Fox 25BB

I have a few of these motors most are new but I've run a few in combat ships RC flying wings. They are great in a full 2 cycle run.
However as a stunt motor they have been a source of problems. I've broken in 2 new ones and they really are not giving me anything near acceptable performance
I really can't get these to slow down, I've lowered compression, tried different plugs, different fuels Sig, Fox,and Powermaster/GMA. I've tried most of the normal props apc/zinger/graupner, even my old supply of Rev-up and top flite props both in 9's 4/5/6 and 10's 3/4/5. Motor really doesn't like 10 inch props, but I was sure that that would be the case. About the only thing that i haven't done is play with the venturi size as it really a little more work then changing a venturi insert.
I really don't fly serious stunt but i'd like to slow these down a little bit so that at least I can watch the maneuver instead of rushing through them.
Has anyone done anything with these motors or are they one that is better left in the drawer. I haven't been able to find anything on them so any info is appreciated

If the Fox responds at all like the OS 25BB's do, you can slow it down with 8-4's. I have several OS-F 25 ABC engines in use for Speed Limit Combat. With an APC 8-6, they are doing about 85-90 mph when opened up. When I tried an APC 9-4, on a wing with about 450 square inches, they wanted to scoot along at 80-85 mph. At my age, the reflexes really aren't up to speeds of 85 or more, and even with a streamer, I'd have been disqualified for being too fast.

Then when I tried APC 8-4's, the level speed dropped to 75 mph. Later on, with some Tornado 8-6's, we were getting slightly faster speeds with the black ones than with APC 8-4's, and comparable or possibly slower, speeds from the white Tornados (also some weird sound effects from those).

We could back off the needle settings with the APC 8-6's and get the speeds to hover around the desired 75 mph area, but that's not the usual way to tune a combat engine. Incidentally, what do those Foxes weigh? I wonder if they would be competitive in C-L Speed Limit flying?

Back to the subject at hand, I haven't ever seen a "9-3" prop, but there are 10-3's you can find and cut down (someone on eBay just sold 80 of the old TF props in 10 by 3 1/2 pitch with a dutch auction, and three of the Isky's & SSW regular messagers wons shares in that auction).

The K.

If it's flying too fast go to longer lines. Each added foot of line length adds approximately .1 second to your lap times. You may have to re-adjust your leadouts back a fuzz &/or add a bit of tip weight if you make a big jump in length. Don't get carried away. Sometimes two feet can make a big difference.
Nils

this is a Golden state Models Yak 9. Considerably better wood and quality then the sterling kit, also a ton lighter. unfortunately it doesn't have adjustable leadouts but there is a little more rearward sweep then the origionals had. I'm flying on 60 foot lines/.015 I've never had any reason to go over that length for a 25 size. There are no problems with motor run upright/inverted, just loves to run away. Tank is uniflow ,no pressure, but it's not a tank problem. I have these motors and am really just playing with them to see if there is a use for them other then just fast 2 cycle flying.
Something just for the heck of it. A K&B20/28 accepts the Fox 25 bb venturi assembly and runs well with it. This motor also willingly swings 10 inch props, only downside is that it is a little heavy the Fox 25 weighs 7 oz W/O muffler, that is if my scale is right

I'm not familiar with the engine, but from what you say, I'd fill the venturi with epoxy and then drill it out to a very small hole and see what happens. If it can't breathe, it can't go fast. You can open up the hole until it suits you, presuming it works. Personally, I would not try flying that airplane with a 4-2-4. It needs to truck along at a fairly constant speed to fly well.

Jim, great minds must think on similar lines. I've just come up from the shop.I've sleeved down the venturi area with brass tubing and JB Weld. In my first post i said that i did everything but reduce the choke area and i had the opportunity to check area with some of the other 25's that i have. This venturi is huge. i didn't measure but went and made an insert from 5/16 od approx 1/4 i.d tubing, the needle valve body is fat enough i think to finish the job, however, if not then a ST valve is just slightly thicker and may do the trick.
These look like bullet proof motors, well built. I still have my origional ones that were bought over 10yrs ago for rc combat. They thrived on synthetic oil, 20% nitro and 8/5 props. These motors are reasonably potent,well made, but oh so heavy when compared to the fox 25 bushing motor

On a 25FP I would put a .238 or a .242 venturi on it with a .135 (OS) needle. I don't recall what the diameter of the Fox needle is for those engines, but the ST needle would be .155 or .157, and then you would go up about 20 thousandths on the opening of the venturi. Ironically, the 25 FP shipped from the factory with either a .257 or a .261 venturi. This is too big and why some are having trouble with the engine. But if you don't change the venturi, you can change the needle. Put in an ST needle and the choke is about right.

Your .250 choke with the larger needle may be a bit small, but it should tame the engine admirable, albeit there will be a loss of power.

I, too, am working with a Fox 25 BB. I acquired it used, but in essentially new condition, several years ago and immediately sent it off to GMA for rework. (Yes, this engine,as well as a Fox 35 that I had George convert to ABC, are now amongst my most prized possessions.) So far as I know, all he did was change the venturi and substitute an OS needle valve. As best I can tell with my rather crude calipers, the throat of the venturi is 33/128 or 0.258".
In bench running for preliminary prop selection using Power Master GMA 10/29 (all castor), of the props I have so far tried it likes a Zinger 9X4 best. Just on the verge of breaking into two stroke, it turns 10,300. A Power Point 9X6 overloads it. A Magnum 8X6 three blade shows some promise (breaks to two stroke at 10,100). The 9X6 APC three blade that I tried because of its very narrow blades is far too much prop.
Yet to be tried are a couple of different 9X5's and a 10X4 Zinger cut down to 9 1/2". My guess is that one of the 9X5's that I am going to try - - Zinger, Rev-Up, and APC - - will turn out to be the initial prop of choice.
If it shows any tendency to run away in the air, then, in addition to propping down a bit I will try adding a BruLine air filter to reduce the effective size of the venturi. Did you see the suggestion that John Althoff posted sometime back about using the air filter to hold plastic disks with various sized holes to emulate different venturi sizes?
I find all of this experimentation interesting and fun, but the only meaningful results will come from putting the little beastie in the air. I will let you know the results of the further bench tests and initial flight tests.

thanks for you reply on the Fox 25. I have been using a zinger 9/5 for my last outing. it's on a Yak 9 and suprisingly it is running fairly well. It is just breaking into a 2 cycle run and leaning in the verticle. As my other post said I basically have a .250 venturi amd if it is a little small, well then so be it. The only thing is I'd like a little more tension on the vertical line.So I guess I'll have to shim the motor out I dont think that it's going to do well on 10 inch props. I have made a 9.5/5 to try though that may be the magic prop.

Can you move your lead outs forward a bit? That should help with the overhead tension.

O.S. Sorry i can't adjust the leadouts on this plane. I built it right after i came back to Controlline and it has fixed leadouts. If anything I have it about 1/8 inch farther back then plans show. The plane is not falling out but it does get lighter on lines then I personally like.

All airplanes have adjustable leadouts. You just need to cut into the tip and move them. check your CG. How does it compare to the plans, and how far back are the leadouts in comparison to the CG?