[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

I know.

Sander

We used the 13mm steel rope too, works well on an electic drill. We named it the serpent, because if it came out of the exhaust pipe while the drill was still rotating, it would strike like a cobra

Frits Overmars a écrit:

A large crankcase volume is good but with a very large case the flow velocity inside the case will become so low that fuel and air separate.

Hello Frits and all the others!

Yesterday I was on the dyno with my 80cm³ engine and now I got the depressing feeling, that I might have reached this point you were talking about Frits.

The maximum power is not bad for the first day on the dyno, over 28hp at the rear wheel, but the power curve is ugly and the biggest problem is the area from 7500u/min to 9000u/min, red marked at the power graph.

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The engine needs about 10 seconds or even more to rev from 7500 to 9000 and it stutters like hell. Is it possible, that caused by the exhaust pulses that arrive at the wrong moments at those low rpm, the flow velocity became so low that fuel and air seperate, as you said Frits?

Some facts about the engine:
- reed valve directly to the crankcase
- 465cm³ crankcase volume
- 34mm carb
- exhaust according to the FOS exhaust concept

The smaller I choose the main jet, the easier it revs over this critical area. The main jet is 140 at the moment and I think I shouldn't go smaller to prevent piston seizure. And even with this main jet it takes these 10 seconds. With 152 for example it doesn't rev over this area at all (on the dyno)

Now of course I brooded what I could do to solve this heavy problem, because the whole engine is unusable so far. And these are the things I want to try now:

- a lot of ignition advance
- a smaller reed valve
- thinner reed plates
- a boost bottle

Best regards

edit: But I don't think, that more ignition advance will help, because the torque in this area is really nearly zero and that let's me assume that it is a carburation problem and not only the "well known power gap" before the power band.

[quote="Daniel A."]

Frits Overmars a écrit:

Now of course I brooded what I could do to solve this heavy problem, because the whole engine is unusable so far. And these are the things I want to try now:

- a lot of ignition advance
- a smaller reed valve
- thinner reed plates
- a boost bottle

Best regards

edit: But I don't think, that more ignition advance will help, because the torque in this area is really nearly zero and that let's me assume that it is a carburation problem and not only the "well known power gap" before the power band.

Congrats on your nice graph, seems like a good starting point!

It could be the returning wave of the pipe arriving at a bad timing. Then it yould hinder proper scavenging in that speed range. If so, more ignition advance will help, as will a bigger stinger diameter, higher crankcase compression ratio and lower transfer timing.

[quote="Haufen"]

Daniel A. a écrit:

Frits Overmars a écrit:

Now of course I brooded what I could do to solve this heavy problem, because the whole engine is unusable so far. And these are the things I want to try now:

- a lot of ignition advance
- a smaller reed valve
- thinner reed plates
- a boost bottle

Best regards

edit: But I don't think, that more ignition advance will help, because the torque in this area is really nearly zero and that let's me assume that it is a carburation problem and not only the "well known power gap" before the power band.

Congrats on your nice graph, seems like a good starting point!

It could be the returning wave of the pipe arriving at a bad timing. Then it yould hinder proper scavenging in that speed range. If so, more ignition advance will help, as will a bigger stinger diameter, higher crankcase compression ratio and lower transfer timing.

I have a similar dip in the power curve on my bigger bore twin.
if its anything to do with the return pulse arriving too early then a possible solution for you may be as I have done:

I am attempting to remove it via a solenoid controlled resonating chamber in the exhaust header (think HONDA ATAC) that can be installed in minutes with no permanent modifications required. ei: its a slip on system.

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Thank you Haufen

Of course it is this wave, but I have never had an engine in which that impacts as bad as here. The reason for it impacting so bad is the big crankcase I think. Unfortunately I can't make it smaller. This is why I will play around with the other parameters now, hoping that it is not impossible to get away this stutter.

Personally I put the best hope in a boost bottle. Let me explain you why. When the exhaust pulse pushes fresh charge back to the crankcase, the pressure inside the crankcase rises. At some point it is so high, that no more fresh charge coming from the carburetor can pass the reed valve, so it travels back to the carburetor and gets richer. So in fact that smaller main jets improved the problem a bit I suspect that a boost bottle could prevent the air passing the carburetor more than one time, ensure the right mixture and solve to problem like this.

@Jeram: Very nice system! I heard of it the first time now and just used google to inform me a bit about it. But isn't it complicated to set up the mechanics and electrics to make it work reliable? I mean, I would not know how to install it within minutes

Regards

Daniel A. a écrit:

@Jeram: Very nice system! I heard of it the first time now and just used google to inform me a bit about it. But isn't it complicated to set up the mechanics and electrics to make it work reliable? I mean, I would not know how to install it within minutes

Regards

Thanks,

I like this idea better than the boost bottle because it removes the problem at the sourse before it passes through the cylinder and crank cases.

