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Running a Fox 35 Stunt

Running a Fox 35 Stunt

The Fox 35 Stunt engine has been in production longer that any other currently available model airplane engine. To my knowledge, it is the only engine still being produced with the cross scavanged, baffle piston design. Modern engines are more powerful, vibrate less, and run on feul that cleans up easier than the high castor oil content fuel required by the Stunt 35. So why run one? Well, to quote the instruction sheet that came with my %0th Anniversary model, ....

The Fox 35 Stunt harkens back to an earlier time... a simpler time, when it was OK to burn leaves but not OK to burn the flag, politics were more civil, you didn't have to show ID to buy a can of spray paint, and kids settled their differences with fists, not guns. Properly broken in and run, the Stunt 35 can be a good engine for many airplanes, and it can also be modified to correct some of the shortcomings of the design. If you have a Stunt 35, I highly recommend that you send it to Larry Foster to have it reworked. He is the one who provided the information on this page. You can also buy a brand new Stunt 35 reworked by Larry from RSM Distribution.

What is the easiest way to improve a Fox 35?

Until Randy Smith came out with the Hi-Zoot crankshaft there wasn't an easy answer to this. Now it's simple, install the Hi-Zoot crankshaft. The biggest improvement gained in reworking a new or good Fox 35 was to reshape the crankshaft to reduce vibration and add counter weight. With Randy Smith's crankshaft this is already done for you. To date I have never found a Stock Fox crankshaft that was straight and ground true. Randy's crankshaft is straight, true and has counter weight added in the proper place. By adding counter weight the motor has more torque which is what we want in a traditional stunt run. By making the bearing surface area true it saves the front bushing. The effective balance of the crankshaft reduces vibration, any reduction in vibration increases power.

How important is the piston and liner seal?

It is the single most important thing for a good running Fox 35. Always check the piston seal on a hot motor. Run it for at least two minutes. Check it right after shutting it off. Pull the prop slowly through compression. It should not loose compression during this time.

How important is truing the mounting lugs?

Unless you have an extremely ridged motor mount it shouldn't affect the running of the motor. If you don't assemble your motor mount with the motor bolted to the mounts then it can cause stress on your airplane. I true the mounts to save the plane not the motor.

Is the Hemi-Back plate worth the money?

Yes! The Hemi Back plate was designed by Marvin Denny. Its CNC machined and is anodized. It gives a better surface for the rod to ride against. Another benefit is it reduces the crankcase area and increases fuel draw.

Is a Hemi-Head worth the money?

Yes! Marvin Denny designed the one for Fox. Although it's not exactly as Marvin designed it, it still helps. Several people have done a better job on producing the Hemi-head than Fox, due to better machine work. There are different Hemi configurations and plug placement but all of them will enhance performance. Some better than others.

What fuel should I run?

I recommend at least 10% nitro and no less than 28% oil. I recommend also running 2 to 5% synthetic to keep the motor clean. DO NOT run synthetic in a motor that has run a lot of all castor in it. I also recommend adding 1 ounce of AERO-1 fuel additive. This will give added protection against a lean run and add life to the motor. The Fox 35 is very thinned skinned and requires a high amount of oil to cool it. This is the reason that you should run it mostly all castor. Castor will carry the heat out of the motor.

Can I run more than 10% nitro fuel?

I personally run 15% in my motors. The Fox 35 runs cooler and produces more power with the higher nitro. The draw back is more stress on the crankshaft. Don't run 15% if your using a prop extension.

Should I run a prop extension?

Should you? NO
Can you? Yes
Remember that the Fox Crank shaft is weak. You take a chance of breaking the crank shaft anytime you run the motor much more if you use an extension.

What Plug Should I Run?

Any plug in a high heat range, such as a Thunder Bolt R/C. I run the Glow Devil 300. The Fox Miracle plug is a hot plug, I don't like the quality.

What kind of muffler should I run?

As little as possible. I do not recommend the stock muffler due to high back pressure. I recommend the RSM tongue muffler.

How do I Break-in a stock Fox 35?

Slowly. Don't run it sloppy rich. Running the motor sloppy rich can damage the rod. Here is what I recommend.

This should give you about 1.5 hours of running. Replace the glow plug and set the first couple of flights richer than normal until you know how it well perform in the air.
Hope this Helps:
Larry Foster

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