Does the head match your other engines?  It may have acquired a low-performance head.

Is it leaking, or is it just low compression?  Figure out where the leak is, if so.

If it's leaking at the piston/cylinder seal, then it's hurting bad.  If you have some spare pistons you may want to try finding one that fits better.  Otherwise running it in is probably the only way to fix a poor piston/cylinder seal (and I wouldn't give you very good odds).  Of course, if it's leaking around the head/cylinder seal, or around the glow button seal, then you've got a more easily correctable problem.

Hello Tim,
  Have had the cylinder off and it appears to be the correct one,but I can't see a number stamped on it. It has two bypass ports but they are parallel at the top, I believe the venom (?) has a stepped top to the ports. The exhaust port is the large single slot rather than the later dual slit type. The barrel and piston are in excellent condition with no sign of any scoring, even the normal pattern that you would see after light running. The copper plated top of the piston is shiny bright. There is a head gasket in place and looks good, the head and barrel were tight. So it all looks good, rightdown to the sub piston induction
  The only problem that is apparent is a slack ball joint. I have had this problem with the later Estes assembled product engines that were being sold for $4.95 a year or two back, but never with a genuine Cox assembled engine. I realise that a slack ball joint points to the engine having been run, but in all honesty this does not appear to be the case here.
  My hearing isn't that good, but I can maybe hear a slight leak when turning over by hand, but I can't be absolutely sure of this (old age has its problems!).
  It all looks as though this is a "real" Black Widow and not one of the later Estes so called Black Widows. Maybe I just have a dud engine, I suppose it did happen with Cox, but fortunately this is the only case I have come across. I will check with some soap solution to see if the head glow seal is OK and also around the head gasket. If this doesn't show anything up then it must be a poor selection of piston and cylinder. I will try some spare pistons that I have around somewhere. Other than that I will try running it in and see if it does get better, but like you, I have my doubts!

Thanks,

Andrew.

   Yes you could get a new piston and cylinder from Cox International for $10 , or you try and run the
original  P/C and see if they improve compression.
 

              Andrew, what I have to offer is truly based on my personal experiences. I like yourself would be concerned with what your experiencing. I have purchased some engines from the late eighties that had the same very symptom your running into. The copper head gaskets can be very problematic at times. I've had brand new engines with three head gaskets in the cylinder as well. Be sure to check that none are stuck together. It surely wouldn't hurt to lap the head on a piece of glass to true it up.   The Black Widow didn't come with the high comp 1702 head so adding this head would surely boost it. My best running Black Widows though had nearly no compression at least that's how it felt but they were truly screamers. I had a 1980's Baby Bee which has the familiar Sure Start configuration which had nearly no compression. I ran it rich regardless for several flights and it still runs well to this day. If your engine is starting and producing rpm's I would just keep running it. I would also check to see if your reed valve is closing off properly and not leaking down. You can remove the tank and place your syringe onto the venturi nipple and check for positive and negative pressure. If it's leaking at all when you draw the syringe back then this could make the engine feel weak in compression. I've also used the Davis Diesel devarnishing brush. This works very well. You place the brush from the bottom up while twirling the cylinder in a up and down fashion. They claim it puts micro scratches in the cylinder.  I use it consistently on my TD's. Davis Diesel states it allows oil to fill in these scratches and boosts power considerably. I guess its similar to a ball hone the way it puts cross hatch marks on a cylinder. Regardless of what they claim, I've used this brush with success on all my Cox engines and it only costs a few dollars. Cox used to state in their directions to use 000 steel wool . Ken