You are going to get a lot of different answers to your question. I have never heard the  answer your friend gave you. I would guess most people would say not enough oil?
Ed

Hi, Robin - I'll go along with Mark on this one and add something from my personal experience.
My first FP .40 did indeed have the runaway problem. I was told that adding head shims would help but I also considered minor head modifications.
Since I had a spare FP .40 head, I decided to experiment with reshaping the combustion chamber mainly by enlarging the existing hemispherical chamber and reducing the width of the squish band. This would, of course, lower the compression ratio considerably.
On the particular engine I had, a radius of .48 inch and a .050 wide squish band worked best. The runs were perfect, yielding a very nice 2-4 break and it never ran away. I was using Powermaster 10/22 fuel and an 11-6 Rev Up prop. I tried a number of different plugs and they all worked well. An OS ratcheting needle valve did the trick.
A friend of mine was having similar problems with two FP 40s and he asked me to modify them. At the time, it was popular to reduce the blowdown by lowering the ports (intake or boost, I do not recall) so we decided to try the head mod on one and the port mod on the other. After many test flights, we decided that although both mods worked, the head mod yielded the better results.
My personal preference is really the head mod. When cutting a plated sleeve (AAC, ABC, AAN, ABN, whatever) you are grinding though a plating interface. Doing so can allow the plating to peel away at some future time, with disastrous results. The head mod on the other hand is quite safe. If you're not pleased with the results you simply replace the head with a new one. If the sleeve mod is not satisfactory, you must replace piston and sleeve ($$$).
I have performed this mod on many engines including the K&B 40, FP 35, LA 40, Bodak 25, Tower 40 and ST 60. I did them for people at no charge and all feedback was very positive. I tried it on the LA 46 but it made no noticeable difference - the engine ran perfectly before and after.

Regarding the addition of more oil, I had a mild runaway issue with a Fox 35. Since I did not have the opportunity to modify the head at the time, I tried extra shims -they did not help. I added castor to the fuel to bring the total content to around 25%, left the head shims in and the runs were right on - a very nice 2-4 break.

As I always state, this worked for me - individual results may vary.

Bob Z.

Hi, Allan - no, not really a runaway problem as more of an bit of curiosity* to see if I could get a more classic run.  Most people are very satisfied with the B25 as is.

I had the motor on my Cardinette and I just felt like experimenting.
Turns out that I could run a higher pitch prop and the engines just got a bit milder.

Bob Z.

*Engineer: Can't leave anything alone!!!!

Randy,
thanks that is the most consice clear listing of possible causes I have seen listed.  I appreciate the facts,, and not "well I was told by,,,,," that you normally get. interestinly enough, it seems to work in line with what I have pretty much discovered out here in the desolute regions of Washington state,, maybe its universal then, lol

I've often wondered why my OS 25LA-S won't break for me. Thanks for the checklist of things to look for .... not a priority, but I'd love to hear it 4-2 break through manuevers.

FlySafe!  Larry (Larrys4227)

Randy , a very good morning to you from France.

As ever ....Much obliged for your usual common sense reply to my query.

Keep well , Robin.

[ Ex-pat Brit in the Charente FULL of ex-pat Brits ! ] 

Hi, Kim - I will gladly share that info with you.

If you have access to a small lathe, you're in business.

I just went out to my shop and modified an ST 60 head so you could see the sequence.

1st photo: Stock head. Note the wide squish band.

2nd photo: I chuck the head by the combustion chamber protrusion and take a small skim cut just to insure concentricity of the head OD to the protrusion.

3rd photo: This is the tool I made. The radius is approximately .48 inch.

4th photo: The actual cutting process - slow and steady to avoid chatter. Moderate cutting speed. Note that the lathe is running in reverse.

5th photo: Voila - the finished product. Squish band is around .050.
Total time around 10 minutes.

Hope you find this helpful.

Bob Z.
 

Hi, Kim - glad I could help.
It will be interesting to see what results you get should you try the mod on other engines besides those I've done.

