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 :-(