Fox .25 BB problem
 

It won't stop when the carburator is fully closed. It keeps on ticking over with a great slow idle, but I can't stop it with the throttle cut on my radio.

Here's what I've checked:

1. Yes, the throttle barrell goes fully closed ... actually beyond fully closed. I've adjusted the stop screw by backing it all the way out, closing the throttle as far as it'll go by hand, then lightly screwing in the stop screw until it just touches. And yes, the throttle arm and servo are set so as to bring the barrel right up against the stop. I can even load the servo with my sub trim.

2. All carb gaskets are there and in good shape. All screws are tight.

3. The needles are tuned for a good WOT, just into 2 cycle, and a good idle with great transition.

I know there must be an air leak somewhere, but where???

Thanks,

Phil

RE: Fox .25 BB problem
 

The back plate housing could be lose and/or the gasket is bad? Could be leaking from the front of the crank?

Enjoy,

Jim

RE: Fox .25 BB problem
 

[link=http://www.flitelinesolutions.com/engines.html]http://www.flitelinesolutions.com/engines.html[/link]

No, in fact it does have a back plate and a one piece front crankcase. There's a picture at the above link.

RE: Fox .25 BB problem
 

Pull the fuel line. They stop every time when you do that.

If it's a real issue to you, remove the carb, clean the mounting area carefully to remove oil residue, and then
put a FINE BEAD of RTV on the seat. Let it skin over for a few minutes then reassemble. You should be fine.

This is a really nice little motor...don't booger it up.

'Race

RE: Fox .25 BB problem
 

I value my fingers a little too much to be doing that.:D

Actually, I can stop the motor by grabbing the spinner with my fingers, but that seems a little dangerous, too.

I'll try that, although it does have a gasket there already, and I've made sure it's clean and the carb is tight. Perhaps it does still leak a little. We'll see ...

Phil

RE: Fox .25 BB problem
 

I'll check/clean/RTV the back plate while I'm at it, although I would think that would cause the engine to run poorly in general.


Of this, I am aware. Runs great, good power and awesome RPM, although it does vibrate quite a bit.

Thanks, all,

Phil

RE: Fox .25 BB problem
 

Phil...be sure and let us know the results of your work on the motor. Having a motor that refuses to quit is a rather desirable problem to have <G>

Quite often, this little quirk can be caused by a minute air leak in the carburetor area. Not enough to disrupt motor operation at higher speeds, but just enough to keep the little bugger ticking over at low idle when you really wish it wouldn't.

No danger in pulling the fuel line. I've been doing it for 30 years, and still have all my fingers, scar free no less! Keep in mind that the prop is mounted forward of the carb, and just don't stick anything you value in that area. Simple enough.

The .25 DOES vibrate a bit more than other motors, but that's due to the iron piston. The upside is that the lapped iron piston/steel sleeve will, with proper CASTOR lube, outlast other motors by a considerable margin. It's not uncommon for a lapped piston Fox to run 10-15-20 seasons and still be going strong. Good value in the extreme.

Keep us posted...

Regards,

Race. (Fox Lover extraordinaire)

RE: Fox .25 BB problem
 

Did you look for a slot in the crankcase to the front bearing? Look on fliteline web site at this page. It sure sounds like your problem.

http://www.flitelinesolutions.com/ez.html

RE: Fox .25 BB problem
 

Phlip:

Not sure the link reference to Flightlines that Badger 5964 is what you want. That reference is for the EZ (air bleed) carbs AND .40 and larger engines. Your .25 has a variant of the MK-X carb.

Try this after the RTV attempt.

The MK-X carbs have coarse threads on the needles and sometimes they tend to be a small source of an air leak. Pull both the high and low end needles Screw then IN first to LIGHTLY touch the bottom and count the turns. Wrap each needle with ONE layer of "Plumbers Tape" (TEFLON). Reinsert the needles and screw then all the way in to LIGHTLY touch the bottem then turn them back out the exact number of turns from before.

NOTE: You want the tape to SLIGHTLY extend past the treads at the needle end as this will help keep the tape from balling up when you reinsert the needles.

Good Luck.

Dan

RE: Fox .25 BB problem
 

A status report:

Sunday, I removed the carb and did a few things. I put the fuel line around the needles as is suggested for fuel leaks on Flitelinesolutions.com, and I also removed screws (cam screw and barrel stop screw) and put a touch of sealant in the threads. I removed the two screws that hold the high speed needle ass'y to the side of the carb and sealed that, tightening the screws, as well.