I am about to install the linkages and solenoids next week on my bike so I will let you know how simple it is.

But in reality all it will be is some automotive central locking solenoids connecting to some rose joint linkages from a remote control car shop.
A programmable ignition or an RPM switch can be used to control them.

the time consuming part is tuning it on the dyno to find the point to open/close and the optimum volume of the chamber to move the gains to the RPM range that you desire.

Should have mine on the dyno in a couple of weeks.

Daniel A. a écrit:

Thank you Haufen

Of course it is this wave, but I have never had an engine in which that impacts as bad as here. The reason for it impacting so bad is the big crankcase I think. Unfortunately I can't make it smaller. This is why I will play around with the other parameters now, hoping that it is not impossible to get away this stutter.

Personally I put the best hope in a boost bottle. Let me explain you why. When the exhaust pulse pushes fresh charge back to the crankcase, the pressure inside the crankcase rises. At some point it is so high, that no more fresh charge coming from the carburetor can pass the reed valve, so it travels back to the carburetor and gets richer. So in fact that smaller main jets improved the problem a bit I suspect that a boost bottle could prevent the air passing the carburetor more than one time, ensure the right mixture and solve to problem like this.

Regards

I think you are right with it being caused by the big crankcase. Did you happen to dyno your engine with the same setup and a smaller crankcase, before?

A boost bottly is typically set up to work in combination with a certain throttle opening in the range of 1/16 to 1/4 throttle. If your problem shows only within that region, installing a boost bottle or rejetting the carb may help you get rid of the symptoms.

If it shows up at a certain speed range, regardless of the throttle opening, then I'd leave the boost bottle idea aside for the moment, and concentrate on ignition timing and the other things already mentioned. Because then, there is a good possibility your problem is caused by a too small pressure difference between transfers and cylinder at the time your transfers open / are open (and thus cylinder scavenging can not be done properly).

In general, there are two main levers on this. First one is shifting the returning pulse to another speed range and / or weakening it (for example by altering exhaust temperature by ignition timing and / or stinger diameter). The other one aims at a higher pressure inside the transfer ports when they open (crankcase-cr, transfer timing).

28 rear wheel HP from a 80 cc engine on the first day certainly is not bad at all; my compliments.
The exhaust pulse that provides maximum cylinder filling at the rpm of maximum torque, causes a maximum disturbance, pushing the cylinder contents back into the crankcase before the transfers close, at 2/3 of that max.torque rpm.
Your power curve is a textbook example: max.torque at 12.500 rpm; max.disturbance at 8.300 rpm.
Yes, the exhaust pipe is the main culprit here. But any changes to your crankcase volume or your pipe are likely to cause a loss of maximum power...

Because of the weak vacuum in the crankcase, the carburation gets upset as well. But this is not a problem you should solve via the main jet. And a boost bottle is only of real value when you have to use a very small carburetter; a 80 cc engine with a 34 mm carb does not need a boost bottle.

Weakening or shifting the exhaust pulse is the way to go. A power valve, an ATAC chamber or ideally a trombone pipe will help, but the simplest approach is advancing the ignition timing in the problem rpm area.

Citation :

I don't think, that more ignition advance will help, because the torque in this area is really nearly zero

That logic is beyond me...
Let's talk numbers: 30° advance at 2/3 of max.torque rpm is safe and if the cylinder filling is really as bad as yours, even 40° advance is safe.

The problem is depending on the rpm, not on the throttle. And no, I did not test the engine with a smaller crankcase before.

@Frits: Let me explain you how I came to this "logic" I assumed, that when the torque is nearly zero, air and fuel must have seperated. And if they have seperated, I couldn't imagine that early igniton would help a lot. But I forgot, that with more ignition advance the exhaust pulse weakens, too. And then maybe there is no seperation anymore and it might work :) Sorry, that was my mistake.

I think we all agree that the problem is caused by a too little pressure difference between cylinder and case, that furthermore leads to a bad carburetion. So the velocity in the crankcase IS very low and we can assume, that fuel and air have seperated, can't we?

I will try 30-40° ignition advance as soon as my Ignitech is working again. In the mean time (I have to find out what part of the Ignitech is broken and get a new one then) I will try a boost bottle, just to test it.

Regards

Daniel A. a écrit:

...the velocity in the crankcase IS very low and we can assume, that fuel and air have seperated, can't we?

