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

dutch fisher a écrit:

Before we go too far into the wonderful world of squishing.
It might be an idea to pinpoint in time your blown ionized gases in relation to the spark event.
Just so we are all singing from the same hymn sheet

I'd say that ionization begins when the ignition sends its high tension to the spark plug, at a time where the squish flow has already reached a substantial velocity. Seems rather basic to me. Have you got a different song in mind Dutch?

Talking about squish reminds me of my teacher/lecturer at the Anthony Fokker aircraft engineering school Bob Kampschuur who studied and worked ai the famous RICARDO institute where it's founder and briljant scientist Sir Harry Ricardo developed squish. RCARDO still is a major player in amongst others combustion engine R&D, one establishment in the UK and at least one more in the USA. Without Ricardo's work the internal combustion engine would not be where it is now. How about that.

Bob van der Zijden a écrit:

Without Ricardo's work the internal combustion engine would not be where it is now. How about that.

Who knows, Bob. Without Ricardo's work England might have lost the war and we would all be riding DKWs now...

Citation :

Who knows, Bob. Without Ricardo's work England might have lost the war and we would all be riding DKWs now...

No way ; better climb back in the trees !

Nothing wrong with DKW, a brand to be proud of.
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If I could change history, I damned sure would.

Maybe the next war will start and end quickly with the right people being hanged while the rest of us attend to working & living.

That Harley Motor Co. received the old DKW motor as " spoils of victory " was nothing for the company to brag about.

But, it did happen, and various lightweight " Harleys " were produced after the war and through 1966.

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The machines enjoy popularity among collectors and I know a few older guys who started out with a new Hummer or Pacer.

These were before my time, but a few years ago I purchased a fine-looking 1962 Scat.

With their high-mounted exhaust, Scats looked like a " scrambler ".

Its Bendix magneto is probably the best component on the bike.

( The same mag was used on the genuine Aermacchi Sprint racers imported into the USA. )

A lot of guys raced these bikes and performed modifications. One of the well known " tuners " was a man in Florida named Puckett.

Today, at vintage events like Davenport, a few races will be run. Seems a waste, but some people like doing it.

So yeah, Harley-Davidson had models with the DKW motor.

The Scat I own is in fine condition and still has its original paint on the fuel tank, and cloth-bound wiring harness.

I'm quiet happy with my original East German MZ RE125/1960 road racer, the first real good rotary disc two stroke racer. Presently I'm getting it ready for some so callded demo racing in Holland. Not with a mocking Fikuni as many Ossi Krauts do but with a genuine East German 35mm BVF M3 type racing carb with separate float chamber. BVF stood for Berliner Vergaser Fabrik. That was also the place where most probably the first flat slide carb was made, designed by the famous engineer Alexander von Novikof, the carb was called the ALNO carb. If I am mistaken please correct. By the way , that's something what a lot of tuners should use on their high revving single two strokes, a separate float chamber. It can be combined by some simple engineering with an integral float carb.
They could discover that a separate float job can give much better carburation because of lack of vibration on the mocking Fikunis and Keihins even with rubber manifold jobs. The famous Pommy job AMAL match box float chamber is meanwhile again available from Blighty at reasonable price. Just look for it at Ebay UK. And ehh, the MZ RE125/1960 can be acquired by someone with reasonable funds...

Sorry, it has to be quite.

Bob, this is stuff I know nothing about, so I Googled it, and got pictures not only of that "matchbox," but of more ordinary looking cylindrical-shape remote float chambers. Is the matchbox superior to these? I had some round-slide Bings on a fuel-burning outboard in the early Sixties that had remote chambers like this. Also, did you bikers ever try floatless setups? The floatless chamber would have an overflow pipe (return to fueltank) set at the height that hopefully gave correct "head."

I was popular with a few old Brit-bike owners here, many years ago, after I encountered one in a bike shop, complaining that his Triumph no longer would idle smoothly because the Amal carburetor slides were worn out (I think those carbs were all rigidly mounted)(and probably didn't have very good air filters). I offered to coat his slides with a light build-up of a sprayed-on heat-cured moly coating. Worked well enough for him that he started sending his pals to me. I've also used this light moly coating on rotary valve cases.

