[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

Jan, what you need is a wide power range: (maximum usable rpm) / (minimum usable rpm). And the power range of a racing two-stroke depends on the pipe characteristics, whereby there is a torque collapse at 2/3 x the rpm of maximum torque.
Engine rpm has no direct influence, so there is no objection to high revs (if the engine revs higher you can use a shorter gearing and have more torque at the rear wheel for a given riding speed).
The problem is: if you want high revs, you need much time-area, especially for the exhaust port. And the higher you make that port, the stronger the exhaust pulses will be, and the more these pulses will upset the scavenging at 2/3 * rpm of max.torque.
You will need to keep the port timings moderate and make the ports as wide as possible in order to create sufficient time-area, like you already planned. And you will need a 'tame' exhaust pipe. You could try a narrow header and / or a shallow end cone.
And have you got a programmable ignition? That could make a lot of difference.
I think the optimum would be real 'racing' port timings and a trombone exhaust pipe.

A good method of creating torque at the cost of sacrifying max power  is an exhaust pipe with a stinger as used already for many years by OLAV AAEN,  the man of one of the most interesting 2 stroke engines ever. Another feature of such a setup reducing exhaust noise.

Frits Overmars a écrit:

Engine rpm has no direct influence, so there is no objection to high revs (if the engine revs higher you can use a shorter gearing and have more torque at the rear wheel for a given riding speed).

Tnx Frits.
besides being incredibly fun to race, my main goal is to have my engine opened up to see if it realy is a 50cc. That would mean I've done a very good job and I want to achieve my goal using only modified original parts as they cost next to nothing second hand (so no programable ignition, I have welded my heads so I can have a central spark , make my own pipes using Coker Racing software, flowed cases, 19mm Dell orto,..)

last year I tried a powerful high revving engine but it's no use : too hard on the clutch and as I can not change the gearing (well, it's allowed so I could but I'm not going to spend my money on having special gears made up for a shorter final drive after the double vario) higher revs mean higher speeds which I can never make on the track as there are hardly long straights.

the moped should attain 70km/h max and pull hard right from a stand-still. I calculated that 70km/h would mean the engine only should rev +/- 8750rpm and I've already tested with 180/130and 170/120 timings and I get the speed I need and it pulls hard once it has some speed but when it's moving slowly it takes a bit too long to get it going again, and as there are many very slow corners I would lose too much time.

So I guess I still need lower timings ? Should I look at timings for carts with fixed gearing ?

@ Bob : any more info on Aaen's stingers ? googeled but couldn't find what's special about them.

JanBros a écrit:

... I can not change the gearing (well, it's allowed so I could but I'm not going to spend my money on having special gears made up for a shorter final drive after the double vario).

Maybe it won't be necessary to have special gears made. The Honda Sky was produced with various wheel sizes. If you can fit smaller wheels, you will have a cheap, shorter gearing.

JanBros a écrit:

Should I look at timings for carts with fixed gearing ?

Jan,

Have you considered a kart style slipper clutch? Engage at peak torque and eliminate the 2/3 dip.

Even with a wide ratio gearbox, you will never pull the RPM's below the level of best acceleration.


Larry

Frits Overmars a écrit:

The Honda Sky was produced with various wheel sizes. If you can fit smaller wheels, you will have a cheap, shorter gearing.

I know, but I specificaly choose the Sky for it's big wheels :  more ground clearance, less trouble in ditches - or otherwise said : I can go where small wheelers can not

Larry Wiechman a écrit:

JanBros a écrit:

Should I look at timings for carts with fixed gearing ?

Even with a wide ratio gearbox, you will never pull the RPM's below the level of best acceleration.
Larry

last year there were 120 mopeds. about 80 of them were Honda's: +/- fifty-fifty devided between Walaroo's and Sky's. All the fastest guys (real MX'ers not like me) rode plain and simpel original Sky's : it realy is a damned good moped for it : pulse directly from the off and reaches 55 easily.

here is a comparison to an original Sky (red) and my latest attempt (170/120°) yellow - (I use my GPS-laptimer as dyno) :

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

only at 55km/h I've catched it back and leave it for dead after wards, but the ori sky is so bloody fast in starting/out of very slow corners . If an original can do it, so should I, or is it solely the expansion pipe that kills the bottom end ?

"Only" a 19mm carburettor seems a bit small, around here even an almost all original 50cc engine pulls much better with a 21 or 24mm carburettor, usually okos(copy of keihin), moded so they don't dry the fuel bowl and using stage6 or similiar good quality jets.

senso a écrit:

"Only" a 19mm carburettor seems a bit small, around here even an almost all original 50cc engine pulls much better with a 21 or 24mm carburettor, usually okos(copy of keihin), moded so they don't dry the fuel bowl and using stage6 or similiar good quality jets.

yes, only as it doesn't need to rev high, it doesn't need a big carb.
the original sky has a 12mm carb, so if you look at the curve's i lose little down low and gain a lot where the original runs out of breath.

btw : the yellow curve is just a first run without much setting up/playing wth ignition. still same vario, just heavier clutch springs.

