95 b.h.p./litre

1973 Black Widow (Cat#150 - Manufactured May 1973-Jan 1996)

Cox Black Widow 049

During the 70s a couple of Cox engineers were playing around with different colored Babe Bee and Golden Bee parts and came up with an all black engine with a red spinner. They hopped it up a bit with a dual bypass cylinder from a Medallion, a black Golden Bee tank and a slightly larger (0.062") venturi intake. The Black Widow was born. These engines were marketed as a High Powered Combat Engine. On later Black Widows the red rubber spinner was replaced by a red anodized aluminum Tee Dee style spinner. In the late 1990s some Black Widows were produced with the a dual bypass slit exhaust cylinder. The slit exhaust was to prevent fires. According to an Aeromodeller engine test done in August 1974 the Black Widow on 25% Nitro output power was 0.08 bhp (60 watts) at 15,000 rpm with a max. torque of 6 oz.in at 9,000 rpm.



Cox Glow Heads

There were 5 types of Cox Glow Heads produced for the Cox 049....

   1. #302 Pre 1955 Standard Head - Hemispherical shape - Low Compression - Smaller dia. threads - used on Space Bug, Thermal Hopper, Space Bug Jnr and Strato Bug Only.[11]
   2. #302-1 Post 1955 Standard Head - Hemispherical shape - Low Compression - Larger dia. threads - used on all engines produced post 1955 until the #325 head was produced in 1980.[12]
   3. #325 Standard Head - Hemispherical shape - Low Compression - Larger dia. threads - used on all post 1956 engines except Tee Dees, QZ, Killer Bees, Venom and Texaco. Replaced 302-1 head in 1980.
   4. #1702 Hi Compression Head - Trumpet shaped - High Compression - knurled top - 2 fins - used on Tee Dees, Killer Bees, QZ and Venom
   5. #315 Texaco Head - Hemispherical shape - Low Compression - 5 fins - used on Texaco Engine only. Allows greater cooling for engine swinging larger propellers.

This is a wonderful site.. excellent is the best word to describe the content..
http://en.wikipedia.org/wiki/Cox_model_engine


Jim

        Jeffrey, I might be able to add some helpful hints. Before you dissasemble the engine try this. Close the needle all the way down until it seats. Then with your finger over one of the vents put your syringe on the other and inflate the tank then listen. This will let you know if the o ring is leaking where it seats and also if the screws are leaking as well. I've even had it leak around the tank itself where it seats against the back plate. In addition, I'm not saying it doesn't work but I've had trouble using the teflon reeds. I came across one that didn't allow the circlip to engage in its groove properly. I don't know why I always check this by using a small pick to rotate the clip after installing which I did to make sure it was engaged. Anyway it came out on a screaming run and almost went through the side of the case. It ruined my best runner to date. Now if you have the style with the plastic reed retainer the above is not necessary. When the engine is apart prior to reinstalling, place your syringe on the back of the tank itself where the small o ring goes and withdraw your syringe creating a vacuum. This will tell if the reed is seating properly and also push the syrings to make sure air is flowing the opposite way. I've actually had reeds that looked somewhat ok but actually where distorted enough to not seal correctly. I would also suggest that you flatten the back of the crankcase using some 400 grit paper and a bit of oil on a flat plate to true it up so it seals better to the tank. I have a small piece of glass I do this on. The screws actually pull material out of the case and cause a bit of a burr. You didn't mention if your backplate was die cast or metal. I like the plastic ones but had several probles with them. I own many with cracks that formed around the screw holes which are virtually invisible until you flew the screw within the hole. These caused annoying run problems . I think it may have been due to crashing and also overtightening. These types will distort and cause leaks if overtightened. A plug thats on its way out can be a problem as well. Even though they light up and appear fine they can be causing poor runs as well. Sometimes you can straighten the element in the plug using a straight pin keeping the coil even and well centered. Using the 35% can be a bit hard on the plugs . I've dealt with this using additional head gaskets if I had an engine that was eating them up. As for pick up tube issues well I'm still trying to figure out how to deal with this problems. Ken Cook