465 cc crankcase volume for a 80 cc engine is large by today's standards but I do not expect fuel separation yet.
Maybe in a stone cold engine at starting rpm... You could put a gas burner under your engine, but do not call me if your workshop has gone up in smoke [Vous devez être inscrit et connecté pour voir cette image].

Daniel A. a écrit:

The problem is depending on the rpm, not on the throttle. And no, I did not test the engine with a smaller crankcase before.

@Frits: Let me explain you how I came to this "logic" I assumed, that when the torque is nearly zero, air and fuel must have seperated. And if they have seperated, I couldn't imagine that early igniton would help a lot. But I forgot, that with more ignition advance the exhaust pulse weakens, too. And then maybe there is no seperation anymore and it might work :) Sorry, that was my mistake.

I think we all agree that the problem is caused by a too little pressure difference between cylinder and case, that furthermore leads to a bad carburetion. So the velocity in the crankcase IS very low and we can assume, that fuel and air have seperated, can't we?

I will try 30-40° ignition advance as soon as my Ignitech is working again. In the mean time (I have to find out what part of the Ignitech is broken and get a new one then) I will try a boost bottle, just to test it.

Regards

Hi Daniel. Hi had a situation like yours in the NS1, when passing through those rpm's half throttle had better power than full throttle, if I pushed full throttle I would not even put the bike in high revs in 2º gear. Here is the video [Vous devez être inscrit et connecté pour voir ce lien]

Today that is solved, mostly due to ignitech at 30º from 6 to 8k, and by playing with needle and needle jet diameters, so, I don't see why you play with the main jet, but maybe I am wrong.

So do you go full throtle when the engine is not on the power band?

Some time ago I had read somewhere that reed race 125 engines would like ~1.3 crank comp. You have less cc, so it might be difficult to reach that value, but still you have less then 1.2 :S

Regards

Hi romeuh,

I just played with the main jet, because it brought maximum power when running in the power band. But as a side effect I noticed, that the stutter problem improved, too.

Other throttle positions than full throttle do not help to solve the problem. Like I said, it takes over 10 seconds to rev from 7500 to 9000 (at full throttle) and it sounds really crazy. If I don't give full throttle he doesn't even rev over this area, impossible.

Regards

I think if it was fuel separation, you'd be lean, so using a smaller mainjet would make it worse... wouldn't it?

i suppose you are using a Derbi/Minarelli engine Daniel?

Some cylinder really run welle on the Derbi with a 85mm conrod...

No, it's a Simson engine
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A little status report:

I just made some fast tests and I tested only in first gear (because my court is so small ^^)

30° advance - doesn't help
40° advance - seems to help a bit, bit it still stutters
15° advance + boost bottle - doesn't help
30° advance + boost bottle - helps, nearly no more stutter :) :) :)

I try to optimize the boost bottle now (the tube is a lot too short) and then I will try reactivating my Ignitech and test different ignition timings on the dyno :)

Daniel A.
Hi, what is your exhaust exit (reduction) diameter?

18mm

Hello.

I have a question, I have seen in some port layout drawings, and I think it happens to be the same in this photo, in this case a powerfull cylinder.

Why is the backward angle of the secundary transfer ports diferent. Why aren't they symmetrical? I had placed two lines following the backward wall direction to show what I mean.

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Swirling effect to achieve better mixture mixing?

Monday morning cylinder.

Daniel A, sometimes the load of the dyno gives you false graphs. Especially when the engine is so small like an 80cc at the lower RPM's.

Have you field tested it to see if that dip is really there?

Jeram, can you use these valves to just join the two pipes together at the headers at low speed. Like a two into one chamber then shut them off at high speed? Save using boost bottles.

uniflow a écrit:

Jeram, can you use these valves to just join the two pipes together at the headers at low speed. Like a two into one chamber then shut them off at high speed? Save using boost bottles.

That was actually one of my first ideas But I had originally decided not to try it.

It would entirely depend on whether the 2-1 would give the desired results, as it would no longer be about delaying the pulse but would also receive a pulse from the ajoining pipe at approximately 180 degrees out of phase.

Which now I think about it again, that could be perfect.

My pipe is tuned for maximum power at 10,500 RPM and I want to increase the hp between 4000 and 6000 where a large flat spot exists.
So if the pulses from the pipe are approximately 180 degrees out of phase a pulse from the ajoining expansion chamber could potentially arrive at the desired time.

I will certainly try putting an interconnecting pipe on the exh butterflies and compare it to standard ATAC chmabers on the dyno so that I can find the optimum solution, thanks Neil.