This is a guide to carburettors as used on Lambretta motor scooters, it is not a guide to how a carburettor works or what jets go where nor is it a guide to trouble shooting problems associated with carburettor problems.
MB DEVELOPMENTS HAVE A RUNNING IN AND CARBURETTOR TUNING BOOKET. AVAILABLE BY MAIL ORDER, THIS EXPLAINS THE WHOLE SET UP PROCEDURE FOR AMAL AND DELLORTO CARBURETTORS WITH EXAMPLES AND SHEETS OF JETS USED AND AVAILABILITY.
INTRODUCTION TO STANDARD CARBURETTORS FOR ORIGINAL ENGINES
The original carburettors used on the Lambretta engine from the factory have always been Dellorto.
Depending on the engine used there have been 6 carb sizes.
We will ignore the early MA and MB carbs as they were quite inferior compared to the later SH type fitted to the series 3 Lambrettas.
The SH1 carbs were available in 18 and 20mm sizes.
The 18mm version was fitted to the Li 125, Li 150 and the Li 150 Special.
The 20mm version was fitted to the GP 125, SX 150, TV 175, TV 200, SX 200 and Jet 200.
The SH2 carbs were only available in one size 22mm.
They were only fitted to the GP 150 and GP 200 engines.
The standard carbs still in use in most cases are very old and worn usually making them temperamental.
You usually find that they work very well, or more so they cause the engine to work one day and not the other.
It is difficult to find genuine Dellorto replacements.
There are some Indian copies of the original carbs made by either Spaco or Jetex.
These are only available for the GP range in 22mm.
Although new, these carbs seem to vary in quality and again can work well or tend to be temperamental.
There are cases of totally the wrong jets being fitted and with some carbs the fuel supply holes weren’t big enough and fuel starvation could take place.
Not a lot could be done to improve or tune up the carbs, there were some conversions but these didn’t really improve the engines speed.
Some jets are available to help set up your engine.
Usually most people just changed the main jet.
MB Developments only keep standard main jets for these carbs.
The most common conversion and is still used today was to bore out the 22mm to 26mm.
We find from many customers that this conversion tends not to work and the engine becomes erratic.
When the carb body is bored out the area where the slide sits down now becomes open dragging in air making running erratic.
If a bored out body is used then the inlet manifold needs opening to suit, the original manifold is not really big enough to do this correctly and gets very thin.
Standard carbs will always limit the power and speed of a Lambretta engine, although saying this I have known some reasonably quick engines using 20 or 22mm carbs.
As a comparison a GP 200 could vary in speed from engine to engine.
Innocenti stated the GP 200 had a top speed of 68 – 70 mph, this was with the rider laid down, now most of you don’t ride like that so expect a true speed sat up to be a little lower!
The Indian GP 200s were slightly tuned compared to the Innocenti item, they had larger cylinder ports and used a big bore exhaust.
To compensate a larger main jet was fitted.
We have known these engines to vary in speed from doing 60 – 65mph to some doing well over 70mph.
I have personally had a cut down using a 20mm carb be clocked at 80mph!
This proves that some speed can still be gained using the standard carbs, but power is limited by them!
Standard Dellorto carbs are very primitive by today’s standard, lets face it the design is now over 40 years old.
MB Developments recommend you upgrade to a more modern carb set up!
As another comparison we did an engine for a customer who was adamant that he wanted to use his 20mm carb on a stage 3 Honda 205 using a Clubman exhaust.
He was warned that the carb would let down the engines full potential.
After running the engine he realised that it didn’t perform very well and suspected it was a jetting fault.
The Scooter was dynoed only to find it had 8 horsepower!
A change of carb to a 28mm pushed the horsepower to 15, showing what difference a carb makes.
There have over the years been a number of carbs used with Lambretta tuning.
The list is endless; Amal, Dellorto, Wal Philips, Keihin, Lectron, Mikuni, Bing and many more, some are good some are bad some are better than others.
Basically any carb between 16 – 40mm could be used and set up to work with Lambretta engines.
Amal have been used because they were made in England (they are now made in Spain), they were easy to get, cheap and easy to set up!
In the sixties Amal offered the MK1 carb, this was solid mounted and quite crude by today’s standards.
The sizes used were usually 26, 28 and 30mm, this carb required a special manifold offered by the sixties tuning shops.
Basically the carb worked OK but wore out.
This carb is now available today but has been upgraded and is now a rubber mounted type, it is called the MK1 ˝, there is no great advantage in using this carb.
In the Seventies Amal offered the MK2 rubber mounted carb and is still used to day.
Again a special inlet manifold is required and is available from most tuning shops.