Floatless carbs are used on the present speedway bikes. These methanol burning 500's restricted to 34mm carbs are good for 75-76 HP and use a lot of fuel. A typical characteristic of a methanol-air mixture is the air-fuel ratio that can be burnt is a lot wider (from lean to rich) than with gas or petrol as we in Europe say. Also a very rich mixture with methanol is happy to be burnt, the real advantage is that thru internal cooling the piston runs a lot cooler and the engine's thermal reliability will improve considerable. Fuel consumption however more or less doubles. Wth increased cr power can go up by 20%. I think that running a carb without float chamber should be possible. Personally I would than prefer a small electric pump driven with a pwm (pulse width modulation) circuitry. Advantage here is that supply and pressure can be controlled very accurately, the continuous lowering fuel level in a tank causes a changing pressure.
It's told easily but I think worthwile trying. I still have quite a few of these pumps available from my "museum". Another advantage of a floatless carb is very obvious the non presence of emulsion in front of the main jet. Have a go I'd say.
The coating you referred to could be the 321D from Molykote, I use it already some 45 odd years. It is sold in aerosol cans and in liquid form, real good stuff.

I have often wondered why the "chainsaw-type" of carb's are never used on 2-stroke bikes. main advantage would be that the main jet is simply adjusted with a screw.

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next to no float-bowl , fuel is pumped in by the cranckcase vacuum. should try it myself on my MX-moped

These pumper carbs as they are called are in fact not carburettors but low pressure fuel injection. If I'm wrong please correct me.
These pumpers have 2 pump diaphragms, one to get fuel into the system and the other one for punping the metered fuel thru the atomizer/nozzle. The big disadvantage is the the two needle jet system only controls ttwo rpm ranges: one low and one high. In engines where a broad power range is used this causes a dangerous lean area between low and high rpm. I used a Keihin pumper in 1974 on Sylvain Geboers works Husqvarna 250 withe the mentioned result. However a very keen Swiss engineer by the name of LORENZ AEBI had a very smart solution to solve this problem: he incorporated a needle jet controlled power jet for the high rpm, used the previous high rpm needle jet for mid range and the low one still as low rpm jet. His carb was used for a very long time on the then dominating Rotax aircooled direct drive 100cc ( 50 x50) kart engines. He called his carb IBEA the reverse of his name. Lorenz is now at least some 80 years old and Lorenz junior is still producing Ibea's, unfortunately with only 2 needle jets which is caused by silly homologation rules. The 3 needle job had a inclinated brass flat slide of 1mm thickness and did an extremely good job in those days. The base of most of the pumpers is Tillotson. Othe manufacturers copied AEBI's idea. Their 28.5 mm type might be a vey good thing on a 50cc. For my own classic racers I have acouple of NOS Keihins in various sizes. Needless to say tey need the AEBI mod. I forgot to mentin these carbs are driven by crancas pulses. They can also do a good job on 4strokes.

Some of the superkart people employ twin vacuum pumps.  One pumps into the (no-float) bowl and the other pumps out of the bowl to maintain the fuel level.

Bob mentioned : " They can also do a good job on 4strokes. "

The Tillotson diaphragm " carbs " were OEM on H-D from ' 66 - ' 70.

Many of their race bikes from those years were fitted with same. There was even a dual carb set-up developed for the KR flat heads.

The devices used Idle & Intermediate adjusting screws ( needles ) AND there was a replaceable main jet.

The main jet was NOT controlled manually. There was no needle for the Main. Different sized jets were available.

For the street bikes, an accelerator pump was also fitted. The big inch motors needed that, or " coming off the line " they would backfire.

To augment the Intermediate fuel supply, a direct passage way from the fuel chamber leads to the Main nozzle. That passage has a check ball. As the throttle butterfly becomes fully opened, internal pressure makes the ball seat and fuel is delivered & metered only through a SECOND passage. Inside that second path is the Main jet.

These " carbs " can be nothing but trouble. They have three replaceable plugs ( welch plugs ) which must be removed when the body needs to be cleaned.

