mk13

Yes you have right, with oil in gasoline the bubbles are significantly smaller (but there is lot of small bubbles).

I use a synthetic oil for 2 stroke from my supermarket (I can't remember the brand).

Yes I have for the moment the open curve from Guillaume. I will try with the solenoid electrically disconnected to set the needle a full throttle and after set the solenoid curve.

I will check the exhaust pressure line tomorrow if I find some leak, and will do the test of disconnect exhaust line from the muffler at full throttle.

​​​​​​Thanks a lot for all your way of investigation!
I hope I will find the root cause in the next days 🤞

1967brutus

Good luck testing, I am pretty sure we;ll find a solution, and this is exactly the reason why we should do this in the open discussion, not via non-public private messaging.

Let the learning continue!

1967brutus

Something completely unrelated to your problem, just sprung to mind: You are basically, of all people involved, running the smallest engine yet in this entire saga.
I have converted an OS FP10, and when doing so, I pulled the head and noticed that these small OS engines have a rather thin cylinder head. For the FP 10 this resulted in a sparkplug that protruded way too deep into the combustion chamber.
Please check if this is the case with your .25 engines, and if so, add an extra washer under the plug. The threaded base of the plug should be, as a minimum, flush with the ceiling of the combustion chamber, or even slightly recessed, since the electrodes will be pretty close to the piston top otherwise.

It has nothing to do with the bubbles, but a protruding spark plug will negatively affect the scavenging process of these tiny 2-strokes.

Jim.Thompson

That makes sense, I should have guessed. Thanks.

mk13

I checked the position of the spark plug in the combustion chamber and it's ok the thread of the sparkplug is flush.

You say you have converted OS FP10, so this engine is smaller than my 25RC or my 25LA ? I don't understand something?

I tried today to set the needle with the valve off at full throttle, and after set the valve on each point of the curve, it's a little better but still bubbles.
So, remove the valve and try to run the engine directly from the tank, and it's not too bad.
So, this afternoon I went to my club to do a flight test, the plane fly not bad en the engine run good, but each time I have engine cut after some minutes and I must do emergency landing (without crash 😅&#129310
I saw some bubbles (always that!) come from the needle. I don't know if I have a leak on the needle seal (I already have a piece of hose on the needle to improve the airtightness) or it's due to emulsion.

The research continu 😅

Bert, what coil do you use on your strobe light to check the timing ignition? (Wire diameter? How many loop?

​​​​

mk13

An other question, how you receive the tachometer signal from rcexl ignition on your Taranis? Do you use a interface module?
Thanks in advance! ☺️

1967brutus

Hmmm if you had bubbles even with the solenoid completely out of the loop... THAT is weird. The only reason I can think of, is some form of heat transfer.

I don't know the exact coil size. I do know it is not very critical, I tested three different Henry-values and they all three worked equally well.
It's a tiny thing (I believe they call it a "choke coil") maybe 1 cm long and 5 mm diameter or less.

Yes, indeed, I did run an OS FP10 on gasoline (1,7 cc, so yes, pretty much smaller than yours), but I never ran it with the solenoid because despite having all the electronics and stuff, that project got halted. I still want to finish it.

1967brutus

https://www.rcgroups.com/forums/showpost.php?p=39141981&postcount=18
Read post #18
The coil in post #1 of that thread is what I also used for the stroboscope.

mk13

Thanks Bert for your reply.

I did some new test yesterday, first on the 25LA engine. I separated the needle from the back cover of the engine to reduce vibration on the needle, it's a bit better, but always after some minutes of running the engine cut from bubbles come from the carb, may be an heat issue?
I don't know how to insulate the carb without disconnect it from the engine?
The LA series with the needle separate from the carb seem to be not the best design.

​​​​​​After I did some new to test on the 25RC without solenoid and a good sealing on the needle and the carb, the engine work well with no cut, just some RPM variation when the plane is vertical. So I did a fly test, but with the gasoline conversion it seems a little underpowered.

I did also a test with gasoline mix only with castor to check if it could change the bubbles problem but it's worse than the 2 stroke synthetic oil, so I come back with 10% of fully synthetic.

So, this morning I buy a new used engine, an OS 40SF, I saw in your YouTube channel than you have already test one (without solenoid I think) and it seems work well, I hope it will be the same for me.

For my strobe light, I put a little coil and it work but I must be very close to the ignition module output (some millimeters), in your video your are at 1 or 2cm. So I will test with an other coil bigger, may be it could change something.
​​​​

1967brutus

Indeed, bubbles appearing on the carb side are a heat issue, and to be honest, in 2strokes I have never seen that before because usually the carb is cooled by th propwash and the evaporating fuel. Only in engines in pusher config it is an issue. Disconnecting CAN be done by making a POM or PTFE distance piece on the lathe, but that is not easy.
Then again, it must be remembered, your engines both are bushed engines, which means there is friction, and thus heat generation exactly where the carb is mounted.
My tests with the OS FP10 were very limited (maybe total 15 minutes of runtime when the project got halted) so I cannot really say much about that.
All my other 2-strokes have ballbearings, meaning the front of the engine remains a lot cooler.

The RC indeed is quite a bit less powerfull than the FP (baffle piston scavenging vs. Schuerle) and that can quite well make a difference.
It should however, be sufficient to fly a light plane like a Robbe Charter for general sports flying.
I would expect it to spin an APC 9 x 4 at 10K or slightly below.
MIND YOU: at this stage (low powered engines) optimizing propeller choice can make a huge difference,and here it helps to understand that in general, a gasoline conversion shifts the "focus" of an engine a little bit towards lower RPM on heavier props.
As an example: My ASP FS30 fourstroke is fairly comparable to your RC when it comes to power. It is mounted on a Robbe Charter of 2,1 kilo (aillerons, no dihedral).
I tested with 9 x 4, 9 x 5 and 9 x 6 props. According to the power calculations, 9 x 4 resulted in the highest output (measured in Watt) but 9 x 5 gave a better flight performance. I only get a meagre 180 W from that engine, but despite that, it is more than enough for flight instruction including landng and take-off.
Castor oil is a purely biological product, without any stabilizing, deterging or other dopes. I expected that, but had no real estimate of the severity.
Considering the SF is ballbearinged, I am fairly sure it will be an improvement. Please check if your SF is ringed or not ringed. If not ringed, check the top of the liner for imperfections on the nickel coating. That is a weak point of the non-ringed SF series, the ringed ones don't have this problem. If the top of the liner is still OK, I am fairly sure the SF will perform well, but keep an eye on running temperatures. In the air, flying, no real problem, but on the ground they get pretty hot on gasoline.
Maybe the video is exaggerating a little bit... Usually my coil has to be within 5~10 mm to pick up the signal, In general, the signal is strongest in the area where the HT lead is connected to the box. I could not see much difference in the vaeious coils I tried. The ignition box is a faraday cage, there simply is not all that much signal to pick up.
If I mount the ignition box in the proper orientation in the plane, close to the skin, sometimes I can pick up the signal on the outer surface of the fuselage. Most planes, I need to open a cover and access the ignition box from the inside. Taping a stick to the coil helps in narrow fuselages.

mk13

I swapped this morning the 20H carb of the 25LA, by a 3A carb found in my club (I think it's a carb from a 40FP).
I secured all possible leak issue with sealant + hose on needle.
Engine seems to work just perfect! (And with only castor oil, no synthetic)

Maybe a 2A carb should be more suitable, but I don't have it.
Tomorrow I will do a flight test with a 9x5 prop.
And maybe I will use the 40SF for an other plane.

​

1967brutus

That is all not too bad...Absolutely not bad.