I'm getting a "found object" Firecat in the air.  It came with a tall, short tank, as seen in the picture.  The whole setup should be apparent, except that there's a DuBro fuel filter in the line from the tank to the engine.

The run is -- strange.

First, the nose of the plane has a resonance with the engine speed when it's 4-stroking, which foams the fuel, which leans the engine out, which lets the fuel settle down, which makes it 4-stroke, and repeat.  I'm not sure if there's much I can do about that (the prop is already in good balance).

Second, the thing really leans out as the flight progresses, to the point that I'm alternating between worry that my nice new-to-me McCoy is going to burn itself up, and thankfulness that I'm using 25% all-castor fuel.

Fuel is Sig 15% nitro, 20% castor that's had a cup of Fox castor added, to bring it to 14% nitro, 25% castor.  Prop is a Top Flite 10-6.

Even 4-stroking, the thing is giving me lap times between 4 and 4.5 seconds (I didn't measure closely).  I'd love to know a good way to slow that down a bit.

Suggestions welcome. 

I'm thinking about replacing what's there with a uniflow, that isn't so tall.  I would hope that'll give me better draw and put outboard edge of the tank closer to the needle.  I have a bit of an issue with that because I'd like to be able to do the pattern with the plane, and there's only 3 1/2" from the leading edge to the back of the engine.  This leaves me concerned that any tank that'll fit will have problems with fuel flow toward the end of the pattern. 

I'm kinda thinking about putting said uniflow tank on the inboard side, but that's not very old-timey.

I ran a lot of McCoys back in the 70's.  Never had much success with metal tanks for what Dan said about pickup.  When I ran them on pressure I had more success but the most success was with clunk tanks.  Never did like wedge tanks.

Ken

I'm thinking about replacing what's there with a uniflow, that isn't so tall.  I would hope that'll give me better draw and put outboard edge of the tank closer to the needle.  I have a bit of an issue with that because I'd like to be able to do the pattern with the plane, and there's only 3 1/2" from the leading edge to the back of the engine.  This leaves me concerned that any tank that'll fit will have problems with fuel flow toward the end of the pattern. 

       Just make the tank depth/vertical dimension whatever is needed to get the desired capacity.    I agree with the general idea.

      Check that the insert is in the venturi. It should be a black oxide-finished insert about 3/8-1/2" long , fit the inside bore of the venturi boss, and maybe a 1/32-.040 wall thickness.  The spraybar passes through it, so it's way down in there.  They are frequently removed to Get More Power. The apparent power level and extreme mixture change from beginning to end is strongly suggestive that this is not present. If it is present,  you can make it smaller, by sleeving it down with brass tubing. That will make a big change in this case, but, you have way more power than you need, and not enough suction, and that would improve both.

   15% sounds OK, and may be the only thing making it possible to needle with the excess venturi.

    BTW, uniflow will not make it worse, but it also won't improve it. The fuel pressure will be the same as it is at the end of the tank with the current setup, just for the whole flight.

I'm kinda thinking about putting said uniflow tank on the inboard side, but that's not very old-timey.

   You want it old-timey, or actually-worky?  Also not a bad idea, but it *should* work with a slightly narrower tank as it is, if the engine is right.

     I don't think there is a lot you can do about the vibration, slowing it down might move it out of resonance, but without extreme measures you are going to have a lot of trouble trying to fight the resonance by beefing it up.

     Brett

p.s. by "extreme measures" I mean stripping the front end down to the balsa and motor mounts, and replacing the doublers with 1/8 hard ply all the way back or beyond the high point of the wing, then rebuilding it. If you even attempt that - which I don't recommend - you will likely find it oil-soaked in all the critical spots, which will be nearly impossible to remove sufficiently to glue on new doublers. Then - you are cutting off the entire nose, making a new one, and grafting it on. And it might not work anyway.

   My suggested solution is obvious, but even less old-timey.

... Check that the insert is in the venturi. It should be a black oxide-finished insert about 3/8-1/2" long , fit the inside bore of the venturi boss, and maybe a 1/32-.040 wall thickness.  The spraybar passes through it, so it's way down in there. ...

What you describe, except in natural-colored aluminum, in down in there.  My, that's still a honkin' big venturi, though, isn't it?  I may neck it down further.  I'm also going to put at least one layer of nylon mesh over the intake -- less old timey that way (particularly because the mesh I have is purple), but maybe the engine will last longer.

   15% sounds OK, and may be the only thing making it possible to needle with the excess venturi.

Hmm -- I shall keep that in mind once I use up this gallon of fuel.  I chose it because it was close, and had lots of castor in it.  The nitro content was definitely a ride-along consideration

... you will likely find it oil-soaked in all the critical spots, which will be nearly impossible to remove sufficiently to glue on new doublers.  ...

That's actually not much of a concern.  The airplane in question had a 5/8" bow in each trailing edge, arranged to make it roll in when you attempted to fly level.  As near as I can tell, that airframe got very little flight time, and I'm guessing from the pattern of repairs that most landings ended with a "crunch".

I think if I'm going to make a new nose for a Fire Cat, I'll just make a new Fire Cat.  I suspect that I could put a 25LA on this one and see it fly just fine; I just want to see if I can get it going with the equipment that was on it when it came to me.

What you describe, except in natural-colored aluminum, in down in there.  My, that's still a honkin' big venturi, though, isn't it?  I may neck it down further.

   As I recall, the insert was pretty reasonable, but I don't have one to double-check. If the insert is present, that is probably not part of the problem.

     Brett