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

The 'modulated' power jet permitted a much wider range of mixture variation.
We started with a 45 jet, as was used in the 'on-off' system.
The on-off system permitted us to use a 42mm carburetor on the dyno without holing the piston at max torque.
By giving more fuel at the most critical point: maximum torque and just before.
With the modular pj we could improve a lot more, especially overrev!!
We went from 45 to 120, the bigger the better....

Hallo Jan and Frits,

thank you for your interesting infos!

I want to use a RTD ignition which got an output for a power jet. But my possibilities don't allow to produce a complete setup for a modulated power jet on the dyno. So i have to stick with the ON/OFF Setup. Jet 45 means Dellorto size 45?
I just want to cut the fuel off at very high revs so that the exhaust gas temperature gets only critical for a few seconds on the straight.

I searched for a Keihin power jet but all i found is a complete VHSG/D carburetor. No spare parts.
You think it's a good idea to adapt the "jet-tube" into the carburetor body (in front of the slide) and the solenoid with the valve outside?

Best regards,
Bernd

Döllinger a écrit:

Jet 45 means Dellorto size 45?

Yes. But the powerjet effect with a 45 jet will be minimal.

Citation :

You think it's a good idea to adapt the "jet-tube" into the carburetor body (in front of the slide) and the solenoid with the valve outside?

That is exactly what can be seen in the pictures on the previous page, isn't it?

Hallo Frits,

thank you for the pictures! Due to a software-bug i could not see them until now. Sorry for that.
They show what i mean. On these carburetors the "fuel-duct" is inside. I need to make that on the outside with a milled aluminium part.
Am i guessing right:
Fuel comes from the floater-bowl and rises through a changeable jet. Then it runs through a drilling that can be opened/closed via the solenoid. Finally it is sprayed into the air stream by a "jet tube"
Regarding the bore in that "jet tube": Should it be straight or better in direction of the air stream?

Best regards,
Bernd

Number 13 in the picture below is the powerjet. It is important to keep the fuel volume between the powerjet and the jet tube exit as small as possible in order to avoid lag.
If I remember correctly the jet tube opening is square, but bevelling the downstream side, like some needle jet screens, may be a good idea.
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@ Döllinger,
To follow up on your private message, here is some more information about the layout of the power-jet in our carburetor. Very similar to the Dellorto one.

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For information, the final sprayer is no longer available at Dellorto, we manufacture them now. Drawing below.

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Hallo Francis,

thank you for your drawings and infos. A really great idea with the float-inlet valve!
I will see how i can adapt that to my carburetors.
Best regards,
Bernd

Francis lovely work, this has brought a question to mind. 
I have a Honda power jet solenoid like that of which aprilia used. The solenoid has failed & the Honda item is no longer available, would you know if there is a replacement solenoid available ?
Kind Regards Rick

Probably at KEIHIN

Hi,

Long time reader, first time posting. I have a question regarding the spark plugs with part of the threads machined away to expose the electrode. Does this modification change the heat range of the spark plug? I race two different classes of outboards. One on methanol fuel and the other on pump gas. In the methanol motor we use the coldest heat range available. A Q55 by champion to be exact.

F888 a écrit:

Hi, Long time reader, first time posting. I have a question regarding the spark plugs with part of the threads machined away to expose the electrode. Does this modification change the heat range of the spark plug? I race two different classes of outboards. One on methanol fuel and the other on pump gas. In the methanol motor we use the coldest heat range available. A Q55 by champion to be exact.

The modified plug body will pick up a little less combustion heat, and the insulator nose will pick up a little more. The resultant heat range might be a little hotter, but I've never taken this into consideration when modifying or fitting plugs.
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I apologise if this has been asked / answered & or mentioned before
Frits what plug brand, model, heat ranges was common to the RSW & RSA Engines

RAW a écrit:

Frits what plug brand, model, heat ranges was common to the RSW & RSA Engines

Plug brand was Denso, heat range was initially 35 and then 34 from 2007 onward.

Thank you very much for you input, Frits. The photos you attached were helpful as well. I will go with Jan's method of 'Try and See'

Hi,

i found Keihin solenoids in the USA and bought them. They are thought for ON/OFF.
I will equip them to fresh carburettor bodies with all the needed ducts.
@ Frits what jet ca. size would you assume for a carburettor with main jet 220 (Dellorto) to get a good effect?

You always used stepper valves or did you use normal solenoids with a PWM-signal as well?

Best regards,

Bernd

Döllinger a écrit:

i found Keihin solenoids in the USA and bought them. They are thought for ON/OFF. I will equip them to fresh carburettor bodies with all the needed ducts.
@ Frits what jet ca. size would you assume for a carburettor with main jet 220 (Dellorto) to get a good effect?
You always used stepper valves or did you use normal solenoids with a PWM-signal as well?

