[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

I.T-Racing a écrit:

What do you guys think of this Engine design With transfer Ports under the Exhaust Port.
My idea is to get a better flow and more gas into the Engine.

You may want to take a look here, I.T.
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A short introduction of yourself and your activities would also be appreciated

Thanks [Vous devez être inscrit et connecté pour voir cette image]

My name is Ivo Trambo born in Germany.
I am 19 years old and 2022 i started with tuning 2 stroke Engines.
I started karting in 2014 since then I've loved motorsport and Engines. I am a good driver with no money so i started tuning my own engines because the Racing teams want 1000 only for a tuned cylinder so i was reading some books and in the Forums.

I started tuning 100cc kart engines because i had no money, with a great success.
With the knowledge i get from Reading books and Forums i found out how to modify engines to get Great HP with a big Powerband.

My first Engine i made was a 1983 100cc TKM Rotary valve it has a Huge powerband it pulled from 5.000rpm to 21.000 without any problems but at 19k the Engine had a big lose of power but it had the Great Low End torque.


Everyone I let drive this Engine Said it is one of the best 100cc engine they ever Driven.

Then i started with Modifying my 125cc TM KV95 Rotary engine. I want to make Max power possible.
This Engine is a Beast. I was driving with the old small Exhaust but i will maybe build my own Exhaust sometime. The Engine has more power than every modern KZ kart and it had also an huge powerband which is bigger then every modern KZ Engine. When i got the Engine the powerband started at 9000 like the new KZ engines, But after some tuning in the Cylinder and Changine The Rotary Valve it now has more Peak Power and the Power started at 7500rpm, after tuning this engine i had with the same Gearing instead of 14500rpm, 17000rpm on the Straight. The KV95 is now 20kmh faster than the modern Shifter karts from SRP Racing in Kerpen on the Straight with the same weight.

Welcome Ivo !

Hi Ivo, thanks for the extensive introduction. Do you happen to have any power curves?
And can you tell me your Postleitzahl?

Sadly I dont own a dyno but i will build a dyno if i have time for it.

I live near Cologne

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Lovely bikes

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I now have a dyno Result from my Friends dyno with my TM K9C engine 1 curve with original Cylinder and 13cc Combustion Chamber and 4° Piston and the other with My tuned Cylinder

I drive now with a 30mm carb. I think with a bigger carb i maybe got a little more power


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I will try this combustion chamber to get a bigger powerband it has 7cc Volume and is designed for 0° Pistons

I would rather use a different combustion chamber shape, but your power curve looks excellent

What a pleasant surprise seen Frits with Alex on his "2stroke stuffing" channel.
I thoroughly enjoyed  both times I watched it!
Surprise no 2 Alex got a FOS cylinder, now I'll have to turn on notifications too.

Panas a écrit:

What a pleasant surprise seen Frits with Alex on his "2stroke stuffing" channel.
I thoroughly enjoyed  both times I watched it!
Surprise no 2 Alex got a FOS cylinder, now I'll have to turn on notifications too.

Friends have been nagging me forever about making a video with me. I kept telling them that I am a writer rather than a speaker and they kept forwarding me requests from their viewers who wanted to see me live. I finally gave in and I must say I started to enjoy it. It helps a lot when good friends ask the questions.
I spent a very nice week with the other 'stars'. What most surprised me when I saw the result on YouTube, was the amount of ethanol we consumed and still managed to converse somewhat cohesively.
I'll be 75 in a couple of months if I don't behave too irresponsibly in the meantime.
Last week, again after much nagging from my friends, I rode (or drove, I'm not sure what you should call it) a quad. I could not come to grips with the thumb throttle; I need to hold a firm grip on the handlebars when I am drifting. But otherwise I enjoyed it.
It's a good thing I do not have any motorcycle riding gear anymore, otherwise I might be daft enough to try that too again.
Alex Degnes: [Vous devez être inscrit et connecté pour voir ce lien]
Jan Schäffer: [Vous devez être inscrit et connecté pour voir ce lien]

Frits, very nice footage of your week in Germany and the explanation of technical items was also clear.