MBD make 2 types of inlet manifolds, one for 28 / 30mm sizes and one for 32 or 34mm sizes both in 150 and 200 stud patterns.
The MK2 offered three sizes of carb bodies.
1. The 2600 series, small body carb used sizes of 22, 24 and 26mm.
2. The 2900 mid body carb used sizes of 28, 30, 32 and 34mm.
3. The 3000 large body used 36, 38 and 40mm.
In the 2600 series the 26mm was the most popular and worked very well on stage 3 and 4 tunes. I ran a 26mm on a stage 3 Suzuki 190, which gave 16 HP.
This carb wasn’t available with chrome slides, but this wasn’t a problem like it was with the larger models.
They were physically quite small and very easy to work with and set up.
This carb could quite easily work on engines up to stage 5 200 motors.
Although filters are available for this carb they weren’t used as they fouled the panel.
In the larger bodied 2900 series the 30 and 34mm carb was the most popular.
Though out the eighties they were very commonly used for stage 4 to 6 tunes.
The 28 and 32mm sizes were used but were not that popular.
Again this sized carb worked very well.
The main problem was the standard zinc slides used to stick in damp conditions, but once changed to the chrome option usually there were no problems.
The only main fault was the locating pin holding the slide used to wear and fall out, the carb rubber if over tightened would split.
The rest of the carb was very reliable if set up correctly but tended to spit back a lot of petrol.
This sized body carb was available in a standard form or power jetted form.
Both types worked as well as each other, most preferred the power-jetted option on the 34mm carb body.
The power-jetted carb wasn’t actually a power jet as it states, as it gave no extra power! Having a power jet fitted allowed the main jet to be reduced in size this then cleaned up low down running.
To compensate on fuel mixture high up the power jet came in to richen up the main jet.
The 3000 series wasn’t very popular and was mainly used in racing circles.
A newer version of the large body is available called a smooth bore but this was no great advantage.
With the introduction of the TS1 cylinder kits Amal carbs were recommended as the inlet manifold was designed for them.
The physical size of the carb meant that the carb touched on the side panels, machining or removing the bell mouth was needed but still the banjo touched.
The only cure on most types of panels was to cut out a large hole.
These, although they worked fine, required a fast flow fuel tap (as do ALL Lambretta engines).
The design of the carbs internals and angle of the carb required the power jet blanking off, at high speeds the engine would suck in air not fuel.
Because of their crude design the Amal carbs should be considered the easiest to set up!
Although we used to use Amal carbs and still sell jetting spares MB Developments prefer Dellorto and Mikuni carbs.
Dellorto have been used because they were made in Italy and were also cheap to bring in through the scooter importers.
Dellortos had the reputation as not being as good as Amal carbs but were in fact better than Amals.
Amals are crude and don’t flow air as quickly as other carbs.
Dellorto offer a large amount of needles and spares compared to Amals, making Dellortos harder to set up!
Dellorto have had over the years a number of different styles of carburettors.
In the seventies a solidly mounted square slide carb was used, this again was quite crude and was actually a four stroke carb but could be set up and worked fine by the standards set then.
The eighties saw a mass influx of 30mm PHBH four stroke carbs, these had a round slide and were again solidly mounted.
It was probably this batch of carbs that gave Dellorto a bad name.
Because the carb was jetted for a four-stroke engine, two-stroke scooter engines ran weak, seized and regularly melted pistons.
It was possible to set the carbs up, I used one for years, but most dealers didn’t know how to set them up and scooter tuning gained its bad name.
The mid eighties saw more 30mm solid mounted Dellortos arrive, these were two-stroke types and used a total different set of jets and again scooter dealers couldn’t set them up! Hence more problems!
These carbs when fitted with correct jets worked OK to a point.
Lambrettas vibrate, it’s a fact of life.
Engines that vibrate then cause a problem as the fuel inside the carbs float bowl turns to froth, this then supplies air into the engine causing the jetting to alter, running weak one minute then rich the next!
This problem only really came about in the mid eighties as people changed from low revving engines using Clubman exhaust systems to racing expansion chambers.
The extra revs with the expansions caused the problems.
Solidly mounted carbs used to wear out slides, needles and needle jets.
The 30mm rubber mounted version PHBH Dellorto carb eventually came into the UK, this cured the unreliable fuel frothing problem, but took a long time for customers to get back onto them as these were better made carbs compared to the Amal design.
This carb is used regularly to day and is our preferred carb for all touring tunes.
Throughout the 1990’s the Flat slide or Oval Bore VHSA and VHSB carburettors became popular.
