🚀 go-pugleaf

Randy Buckland (rcb@ncsu.edu) wrote:
: I am currently having engine problems with a repaired Fox 40. The engine
: was crashed in such a way that it destroyed the carburator and
: sucked a lot of grit into it. I have replaced the carburator and cleaned
: the engine out and installed it in a new plane. The problem is that I can't
: get it to run for more than about 2 seconds. If I squirt fuel into the air
: intake, the engine will start but does not appear to draw any fuel from the
: line and just stops after a couple seconds. The lines are clear since with
  
  If you want to really test your pressurized tank, fill it with fuel, pull
  the lines off the carb and muffler -- blow into the pressure tube (be sure
  to point the carb line away from your face!).  You should get a fair stream
  of fuel peeing out.  If you have to blow hard enough to puff out your cheeks,
  you have a kink in the lines, usually the clunk line or feed line.  If you
  only get air out,  you've probably reversed the pressure and fuel lines (a 
  .50 probability on my models).  If you get bubbles, you may have punched a 
  hole in the feed line.  These holes will only occur at the most inaccessible
  places in the fuel system...

  A better test is to pinch the feed line shut, blow up the tank, pinch the 
  pressure line shut, wait a few seconds and open the feed line.  The fuel
  system should be able to hold the pressure and squirt fuel out;  it's pretty
  easy to blow up a plastic tank.  From what you describe, you probably have
  a gross fuel feed problem (new installation, no fuel draw).

: 
: 1. The model is built in such a fashion that the tank is fairly low relative
: to the engine. The engine is side mounted with the fuel intake on top and
: the intake is level with the top of the tank. Also, the plane is a tail
: dragger and that places the tank lower than the engine when on the ground.
: I can't get the tank any higher or the engine any lower, do I need a pump?
: 
   
  If the engine's worked fine before, you probably are getting sufficient
  pressure off of the muffler.  It is possible to find points on a muffler
  or pipe that have little/no/reverse pressure, but you indicate the engine
  used to work.  You should be able to at least get the thing cranked!

  Tank placement is too low if the engine gets too rich when you fly inverted.
  Generally, a pressurized feed system cures most of your ills.  It galls
  me to see people go out and pay as much money for a Perry carb and pump as
  they would for a decent engine, turn around and slap 'em on an engine that
  was a dog to begin with.  (I can't offer you any reason why you would want
  to do that!) 

: 2. The piston and liner got scratched in the crash by the grit and when I
: squirt WD40 into the cylinder and turn the prop, I get a little escaping 
: gas as the piston approaches the top. As I loosing so much compression that
: the crankcase suction that is supposed to draw fuel in is failing? How can
: I test this and what is the solution?
 
  It's harder to totally trash a ringed engine with grit, anyone flying in
  Florida becomes an expert in grit!  An ABC will eat itself in a heartbeat
  with a mouthful of sand.  ALWAYS, ALWAYS, ALWAYS, tear down any engine 
  that has screwed itself into the ground!  Pull the piston, sleeve and crank
  (mark the sleeve to the housing with an alignment scratch to the notch in 
  the sleeve.  If there isn't a notch, mark the side of the sleeve too, not
  the top (you'll screw up the head seal).  Clean the main bearings, housing, 
  internal bits.  Check for interesting scratches in the sleeve, wear on the
  rod bearings, main bearing rust, and look for any bits of metal in the
  crankcase (although these usually get sucked into the cylinder, where they 
  spend the rest of their time having real fun...).  If you have a replacement
  ring and the one you're running is kind of old, replace the ring.  If you
  don't have a new ring, resist the temptation to take the old one out, it
  may be brittle from a lean run and you'll break it.  Besides, the odds of
  putting it back in upside down follow the same 50-50 rule as tank feed
  problems.

  When re-assembling, everything goes back together in exactly the same position
  that it was originally in.  The rod and piston can be inadvertently reversed,
  so you may have to mark the back of the rod, note the position of the ring
  notch, etc.  If you aren't sure you got everything back together in the right
  orientation, treat it like a new engine and break it in on a test stand. You 
  will probably shorten the rod bearing life by putting the rod in backwards,
  the ring will have to take a new set -- something an old ring may not want
  to do.  Take care installing the sleeve; watch out for any metal burrs you
  may dislodge and drop into the crankcase housing.  I usually lightly
  lubricate the outside of the sleeve, deluding myself into thinking that it
  will be easier to get out, the next time.  As you put the engine back 
  together, rotate the crank while you listen/feel for grit.  If you think
  you've found some, pull everything apart and dribble another can of WD-40
  through the bearings.

: 
: 3. I have set the needle valves according to the Fox literature but am
: uncertain if they are right. Can someone with a properly adjusted
: Fox .40 BB engine measure the amount of exposed thread on both needles
: so I can check my setting? also, does the end of you high speed needle
: have a bit of metal sticking off of one end or is the end of the needle
: flat?
: 

  They're all different, counting threads is not reliable.  Setting up
  according to Duke's instructions is probably the only way to make a Fox
  engine run properly.  I've never seen a needle valve with a piece of
  errant metal on it, although you will find them with a flat spot on the
  tip.  Needle valves are cheap, when in doubt get a new one (especially
  after an air to ground density transition).

: 4. Is there any parts on the crankshaft directly below the carburator
: that help the fuel draw that might have been damaged in the crash?
: If so, how do you remove the bearings and crankshaft to check this?
: When I cleaned the engine, I could not find a decent way to remove
: these parts without major force being applied.

   You may need a small wheel puller to get the collar off the front of
   the crankshaft.  Pull the rear plate off and have a look.  You have to
   pull the sleeve to get the rod clear of the crank pin, then pull the
   front collar off to get the crank out the back of the engine.  Sleeve
   removal can be fun in a heavily castored engine.  You may have to heat
   it in your oven (wear mitts) in order to loosen it.  A succession of 
   #11 Exacto blades, small screwdrivers, bigger screw drivers, can be used
   to pry the sleeve from its top lip.  I'll also use a piece of hardwood
   between the sleeve intake and exhaust ports on top of the piston, using
   the piston to push the sleeve out (be very careful of using too much force,
   you may bend a rod or wrist pin!).  Duke may know his engines, but I 
   don't use castor anymore!.  I won't bother beating a main bearing out of 
   an engine unless I have it's replacement.  They can usually be cleaned
   in place.
   
: 
: Any other ideas? or is it time to buy a new engine?
: --

   If you don't feel comfortable tearing down and rebuilding an engine, the
   chief advantage to a Fox engine is their excellent service.  Send it back
   and have it overhauled.  (This won't fix your fuel feed problem, but you'll
   know the engine is good for another couple of hundred flights).

: Randy Buckland                               "It's hard to work
: North Carolina State University               in a group when you're
: randy_buckland@ncsu.edu (919) 515-5491        omnipotent"       -- Q

----------------------------
Bill Chidester    KE4WS
bill@c3177208.ssr.hp.com