Similar mods to the LA46 were suggested by an Australian stunt expert. The objective was to improve the 4-2-4 performance. I have done the recommended mods and it does work surprisingly well.
Doubtless this will be condemned by those who say that engines such as this should be left stock and run in a wet two stroke. However there are lots of people who fly for fun and many old timers prefer the 4-2-4 break to the more modern way of running an engine. Each to his own as far as I am concerned.
Do NOT even think about attempting such modifications unless you are a competent engineer. Many engines are ruined by people who don't know what they are doing! If you are competent, then this modification is good if you prefer the older style 4-2-4. The article is contained in one of the old British AMI magazines and also in one of the Australian journals.

Andrew.

I once, many years ago, did this same modification to a Yamaha 350 that had an abrupt power band.  It tamed it considerably and it was much more pleasant to putter around on but I never thought of doing it to a model engine.  Interesting idea. 

The piston has been modified by creating sloping ramps at the two transfer port locations..............

From memory Derek Pickard did a write up in ACLN issue 70 and stated that the above modification made the engine 4/2 better.
Ah, found it .....

"Stunt engine specialists have been modifying these
engines since they have been on the market and fliers
report good results. The modding work is made easier by
the lack of a third transfer port so the specialists
invariably do no more than raising the tops of the two
transfer ports and adding a couple of head shims. The
motor will then run well on 10% nitro and give even better
4-2-4 control. And all this can be done for a very
attractive total price."

"If you can use a toothbrush, you can modify this motor
for stunt. The work involves little more than taking it
apart, using simple tools carefully and cleaning before reassembly.
Take off the head and have a good long look at the way
the ports are arranged in relation to the piston. Gently
mark the piston top for the widths of the two transfer
ports. Pull everything to pieces and have a look at the
way it goes together for subsequent reassembly. Then
thoroughly clean all the parts to avoid nasty bits sticking
onto them and subsequently cause grinding damage.
What we’re out to do is raise the effective transfer port
timing. It needs only a small amount. The normal way of
doing this is to file/grind the top internal surfaces of the
two ports being careful to maintain the roof angle and
ensure both ports heights are matched. if you feel
confident to do this then do so without holding the
cylinder in a vice or forcing the piston temporarily back in
position (to check height levels) without prior cleaning.
The amount of metal to be removed is very little. Just
20thou (half a mm) lifts the timing from 116 degrees to
124 degrees and 30 thou takes it to a useful 127 degrees,
which is about where we need it to be. Don’t get too
ambitious here and the very maximum is 40 thou (one
mm), which lifts the transfer timing to just over 130
degrees."

"For those who dislike such work or can’t hold the liner
with confidence, an easy alternative is to ‘notch’ the
piston top where it relates to the two transfer port
windows. The marks previously put on the piston crown
are where the top can be notched down to open the ports
slightly earlier. This is done with the piston being handheld
to enable a small fine file where the notches are
carefully filed into place with a shallow angle. But due to
the way the top of the piston sides form the vital working
seal against the liner; nothing too deep can be risked.
Don’t extend the notches down more than 20 or at the
very most 25 thou and be careful that both are the same
depth. This should raise the transfer timing to around 126
degrees and help the 4-2-4 stunt run."

That is indeed the man, Derek Picard! The mods were done to an LA46 NOT an LA40, so the results may not be transferable? The comment about losing power from Tim, is not so with the LA46. It has about the same grunt as the conventional set up with an APC 12.25 x 3.25.
For years I ran my LA46s bone stock as per Brett Buck's advice on not messing with engines. I came across the Derek Picard article again last year. As I regularly repair old spark engines and basket cases, I decided to give it a try, as I had a fair number of spare P/ls. The work did not take long and I was very surprised at the results, using Derek's suggested set up. I gave it a try and was most surprised at the results. A great 4-2-4 break and lots of power to spare. In fact I could tell no difference in power to the conventional LA 46 set up. No, this wasn't the head gasket patrol. I used two as I do for normal LA46 operation.
If you prefer the older 4-2-4 break to the more modern high revs, low pitch approach, then this is definitely to be considered. I did try an LA 46 without the mods and used Derek's set up  and no way was it as good as the moddified LA46. It was more prone to winding up in a breeze, but then most older 4-2-4 suffer this problem in spades.
I would not agree with the article as printed as it was saying that any one could do the mods. Having seen the ham fisted results of people that have not a clue, doing mods. I cannot agree that anyone should attempt this. Those with the necessary skill will finish up with an excellent 4-2-4 engine.

