Have just bought a 26mm Spaco carb?

Hi All,

Have just bought a Spaco 26mm carb to replace my 20/20 on my PX125 disc with polini177 and sito exhaust.

I’m not sure what changes to make…at present my 20/20 carb is setup with a 107 main jet…do I just swap over the carb and fit the 107 main jet to the 26mm carb or do I need to change other things such as the BE mixer jet etc…

Any tips on fitting the carb will be well appreciated.

Thanks and Regards,



the 107 main jet is way too small for the 26.
You didn’t specify your sert up if your cranckcases are ported race or not race cranckshaft…
anyway if you are running now with a 107 and it is ok your main with a 26 should be around 118.
Get also 115 , 120, 122.
Start with the 122 and go down till you get the full trottle clean (it is easier and safer going down from rich to get it right) or if you don’t have „good feeling“ do the plug chop technique to check the spark.
(of course if you get full throttle clean with 122, that I doubt since your Sito plus, stay with 122)

Hi Curare,

no porting done as yet…26mm came with:

Came with :

120 Main Jet
BE3 mixer tube
160 Air connector
55/100 slow running jet
60/100 starter jet

Is it just the main jet I change or any other jets plus If I decide to buy a Simonini will I have to make any further changes or does the sito and simonini require same jet.

Thanks and Regards,



first disconnect the throttle cable and start the engine. If start at normal rev it means you have to do something with the cable.
But there is also a screw with which you regulate the slide opening. It could just be that it is too screw in.


I have changed the carb but when I start her up she screams at full rev…choke cable looks fine and throttle cable is moving freely but I did notice that the slider inside the venturi is open a bit in other words it does not close fully…is this normal+±…anyone think was is causing high rev at startup.



…considering your set up I think you can start with the 120 main that you have and then if rich go down untill you get the clean high rev. (I think it will be around 118 or 115, but you have to try I don’t have experience with 26 and Polini with no port done and normal crank)

Get the main jet right and then examine how your engine feel through all its range, then if not regular in a range section can modify something, not before, since it could go ok like this.Yes you have to fix the air screw to get the best transition and fast response from iddle to 1/8.

When you change for a Simonini you would need to change the main jet with a bigger one (one size bigger should do it).


it was the throttle cable…it is closing fully now…I did notice that my autolube does not seem to be working now although the guy who sold me the carb said it was…anyone know how I can check this?