Cambridge Lambretta Workshops

Basic carburettor settings

Tuning a carburettor on a two stroke is probably one of the most important jobs to get correct, but can also be the most difficult. Having your engine set up professionally by a competent rolling road operator is vital to the safe running of your engine.

The table below is a guide and starting point for the most popular set ups and carbs used. Please note this is a guide only as every scooter is individual and needs different jettings. Smaller SH20/22 and PHBL jettings are for running with a GP200 sized airbox, with a hi flow filter. PHBH are for open mouthed, or when using our breathing sweet filters.

The are base settings, by this we mean they shouldget your scooter up and running, and be acceptable for running in. During this period you should keep a close eye on spark plug colour you can use our guide here. To fine tune the jetting to your scooter, you should have your scooter dynod, or you can read our fine tuning your carb guide if you feel comfortable with jettign and setting up your own scooter.
(if you are reading these instructions on paper, the other instructions that we refer to are available from our web site at www.lambretta.co.uk/technical.html)

Factory settings for standard Lambrettas with no modifications

Model	Carb fitted	Choke Size	Slide	Needle	Atomizer	Main Jet	Pilot Jet	Starter Jet	Float	Air Screw
Standard Lambretta set ups and jetting
150 Li I	MA 19 BS5	19mm	65	D18	260 B	95	40	55	6.5gms	1/2-2 turns
125 Li II	MA 18 BS5	18mm	50	D20	260 oB	93 or 73	35	55	6.5gms	1/2-2 turns
150 Li II	MA 19 BS5	19mm	50	D21	260 B	102 or 78	40	55	6.5gms	1/2-2 turns
125 Li II Later Type	MA 18 BS7	18mm	50	D20	260 B	73	35	55	5gms	1/2-2 turns
150 Li II Later Type	MA 19 BS7	19mm	50	D21	260 B	78	40	55	5gms	1/2-2 turns
175 TV II Early	MB 23 BS5	23mm	70	E15	260 B	110	40	60	6.5gms	1/2-2 turns
175 TV II	MB 21 BS5	21mm	70	E15	260 B	88	40	55 or 60	6.5gms	1/2-2 turns
175 TV II Later Type	MB 21 BS7	21mm	70	E15	260 B	88	40	60	5gms	1/2-2 turns
125 Li I	MA 18 BS5	18mm	50	D16	260 oB	92	35	55	6.5gms	1/2-2 turns
125 Li III	SH1 18	18mm	5914-1	-	5899-1	99	42	50	5gms	1/2 turn
150 Li III	SH1 18	18mm	5914-2	-	5899-1	105	45	50	5gms	1/2 turn
Li 125 Special	SH1 20	20mm	5914-2	-	5899-4	98	45	50	5gms	1/2 turn
Li 150 Special	SHI 18	18mm	5914-1	-	5899-2	101	45	50	5gms	1/2 turn
175 TV III	SH1 20	20mm	5914-1	-	5899-2	106	50	50	5gms	1/2 turn
200 TV	SH1 20	20mm	5914-1	-	5899-2	108	48	50	5gms	1/2 turn
SX 150	SH1 20	20mm	5914-2	-	5899-5	102	45	50	5gms	1/2 turn
SX 200	SH1 20	20mm	5914-1	-	5899-2	103	48	50	5gms	1/2 turn
GP 125	SH1 20	20mm	5914-2	-	5899-4	98	45	50	5gms	1/2 turn
GP 150	SH1 22	22mm	7895-2	-	5899-4	118	45	50	5gms	1 - 2 turns
GP 200	SH1 22	22mm	7895-1	-	5899-2	118	45	50	5gms	1 - 2 turns
Lui 50	SH1	mm		-				50	5gms	1 turn
75 S Vega 75 SL Cometa	SH1 20	20mm	5914 - 1	-	5899 - 6	68	45	50	5gms	1 turn
J50	SHB 18/16	16mm	6493 - 2	-	-	60	40	50	3.5gms	3/4 turn
Cento 100	SHB 18/16	16mm	6492 - 1	-	-	70	40	50	3.5gms	3/4 turn
J125	SHB 18/16	16mm	6492 - 1	-	-	72	40	50	3.5gms	3/4 turn
J125 Starstream	SHB 18/16	16mm	6492 - 1	-	-	72	40	50	3.5gms	3/4 turn
Model	Carb fitted	Choke Size	Slide	Needle	Atomizer	Main Jet	Pilot Jet	Starter Jet	Float	Air Screw
Modified / Tuned / Kitted Lambrettas, base jetting details
Budget 175/185 kits Ancilotti	SH1 22	22mm	7895-1	-	5899-2	118	45	50	5gms	1 - 2 turns
Budget 175/185 kits Ancilotti	SH1 22	22mm	7895-1	-	5899-2	125	48	50	5gms	1 - 2 turns
Budget 175/185 kits Ancilotti	PHBH	26/28/30mm	30	X7	AV264	100	45	50	5gms	1 - 2 turns
Budget 175/185 kits Expansion	PHBH	26/28/30mm	40	X7	AV266	120	48	50	5gms	1 - 2 turns
Latest Ultimate Versions of Mugello kits, as supplied by Cambridge Lambretta. NB if you kit has been supplied by another dealer, we would suggest you contact them for jetting advice as we modify our kits. We do not always display our exact jetting details on this page as we can only offer our customers more exact advice due to the differences in kits.
Mugello 186/200 Ancillotti	PHBL 25mm	25mm	45	D22	AQ264	110	50	50	5gms	1 - 2 turns
Mugello 186/200 Expansion	PHBL 25mm	25mm	45	D22	AQ266	120	55	50	5gms	1 - 2 turns
Mugello 186/200 Ancillotti	PHBH 26mm	26mm	45	X4	AV264	100	50	50	5gms	1 - 2 turns
Mugello 186/200 Expansion	PHBH 26mm	26mm	45	X4	AV268	110	50	50	5gms	1 - 2 turns
Mugello 186/200 Ancillotti	PHBH 30mm	30mm	45	X4	AV266	105	53	50	5gms	1 - 2 turns
Mugello 186/200 Expansion	PHBH 30mm	30mm	45	X2	AV268	120	55	50	5gms	1 - 2 turns
Imola 186 Ancillotti	PHBH 30mm	30mm	50	X4	AV266	110	55	50	5gms	1 - 2 turns
Imola 186 Expansion	PHBH 30mm	30mm	50	X4	AV268	120	55	50	5gms	1 - 2 turns
Mugello 225 Ancillotti	PHBH 26mm	26mm	45	X4	AV268	110	53	50	5gms	1 - 2 turns
Mugello 225 Expansion	PHBH 26mm	26mm	45	X4	AV268	120	53	50	5gms	1 - 2 turns
Mugello 225 Ancillotti	PHBH 30mm	30mm	45	X4	AV268	115	53	50	5gms	1 - 2 turns
Mugello 225 Expansion	PHBH 30mm	30mm	45	X4	AV268	125	53	50	5gms	1 - 2 turns
Monza 225 Ancillotti	PHBH 30mm	30mm	45	X4	AV268	118	55	50	5gms	1 - 2 turns
Monza 225 Expansion	PHBH 30mm	30mm	50	X4	AV270	130	55	50	5gms	1 - 2 turns