Regards,

Andrew.

Thanks guys! I will give it a go in the Smoothie next weekend (it now has otherwise identical stock LA40) and report back. If it has the right kind of stunt power it may find a home in the 52 Nobler that is about to maiden. Will start with APC 11-4 and see what it thinks. It is still a modern engine and probably will be closer to a wet 2 than 1960-type 4-2-4.

Here in NorCal we are doing our best to imitate your English summer weather, cool foggy overcast with a bit of rain and heavy mist, at least along the coast. So, my experiments should be transferable to your locale.

There is little doubt the LA40 is not as out-of-the-box great as the LA46, but its not bad, either. I have two that I run with APC 11-4 in a not quite wet 2, with lap times around 5.4 on 60-foot eye to eye lines (Smoothie and now a Half Fast III combat model from the 50s). This does not provide a 4-2-4 "stunt run" but is more like the modern higher-rev flat pitch "stunt run." I am a believer in running modern engines as they were designed to be run.

That said, it would be fun to have a 4-2-4 run from a modern engine. If the wind is not too much on Sunday I will give it a go and write up the results.

Tapering the outside of the cylinder smooths the flow into the compustion chamber.  If the lower edge of the intake port is left as is, the timing will not be changed, although more fuel/air will enter the chamber.  If the lower edge of the intake port is moved down, the intake period will be increased.  George Moir wrote about it in the March, 1955 issue of MAN when he described the changes he made to convert a Fox .29 into a winning B-TR engine.  A picture of the modified cylinder is shown in the accompanying picture.

Randy, I agree -- I was just illustrating an extreme instance of removing the sharp edge (tapering the intake or bypass) from the lower outside edge of the port.  Breaking or smoothing the outside edge of a port should improve the flow into the cylinder.  If the upper and lower edges of the inside of the cylinder remain unchanged, the timing will be unchanged.

Mike:

First, be sure to enthuse about the performance the next time you're at a contest with Brett Buck.  Perhaps at arms length or more.  Please send me a movie.

   I am not against modifications or changes - if you tried it stock first, and then attempted to correct the issues you found. Ask Randy - David, Ted, me, and maybe Paul Walker were/are on the permanent poo-poo list for going off the reservation from Randy's excellent stock system setup for both the PA40 and PA61.  

  What frustrates me is when guys take good engines out of the box, then either send them to some supposed expert to get "fixed", or take a known good system and then make significant changes because they heard it might be better, or because they want to brag to their buddies about their "Joe Blow 56".  Then complain that they don't work. Usually this uses ideas from the late 70's/early 80s (drop the liner, raise the intake, add 400 head gaskets and use the biggest prop you can find) that didn't work very well even then when there was little alternative. The ST60 wasn't a particularly good stunt engine, but it was a lot better than a 45FSR that got hacked up by every guy with a Dremel tool. We even saw an example of *reverse blowdown" in an otherwise perfectly fine ST51 fully documented on SSW.

      A good engine run used to be a very rare thing, and I just hate to see people making the same old mistakes we made back in the supposed "good old days" when far better approaches are easily available and well-documented.

    I actually have no idea whether the 40LA modifications described here are necessary or beneficial. I also don't know what these mods were intended to fix.

    Brett

Both the  LA 40  and 46 can easily be setup to run a 4/2, although it is a higher RPM 4/2 than a classic fox or  ST run
The 40 will run a nice 4/2 on an 11 x 4.5  prop, the 46  will do this nicely on a 46
Both need to have the venturie sized properly in conjunction to the needle valve assembly, this has been the biggest cause of bad runs I have seen with OS FP and LA engines. and others . Most of these is because of way too large venturies or too thin NVAs
They also respond better to the NVA setup in the front of the engine, and some will need 1   one   extra head shim
Retime is also and option, but you really need to know exactly what your doing, I have seen way too many engines killed by bad custom work on the sleeve and piston.
This is best left to experts and is not necessary to get a nice stunt run

Randy