I put a little sealant on both top and bottom of the gasket that goes between the carb and crankcase, and pressed the carb down as tightly as possible while tightening the set screw on the side (I put a little sealant on those threads, too).

Monday, I fired it up and fussed with the needles to get the performance back to what it had been before my disassembly. I also swapped my throttle linkage to the inner hole on the carb arm and used my travel adjustments on the transmitter to reconfirm that I'm getting full WOT and completely closed with the throttle cut switch. I set my closed throttle position on the transmitter to one click of sub trim above load on the servo.

Just a few minutes ago, Tuesday, I ran it again. I set my idle trim to 10 clicks up from closed throttle. FWIW, prop is TF PP maple 9-4. WOT tachs at 13,500 RPM and when I pull the stick to idle, it comes to about 5,000, then slowly drops over the next 30 seconds to 4, 000 and eventually evens out around 3,500. Anytime it will rev right back up without hesitation or bogging.

If I hit the throttle cut and hold it, or, accomplishing the same thing, pull the throttle trim back to zero, the idle drops a bit more, to around 3,000 RPM. I can kill it easily by putting my finger over the exhaust outlet (I'm using a 1/2" ID silicone extrension, the large Dubro one to direct the gunk away from the bottom of the plane, so no risk of burns).

What I'm thinking is that the throttle barrel is too loose (too much clearance built in by the machining tolerances) in the carb. I think the idea of an air leak at the front bearing and crankshaft (which was also suggested by the guys at flitelinesolutions) begs the question, "Where is the fuel coming from?"

I dont think I'll be able to fix this problem without replacing the carb. Right now I'd be happy if I could get it to idle at 3,500 RPM immediately after pulling the throttle down, rather than having to wait for it to slow down when it feels like it. I think I also have another problem mentioned on Flitelinesolutions.com, that is that the throttle cam screw is worn just a bit and the throttle barrel will float side-to-side, varying the idle mixture willy-nilly. But, their idea of inserting a piece of a ballpoint pen spring to hold the throttle biased to one side is problematic with my engine because the spring keeps the throttle barrel from fully closing, which would kick my idle up to about 8,000 RPM!

Anyway, sorry for the long post, and thanks for your suggestions. I'm still open to try a few things before I give up. I may just fly it like it is, though.

Phil

Edit: PS: Fuel is Omega 10%, castor/synthetic blend. Ran it again after writing the above, and three clicks richer on low end needle let the speed come down to idle a little quicker, still with a good transition.

RE: Fox .25 BB problem
 

Phil..

I believe I might have a brand new Fox .25 carb tucked away...let me see if I can find it.

PM me if you're interested.

'Race

RE: Fox .25 BB problem
 

I must regretfully confirm that advice. Never suffered any harm from pulling fuel lines or other activities behind the prop. Sadly I have not had such happy results from fiddling with spinners or anything else in front of the prop.

jess

RE: Fox .25 BB problem
 

The fuel is coming from the same needles and jets as when the throttle is open. On many carbs the barrel is slightly open on the bottom when closed on top, also there is sometimes a notch on the bottom. This is especially true with an air bleed carb. I do not know it you have an air bleed or twin needle carb. Also you are not getting near the the 15,000 RPM you should be getting form a 9-4 prop. I don't know how long your exhaust extension is, but have you tried it without it? It could be that excessive backpressure is causing poor performance and too much fuel pressure.

RE: Fox .25 BB problem
 

Yes. Same thing happens.

It's a twin needle carb. Maybe I've not leaned it out enough on the top end to get the 15K RPM that it should, but I'm trying to be careful and let this engine last as long time. I'm not racing, in fact I've got the engine on a PT-19.


Phil

RE: Fox .25 BB problem
 

Yahoo!!! :D:D:D Fixed it!

Carb arrive in today's mail. Got it installed tonight, and fired it up. Aside from being hard to start because the spray bar was clogged (I clearded it out with a fuel bulb and a little pressure) we're talking WOW, what a difference. Talk about throttle response. It got too dark to tach it, but it revs, comes down to an even lower idle, and ... it stops when the throttle is closed. I couldn't be happier.

Picture shows three obvious differences as well as different needle valves:

Edit, oops, forgot to upload the pic.

RE: Fox .25 BB problem
 

Great News!!!!

I'm happy everything went well!


'Race