Cox problem solved -- but why?
 

I mentioned some time ago that I was having some damned frustrating problems with an old Cox Black Widow.

It would run okay for a few seconds after starting but would then begin to labour and the revs would start to die off -- as if it were too lean. The problem was so bad that it would only run for about a quarter tank before dying -- even if it were started running at a blubberingly rich four-stroke.

I stripped it down and blew out the entire fuel system till it was spotless and tried again -- no difference.

Fitted a new glowplug -- no difference.

Reseated the ball-socket -- no difference.

Replaced the fuel pickup tubing -- no difference

Replaced the little gasket on the intake venturi -- no difference

Tried adding some head gaskets -- no difference

Changed fuel -- trying MSSR and CoolPower oils at 20% and 25% with nitro from 0% to 35% -- no change.

Richening up the engine when it started to sag would simply result in it breaking into a four-stroke without gaining any revs.

Without richening it, the engine would run for about a quarter of a tank -- alternately bogging down then picking up revs again for a while.

I eventually gave up and figured that the poor old thing was just plain worn out (it has had a pretty long and hard life).

However, this weekend I really wanted to get a little foamy into the air so I broke out the BW again and it was still misbehaving.

In blinding flash of "what if?" -- I decided to try the castor-based fuel I mixed up a few weeks back to see whether the Castrol R30 oil would mix with methanol.

Guess what -- that little engine runs like a dream now -- even with 0% nitro and 25% oil.

What gives here?

I know most people advise that the Cox engines should be run on castor-based fuel but the difference is absolutely unbelievable!

Perhaps this Cox really is pretty worn and the castor simply provides a better piston/liner seal than the synthetic oil does so that as it warms up less compression is lost?

Maybe it's that the synthetic was simply not providing enough lubrication or cooling and that meant the BW was overheating despite the fact that 25% oil was being used.

I tend to favour the former actually -- because I know this engine used to run very well with synthetic oil but that it just got gradually worse and worse over time.

So there's a new entry in the flight log: if all else fails, try castor-based fuel :-)

Of course now I have the problem of ugly orange-brown oily stains appearing everywhere :-(

RE: Cox problem solved -- but why?
 

It's hard to say without seeing it run but it sounds like your crankshaft is binding in the case. Either too tight mechanically or really gummed up with oxidized castor residue. Take the crank out of the case and clean/inspect everything carefully.

If that ain't it, like you said, it may just be plum wo' out!

RE: Cox problem solved -- but why?
 

This is something that has puzzled me before but how do you do that?

I was always afraid of busting the thrust plate off the front of the crank.

RE: Cox problem solved -- but why?
 

No, the crank is quite free -- perhaps a little too free :-)

I'm pretty sure I can detected some lateral play at the prop driver end :-(

And like Renegade, I was wondering how to remove the prop driver from the crankshaft -- is it simply an inteference fit or is it threaded?

RE: Cox problem solved -- but why?
 

Just pressed onto the splined end of the crank. Just put in a screw (after taking the tank and piston/cylinder off) and place the back of the case on a wood block, and rap the crank out with a small mallet.

If you need a new case, let me know I can send you one.

AJC

RE: Cox problem solved -- but why?
 

Renegade- yup, I know the feelng. it's pretty easy to do, but I had to be told by these guys here as well. I wrestled with one for a few nights before I broke down and asked for help.

For Reedys-

-take off the fuel tank/reed retainer so you can see right into the back of the crank housing.
-I set it on the anvil of my bench vice with the back side down (so the drive plate is facing up)
-get an old prop screw and thread it in about 1/2 way and gently tap the head of the prop screw with a hammer. The crank with slowly start to push out of the drive plate until it finially just drops out the back end.
-take off the prop screw and you can pull the crank shaft out of the back of the case.

For the TD's its the same, just take off the back plate and follow the same steps.


to get the shaft back in-

-slide the crankshaft into the case
-line up the indents on the drive plate with those on the shaft
-Push on the drive plate and put the assembly drive plate down on the anvil
-take a dowel (wood or metal) and tap the back of the crankshaft with a hammer until the drive plate is snug.
-I usually make sure everything is tight by putting on a prop and tightening it up all the way, this pulls the shaft up tight in the drive plate.

Hope this helps,

as always,
t

RE: Cox problem solved -- but why?
 

Woo!

Thanks guys, just the info I needed!

RE: Cox problem solved -- but why?
 

I have !always! found that the back of (used!) cox reed crankcases are never flat- so i layout a sheet of p600 wetand dry on glass and gently lap them flat. It never ceases to amaze me how much material needs to come off. it is also a good idea to countersink the screw holes very slightly too.
An air leak between the backplate and crankcase could cause running just as you described.

J.M

RE: Cox problem solved -- but why?
 

HI XJET! Maybe the extra oil is helping to seal off a crankcase pressure leak out the front end?

RE: Cox problem solved -- but why?
 

You might well be right -- it does leak a bit of fuel/oil from behind the prop-driver.

RE: Cox problem solved -- but why?
 

One other thing to check is varnish on the cylinder walls. If the inside of the bore is medium brown or darker, use 0000 steel wool or the bore brush sold by Davis Diesel to clean it back to bright metal.

The varnish actually gets sticky at running temperature and causes all sorts of bad running and overheating.

RE: Cox problem solved -- but why?
 

XJet,

Have you tried switching back to synthetic mix to see if the BW returns to its old habits? Just a thought-and it would settle the debate once and for all :D

Derek

RE: Cox problem solved -- but why?
 

No, it's clean as a whistle inside with no sign of castor-oil gumming -- which stands to reason since this is the first time I've run it on castor-based fuel :-)

RE: Cox problem solved -- but why?
 

Good point -- I'll try that next time I'm at the strip.