Dell'orto carburatori piccola aggiustamento 

If you have just fitted your new carb, or If your scooter was running well but, isn't any more then read on. During periods of particularly warm or cold weather, you may find that your scooter is not running as well as expected. This is due to air density, namely the amount of oxygen in the air. Two main things affect air density, temperature and altitude. The same volume of air when cold will have much more oxygen in it and therefore will burn far better. So throughout the year you will need to tweak you carb a little. 


This is where the mixture screw comes in. This small screw located at the side of the carb can make a suprisingly large difference to the way your scooter runs. Typically the carburettor should be set up to allow a little room for adjustment in either direction. If you find that the screw is fully in, or fully out, then it's time to change to the next pilot jet. By doing this, it should put you back in the middle somewhere. For info on that click here. 

The mixture screw is not always on the same side as the one shown in the photo above. In fact you can specify which side you want it on, due to access restrictions and such. If you look at the photo you can see that this carb has been stamped BS at the end of the model code. The S denotes that the mixture screw will be on the left, as left in Italian is 'Sinistra'. If you have a D spec model then your screw will be on the right, Italian for right is 'Destra'. More on Dellorto carburettor model codes can be found at the end of this chapter.

Adjustment couldn't be simpler. Go for a short run to get things up to temp first. Then if you want to lean the mixture off, turn the screw clockwise or anticlockwise to richen the mixture. Begin with small movements of about a quarter turn at a time to start. It is quite sensitive. Check that the throttle cable has about 1mm of free play in it when closed. Then turn the idle screw in to obtain a fast tick over of around 1400 RPM. Then adjust the mixture screw to obtain the most even running. Now take to idle back down to normal. If you find, that whilst sitting ticking over, your scooter revs up and down slightly on it's own, then this is a sign that it is too lean. However, If it's lumpy and uneven this could indicate it's too rich. If you find you are unable to obtain an even idle then you will need to change the pilot jet. More info on that can be found here.

For Lambrettas running the carb on the left side, as in the 'original' side, then I find it preferable to have a BS carb with the mixture and idle screws on the left hand side, as featured on this Dellorto 28mm PHBH BS, picture below. It makes it easy to adjust with a stubby flathead screwdriver.

The idle screw is the larger one and simply adjusts the tickover rate. Sometimes these will have a plastic thumb wheel to make adjustment a little easier, see example right. Note the rear exit fuel tap, it can get very tight in there without one. 

If you are just looking for a quick jettting guide for a Lambretta with Dellorto PHBH carb then follow one of these links below. 


Lambretta Dellorto 26mm  Lambretta Dellorto 28mm Lambretta Dellorto 30mm

more on Dellorto carburettor codes

S - "Sinistra" - "left" in Italian - Left hand idle mixture and idle speed screws.

D - "Destra" - "right" in Italian Right hand idle mixture and idle speed screws.

A - Generally clamp type fitting. 
B -
Generally rubber sleeve mount type fitting. 

So a PHBH26 BS carb, like this one, is a 26mm rubber mount carb with the mixture and tickover screws its left, as you look at it front on. Other letters have no specific meaning. 

Mikuni Carb info

This area is under construction - check back soon


Some infomation on mikuni carbs can be found here for now. 




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