The small check ball for the additional Intermediate supply is under one such plug. Obviously, prying out a metal plug and smacking a new one into an aluminum die cast body can lead to trouble. It's easy for the body's plug area to lose metal, or become slightly out of shape. The thing needs to seal perfectly and not allow any leakages or the fuel metering is upset.

Today, the majority of those cycles from 1966 -1970 will be fitted a Bendix carb. Bendix was OEM from 1971 until Harley began using a very similar version made by Keihin. These use a normal float and work well.

There were guys trying to run diaphragm carbs on two-stroke dirt bikes in the 60's & later.

Personally, I never saw these guys having much success.

As Bob stated, there was a problem with versions having only two needles.

That's right. Blasting into a turn, shutting off, and getting right back on the throttle requires a steady ( correct-to-rich ) mixture throughout the entire RPM range being delivered, or else !

Think about this : These carbs were good ( and NEEDED ) for chainsaws. When going full blast through a tree / log it's WFO with full fuel & oil being delivered. When the cut is finished and the " trigger " has been released, the load on the motor quickly drops to a bare minimum. It's mostly a situation of ON with load, or OFF with no load. On a motorcyle hauling-ass into a turn with the transmission gears being knocked down a cog or two, the load on the motor will be increased greatly. It needs plenty of fuel and lube to keep it cool & lubed. When getting back on the gas, the fuel must be delivered in correct proportions for smooth acceleration. That requires a multiple system of over-lapping " jets ".

Bob van der Zijden a écrit:

These pumper carbs as they are called are in fact not carburettors but low pressure fuel injection. If I'm wrong please correct me.

Beeing a long time silent reader of this long and very informative thread , I might finally have found an area where I hope I can make a small contribution.

I race radio controlled cars with 23cc 2-strokes, and we use Walbro WT-series membrane carbs. RPM range of the engines is abt 8-22k rpm, and the carbs handle it pretty well even if it's not just on/off on the throttle.

In these carbs, one membrane works as a fuel pump operated with the pulses from the crank case. The other membrane works as a 'float chamber' with atmospheric air on one side, and a chamber of fuel on the other. The membrane operates the needle that lets fuel into the chamber via a lever and needle. Modifying the lever changes the volume of fuel in the chamber and characteristic of the carb. However, it is actually the pulses in the intake that sucks the fuel into the carb and mixes it with the passing air.
A butterfly valve placed downstream of the venturi acts as throttle, and there are a number of small holes in a row in the bore that controls the fuel flow from the L needle as the butterfly moves past them when the throttle is applied. These can be modified to adjust the fuel flow up to abt 1/3 throttle opening. After that, most of the fuel flows through the main H jet and the hole for that is placed in the venturi.
In some cases the holes in the carb may be to small for the required fuel flow for the application, so the holes are restriting flow, and not the needles. In some cases it's the 'feed line' to the 'float chamber', and in other cases it can be the holes to the H or L fuel paths.

Some of these carbs even has more of a 'ball valve' type of throttle. Others also has a built in 'accelerator pump' (extra fuel is pumped in when throttle is applied). There are a lot of variants of these carbs, so some may work diffrently.

There are a number of Walbro manuals availible on the internet, so those interested in the details can google to find more information.

It just turned Wednesday June 3 in Thailand: Jan Thiel, Happy 75th Birthday!

Happy Birthday, Jan!

Happy Birthday Monsieur

Happy Birthday, Sir !

Happy Birthday. Your accomplishments are much admired !

Happy Birthday dear Jan and thanks verry a lot for your posts in this genial forum.Long life at you and all of your familly.Best regard.Zézé

How much did the development of the 125 contribute to the RSV 500?
Have you (Jan and Frits)worked on it?
Any info for the 500 or links are welcome!
By the way,I have introduced myself
I just changed my user name because i could not log in anymore with my old one 3xp4tpsmtt...

No, I never worked on the 500, but I saw everything that was done.
Frits never worked at Aprilia.

Mr.Thiel,i do not want to bother you with too many questions again If there is anything
you would like to share...
Like... was it scaled up 250 with technology from your 125 cylinders(i guess so...)
Was it really 500 or 4+
So Frits was your personal assistant.

Panas a écrit:

So Frits was your personal assistant.

That's right Panas. I could fry the best chips that Jan ever tasted. He said so himself .