I picked up a brand new L&J Fox and it needs to be broken in so I thought I would use a stock cylinder head for the break in. When I took the L&J hemi head off, it didn't have a head gasket. Was this normal?

   Probably stuck in the head, but maybe not. The goal is to jack up the compression  - to a *moderate* compression level - from the extremely low stock configuration. But why would you take it apart and change heads to break it in?  I would very strongly recommend putting it back together, taking great care to get the head screw torque even, and then just run it as per the instructions Larry provided.

     Brett

Thanks Pat, I've never seen that before except on Enyas-no head gasket. Unlike a stock Fox head, there is no channel around the combustion chamber on an L&J hemi head to hold a gasket so it is easily apparent if there is a gasket present.

I have the engine back together with a stock head and will break it in and fly it. It feels good. Later I might put the L&J on it to see the difference. With the L&J head there is so much compression that it feels like a diesel.

Thanks Pat, I've never seen that before except on Enyas-no head gasket. Unlike a stock Fox head, there is no channel around the combustion chamber on an L&J hemi head to hold a gasket so it is easily apparent if there is a gasket present.

I have the engine back together with a stock head and will break it in and fly it. It feels good. Later I might put the L&J on it to see the difference. With the L&J head there is so much compression that it feels like a diesel.

      It's fine that way, you want that. That's probably the only modification that gains any significant power., causing it to hold a 4-stroke to much higher RPM, and increasing the break (which is mostly noise and very little power change with the stock system) .

      Brett

p. s. If you do later change heads you will have at least some running to do before it settles down again, maybe 30-40 flights or so to make sure. The problem with doing any change to a Fox is that it is so light and flimsy that it changes shape every time you change anything, like tighten the head screws. The rule for Foxes is that it will run the same way nearly forever, but any change, even a seemingly innocent one, and all bets are off.

This engine didn't have any preservative oil in it, just a little residual dried up castor. I'm well aware of the difference. And I'm not worried about wearing my cylinder heads out by switching them.

Here are Larry Foster's words on breaking in his engines.

    I formatted the text above, and made minor edits for typos, otherwise, Larry Foster's instructions:

"Most Asked Question About the Fox 35"


I have been reworking Fox 35's since 1985. I've been asked a lot of questions about the Fox 35. Here is some of the most frequent asked questions and the answers I give. Let me state that my way is not the only way to have success with a Fox 35 but what I recommend from flying this motor, hundreds of hours of testing on the bench and reworking hundreds of motors.

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 lose compression during this time.

How important is truing the mounting lugs?

Unless you have an extremely rigid 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 then Fox, due to better machine work. There are different Hemi configurations and plug placement but all of them well 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 w[ll 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 at mostly all castor. Castor well carry the heat out of the motor.

Can I run more than 10% nitro fuel?

Yes!

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. 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!

Do not run it sloppy rich. Running the motor sloppy rich can damage the rod.

Here is what I recommend.

Use a 10X4 Prop (not wood as you will most likely break it)
Fuel the motor with 2 ounces.
Set it in a fast four cycle but do not let it break into a two cycle...
Let the motor cool down completely
Repeat this 5 times.
Total fuel 12 ounces.

Using the same prop fuel the motor with 4 ounces.
Again run it in a fast four cycle but do not let it break into a two cycle.
Let the motor cool down completely
Repeat this 4 times.
Total fuel 20 ounces.

Change the propeller to a 10X6
Fuel the motor with four ounces of fuel.
Turn the needle valve out one turn.
Set it to run in a strong four cycle but do not let it break into a two cycle, also do not run it sloppy rich.
Let the motor cool down completely.
Repeat this 5 times.
Total fuel 24 ounces.

This should give you about 1.5 hours of running.
Replace the glow plug
Set the first couple of flights richer then normal until you know how it well perform in the air.

Hope this Helps!

Larry Foster