Be warned carburation is the most single item to perfect and it is probably one of the topics that creates so many debates and differences of opinions. I could name you three different shops, most would argue among the best in the UK, all three would give you different jetting settings for the exact same set up. So why the big difference, well its all down to jetting needs to be done to an individual scooter, no two set ups are ever exactly the same. This is why it is so terrible hard to give you a quote of jetting and say there you go, in practice the best way to have your scooter jetted correctly after any type of tuning work would be on a dyno or rolling road.

Because of this I am not going to get into the the depths of this carb is best, that carb is rubbish debate as I open myself up far to much from attack. Unless I sit on the dyno for hours on end with hundreds of different carbs and jets on dozens of different tune / kit types the information I would giving out would be worthless. One dealer recently opened a debate on the tech help pages stating carb type X was best, but unless that dealer had tried ALL other types of carb on exactly the same scooter under exactly the same atmospheric and other conditions, the statement could only at best be their opinion and not the truth, no matter how well the carb performed. OK the basis of this page will be to build a data base of jetting specifications up on. This by itself is no mean task because of the things mention above.

All jetting and settings contained within these pages are for your guidance only and should be considered as base settings for you to start with, whereby you would continue to try to find the correct settings.

Please note carburettor jetting for the standard Lambretta carburettors
are already in the Workshop Section for standard carbs.

To select jetting details for performance kits, please choose your kit below, and any details we have on file will then be displayed. (at present where we have no details, no link will be found)

Kits for small block engine cases, 125/150/175
175cc kit
SR kit
Mugello kit
GT kit
Imola Kit

Kits for large block engine cases, 200
TS1 200
TS1 225

You can help us to help others by submitting your jetting details on the LCGB forums.

When jetting your scooter, the engine should be up set up properly, including correct timing etc, make sure you run the scooter first so it is at full operating tempreture. Make sure you have a clean air filter, all hoses and connections tight, and no other problems. We recommend that you use some masking tape around the throttle housing, mark out the starting position, and the full throttle position. Half way between these make another mark, and then divide this mark again. You are looking to get idle (no throttle) one quarter, half, three quarters, and full throttle positions marked out so you can tell what is affecting the running in which particular throttle position. As well as seeing how your scooter runs you can use the spark plug colour as a guide to what is happening. Follow the guide below step by step, do not move onto the next step before you are happy with the outcome of any changes you have made and have the step you are on correctly set up. Changes should be made one at a time, otherwise you will never know how or if what you have changed has worked. One thing to remember though is that if you vary the use of your scooter, you will need to change the spark plug type (heat rating) and your jetting to have a perfect set up! It is probably best to choose you main use of the scooter, and jet accordinginly!

Idle to one quarter throttle
This is controlled by pilot jet, air screw and to some degree by the cut out in the throttle slide. First thing we will concentrate on is the pilot and air screw mix. If the engine is running poorly just of idle, the air screw can be turned in or out to change the air fuel mixture. If the adjustment screw is in the back of the carb, screwing it out will lean off the mixture, while screwing it in will richen it. If the adjustment screw is at the front of the carb, this will work the opposite way around, i.e. screwing it inwards will weaken it. If by turning the air mix screw less then one turn out, or more then 2 and a half turns out the scooter does not operate properly, the pilot jet will need to be changed.

Roughly speaking if your scooter revs up and down on its own it will be weak, if the scooter ticks over unevenly and lumpy it will be to rich. (I.E. PHBH carb has the air screw at the rear, the air screw is screw in to richen it, if you have to turn it less then one turn in you will need a bigger pilot jet which will richen the mixture. Fit the new pilot jet and repeat these steps again.)

The slide affects carburation between 1/8th to 1/2th throttle, ie just as pulling away. It especially affects it between 1/8th and 1/4qtr but has a lesser effect up to half throttle. The slides come in varouis sizes, and the size is determoned by how much is cut from the back of it. The larger the cut away the leaner the mixture. If when you pull of gently your engine stalls, the slide will be to lean. If when you set off the engine splutters, it will be to rich.

One Quarter to three quarters throttle

The needle and needle jet (in the SH series, and the Jetex, they only have an atomiser and no needle) affects carburation from one quarter throttle to three quarters throttle. To a lesser extent the slide will as well, but this should have really been set up in the last section. DellOrtos supply a wide range of needles and needle jets for the PHBH range, so the are a hugh number of variteis you could use to fine tune your scooter. In pratice Lambrettas tend to run better on a limited number of these, X2 to X25 needles could be used. The AS and AV needle jets from 262 to 266 tend to work best.

The needle jet differs by internal diameter and length. The AS range is longer then the AV, the longer the needle jet will cause the mixture to weaken at just over one quarter throttle and unde accleration. Within the AS range you can get different sizes, the lower the number the leaner it is. I.E leanest needle jet for a Lambretta is AS262, the richest AV266. You can buy other needle jets, but these tend not to work on Lambrettas.
The needle controls how much fuel is pulled up into the carb venturi and can be the hardest to get correct. The thinner the taper, the richer the mixture, needles also differs in taper, which is designed to fine tune the different mixtures at different throttle positions. The needle can also be adjusted by repositioning the clip on the needle, this has four positions. The highest clip position will be the leanest setting.

OK to set the needle, and needle jet, accelerate through the gears unitl the throttle is at half throttle, a slight up hill grade is the best place to do this. After a few seconds of running at half throttle, pull the clutch in and stop the engine. Do not allow the engine to coast or stop without the clutch being pulled in. Remove the spark plug and look at is colour, it should be a chocolate brown colour (or tan). If the plug is white it is to lean, or if it is dark brown or black, it is to rich. Also you could tell while running, if your scooter feels as if it has a flat spot on and around mid range, or it does not start to perform until the throttle is fully open, it will be to weak. If to rich the scooter will splutter at this position, but it will probably clear as you go onto full throttle. Again as we have stated, change one thing at a time, run the scooter down the road again, and then check.

Three quarters to full throttle
This is primarily controlled by the main jet. This operates when the needle is high enough out of the needle jet, which then allows the main jet to take over. The higher the number of jet, the richer it is. It is best when setting the main jet to start rich, and come down one step at a time to get it correct.

With the main jet, running to weak will give symptons of if you run on full throttle and slow down, your engine will feel as though it suddenly speeds up a little! Also the spark plug colour will be grey to white, this is very dangerous for your engine. If rich the engine shoud splutter and 4 stroke. OK to set the jet, most kit or carb suppliers would have recommended a base setting, fi correct it should be on the rich side. Run the scooter again, probably in third gear at just over three quarters throttle is best, these is no need at this stage to hold your throttle fully open. Again as before wht the needle settings, pull the clutch in, kill the engine and stop. Check the colour of the spark plug. Hopefully you will be choclate brown or darker. If running in best to be on the darker side, if trying to perfect, then chocolate brown. Contiune to lower the main jet size, one size at a time and re run the scooter to check operation and plug colour unitl you are happy you have the correct size.

Final checks
On your first outing with a new jetted set up, ideal it would be best to start of at a slower pace, and gradually increase speed once you are happy that every thing is running fine. If the scooter feels weak in any throttle position, stop and adjust your settings until it becomes richer or correct. Keep an eye on your spark plug colour until you are totally happy it is the correct colour and all is fine.