<span style="font-size: medium">I finally started my new O.S. FS40 four-stroke yesterday after almost 10 years in the box. After priming it started right away, but as I leaned it out at full throttleit started toknock. I backed the throttle down and it went away. At full throttle and a rich needle valve setting, there was no knock. Anyone have any thoughts why it's knocking? Is it a new engine breakin issue? I don't see how any kind of knocking can be good. Thanks!!
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If it was a true mechanical knock in the old sense (loose connecting rod), the sound would not go away with throttle manipulation. I suspect that you are hearing the onset of what old timers used to call "spark knock". By manipulating the throttle and mixture controls, you are altering the ignition timing on a glow engine. The four-stroke glow engine is quiet enough for you to actually let you hear the spark knock, even though there isn't any spark.


Ed Cregger

You are making it too lean - That's why you threw a prop

In their queste for ever more power, the compression ratio went up and up, until engines started to throw props because of pre-ignition of the charge. If you want a sweet running engine, either use a colder plug, add some 3% acetone to the fuel,  or reduce compression ratio. Adding some water to the fuel also helps. This added water however spoils the brew quickly, so use up that fuel the same day!

<span style="font-size: small">Thanks for the quick response!!!
Ok, I'll back off the needle valve a bit. I was concerned because O.S. FS91 doesn't do it. Ya gotta love the sound of a four-stroke.

In regards to the prop being thrown -The higher RPMs from a lean mixture will cause the prop nut to loosen? I'll be sure the tighten it as much as possible.

Thanks again!!</span>

I've never tried using aceton in the fuel myself, what is it supposed to do?

<span style="font-size: small">What do you mean by "colder" plug? Also, what is the ratio of acetone to a gallon of fuel?</span>

No, the lean mixture will cause the mixture to detonate before the piston reaches the top (that's the knocking sound you're hearing). When you get too lean, the exploding mixture will stop the piston before it gets to the top and push it back down (a backfire). The sudden reversal of the engine is what is throwing the prop - it will also ruin the engine if it happens too much

Only use a OS "F" type plug in a 4-stroke (or other brand of 4-stroke plug)

This has always been the problem with the "old non Surpass" engine TOO HIGH A COMPRESSION RATIO (CR). Compound this with the fact that when it was in production the recommended four cycle fuels had about half the oil content (12%) of the fuels recommended today (18% TO 20%). Drop down to a fuel that uses about 12% oil by volume. This will help effectively lower the CR as oil is not a comprisable fluid unlike fuel vapor (in the gaseous state). The true solution that I found to work very well was the replacement of the original mild cam to a Surpass cam. The increased dwell angle (overlap) effectively lowers the compression ratio at low RPM ( high loads) yet allow for the superior breathing at high RPM levels. This is a win win situation as far as performance goes for this engine you get both better low end and high end performance. This cam is a direct replacement.

This is not just my fix but one I first read about in Clarence Lee's engine clinic in the now defunked magazine RC Modeler.

Friends don't let friends fly nickel,

Konrad

What fuel are you using in the engine. If it is knocking from detonation, it sounds like your fuel has too much nitromethane in it.
i would try some 5% and see how it goes.

I am still running a old SAITO 80 with high compression and it liked 5% nitro glow fuel just fine. It has never had any of that special 4 cycle fuel in it. Just regular airplane fuel with a blend of synthetic and castor oil.

My old OS FS91 Surpass liks 5% just fine too.

I'm waiting also to find out what the acetone in the fuel does...

Uh Ohhhh.... I'm using 15%. Sounds like that may be contributing to the problem.

A colder plug will help pull heat away from the combustion chamber. Excessive heat, load, and pressure are what cause motors (any) to knock.

No matter how tight you go on the prop nut, and/or jam nut....it's not enough to stop a prop from being at least loosed...and likely thrown. It is not cool when this happens....so make sure you and your buds are behind the plane the whole time until you get the needle figured out! I've had them fling near 20 yards away before...lotta energy.

It might not be a bad idea to grab a tach...get to peak, then back off a few hundred RPMs. Remember to make small adjustments, and wait a few seconds in between adjustments...let the motor respond

Acetone in fuel stabilizes combustion. It also helps to evaporate the fuel, and prevents prop throwing by increasing the CR the engine can be run at.
@ Rduhb,
ratio is 3%, that is three parts of acetone in 97 parts of fuel.
Me, I'm European. Don't ask me about ounces and pints, or gallons.

I remember on my old FS91 was when I had taken out the glow plug and later just put it back in finger tight. A long time Later I used the engine on an airplane. and on its maiden flight take off flight the glow plug came loose and that sucker threw the prop and spinner. I was lucky and made a landing Ok, but it was exciting though. I found the prop and spinner, but the two nuts had disappeared. so i had to get new ones of course. I tightened the glow plug down like it was supposed to have been done, and the engine worked fine from then on.

I remember using a FS40 engine back around 1990 in a airplane and I only ran 5% nitro fuel. It ran just fine for me. I never tried running a higher nitro content fuel at the time I owned the engine.

It is running correctly

Try to tune it so it doesn't knock

Engine knock comes from preignition, the fuel explodes before the end of the compression cycle. It is caused by high compression and a fuel that burns too fast. We get confused but high octane auto fuel eliminates knock by burning slower. And you may be getting some backfire to throw props. Get a lock nut from the hardware store for safety.

You can also easily get knock if mixture is too lean. To high a ratio of oxygen to fuel vapor causes fast ignition. I suspect this is part of your problem since problem goes away when you richen up.

Here are some thoughts.

Run it rich, especially if it is new and in break in. The knock may go away when you finish break in.

Check the valve setting.

Colder plug may slow down ignition.

Add some castor to fuel. Burns slower.

Use 5% nitro. Burns slower and should be all you need, especially for a young engine. Don't worry about nitro to sweeten up your idle. A new engine will idle a bit fast until it breaks in.

Personally, I am concerned about adding things like acetone or water. Castor is much safer.

My thoughts anyway.

Tom

I don't know that you can use a colder plug on a 4-stroke. They need the extra heat to get past the second stroke

<font size="4">Thanks everyone for the comments.

I started it again this evening. Ran it rich without any knocking. I'll get some 5% this weekend. I've got it installed in a"Pietenpol". It's a Balsa USA kit that I bought from a guy that had it for 25+ years. SHOULD fly great with this engine.

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Rich is good, lean is bad. Ihave noticed the same thing from my Mag .52 four stroke if Ilean the needle too far.

Sure you can the K&amp;B 4C is an excelent colder four cycle plug that works great on high compression engines, or high nitro.

This engine will run fine with no knocking with regular 10% or 15% nitro fuel and the OS "F" plug that came in the engine if you keep the mixture a little rich

Heat range and the persistance of heat are two completely different things.

The persistance of heat is a function of thermal inertia and requires more mass in the glow plug element than we are accustomed to with standard glow plugs. One can have a glow plug that stays hot longer, but which may or may not be as hot during normal running.


Ed Cregger

how does a compression ignition engine ....knock?


I do understand the concept of knock, its just that in the absence of a spark to control the ignition point, in every power cycle there will always be knock.
Are you saying that its excessive?