Lambretta LD Carburettor | Lambretta Journey

Lambretta C / LC / D / LD Carburettors and information

Note on images below – click on image to increase size

125C Zenith Carburettor (Image1)

125LC Delloroto Carburettor (Image2)

125D / 125LD Delloroto Carburettor (Image3)

125LD Dellorto Carburettor (Image4)

Late Models – D & LD Carburettor (Image 5)

MA18 / MA19B Late Lambretta Carburettor (Image 6) With Side Panels & Air Intake / Cable Operated

Often found on a MK1 Early D / 125 LD (Image 3 above)

Noticeable by push button (Item 20 known as a Tickler) to push float down / flood chamber.

00412036 – MA18 B2 Carburettor (Without stainer)
00412107 – MA18 B3 (1/2) Carburettor (Without strainer)
00412111 – MA18 B3 (3) Carburettor

Blend 5% spark plug short thread 225 Bosch scale Sae engine oil 15/50 400 gr. Transmission oil sae 140 100 gr.
Carburettor MA18B2 max jet 75 min jet 40 valve 50.
Carburettor MA18B3 max jet 70 min jet. 45 valve 75
Zenith carburettor 18 max jet 102.
Advance ignition Marelli 34-36.5 mm on the circumference of the flywheel. Ignition advance Filso 32-34 mm on the circumference of the flywheel.

Dell’orto MA 18-19 B3
Float Needle
Carburettor needles Dellorto
type: D1
used with Dellorto-carburettor: Dellorto MA, Dellorto PHBL, Dellorto UA, Dellorto VHST
diameter shaft (A): 2.45 mm
diameter tip (B): 1.5 mm
length taper (C): 15.4 mm

Main Jet (Fits under Atomizer)
Main Jet / Small thread jet (5mm) for DellOrto carburettors
Thread size – 5mm (wide)
Head size 7mm
Full height 8mm (from head to bottom / length)
Head part height 3mm
Then choose your size – standard ones include
Main Jet – 68 MK III Part No 00412100 – LD125;
Main Jet – 72 for MK 1 LD150 ;
Main Jet – 75 for MK II & III Part No 00412017 – LD150

Needle Float Part No 00412040 – Main Needle Jet D.1

Often found on a MK II / III
(See image 5 & 6 above)

00412122 MA19 B4 – Model D150 Carb
00412138 MA19 B4 – Model LD 150 Carb
00412143 MA18 B4 – Model LD125 Carb

Cable operated (Cable near the passenger seat)
Note differences in D and LD Models (Air intake & Choke. Due to no side panels, the choke is operated via a pull down Lever action)

Dell’orto MA 18-19 B4
Float Needle
Carburettor needles Dellorto
type: D3
used with Dellorto-carburettor: Dellorto MA, Dellorto PHBL, Dellorto UA, Dellorto VHST
diameter shaft (A): 2.42 mm
diameter tip (B): 1.5 mm
length taper (C): 14 mm

Lambretta LD D C LC Dellorto Carburettor

MA19 B 4 Part No 00523238 (Complete carb – without filter 150D)
MA18 B 4 Part No 00412143 (Complete carb – without filter 125D)

In this news post, a Lambretta Dellorto MA19 B4 / MA18 B4, normally fitted to later Lambretta models, but found on some early ones, if someone has swapped it out. Not to be confused with the earlier model MA18 B3 – MA19B3 carburettor.

Note – This news post will be added to and include sizes to look for parts. For example,

If looking for a Main Jet and it was not available at a main dealer you would research online (eBay):

Main Jet / Small thread jet (5mm) for DellOrto carburettors
Thread size – 5mm (wide)
Head size 7mm
Full height 8mm (from head to bottom / length)
Head part height 3mm
Then choose your size – standard ones include
Main Jet – 68 MK III Part No 00412100 – LD125;
Main Jet – 72 for MK 1 LD150 ;
Main Jet – 75 for MK II & III Part No 00412017 – LD150

Needle Float Part No 00412135 – 2265 Main Needle Jet D.3
Adjuster Screw Part No 00412042 – 2115
Starter Jet (No 75) (Choke Jet) Part No 00412139 – 2119
Atomiser Part No 00412097 2058 – Atomizer number 255B

Carburettor Servicing
If you have not serviced (stripped down) either of the two carburettors that you have tried to use and you have never had them running, this may be worth doing. Some or one of the Jets may be blocked and may need a review, or the float may be stuck. Note: do not use small needles to clear out jets as damage can occur easily. Ideally you would get a carb cleaner fluid and soak the parts and then use a high pressure air spray can to blow out the holes. This is not always necessary though.

Gaskets

Carburettor kit seals can be problematic at times and after cleaning, some new gaskets kits never quit fit right and you may be left with fuel leaks. I use (yellow tube) Hylomar Fuel Resistant sealant for this and if I have any problems in this area.

Main Adjustment Screw

If you have not stripped a carburettor down before and looked at some You Tube videos, most will talk about understanding how many times the adjustment screw needs turning and to take note of where it is at the moment. However, if you have never had the carbs working, this is irrelevant for you, as someone may have already been playing around with adjustments, which might be why neither are working at the moment. Most mixture screws for carburettor, when screwed in all the way, start with one and a half turns outwards, which is also what Lambretta LD books recommend, then once running, you may decide to play around with it from there!

