Gary – Lambretta LD D C LC Carburettor MA18B & MA19B

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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 – MA19 B3 carburettor that has a choke lever and normally accessible via an open flap on the side panel (Italian models)

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Hi Gary,

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I have three Lambrettas fitted with Varitronic ignitions that did not need to make modifications to the carburettor or replace any jets.

If you have had the Varitronic working with a different carburettor, that’s great news. Sometimes a Varitronic can be difficult to get going on the first time installing and they require a number of kick over to get started.

I apologise if some of this is basics, as I have no idea of your knowledge of carburettors, but I will cover it anyway. I have also added a few comments that may not apply, but they may be worth a review at some stage.

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.

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.

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 your problem, 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 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
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
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 – (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. 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

Lambretta Journey

Note: Missing is the removable brass screw with a silver washer that is above item 6 (Float Needle), 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 part on the image above. This does not seem to be a part that is available or shown in any manuals. This also needs cleaning.

Lambretta Journey