Hi, Paul.
I was not aware that SAITO made a .61 CID spark ignition engine.
The biggest issue I have is the weight.
Checking the website, the engine and ignition system comes out to around 20 ounces.
When time permits, I plan convert one of my SAITOS to spark with a system of my own design.
I’ll keep you apprised of the progress.
Bob Z.

I did some research some years ago because if you think about ideal engine for our use, both electronic ignigion and 4-stroke are very high on the list.
I found that the lightest system would add about 100g/3,5oz, nothing catastrophic. You see, many details necessary in RC-use, like heavy cables to avoid radio interference, safely oversize battery (something like 100mAh per flight is enough) etc. are not necessary for us.
Also, with well controlled ignition both engine noise and fuel consumption go down, that means considerable weight saving, too.
But then, you’d need something similar to Igor’s system to control it, and someone like Igor to make sense to it. L

Thanks for the feedback, guys. The idea is indeed not so stupid. Cheers! The simple motorcycle ignition are based on the rpm. The higher the rpm, the earlier the ignition. Engine load can be added, but this is usually taken from the amount of throttle. Not something we can use... But being able to define the ignition timing in function of the rpms seems to help to control the engine... 11cc seems a decent engine size for our purpose...

Well, I’m finally getting a chance to put in my two cents.
I’ve been tinkering and experimenting with four-cycle engines for around thirty years.
I own many and have indeed learned a lot. *
My own personal favorites?
1)    SAITO. They make only four-cycle engines and their products are outstanding.. My present accumulation numbers well over twenty and they have performed flawlessly. I had one warranty issue with a SAITO 30 due to premature camshaft wear and Horizon Hobbies made it right immediately. Generally speaking, the SAITOs are powerful, reliable, durable, consistent and easy starting. The cams are driven by a simple double spur gear setup. My SAITO 30, my all-time favorite engine, still runs perfectly after over 1,000 flights. I performed one valve adjustment after initial run-in and never touched it again. It still has the original glow plug.
2)    ENYA. I have a few of them from .35 up to 53. Extremely powerful, low maintenance and easy starting. They have a unique feature in that they can be reverse run simple by re-timing the rear mounted spur gear cam drive.
3)    OS: The small Surpass and non-Surpass are fine performers. I’ve had terrible results with the Surpass 40. I’ve had three of them and they were all awful. The rest of the OS 4S engines, Surpass and non-Surpass seem to be OK.
 So, to the subject at hand.
Fuel is very important in these engines – trust me, I’ve tried many different formulae.
For example, zero nitro in ratios similar to Paul’s. I could never get a consistent run; needle settings were difficult. I tried zero in all of the above listed engines. None yielded satisfactory performance.
When I was working in Italy, I hooked up with some of the pilots in the Milan/Monza area. They are absolute wizards with FS engines. Discussing fuel, they told me to never use less than 10% nitro. Some used as much as 20% when available. Bob Reeves confirmed this as he regularly used 20%.
 
Next, the idea of spark ignition for FS engines. Many years ago, I tried a regular gasoline/castor/synthetic mixture in an ENYA 46 on a test stand. I installed a Champion long reach glow plug and ran it. It ran perfectly BUT, it would not continue running with the battery removed.
So, I’m converting one of my many SAITO 56 to spark. I’ve designed an extension operating off the front of the camshaft and an adapter for the points. Coincidently, the points for my 1967 ALFA Romeo are a perfect match.
For the record, I have not tried any sort of electronic ignition. I’ve been using a standard Kettering system with very good results. Points, coil, condenser, switch and three Energizer lithium cells. I have this setup on my 30+ year old Viking with never a failure. I can usually get around twenty flights on a set of cells. The plane is powered by a 1948 OK Super 60 and has well over 2,000 flights. Still the same fuel mixture: 75%/12%/13%. I have run this mixture for nearly all the flights. Originally gasoline/castor but now gasoline/castor/synthetic. I’ve had best results with AC, Crown and Blue Flame plugs.
 
More to come.

* Many four cycle engines coming up for sale soon.