Tips to look at:

Spark Plug
Although not obvious, after kick starting, it may be worth checking the spark plug status and as well as checking the gap size, check if the plug is dry, slightly wet or very wet. If it has fuel on it and it looks very wet, you have too much fuel getting to it. Sometimes drying the spark plug, waiting a minute and trying again aids in this area and getting it started to review further. Some people say kick it over a few times after removing the plug. I tend to clean the plug, leave it a short while and try again. This does not cure the problem long term though as there are problems else where and you may still need to service the carburettor.

Please review these two page for tips and tricks on reading the spark plug, which may also aid in identifying a possible carburettor fuel issue:

Piston Ring / Cylinder Head
Probably irrelevant for most problems, but consider at some stage removing the head and looking to see if you have carbon build up on the piston and if you do, remove carefully with a solution. If you recently rebuilt it, the piston top and head is probably already nice and clean of carbon build up.

Choke
Remember to only use your choke when starting for the first time. Do not use your throttle when starting from cold.

There is a choke jet (Not the main jet) on the carburettor that is activated when opening the Lambretta on choke, to give it a richer fuel and aid it in starting from cold.

Float Issues (Stuck?)
Sometimes a slight tap on the side, where the float is, can help, if the float is stuck and you have fuel inside the carburettor.

Remember also, the carburettor needs to be level. If its at an angle, the float may get stuck easier.

There may be a build up of muck at the bottom, where the float is and the float needle may be getting stuck in it!

Working with a different non Lambretta card
Sometimes getting the Lambretta started via any working carburettor and then installing the faulty one straight away aids a little in fault diagnosing. Having the working carburettor running on it and then swapping it over straight away, you have a good amount of fuel where it should be, so this can sometimes aid in getting a faulty one working, then once running you can look at screw adjustment if required. Again though, the carburettor may benefit with being cleaned / serviced inside.

Air Leaks
Although unlikely, please check for air leaks around the carburettor (bracket) area if you can. Washing up soap can sometimes help (bubbles when you kick it over) Also ensure the body is not loose with bolts securing parts in place on the carburettor. The main culprit is of course the two main screws on top, that secure the float and top body part in place. This would be more obvious once the Lambretta has started and fuel starts leaking out.

Carburettor Broken Down Information (Use with image below)
Late models – Lambretta Dellorto MA19 B4 (150D/LD) / MA18 B4 (125LD)

The carburettor consists of these main components:

Pilot Jet – Item 13 below – (No. 40 / 45 / 48) – tiny jet screwed up inside. The higher the number, the more fuel you will allow in. A very small screw driver is required to get to it. Check and ensure it is there and the hole in the jet is clean and clear. If you have purchased a second hand carburettor, who knows, someone may have removed it before sending it to you! Original Dellorto Jets do have the number on the tip (40; 45; 48), but it’s very difficult to see with the human eye. A number of mobile phone cameras can pick up the number. Don’t get too close to it; focus and then zoom in on the tip. See Note5 (Pilot Jet Adjustment Screw) below for further information on sizes and why you may need a bigger or smaller Pilot Jet. The pilot jet supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

How do jet sizes affect my carburation?

The larger the number is on the jet, the more fuel it supplies to the engine (“richer”).

The smaller number is on the jet, the lesser amount of fuel that will be supplied to the engine (“weaker”).

For example, if you fit a performance type exhaust to a standard Lambretta, then you will probably need to increase both the sizes of both the main jet and the tick-over (“idle”) jet by a few numbers to increase the amount of fuel needed for perfect running, as more power is produced.

If your standard tick-over idle jet is No.45, you may need to increase this by a few jet numbers to No.48, or perhaps to No.50.