JanBros a écrit:

here is a comparison to an original Sky (red) and my latest attempt (170/120°) yellow - (I use my GPS-laptimer as dyno) : [Vous devez être inscrit et connecté pour voir cette image]only at 55km/h I've catched it back and leave it for dead afterwards, but the ori sky is so bloody fast in starting/out of very slow corners.

Remember a couple of days ago when I mentioned the Range-function that will be implemented in future versions of the EngMod2T two-stroke simulation program? You just gave a perfect example of a situation where you should concentrate on Range rather then horsepower. You can find an explanation here:
[Vous devez être inscrit et connecté pour voir ce lien]

Hello guys! I am having problems with my 70 cc racing piaggio scooter and i would be happy if anyone could help. The problem is im curently jetting the bike i am using a Dellorto VHST 28. A Normal main jet size for this kind of tuning and carburetor is from 115-140 (This is what my colleges are running). I am curently at a 172 and its still runing lean. What could be the cause for needing such a big main jet ? I searched for leaks where air could be drawn but i didnt find any i also sprayed everything with brake cleaner but the rpm didnt rise. Idle is also normal no rpm rising.

Put the Vhst in the bin , get PWK28 and forget your jetting issues ...

ice t a écrit:

I searched for leaks where air could be drawn but i didnt find any i also sprayed everything with brake cleaner but the rpm didnt rise. Idle is also normal no rpm rising.

If you probe without main jet and lean.. is a small atomiser issue (fuel supply? needle valve diameter?)
To really good settings you must many parts changes. Needle jet, jet needle, throttle valve. If not, you will be very lucky man :) The Dellorto carbs is not bad :)

piaggio has very small reeds...and its very hard to proper jetting vhst carb. pwk is much much better...vhst need special setup (jets, needle, float...)

Seb4LO a écrit:

Put the Vhst in the bin , get PWK28 and forget your jetting issues ...

More power with a Dellorto Seb you just have to understand the "Dellorto Way"

I borrowed the same Dellorto from a friend but a TRT (2fast) version, with changed atomiser,needle, floats and so on and i am now stuttering with a 172 and a 156 is to small which is a lot better. I need to buy more jets but i think it solved the problem. But it was rich in the middle so i still need to buy another needle Almost all of my friends are driving dellorto vhst and they are running excellent, some of them only had to change the main jet and done. Some day i will try a pwk

Vhst are CRAP lozza , just good for city mopped :)

ice t a écrit:

Hello guys! I am having problems with my 70 cc racing piaggio scooter and i would be happy if anyone could help. The problem is im curently jetting the bike i am using a Dellorto VHST 28. A Normal main jet size for this kind of tuning and carburetor is from 115-140 (This is what my colleges are running). I am curently at a 172 and its still runing lean. What could be the cause for needing such a big main jet ? I searched for leaks where air could be drawn but i didnt find any i also sprayed everything with brake cleaner but the rpm didnt rise. Idle is also normal no rpm rising.

It depends on which version of VHST 28 you use. There are many versions of this carburetor. If you want to use the VHST I advise you to use the configuration VHST 28 CD - R.9382 Aprilia “SP-CHALLENGE” racing, was developed for the Aprilia RS 125 SP.
If you provide me your email I can send you the Dell'Orto documentation.

ICC karts have no trouble making nearly 50hp with them Seb.

No, in KZ1/2 it's a VHSH 30 CS.
And 50hp without power jet and variable exhaust timing, with 30mm carb, normal fuel, max 199° exhaust timing and fixed ignition

OK, KZ engines make a lot of HP with a Dell'Orto, but VHSH it's another matter from VHST.

Mr Thiel, Mr Overmars

we can see the plug to piston distance is 7.60 mm on the rsa head
how do you calibrate this distance, is there a calculation, or is the result off the shape and volume head ?.............

It is the result of the combustion chamber shape, Pagi.

Frits
i read many time this topic and i did not find the answer to my question. So, can you explain me what is the advantage to use a stringer in the exhaust pipe, what does it improve ? Torque? Max power? Larger power band? all of that? or mainly one first ?
Thanks for your answer

Pierre: frits:

Mr overmars (last question about the head i hope !)
on the rsa head the offset is 11.23mm, how to determinate this value when drawing a dome head.....
keeping the same combustion dome radius (7.86 mm if i remember well) ?????