If I remember correctly the Keihin powerjet solenoids are 'normal' On/Off valves that were triggered by a 13 Hz PWM signal in the Aprilia racers. They used a #220 main jet and a #120 powerjet, so you might want to start with this ratio of  220/120.

Hallo Frits,

thank you for your infos.
The carburettors i use have originally no powerjet and run a 220 main jet.
So you think the mixture will not be to rich when adding a 120 jet?
Sorry for my question, but you surely have an explanation.
The plan is to laser-weld a carrier for the solenoid and the valve onto the body and a part to srew the power jet into the float bowl. Then connected with a small duct.
The valve will be done like Francis posted. Very good solution.
@Francis: On your carburettors its an ON/OFF solution or pulsed?
Best regards,
Bernd

Döllinger a écrit:

The carburettors i use have originally no powerjet and run a 220 main jet. So you think the mixture will not be to rich when adding a 120 jet?

If the engine is happy with just the #220 main jet, adding a second fuel circuit with a #120 jet will probably drown it.  
You might try for example a #210 main jet and a #80 power jet, or a #200 main jet and a #100 powerjet.
Together these will have the flow capacity of a #224 main jet.
Do not switch the powerjet off until you are well past maximum power rpm.

Hi,

We never used ourselves, the electronic powerjet on our carburaters, since in the category SUPERKART, the regulation prohibited any electronics.
We have adapted the operation with a pneumatic control, it is for this reason that the design of our carburetor, at this place is slightly different from the APRILIA version, in order to be able to use the two solution, bellow or solenoid.
There is still a lot of things I do not know about APRILIA's know-how, especially the exact mode of operation of this solenoid.
I heard a lot how it was frequency driven, as to how often it was excited and how ???.  

At one time I worked for MAGNETI-MARELLI carburettors, at the good time of the indirect injection, the injectors were driven at the rotation frequency of the engine and the quantity of gasoline injected was adjusted by varying the cyclic ratio, opening time / closing time.

The 120 jet placed upstream of the powerjet is not representative of the volume of gasoline injected, fortunately, because in injecting 120 additional gasoline point, on a main jet of 220 !
That's a lot, right ?  

Unless I'm mistaken,
The power-jet on the carburetor is intended to correct, to a small extent, the air / fuel ratio, locally and temporarily, in certain phases of operation of the engine and the carburetor, itself.
In particular to try to suppress the detonation and temporarily allow an over-revving, by increasing the temperature of the exhaust gases.

The KEININ solenoid is electrically comparable to an injector, without the needle valve.
So, a priori, nothing prevents him from having it run in frequency.

Regards.
Francis.

I agree Frits

Hallo Frits and Francis,

@Frits:
so i understand that you can NOT count together the 2 values of the jets.
Thank you for your infos! If you have something else in mind please write is down, as everything helps.
Another question: In the exploded diagram of the RSA carburettor: Part #13 is this only a normal jet or is this part something like an atomizer?
I think you want only fuel in the PJ, not atomized like on the main jet system.


@ Francis:

So you think the amount of fuel is more regulated by the valve itself? (or the difusor)

Too bad you don`t produce these carburettors any more.

Best regards,
Bernd

Döllinger a écrit:

Hallo Frits and Francis,

@Frits:
so i understand that you can NOT count together the 2 values of the jets.

Dellorto Jets are engraved in 1/100 of mm.
If you want to get the equivalent jet to the sum of two, you have to add the sections and recalculate the equivalent diameter.

Citation :

Another question: In the exploded diagram of the RSA carburettor: Part #13 is this only a normal jet or is this part something like an atomizer?
I think you want only fuel in the PJ, not atomized like on the main jet system.

No, it's just a pipe to collect the fuel at the bottom of the bowl.

Citation :

So you think the amount of fuel is more regulated by the valve itself? (or the difusor)

The fuel is only regulated by the valve.

Citation :

Too bad you don`t produce these carburettors any more.

Are you ready to pay more than 5 000€ + VAT for a pair ?

Hi Francis,

The fuel is only regulated by the valve. a écrit:

OK, so you regulate by the opening cross section or the time? How you do that with a pneumatic bellow?

Are you ready to pay more than 5 000€ + VAT for a pair ? a écrit:

Puh, that´s a big amount, but you told me they were expensive. I will try to find my own solution. Also with your help here. Thanks for that!
But if you would produce them later in time (1 or 2 years) or perhaps even in a bigger number (by interest) it could get serious.
Best regards,
Bernd

Döllinger a écrit:

Hi Francis,



OK, so you regulate by the opening cross section or the time? How you do that with a pneumatic bellow?

Sorry, no comment.

@ Francis,

no Problem. I don't want to urge you telling your secrets! You told me lots of highly interesting things.

@Frits: With the Keihin ON/OFF silinoids: With the 13Hz Signal you managed to only slightly open the valve so you had a comparable effect to the stepper motors?

Best regards,
Bernd