If you do an engine with a "squarish" head, like an LA or modern ST,  you need a lathe with a 4-jaw chuck.

If one is not available, there is a simple way around it. Chuck the head as in the second photo and take some light cuts until you have a cylindrical section thay will center up in your 3-jaw chuck. Then proceed as I've shown.
The amount of metal removed is inconsequential and your engine will defintely have a different look.

Let us know your results.

Bob Z.

Dunno about the hemi-head etc. Sometimes even cutting the head, seems to noticeably reduce engine power. I'm experimenting with a few cut (modified) engines at the moment. Trouble is toning the FPs down seems to take edge off the power. We, in Philly, have seen unmodified (except for an extra head gasket or two) FP engines lug overweight planes spectacularly. For instance, an oil soaked Oriental we know, weighing in above 52 ounces. The modified engines don't do that. I've a thick winged overweight mediocrity my modified FPs won't pull over the top. Forget about the pattern. But a stock FP with an extra head gasket on the plane does the pattern with no problem. We run Foxes hard in Philly Flyer land. Year after year. (Another worthy controversial topic). When one of our top 10 fliers sees these moded FPs down on power, he says put a Fox 35 in it. He's not kidding. A good running Fox appears to nearly match the performance of modified FPs. Which begs the question. If you're going to tone down the beast, why run it.

Well, soon as the weather breaks (the wild winds especially) and I finish my summer ritual of traveling all over the country visiting family, I plan to continue doing empirical comparisons.  As the data (subjective experiences?) rolls in, I'll probably have more to say.

The gentleman who modified my engines is a well known engine man. He is very generous and skilled. I am very appreciative. Now it's a matter of observing the characteristics of these engines. Like I said, time and weather has not permitted me to do this fully as I want.

On the other side of the issue are club members who have used successfully unmodified FPs. They are advocates of these engine and frankly I've seen them run and run well. Usually on full bodied stunters. But some on profiles as well. Many of my profiles have also used unmodified FPs and run well, for a time. I had two engines modified tho, because of a season (2007) of frustration. It appeared to be the classic FP syndrome. It took quite a bit, but those engine/airplane combinations were eventually resolved. One was a case of soft nosed ARF causing weird vibes. Another was a clogged muffler and a pin hole leak in the tank. On the other hand my new Twister with a stiff front end has flown with a great FP40 engine run from the get go. Even on uniflo. It's a powerhouse lugging a somewhat overweight tubby (46 ounces) with authority. It's a stock engine setup with an extra head gasket etc.

I'm in neat position, actually, since folks on both sides of the discussion are available to me for observation and advice. So. I get to play all this out at the field and at least see how the stuff I build and run responds. In fact, Bobby Zs original hemied engines are around in our club and in use.

I also have a Smith FP40, and it's the best running engine I've ever had (tho I've only run it on profiles, so I don't know about any possible burp upright/inverted, that others talk about).  It is happy 4-4, 4-2, 2-2 at a wide range of RPM and props.  One interesting thing that happened early on is I had it in a profile with the bottom of the case pressing against the inboard doubler, which was not cut out for the engine.  The run was not as smooth as it had been on another plane.  A modelling buddy with sharp eyes noticed a hairline crack in the paint on the inboard side of the doubler where the case was pressing against the opposite side.  I cut a small oval hole in the doubler to eliminate any contact with the case, and voila!, instant run improvement.   

I recall hearing that Ted Fancher had engine run problems (not an FP40) in one of his Classic planes that were eliminated by trimming some wood to allow a gap between case and nose block. 

Recently I took the engine apart out of curiousity to identify the mods, using a mic to compare with a stock engine, and I'll be darned if I could find anything.  The head is stock, and everything else as far as I can see.  Maybe there is some sort of magic voodoo ritual involved!   

Kim

Back in prehistory when I was flying Fox 35's a lot, I would start noticing runaway at about the start of the square 8's.  The top end of the engine would be brown and rubbery with castor crud.  Clean this off and runaway went away.  So I think runaway has to do with heat build up, at least in some instances.