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

Hello,

Is there a good source to get (new) carbs with inclined fuel chamber from? I am looking for one with 34-38mm in diameter. I've already looked at Mikuni, Dellorto and Keihin. Without success.

I know that Aprilia RS 250 and Suzuki RGV 250 have got 34mm Mikuni(?) carbs with inclined chambers, but I'm not sure if these relatively old carbs are well to adjust

Regards,
Daniel

Of course but nothing to do in this topic  

All that you wanted to know on Aprilia RSA 125, and more

Or have I misunderstood something? After all, it's not the first question that is not related to the RSA 125.

But sorry if my post might be really wrong here.

Be cool, Seb, be cool...

If necessary, Daniel will create a new topic...

No problem, Daniel.

Frits Overmars a écrit:

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thanks Frits

can some one tell me the power gain from a VHSB 39 carb to a 42 like this on a 125 rotary valve engine like the RSA?

Thanks Manuel

Manuel bigger gain seems to be in the mid range.

Hello all.
I have a question for Frits and Jan but please anyone feel free to comment as usual.
A while back on this thread Jan said he'd have liked to have built another 125 with two discs and two carbutettors, if I remember correctly.
Now with the lambretta scooter, we have this major problem where the frame spar, a tube, runs down the centerline and makes it difficult to mount a decent reed and inlet tract. Really, it should have been a parallel twin cylinder engine all along, as Rumi later did.
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**The parts in the images are not my own, and I have photoshopped them**

(note: neither the engine mounts nor the cylinder are on the frame centerline!)
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So I have an idea, that my cylinder will have two carburettors and two reeds, either sharing some inlet tract area, or with a divider (two inlet tracts), or with two boost ports (C ports).
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Jan also stated that the preferred intake method is Disc > Crankcase reed > Cylinder reed, and it's easy to visualise why, however the factory crankcase does not take a reed easily at all. And so we left to persue the cylinder reed as best we can.
So the final concept may look like this:
Single inlet 38mm
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Twin inlet 27mm
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I'd love to hear your appraisal on this concept, if it is worth prototyping. Many thanks in advance.

nick gill a écrit:

I'd love to hear your appraisal on this concept, if it is worth prototyping. Many thanks in advance.

It may work Nick, but please don't ask me to look at it again. It hurts my eyes..  The aircooled cylinder, the asymmetrical cooling air delivery, the spark plug position, the diversions in the inlet tracts.... I understand that some people enjoy tuning vintage bikes and scooters, but I do not fancy dealing with problems that have already been resolved. Brrrr....

Maybe Jan has a more open mind. After all, he is even more vintage than I am  

Frits Overmars a écrit:

nick gill a écrit:

I'd love to hear your appraisal on this concept, if it is worth prototyping. Many thanks in advance.

It may work Nick, but please don't ask me to look at it again. It hurts my eyes..  The aircooled cylinder, the asymmetrical cooling air delivery, the spark plug position, the diversions in the inlet tracts.... I understand that some people enjoy tuning vintage bikes and scooters, but I do not fancy dealing with problems that have already been resolved. Brrrr....

Maybe Jan has a more open mind. After all, he is even more vintage than I am  

I am, but I really do HATE aircooled engines!

Frits Overmars a écrit:

I understand that some people enjoy tuning vintage bikes and scooters, but I do not fancy dealing with problems that have already been resolved. Brrrr....
Maybe Jan has a more open mind. After all, he is even more vintage than I am  

Fair enough. Very often working with scooters feels like polishing a turd but the market is very large and very healthy.

Like I said those aren't my parts, just illustrative images I mocked up.
The prototype is modeled as a water cooled cylinder with a central plug head & squish band, 60mmx60mm, expansion chamber etc. The only thing I can't produce is rotary disc timing as the machining is $10K for a 1-off. Thanks for your input gentlemen.

nick gill a écrit:

The prototype is modeled as a water cooled cylinder with a central plug head & squish band, 60mmx60mm, expansion chamber etc.