Typical problems and cures

Engine cuts out
Main jet blocked, Tap or tap filter blocked, Carb filter blocked, Air inlet in tank filter cap blocked, Water in fuel, Float chamber filled with two stroke oil (caused by not turning fuel tap off). Possible ignition fault also
Engine will not tick over unless choke is on
Pilot jet or pilot feed passage is blocked, Pilot air screw not adjusted correctly, Worn throttle slide, Carb top loose
Engine ticks over but will not rev
Throttle cable broken, Throttle slide detached, Silencer blocked, Ignition fault
Engine "4 Strokes"
choke on or stuck on, Choke cable to tight, Air scoop under seat blocked, Air filter needs replacing
Engine misses or backfires
Dirty or faulty spark plug, Air hose detached or split, Air filter missing. See also faults for engine cuts out.
Engine will only run with choke on
Crank case oil seals blown, Main jet blocked, Air filter missing, Hose split or missing
Engine will not start when hot,
Float needle needs replacing, Throttle slide worn, See also faults for 4 stroking
Engine smokes
Too much oil in your petrol, Drive side oil seal blown allowing oil to leak into crankcase, Choke stuck on
Engine will not switch off when hot
Overheating (need decoke), Wrong grade of spark plug, Engine running weak - air filter missing other air leak at manifold or carb top, Exhaust pipe loose
Throttle does not open fully
Incorrect fitment of slide spring, fuel tap interfering with carb tie rod ass, Throttle cable detached at handlebar end.