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Feb 25, 2020, 09:22 PM
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Pierre_de Loop's Avatar
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Fox 40 BBRC 24098


I have a Fox 40 and some questions about it. In the picture you will see a drive hub that looks like it is intended to drive a gear. The engine came with a spinner that is notched like the drive hub. I can imagine using the spinner, but not with the drive hub pictured. Im guessing the hub is for use without a spinner, but it would be helpful if someone could confirm this.

Also pictured is the only muffler I have for this engine. I gather from reading that a lot of folks used the Fox Quiet Muffler rather than the one provided in the box. Im tempted to try to locate an original muffler that came with the engine, but Id welcome opinions on it.

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Feb 26, 2020, 05:41 AM
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I TOBOR's Avatar
That would have been called the "40 deluxe" model having the large diameter prop driver as an option for a spinner
Feb 26, 2020, 06:06 AM
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1QwkSport2.5r's Avatar
The OPs engine is the bigger B-cased version with a lapped or ringed piston. The carburetor on the OPs engine is the MkX version, though that engine was made for a long time and came with one of three different carb designs.
Feb 26, 2020, 09:30 AM
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Remove the drive hub and run the spinner backplate. You can't use both at the same time. The spinner backplate fits perfectly and is real close to the engine. It is made by Fox and came with the BB deluxe.

That muffler is not stock. The stock Fox muffler came in either "tilt up" or "tilt down" configuration. I prefer tilt down mostly because when you fuel up, the excess that runs through the pressure tap won't run down into the cylinder. You might get a muffler from MECOA or a swap meet, or online. Obviously, you won't get one from Fox.

The .40 is a great motor. However I have always had problems with the crankcase backplate leaking air and causing the motor to run lean and erratically. Easy enough to fix. Just pull the cover and gasket, clean everything real good, spread a super super tiny thin layer of high temp gasket sealer, then button it all up and don't touch or flip the prop for 24 hours (you will still need to use the gasket). I get real consistent runs with mine. It will also idle really really low.

Feb 26, 2020, 04:57 PM
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Pierre_de Loop's Avatar
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Thank you all for the replies. I like the idea of hunting for a tilt down muffler.

I soaked this engine in Evapo-rust , then fuel , before adding a little Marvel Mystery Oil and turning it over. I'll try running it when I get the chance and be aware that the back plate may need some attention. Thanks for that tip!

Indeed, I found a paper in another box with an exploded view of the MkX corroborator.

Any advice on setting the needle valves?

Once more, thanks.
Feb 27, 2020, 07:46 AM
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Just set the high speed needle rich and dial it in as you get more familiar with the engine. Keep in mind that Fox needles are VERY coarse and 1 click can mean the difference between rich or lean.

If you set the high speed needle and it runs fine on the ground then goes way lean in the air, that's your cue that the backplate is leaking air. It is also a good idea to put a small piece of fuel tubing on both needle valves to help keep them from spitting fuel around the needle.

Fox engines are diamonds in the rough.

Feb 28, 2020, 08:32 AM
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earlwb's Avatar
Well back in the day, they used to use a belt like strap for starting engines, especially in boats or cars or helicopters, etc. But some guys used it for airplane engines too. You pulled the strap back and forth thus turning the engine over. The teeth on the prop hub was to facilitate that method without much slippage happening. Then electric starter motors started becoming more popular so then the strap or belt turned into a loop so you could use the pulley slot in the starter motor hub for it instead. The teeth on the prop hub are not there for using as a gear. An aluminum gear would wear out in a few minutes of run time if you tried it as a gear.

But you can use the spinner backplate or the prop hub, your choice. Either way works. It depends on the model and how you need to setup the engine for it.
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Feb 28, 2020, 05:24 PM
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Pierre_de Loop's Avatar
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This is a real help to me.
Thanks a million.


I'd forgotten all about rip starts, but recall seeing them on cars...thanks for that explanation!
Feb 29, 2020, 04:23 PM
Registered User
Tilt up is normally used for inverted or side mounted engines.

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