61- Conversion of 4 stroke carbs into 2 stroke by Lasse

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61- Conversion of 4 stroke carbs into 2 stroke by Lasse

Post by hugomez » Tue Aug 07, 2012 2:42 pm

Hi All,

Here a very interesting tutorial that Lasse just sent me 10 minutes ago. He managed to convert two 4 strokes carbs into 2 strokes carbs.

You can download the document clicking at the blue text below, but because is just text and no pictures, I have also copied the complete content below for you to read here if you want.
Transforming 4 stroke carbs to 2stroke.zip
Congratulation Lasse and thanks for the explanation!!!!


Transforming 4 stroke carburetors into two-stroke carburetors & how I managed to do this…

The "story" started with me acquiring a derelict "Junior Corsa" which had been lying untouched for 49 years!

It had 22 mm DellOrto UB-s carburettors set up for alchohol, with large meggas, & motor running without flywheel, utilising "total loss" racing ignition system.

After recommisioning the motor, I changed the carbs as the following: - main jet 95 - idle 50 - needle E1 - slide 70 - emulsion-tube 262 - later 265.

I expected good running, but in fact I just got lots of power, at revs in the vicinity of 7-8-9000 revs!

Midrange were terrible, major hesitation when throttling on, motor dying at revs under 5000 - bike impossible to use in traffic.

Then, in desperation, I acquired a pair of 20 mm DellOrtos UB b four stroke carburetors, because UBs 2 stroke carburetors were impossible to find!

4 stroke UB carburetors are plentifull to get, and even spareparts for these 60 year old instruments can be had, for example from "Eurocarb" in the UK.

Why I bought these carbs, and took the chance? - See below!

I thought about the Bumble Bee – heavy, underpowered, clumsy - & in theory, unable to fly! Fortunately, the Bumble Bee cant read, so it never knew it couldnt fly – so it just took off…

I started from scratch, knowing very little about the differencies parting the two carb types, and to start with, I needed to know just HOW bad they performed on a two-stroke motor – in this instance, my quite “wild” and unmanageable “Junior Corsa Earles” from 1955.

Maybe you guessed it – they were absolutely useless, even if the bike did start.

By opening the throttle, the motor hesitated or died, for suddenly to “wake up” again, then die completely – and beeing impossible to re-start – it looked like a lost case, really.

After etablishing all the “wise men” were right, about it couldnt possibly be done, I started to think!

How should I transform these carbs? Could it be done?

First I took a good look at the emulsion tubes – they beeing short on my two-stroke carbs, but with nozzles protruding about 1 ½ mm up in the airstream of chokes.

Both versions of these UB carbs used emulsion tube number 265 – where the 4 stroke were much longer, and had a number of tiny holes in the shaft, the two-stroke just had a plain un-holed shaft.

Furthermore, the 4 stroke version had not its nozzles sticking up in the airstream!

So, my first priority were to let these 4 stroke nozzles stick up, at approx. 1 ½ mm, just like the two stroke nozzles.

But how? – Simply by removing 1 ½ mm metal from the emulsin tube shafts “bottom ring” where it should sit tightly in its seat, in the lower jet-end of the carburetor Body.

By doing this, I were allowed to screw the emulsion tubes higher up, into the threaded holes near the airstreram in the choke inlets, so their nozzles showed same height as the two-stroke nozzles.

As I had been plagued by “mid range hesitation” in my former 22 mm Ubs carbs, caused by what I believed to be fuel starvation, I went from mainjet 90 to mainjet 100, before I once more tried these 4 stroke UB 20 mm carbs.

The motor started well enough, but could not really pull, because everytime I tried to open the throttle, the motor died!

Think again – what next?

I decided to raise the floatlevel by a full millimetre – and did this, simply by cutting two gaskets, each of 0,5 mm – to elevate the lid, containing the float valve.

Now, the motor ran better – but far from perfect…

Think again – now more radically.

I needed a slide, with a lower “cut-out” to increase the velocity of the airstream through the carbs Orifices, where the emulsion tube nozzles are placed!

BUT, its impossible to get different slides for 60 year old carburetors – so I decided to bite the bullet, and radically change the slides I had!

From the bottom of the slide I took away 1.25 mm in order to lower it, and make the exsisting cut-out sit lower in the carb body.

At the same time I lenghtened the “spur” in the side of the slide by 1.25 mm, in order to let the throttlestops deliver the same idlespeed as before, as reference.

Remembered also, to lift the needles to top notch (because the slide now would sit 1.25 mm lower)

I were really nervous, because this “bodge” were a “non reversal” step...

Would the bike start? – would it idle? - and most crucial, would the dreaded hesitation when winding on throttle be a thing of the past?

After slightly tickling, the motor started instantly – so long so good.

But, to my utter elation, I could wind on throttle without any hesitation, and the bike just stormed forward, it even pulled satisfactorely from as low as 4000 revs in fourth gear!

From 5000 revs. It really accellerated – pulling strongly and cleanly up to 9000 revs!!

In neutral, I could just fling open the throttle, & the motor reacted instantly – success – success…

Idle became steady and stable at 21- 2200 revs.

So, I became more than satisfied – its like having a completely transformed “Junior”! Fast and tractable, good in traffic, & with a really good turn of speed – easily over genuine 100 Kmt. – and this, with the bike still in “running in” conditions!

So, sampled together: Raised emulsion tubes – 100 Mainjets – 50 idle jets - raised float-level – radically changed slides – needles in uppermost positions – idle control screws just one turn out.

Going from 22 mm UBs 2 stroke twin carbs, to 20 mm UBb 4 stroke twin-carbs.
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My little Formichino here: http://www.formichino.com/pictures1.html