Thanks Frits what combustion Design can you recommend?

Here is a picture of my ports from the kz engine. After porting it has a huge exhaust Port
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I.T-Racing a écrit:

Thanks Frits what combustion Design can you recommend?

You will need the fastest combustion you can get, Ivo. Then you do not need to give much ignition advance that would otherwise cause a power loss because of a pressure rise above the piston before TDC. A fast combustion rate also helps in keeping the piston temperature within limits.

But I suppose you were asking about the combustion chamber shape. The best way to speed up combustion is with effective squish, as indicated by the arrows in the first picture below.
The second picture shows the Aprilia RSA combustion chamber geometry. But if you are taking part in official karting competition, you will probably be restricted to a lower permissible compression ratio.
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Hello,
i have some questions for Frits regarding the RSAs carburetor.
Compared to my VHSJ 42 carbs there are some obious differences i´m thinking about.
First thing is the really flat slide compared to the oval/ H-shaped slide of RSW carbs.
Further i can´t detect a starter/choke fuel circuit. Neither i see an intake bore in the carburetor bellmouth nor a starter jet.
There seems only to be one inlet bore on the bellmouth side branching inside, between the 2 body parts of the carburetor.
In my thoughts one bore towards the main circuit atomizer and second towards the idler jet with an air-screw to regulate.

I want to machine my own carburetors for my 250 rotary engine and don´t want just to make a "replica" of the VHSJ.
The needed parts don´t seem to be very difficult to machine when you got the right dimensions.

The powerjet system seems quite the same and i got  some new Keihin solenoids in stock which Dellorto used, too.
Perhaps you can tell a little about the intentions and progress between the RSW to the RSAs carburetors.
Having the RSW carbs is a good starting point for a construction with Solidworks, but i don´t have a pattern of the VHTC carburetor.
Giving advises and thoughts regarding the RSA carbs would be very kind.
The 250 is now near Dresden for a few weeks, so i have some spare time to realise new things until i collect the bike
Best regards,
Bernd

Döllinger a écrit:

The 250 is now near Dresden for a few weeks, so i have some spare time

Ich bin auch in der Gegend und habe auch etwas Zeit. Und du hast eine Privatnachricht.

Hallo Frits,

ja, sehr gerne. Könntest du mir bitte nochmal deine Nummer schicken?
Die in der Nachricht funktioniert leider nicht.
Viele Grüße,
Bernd

Döllinger a écrit:

Hallo Frits, ja, sehr gerne. Könntest du mir bitte nochmal deine Nummer schicken?
Die in der Nachricht funktioniert leider nicht. Viele Grüße, Bernd

Mein Fehler, ich hatte die Handy-Vorwahl 6 vergessen.

Frits Overmars a écrit:

...... I must say I started to enjoy it....

Does that mean we will  see you decipher the 2stroke for us more often on video?  

Panas a écrit:

Frits Overmars a écrit:

...... I must say I started to enjoy it....

Does that mean we will  see you decipher the 2stroke for us more often on video?  

I couldn't say Panas. I am not the one making the videos.

Frits Overmars a écrit:

Panas a écrit:

Frits Overmars a écrit:

...... I must say I started to enjoy it....

Does that mean we will  see you decipher the 2stroke for us more often on video?  

I couldn't say Panas. I am not the one making the videos.

But..But..it could be couldn't it?

Panas a écrit:

Frits Overmars a écrit:

Panas a écrit:

Frits Overmars a écrit:

...... I must say I started to enjoy it....

Does that mean we will  see you decipher the 2stroke for us more often on video?  

I couldn't say Panas. I am not the one making the videos.

But..But..it could be couldn't it?

I simply don't know.

It is Saturday, June 3 in Thailand
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Hallo,

i´ve got a question for Frits regarding my fathers Maico 250.
Two weeks ago i competed two races with the bike. During the competition i noticed a "starting issue".

