Hello All,
I am sure this has been done to death, but I would like to summarise the knowledge base. I have acquired 3 FP40s that are well used, but still have excellent compression. These are R/C models and they have all been fed on lots of castor. There are two ABN and one iron/steel engine (Ouch for the ABN models!). I have spent some time cleaning the castor crud both internally and externally and they still look good and still have good compression. So I intend to run them as follows.

1/ In profiles with RIGID front ends and some anti foaming agents in the fuel. Haynes type R/C tanks. Uniflow with or without pressure (what works!)

2/ 5% nitro and maybe 25% oil (a mixture of synthetic and castor).

3/ A 3 to 4 inch pitch prop and diameter to suit the model.

4/ Free flow tube muffler (because I don't like tongue mufflers. They are too noisy for me).

5/ Launch rpm to get a suitable airspeed, but probably 10,000 rpm plus, to get the engines closer to their design rpm.

6/ Line length to suit lap time.

7 Venturi ........well the FP20 venturi with OS needle valve and the FP40 venturi with ST needle valve have about the same free area. The FP40 with OS needle valve gives much more free area.

8/ Possibly one more head shim or maybe two depending on how they run.

9/ I intend to use them in a fast 2 cycle mode, 4-2-4 operation seems to be asking for trouble, which I can well do without!

 There seems some conflicting advice about venturis, some say a large area  is required , presumably to increase fuel / oil flow and hence promote cooling. The other school says small free area venturi to restrict fuel flow and hence throttle the engine back. So here we have a conflict, both ideas have virtue, but can they both be correct?
The dreaded FP40 runaway is generally ascribed to thermal runaway and this seems reasonable, hence the profile idea with everything hanging in the breeze, to promote cooling! Maybe even use high temp paint to increase the radiative transfer of heat (every little helps!)
One thing that puzzles me about the very real runaway problem is the following. I have seen plenty of R/C FP40s in action. If you do some violent maneuver on say 2/3rds throttle, then I have never seen one runaway, must be a lesson there, although I can't figure it out, unless you use the R/C carb for Stunt with it wired 2/3rds open!!! Don't laugh, no one has been able to tell me why it doesn't run away, in conditions that are very close to C/L maneuvers.
The engines only cost me a few bucks and I can always use them on the dark side if they don't work for C/L. Any comments would be welcome before I go down this well trodden route. Some people seem to have cracked it. The Philly fliers use stock engines and the larger area carb yet others have done it with the FP 20 carb! Comments Please?

Regards,

Andrew.

Andrew, I have bought many FP 20/25 venturis from Tower and, despite what they say on the web site, the ID is 6.5mm. The one they sell for the LA 46 measures 7.1mm.

I tried the FP 20/25 venturi in one FP 40 but the engine struggled in a fairly porky model. Several people in our club had success with the small venturi that used to come with the FP 35 and FP 40. This is 6.75mm or 17/64" ID. I tried the 17/64" venturi in the FP 40—was happy with the engine run but it still struggled in the model so I replace the engine with an LA 46 with the large, stock venturi—happy days.

In all cases, I use the OS NVA—3.5mm OD. The stock ST NVA is 4mm OD and the crankcase and venturi will need drilling out on your FPs if you take that route.

I suggest that you get enough FP 20/25 venturis to convert the engines plus one. Enlarge one to 17/64" ID and try it along with the stock FP 20/25 venturi.

The stock OS FP head gaskets are 0.016" thick. I am very wary of adding extras because of the impact on overall timing. There are 0.010" thick head gaskets available from several sources and I would mix and match the thicknesses if extra gaskets prove necessary. One 0.016" is stock, two .010" is a small step, one 0.010" and one factory 0.016" is a further small step before you get to 0.032" with two stock gaskets. I had a hemi-head on the FP 40 in the example above.

The attached article offers a few other comments that may prove useful.

Hello Geoff and Chris,
Geoff thanks for the info on the ID of the OS venturis. The one for the 46 LAS was in the past listed as the one for the FP40. I do realise that the bores of the OS venturis seem to vary quite a lot. I am not bothered about redrilling the FP40 case to take the ST NVAs, but thanks for reminding me!
The adding of some head shims is a deliberate ploy to alter the timing of the engine. A simpler way than recutting ports or moving the linerup or down. I thought the standard OS hesad gasket was 15 thou and the ones that len supplies are 10 thou. I have both and will use them to best advantage by making the smallest possible change in gasket thickness to cure the runaway.
Chris, thanks for Len's Stuka Stunt Works write up of the FP40. I read it a number of years ago and took some heed of what was said there.
I would have thought by now that there would be a consensus on how to run the FP40 for stunt, it shouldn't be rocket science. I know that Len goes on about varying deck height on these engines, but again that is something that can be altered.
I like a constant two for my stunt runs, so that shoulddn't be difficult to achieve, even if you depitch some props to 2 inch pitch and let the engine run as it wants to! I have had a conversation with a gent here in the UK that gets perfect 4-2-4 runs from an FP40! He swears that Flair yellow spot fuel (5% nitro)  coupled with a Taylor idle bar plug is all you need and he runs a 6 inch pitch prop! Maybe this is another one off that works for his particular engine?

Regards,

Andrew

           I recently took out my FP.40 powered full bodies P-40 stunter. I only have a 4 oz. tank installed and the plane won't make the pattern. Seeing everyone posting about changing venturi sizes I decided to give it a go. I discovered that the LA.25 venturi installed in the FP.40 gave me probably the worst running engine I ever experienced. The plane had absolutely no power, the needle practically did nothing for the run as there was little or no change audibly unless you really backed it out. The venturi size measured .256". The venturi I removed was .270". Now some state that this combo works for the LA.46. I can tell you it sure didn't work here. The .270 venturi is back in and I'm going for a larger tank. My prop was a 11 x 4.5 Thunder Tigre. Randy Smith tongue muffler and wedge tank no uniflow. The plane runs absolutely beautiful with the large venturi. There is obviously a point where too small is too small and I certainly found it. I've noticed this with my adjustable venturi setups on my speed limit combat equipment. Closing down the venturi may get you the economy, but you will have no power.Ken

                     Andrew, the findings I had were somewhat unexpected. Our Fp.40 specialist Dennis, is currently at the Nats. One of the other members had mentioned that I made too drastic of a change in downsizing. I should've started with .010" increment and tried from there. The majority of the Philly Fliers are using the .270" venturi. My engine has one head gasket installed and I never experienced the runaway problems. However, I do have another that I just can't satisfy no matter what I try. That engine is mounted on a original Tudor profile. Running it extremely rich it usually doesn't runaway but the fuel is just hosing in at just a tad over 5 oz's to do the pattern. I will look closer at the venturi on that one. It certainly may be possible I've mixed some up with my speed limit engine venturi's. The majority is using Thunderbolt long r/c plugs. I use Enya #3's and I see Dennis using Enya #3's as well at times. Dennis almost exclusively uses the APC 10.5x 4.5 on his Fp and Tower.40 engines. He also uses Powermaster fuels in the 5% nitro 11/22 oil.