I have a question about an FP .40.   The problem is, when applying down elevator  the engine is hesitating rich.  Of course this happens at the worst time. Like when entering the outside portion of the vertical 8.  So  when this hesitation of power occurs, line tension drops sharply, and I cannot complete the maneuver.   I am sure there are a lot of details that need to be brought out, but I am hoping that I just need to continue to move my tanks position.   It is flying slightly faster inverted than upright.    Any tips tricks for helping with this would be appreciated.

Use a unifllow tank, if it isn't already.  Check if it runs richer upside down.  If it does, raise the tank in steps.  The fuel volume is not at the same level as the internal volume, or the bypasses may be too big.
Depending on the size of the plane you might try a smaller venturi, as little as .25in diameter, and 10/4 prop.  The FP is designed to run at higher rpms.  Overloading it to try an get a Fox type 4-2 run can make it very fussy. The APLP10545  10.5x4.5 often times works better on an  FP40.  A taller venturi might help.  The OS venturis are barely tall enough.  The motor may be spitting fuel in level flight and then sucking it all in on outside maneuvers.

How about bubbles in the fuel line?

    The engine should richen up when the nose points down, but not to any detrimental extent. Give us all the info on line length, take off RPM, plug you are using. Like Phil mentions, on a light stock twister, you may not have the engine RPM up to where you need it. A good hot plug will help. Your tank may be too high, also. If you have an APC 10-4, try that. It will still make a nice run for you.
  Good luck and have fun,
   Dan McEntee

The Olde Tornado / Grish 11 x 4 works pretty well , if youve got one .

Say Static you point the nose up 30 degrees or 45 degrees , Do you get the same rev shift the opposite angle inverted , or the same to horizontal .

Running a smaller venturi gives it less of a "break" (even if it's in a wet 2 it'll still break a bit, and you still want that).  So don't go overboard -- but too much venturi will also do Bad Things.

Running one layer of pantyhose material over the venturi will not only filter out any bugs and dirt that might want to get into the engine, it'll also catch any errant fuel droplets that are trying to spit out and get lost.  If your venturi is a bit too big, running more than one layer of mesh over the inlet will let you tune the effective venturi size -- I do this on all my engines (intentionally starting with a slightly oversized venturi) to get the run dialed in to where I want it.

Let me add some details.    Fuel Powermaster 15% with  22%  synth castor,  APC 11-4 prop,  Uniflow 4 0z.

I only just now noticed the bit about the prop.  Down here in the lowlands that prop would, if anything, be considered a bit small.  Up there, I would hesitate to go much larger (up to a point, on those engines smaller prop = more power).

In addition to all else that's been suggested, try putting a bit of fuel tubing from your uniflow vent to the engine backplate.  This will shield your uniflow input from pressure changes with wind an maneuvering -- if that's your problem, it'll go away.

The engine pic does not show the line from the uniflow to the muffler, but I used it when I flew.  Plug is an OS A6

My twister was powered by an FP 40, I used an 11x4 APC. I used 10 22 fuel half synth and half castor. I used either muffler pressure, OR uniflow open to the air BUT and it matters, I put the uniflow pickup on the inboard side of the airplane. I never got it to work well with the uniflow vent on the outboard side, NEVER.

I prefered muffler pressure and I seem to recall that I was using the smaller venturi with a Supertigre needle assembly. I recall launching at about 9800 to 10,000 rpm but that varies, you are looking for a wet two stroke. trying to run less rpm is problematic.
there are lots of threads on here about the FP,, might pay to do some research

A6 is a cold plug try an A3.


MM

Because you're at high altitude, I'd suggest an email to Gordan Delaney in SLC. I know he's used the .40FP (for which the Pathfinder was designed). He'd be a better source of info than us flatlanders.  Steve

A6 is a cold plug try an A3.

MM

It may not be A6 but No.6 that was called A3. No6=A3=hot plug.
Cold one is No.10 that was called A5.

It is an A3, so it is hot.   My props arrived today.  Should get some flights tomorrow.  I like the looks of that 10.5x4.5.  Has a different shape than the typical sport prop.  I think it is classified as a free flight prop.

I added 3 degrees offset to my 40fp Twister. I worried that it was too much, but after a few months of trimming and flying, and one successful contest, I like it. It always comes right back from a weak wingover or wind-blown corner if I bang it too hard. This is the tightest plane I've ever flown all the way through the pattern. I use a 10.5 x 4.5 APC. The best balance point for me is 1/8" in front of the spar edge.

Right now it weighs 41.7 ounces, and I might add a heavier muffler. A little more weight is better in the wind and it has plenty of wing area to carry it. The 40 is a good combination. I started with a heavier FP 35, but it was just on the edge of not quite enough horsepower, and the 40 upgrade really gave it more authority. Also, my leadouts are aft of where Walter's plans were drawn. It hangs very slightly nose down from the neutral leadouts.
Rusty