The VHSA came in two body sizes, the 28mm was the smallest and used a smaller carb mounting rubber the same as the 30mm PHBH types. The VHSA 30 and 32mm had a larger sized body and used the same carb mount as the Amal 2900 series. The VHSB type carbs where again physically larger still! These came in sizes of 34, 36 ,37, 38 and 39mm these used two types of carb mounting one with one groove and one with two grooves.
The flat slide type of carbs came around as mentioned in the early Nineties.
They could be considered the top of Dellorto's range.
As all previous Dellorto carbs were round slide types these vary because now the slide is a strange flat shape hence the name flat slide carb.
The carbs venturi is now an oval shape to improve airflow! So some times these carbs are called oval bore carbs. These carbs seem to work better than their Amal 34mm rival.
These carbs have a much larger air intake bell mouth which leads into the venturi were the slide sits, as it exits the carb the venturi expands to 34mm for the 32mm carb therefore supposedly breathing better!
This works for all the VHSA and VHSB type carbs though out their range so you could say a 32mm is actually a 34mm carb.
With the introduction of the Dellorto VSHA and VSHB flat slide carbs in the nineties, on a TS1 engine the carbs would now fit under the panel (With a minor modification to an over flow pipe on the float bowl)!
Amal carbs became a bit primitive in their use, added with the fact they touch the panel MB Developments were one of the first to introduce the Dellorto VHSA and VHSB flat slide carbs, mainly because they fit under the panel work.
We found that the 36 – 34mm carbs were physically large but still fitted under the panel.
MB Developments noticed with all engines fitted with 34, 36 or 38mm VHSB carbs that a Lambretta engine couldn’t flow enough air giving a lot of spit back in the process at full throttle.
This was noticed on our race teams engines, these engines were doing between 25 and 34 horsepower but still couldn’t cope with the amount of air and fuel available!
As time went on we phased out the larger carbs on road scooters for the 30 –32mm smaller version with no power loss but much improved the engines all-round performance and the engines ran cleaner.
MB Developments found out that flat slide Dellortos are faulty from the factory, which results in fuel starvation, all flatslide carbs need blue printing to prevent this.
DELLORTO CARBURETTOR RECOMMENDATIONS FOR TUNED CYLINDER KITS
Carburettor sizes vary from engine to engine depending on what the engine is going to be used for.
MB Developments recommend three Dellorto carburettor complete set-ups.
25MM 150 / 200 PHBL CARB KITS
MB Developments designed this kit in 1986 for Mark's own series one, but the kit was only introduced in the early nineties.
The reason for this was, people were still using big tunes with big open bell mouthed carbs because that was the norm and all that was available.
Mark realised that a lot of people were going away from tuned engines and restorations were becoming popular.
But the problem with restorations is the scooters tend to be slow, as the only carb choice was the standard carbs already mentioned.
There seemed to be a market for mild tuned engines with a slightly larger carbs but still using the original air box.
The only carb anything like a standard size was the PHBL carb bodies.
The secret to these carbs fitting was to make a special manifold.
There was very little room to fit the carb between the cowl, filter box, engine mount and panel.
But a new inlet manifold allowed the carb to fit in place to the original item.
This then left scope within the engine to tune accordingly.
MB Developments recommend this carb set up if you want to use your air filter box. It will work on standard, mild tunes and has been known to work on stage 6 motors.
Although the main carb body size used is the 25mm they are also available in 22, 24, and 26mm sizes and all fit on the same rubber mounted inlet manifold.
The 26mm are an oddball as they come with 4-stroke jets, these can be modified but MB recommends the 25mm size.
Our inlet manifolds and carb kits only use the rubber-mounted version as solidly mounted carbs are available but we don’t use this carb for reasons already explained.
Two inlet manifolds are available, one for 150 and one for 200 studded cylinders.
MB Developments offer this kit to suit mild tuned motors such as a stage 2, 3 or 4, 150 – 175 conversions and stage 3, 4 and 5 200 conversions. Or they can be a direct replacement on a standard engine.
Although these carbs are 3mm larger in bore size don’t expect masses of power! The great advantage is that they are a modern carb with jets freely available so you can set up each engine and adjust the carb to suit, unlike a standard item.
The MB Developments 25mm 150 or 200 complete carb kit for use with standard air filter box, consists of:
25mm PHBL on a nice SX200
25mm PBHL on a Series 2
Over the years when using the air filter box in a tuned motor we found that not enough air gets into the engine.
There have being different air boxes over the years but all types gave problems especially when using the PHBL set up!
An old conversion was to cut out the bridge in the air scoop as per GP air scoops.
We found that this made no difference at all.
The next step was to cut out the elbow in the top of the air filter box.
Again we found this made no difference!