Pilot Jet Adjustment Screw – Item 10 below, also known as idle mixture screw – (Adjustable on the outside and has a small hole in it / is horizontal to the carburettor / also known as a mixture strength adjustment for the pilot jet). With it being completely screwed in, its normal position is turned (unscrewed) outwards around one and a half turns. Note1: the Lower the number of turns, the pilot jets let in less fuel. The Higher the number, the pilot jet allows more fuel in. Note2: The adjuster screw allows extra air to reach the small chamber above the jet when it is unscrewed. Note3: If it is over-tightened at any time (spring taken off), its conial end will become damaged and the regulation will be upset, so please treat with care and leave the correct spring on it. Note4: It is also known as an air screw which works with the pilot jet to enrichen or lean the low idle air/fuel mixture. Note5: If you must turn it out more than 2.5 turns you may want to go up to the next larger pilot jet (from 40 to 45 or if you are at 45 then from 45 to 48). Visa-versa – If you are using a pilot jet size of 48 or 45 and need to turn it under 1 turn, you may want to go down a pilot jet size.
Main Jet – Item 17 below – (68 MK III LD125 – ; 72 for MK 1 LD150 ; 75 for MK II and III LD150 – for Carburettors – MA 18 B3 & MA 19 B4 (Screws under the bottom of the atomiser) this controls the fuel when you open up the throttle (supposedly between 50 and 100 percent power) In some cases where pinking is experienced, a larger main jet may help. Pinking is often related to spark plug and compression ratio though. It is possible that someone may have installed a 68 MK III LD125 Main Jet or you purchased a LD125 Carburettor, so you may want to check and replace the Main Jet with the correct LD150 (See above for specs – yours is a 75 – normally available at various places to buy). The wrong one should not stop it from kick starting though! To identify a 68; 72 or 75, they are clearly marked on the bottom.
Atomizer – Item 14 below, also known as a Needle Jet (Click here for more info on Dellorto) – (Big opening is where the Jet Needle goes into) that has minute holes through which the petrol escapes in the form of mist directly into the air intake. Fuel is regulated by the main jet and the atomiser outlet areas which is also impacted depending on how far the needle is located inside the atomiser. The more the throttle is open, the more mist / fuel is released in this area. The atomiser atomizes the liquid fuel in a carburetor, which greatly increases the operating efficiency of the engine and reduces the amount of objectionable exhaust gas emissions. The atomiser may look fine, but it can be worn by the needle and at times the needle and atomiser may need replacing. You should still be able to start it up if there is no obvious signs of it being faulty. Faulty needles and atomizers (Combined together) allows too much petrol to pass. These are very difficult to determine if they are damaged. Needles are easy to find online. Atomizers are slightly harder.
Needle – Item 8 below, also known as a Throttle Needle) – Moves up and down the atomizer, when the throttle is open (between 20 and 80 or 15 to 60 percent of throttle). At idle, the needle sits inside the atomizer. As you use the throttle, the needle comes further out. When your at full throttle, you come off the main jet. If fuel consumption is found to be excessive and the ignition timing has been checked, then the fuel system should be examined for possible leaks and the throttle needle lowered in it’s clip. If there is still no improvement, the needle, jet and atomizer should be replaced (Large fuel usage not to be confused with excessive open throttle causing it!) The diameter of the needle jet (2.55mm) remains unchanged on all LD’s.
Throttle Stop Screw – Item 16 below – (Second adjustable screw on the outside – it is the angled screw with no holes in it) When dismantling the carburettor, the cut away portion of the piston (Item 7 called Throttle Slide) needs to face towards the air cleaner. It is prevented from rotating by this spring loaded screw, which acts as an idling speed control. When screwed in, you will see it put inside the carburettor and the piston needs to be able to slot in via this screw, to stop rotation.
Starter Jet – Item 15 below – Used when choke is engaged. The throttle must not be used when starting a Lambretta up from cold. When the Choke Assemble (Item 3) is pulled up, the Starter Jet is used. See tip below.

TIPS & Further Info

For these carburettors, I unscrew and remove / pull out Item 3 (Choke Assembly x3 parts) all together and kick over to get started. Once warm and after a minute or so, I place the assembly back in (Just the lower part – not the spring or upper part) The lower part may be pulled out using something like a paperclip that has been bent to aid in it’s removal. Sometimes it is better to have the Chock Assembly completely out of the way / removed to get it started for fault diagnostics. Once the fault is rectified, you can put the complete 3 part choke assembly back on and add the pull cable.

Old carburettors can be temperamental and require inspection more frequently than you may desire. so please bear this in mind. Remember to install a fuel filter in the line and ensure enough fuel is getting to the carburettor. Sometimes, bypassing the fuel tank tap is required.

If you can see an air gap in the fuel line, this does not mean fuel is not getting to the carburettor, so please do not be alarmed by air gaps in the fuel line.

Facebook
If all this fails, for other tips, consider posting on Facebook (Lambretta Shaft Riders) for some further comments.

Some basics

Lambretta MA19 Dellorto Diagram (Most common one talked about)

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Note: Missing is the removable brass Needle Seat (Nut with extension for needle to sit in) with a silver washer that is above item 6 (The Float Needle above). The Float Needle enters this hole and fuel comes through this hole as well and to the chamber, which raises the float needle. You can see the 10m nut (which also has a silver washer) part on the image above. This also needs cleaning. Part numbers if required are as follows. Alternatively, buy a second hand carb for £30 to £40 from e-bay and butcher it.

Image below – Float Needle Seat – Innocenti – 00412052 (Does not come with silver washer)
Image below – Float Needle Seat – Dellorto – 0227500_33 (Does not come with silver washer)

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Rejetting and why you may need to do so

Why Rejet? To avoid engine seizure its extremely important to make sure that your idle speed is not too low and the mixture is not too weak .

When you deaccelerate from your high speed and shut off the throttle, the engine is still being driven at a high rpm. However, you now only have a small amount of petrol and oil flowing through the idle jet to lubricate and cool the piston and rings . As a result, you risk seizing engine.

See here (Not an LD carb, but principles are the same) for information on how to rejet

Scooter Center Carb Rejet External Link

A clean spark plug means you can skip the rejet. A black, sooty-looking spark plug means you’re running rich with too much fuel in your mix. If you see white residue, that means you’re running lean with too much air. In both cases, you need a rejet.