A little data: 1 cylinder 250cc, 54mm stroke, 76mm bore. 38mm flat side Dellorto, rotary timing: 48mm BTDC to 27mm after TDC.
Head: self constructed, 0,7mm squish, upscaled RSW dimensions. compression 12,5:1
Exhaust timing 193°, transfer timing 130/132° Ignition: programmable PVL
Fuel:  50/50 Aral 102 /Avgas. With less avgas the engine got serious detonations at the end of straights with more than 400m.

During the meeting the engine was running well with even more power compared to the watercooled 250cc CUP-Maico. No noticeable fading when getting hot, our Maico drove out a 17sec gap to the watercooled Maico. So the engine is Ok for the most part.
But one thing is difficult to handle. If the revs sink below 6000rpm, even a few seconds, the engine doesn´t rev up, like being far too rich.
After 2-300m the engine is free and working well. That´s a problem if the starting procedure is taking a few minutes. You can´t hold the revs up to e.g. 7000rpm like about 2 minutes.
The ignition point is 30° BTDC  at low revs. Carburetor setting: DQ 268 atomizer with K30 or K52 needle, depending on conditions Idle: 40 Main jet 180 to 190 depening on conditions. Slide: 40, Spark plug NGK B11EGP
I thought about a push button controlled powerjet to lean the mixture during the first few hundret meters of the race.
Or is it better to play with the exhaust gas temperature to ease the problem.
Frits, what are you thoughts?

Best regards,
Bernd

Döllinger a écrit:

1 cylinder 250cc, 54mm stroke, 76mm bore. 38mm flat side Dellorto, rotary timing: 48mm BTDC to 27mm after TDC.
Head: self constructed, 0,7mm squish, upscaled RSW dimensions. compression 12,5:1
Exhaust timing 193°, transfer timing 130/132° Ignition: programmable PVL
Fuel:  50/50 Aral 102 /Avgas. With less avgas the engine got serious detonations at the end of straights with more than 400m.
one thing is difficult to handle. If the revs sink below 6000rpm, even a few seconds, the engine doesn´t rev up, like being far too rich.
After 2-300m the engine is free and working well. That´s a problem if the starting procedure is taking a few minutes. You can´t hold the revs up to e.g. 7000rpm like about 2 minutes.

Hallo Bernd,
First a remark about the avgas. I hate the stuff. It's dirty, its energy density is low, it's slow-burning, it requires a lot of ignition advance and many rulebooks do not even allow using it. If you engine needs it, something is not right. In your case I suspect that the combination of 193° exhaust timing and 132° transfer timing leaves insufficient blowdown angle.area.

The carburation propblem you describe indeed seems to indicate that the engine is drowning. The carburation setting has so many variables that I cannot give a straight advice. But one thing you did not mention is the float level. Maybe the float needle does not close well enough, either because of vibration or maybe because of fuel pressure. Do you use a fuel pump?
You might also take a look at several carburation setting videos from my German friend Jan Schäffer (so they are in your native language). You can find them here: [Vous devez être inscrit et connecté pour voir ce lien]

Frits Overmars a écrit:

melvyn trevor a écrit:

Frits, Many thanks for the prompt reply you have explained a lot for me, re-reading the post I can now see my confusion, would velocity be more appropriate, factoring in the driving pressure and exhaust diffuser action?
Grateful thanks, Mel

Yes Melvyn, asking about the maximum velocity of gas flow makes more sense to me than asking about the maximum efficiency of the flow. The picture below shows a velocity profle of blowdown outflow. Note the effect of a radius at the top egde of the port. The maximum velocity at this point can be Mach 1. This will not be influenced by exhaust diffuser action, as the pressure differential during the onset of the blowdown phase is supercritical; lowering the downstream pressure will not raise the flow velocity any higher.
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Hello Frits, is this area likely to behave like a De Laval nozzle exceeding Mach 1? Thank you