This then told us jetting faults came down to the hole size feeding the air box through the frame, which was not large enough!
Removing the air bellows cured troubled engines, again showing a problem when using air boxes!
We advise drilling the sides of the air box, how many really makes no difference because any hole will allow more air into the box than none at all.
Our jetting in booklet says our preferred number.
Drilling in the side means they are not visible from the outside and makes the job much neater!
If using a PHBL carb kit with no holes in the air box the main jet would need to come down to around 80 – 82, when the air box is drilled a main jet of 92 –100 would be normal showing the difference air can make in the system.
We have also found over the years a well-set up 25mm carb kit doesn’t seem to perform as you would expect!
This comes down to possibly the length and change in direction that the air has to take to get into the engine.
But also PHBL carbs tested on down draft manifolds don’t perform as well as the Amal MK2 26mm offered years ago.
But the PHBL set up is for the moment the best set up to use the air filter box and looks original which people still want.
But the conversion has its good points over the standard SH1 carb, it is modern, fully adjustable and most dealers now keep spares for it!
If used for some time the carb mount gets weak, especially if the mounting is over tightened.
This could also be due to petrol softening the rubber.
This forces the carb towards the panel and the air bellows will touch it, some times the carb will come off the manifold.
We have found if you move the air hose higher than its original position this lifts the carb up and supports it better!
If fitted correctly in the first place you should have trouble free miles ahead of you.
There are some differences between models of scooters, some fit and clear the panel some just touch.
This comes down to brackets welded to the frame and engine mount differences we can do nothing about it!
28/30MM 150 / 200 PHBH CARB KITS
MB Developments have used this set up for many years in various specs.
The solid mounted carb was very common and were usually fine on tuned engines if used with a Clubman style exhaust.
These exhausts only actually revved to a maximum of around 7000 – 8000 rpm and gave a smooth power output.
At these revs internal vibrations were usually not high enough to froth the fuel, although fuel did use to come out of the over flows.
When using expansion chambers vibrations came in at different rev ranges.
The introduction of the rubber-mounted body made this carb set up ideal for most tuned Lambrettas.
The carb sizes of 28 – 30mm are a perfect size for stage 4 and 5 tuned kits.
If used with smaller tunes the inlet port size is not large enough and the carb can not breathe correctly.
But we still see these large carbs fitted to standard or mild tunes, they would ideally be better fitted with the 25mm kit as suggested.
MB Developments recommend a 28mm on 175 or 190cc engines and a 30mm on 200 or 225cc engines.
Jetting tends to be very similar on engine to engine. The smaller carb would tend to use a slightly smaller main jet and a weaker needle positioning.
As time went on Scooterists matured, people didn’t want open bell mouth carbs with petrol spitting back either down the panel, engine or passengers foot!
MB Developments realised the need to make clearer running tuned scooters.
The 25mm had filled in this gap but power out put didn’t go up as expected.
The 30mm carb size put up power output by at least 5hp over a 25mm version.
If the engine and cylinder ports were done correctly then a good engine wouldn’t have too much spit back!
But all engines do have some spit back whether it is low down setting off or at high speeds. A lot of spit back is usually caused by over jetting.
MB Developments have over the last few years introduced Ram air filters to clean up carburation.
One such invention is our remote air filter, this uses the GP 200 air hose with a bonded Ram air filter, this design fits onto the latest 30mm PHBH carbs and fits where the air box used to be.
Fitting a 30mm carb and using the original air box is impossible! The remote filter just takes the place of the airbox.
MB Developments have for many years under stood the need for filtered carbs.
Yes, we have tuned thousands of Lambretta engines using open bell mouth carbs and they can be very reliable, but as time goes on and MB Developments are committed to helping and keeping customers any long term complaints comes from high mileage engines wearing out when if used with filters these problems wouldn’t occur.
The 28 / 30mm carb kit is an excellent carb kit for touring style engines as well as engines used every day.
The carb can allow engines to get over 20hp but give good fuel economy with acceleration and speed.
This is MB Developments preferred carb kit for 175 / 190 / 200 stage 4 and 5 tunes.
This carb kit works as a package to back up our touring tuned cylinder kits.
The MB Developments 28/30mm 150 or 200 complete kit, consists of:
None really it's an excellent trouble free kit, just keep a spare mounting rubber as these can fail especially if it has been over tightened in the first place.
30 / 32 VHSA 200 FLATSLIDE CARB KITS.
The VHSA 30 and 32mm carbs are recommended for use on either cast iron cylinders or TS1 cylinders.
If they had been around in the eighties we're sure they would have been used for the stage 5 Suzuki and Honda conversions along with the Amal 34mm.
