Hello,

Recently, I mounted a 40+ year old Fox .35 on a modified Twister that I've had hanging in the shop.  I just wanted to try it out.  I haven't used the Fox in many years.  So many years that I can't really remember the history of the thing.  I know that I used it, briefly, on a Sterling Super Ringmaster back in the 1960's. Beyond that, I'm not sure where I got it, if it was used much or at all before I got it and so on.  The engine turns freely, has good compression and starts easily.

The few flights I made with the Fox/Twister combination weren't too bad after I got the mixture about right but at the end of each flight, the underside of the wings were covered with a black, oily exhaust residue.  Does this indicate the motor not being fully broken in?  Is the black residue caused by internal engine metal (aluminum?) mixing with fuel (wear)?  Is this common for the old Fox engines?

I've seen this black oil before on other engines and have heard various theories regarding the cause but am not sure what the answer is.  Perhaps someone here will know?

I'm using Fox 29% all castor/ 5% nitro fuel for this engine. Four stroke needle setting at take off and during level flight.  Two stroke kicks in when the plane's nose points up.

Thanks,

Joe

Hi Joe - The black is very serious, as it consists of aluminum chips floating in the oil.  Wipe the oil off the wing,  remove the muffler and run it on the ground for a bit.  If the oil comes up clear, the problem was loose muffler bolts.  If not, then pull the engine and remove the backplate.  The rod can walk backward off the crankpin and scuff the backplate.  If it does this, there will be a wierd triangle shape, resembling the rotor of a Wankel engine, cut into the backplate.  The fix is to either install an anodized backplate, or take a facing cut on the old backplate to true the surface, and install a tinplate cover with JB weld.  Either method will give a hard surface for the rod to bear on, and stop the scuffing.  While its apart, rock the shaft over BDC to see if there is excessive rod wear.  If so, replace the rod.  If the play persists, look at the pin bosses in the piston to see how worn they are.  Next rock the prop tips toward and away from the head, and estimate the crankshaft play.  Any more than 1/16 is suspect, over 3/32 means the main bushing is shot, and its time for a new case.  Let us know.  Tom H.

I thought that was normal. All mine do that all the time. I then sell these gems on ebay.Want to buy some good motors cheap .   Just funning ja
EddyR