In that case there is hope. Are you familiar with the AerMacchi Harley-Davidson RR500 racer of 1976? It was a 500 cc twin, which required a 48 mm carburetter for each cylinder. As these were not available at the time, each cylinder got two reed cases and two 34 mm carbs. For your purpose you could spread the two reedcases a bit wider apart, and you'd have to find vertical carbs so you wouldn't need those long curved inlet rubbers. You may want to take a look at diaphragma ('pumper') carbs.
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Frits Overmars a écrit:

Are you familiar with the AerMacchi Harley-Davidson RR500 racer of 1976? It was a 500 cc twin, which required a 48 mm carburetter for each cylinder. As these were not available at the time, each cylinder got two reed cases and two 34 mm carbs.

Thanks Frits. Just the sort of verification and encouragement I needed. I will research the RR500.

Inlet-to-Transfer ports: ( I know them as Boyesen ports but they probably have other names, Yamaha "OW" etc)
Do you think the A ports should be able to receive air/fuel directly from the Left reed or should it be forced to take air/fuel from the crankcase only.
If YES, then should the Left A port be able to recieve air/fuel from the Right reed also, or be excluded from doing this?
How should the C port be considered with two separate reeds? Two C ports is my instinct.

What are your instincts Frits?
!!!!  Thanks again  !!!!

All transfer ports should be able to receive mixture directly from the reeds, like they do with crankcase-mounted reeds.
It is not likely that the left A-port will receive much mixture from the right reed, but it should not be excluded from doing so.
In the AMF-Harley-Davidson there were two C-ports which in my view would be the logical way to construct.

I occasionally wonder if we would have had more fun if the 500's had been limited to 2 cylinders  

GrahamB a écrit:

I occasionally wonder if we would have had more fun if the 500's had been limited to 2 cylinders  

I wonder what would happen if bump starts were brought back to MotoGP. That would teach them foul-strokes!
No, seriously, bump starts are too dangerous compared to what we have now. And Rossi would win anyway; he's twice as big as Marquez and Pedrosa.

Frits Overmars a écrit:

No, seriously, bump starts are too dangerous compared to what we have now. And Rossi would win anyway; he's twice as big as Marquez and Pedrosa.

 Clearly Cal would win.... Dani would never leave the grid  and Randy would discover a new way to crash  

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nick gill a écrit:

Frits Overmars a écrit:

No, seriously, bump starts are too dangerous compared to what we have now. And Rossi would win anyway; he's twice as big as Marquez and Pedrosa.

 Clearly Cal would win.... Dani would never leave the grid  and Randy would discover a new way to crash  

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Nick,
      A twin carb scooter,what a great idea.Having built quite a few twin carb two strokes starting in 1974 I can assure you it will work.A 60x60 engine with max power at say 11000 rpm could use 2x 32mm carbs.Keep the reeds and carbs as close to the cylinder as you can.You could put reeds on the side of the crankcase,Suzuki did on their XR70RV 500 in 1986.A twin carb two stroke has won a world championship,in 1975 Harry Everts won the 250MX title on a Puch.This might inspire you [Vous devez être inscrit et connecté pour voir ce lien]

CRECY a écrit:

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Thanks for the encouragement CRECY. Frits will pass out when he sees the inlet length on that '76 Puch   I will model it up and see if it solves more problems that it creates, and then we'll see. Cheers !

nick gill a écrit:

Frits will pass out when he sees the inlet length on that '76 Puch.

Not likely, Nick. I took that Puch apart to the last screw in 1976 when I was technical editor of the Dutch motorcycle magazine Moto 73.

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For those with long memories a short bloke seem to have little trouble, Ian 'Buster' Saunders here in Australia did many push and standing starts(on a GPz900 was frightening to watch)

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I can't find the pic of the engine but runs a belt driven rear disc with a 42mm VHSG and the  BSG cylinder that got a large tick of approval from Jan Thiel.

Institute of TwoStrokes a écrit:

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I can't find the pic of the engine but runs a belt driven rear disc with a 42mm VHSG and the  BSG cylinder that got a large tick of approval from Jan Thiel.

BSG: big money spent, and a pretty exacting port replica of the RSA cylinder from what I've seen. I tip my hat to them.
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All the hard work is done for you though.........................

Institute of TwoStrokes a écrit:

All the hard work is done for you though.........................

Ha! yes, building cylinders is easy   , and so is the solidworks ! Everyone should build one
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Funny thing is, often when you talk to motorcycle engineers, they have a secret scooter shame  

Looking at the rendering I would urge you to read parts 1 & 2  

Frits, any news on the 50cc low compression engine? How did the test go?