The 30 and 32mm VHSA use the same inlet manifold and mounting rubber as the Amal.
The VHSA carbs have been used lately to replace existing Amals already used.
Personally MB Developments would recommend the 30mm PHBH for road touring engines given the benefits of easy jetting, wide spread of power and clean running, but if you want your stage five-cast engine to go faster then the 32mm VHSA would be recommended.
These days fitting this sized carb to 150-studded cylinder is considered a bit over kill.
Basically the 150 engine should possibly be limited to the 175 stage 4 or 5 conversions using the remote filter and 28/30mm carb.
It is quite viable these days to upgrade to a 200 engine with cheap Indian parts rather than wringing a 150’s neck to keep up with TS1 style engines.
This carb would fit to either an existing Amal inlet or a new inlet manifold and MBD offer manifolds for both 150 and 200 studded engines.
Again a Ram air filter is available but on a cast engine the panel would need cutting for the filter.
The VHSA is ideally suited to a full body worked TS1 scooter.
The carb is slightly smaller compared to the Amal carb, giving it more room within the close space between the inlet manifold and side panel.
The only modification is to cut a small air bleed tube off from the bottom of the float bowl.
Even so the Dellorto carb is very close to the panel.
Because of this MBD recommend a hole for the carb to breathe.
As the carb is so close to the side panel MBD developed a Ram air filter to bond to the side panel therefore making the side panel an air filter.
This type of filter richens up the air mixture so lower jetting may be required, and a hole in the panel would be recommended.
The best way to filter the TS1 engine would be to use a full sized Ram air filter and cut a hole in the panel.
MBD recommends this for a long lasting TS1 Engine.
The MB Developments 30/32mm flat slide 200 or TS1 complete kit, consists of:
All these parts are available individually.
From the factory all flat slide Dellortos are faulty, that is a fact that we state and we are surprised none of the other dealers have picked up on this as they all sell them.
The symptoms of this fault are:
When trying to jet the carb in to your engine it may be OK one day and not the other.
One day your jetting my seem weak the other it may seem rich.
These problems may disappear when the tank is full and may reappear when the tank gets low.
You may notice the engine cutting out when going onto reserve, when stopped the bike fires up and again runs normal.
In worst cases engines have seized for no reason and others have terminal problems with holed pistons.
In these cases up jetting doesn’t cure the problem and will melt a piston again.
All this depends on how you drive!
It’s usually the fast riders that get these problems.
But lets face it if you have a flatslide carb then you should be travelling fast!
The standard needle valve of 2.5mm is too small to start with.
We have seen some dealers increasing the float needle valve to let more fuel into the float bowl when the engine is running.
This would be the correct way to go to improve the problem, but the hole in the carb supplying fuel to the needle valve is not large enough, so this needs drilling out.
MBD offer this conversion to existing carbs already out there.
All flat slides sold by us have this done prior to sale so it’s not a recurring problem with our carb kits.
All flat slide Dellortos have a auto lube pipe in the side of the carb body, the carb usually comes with a rubber blank but this usually comes off then this allows air straight into the engine causing jetting problems! This hole is blanked off in our blue printing process!
The other main problem with the VHSA carbs is that a very rich needle is fitted and if you fit it as standard the scooter will not even run!
This usually occurs when trying to save money and buying from European dealers.
The standard carb, apart from needing the blue print, requires re-jetting all done by MBD in the carb kit.
Once a carb has been blue printed and jetted in there are very little problems.
The slide runs in a plastic sleeve it is by no means a tight fit and after some miles it may seem sloppy this is normal and nothing to worry about!
The VHSA 32mm is our carb recommendation for all TS1 engines.
As time goes on technology gets better and we at MB Developments like to stay in the forefront of this technology.
I know we are working on old engines this doesn’t mean we are stuck in the Sixties.
We move on and want to offer the best products and conversions going, that’s been Marks principle since the start of MB Developments.
After some very positive testing this year with Mikuni TMX 35mm carbs, MB Developments are pleased to announce we have taken the plunge and now stock a full range of needles, slides, main and idle jets for the TMX 35mm Mikuni carb.
MB Developments have been using the flat slide Dellorto as the top of the range carb for a number of years.
We have found when using them on vibrating Lambrettas that the slide wears and can effect jetting.
We have not personally liked the 34 –39mm carb because of the variety of needles and needle jets available causing us a great big headache on the phone.
When you consider the cross-sectional area of the oval bore VHSB carbs a 34mm is more like a 37 or 38mm carb and a 39mm is more like a 43mm carb, for this reason we find them over kill for road engines and their physical size is a waste of time.
For this reason and a need for a better quality carb we have invested a great deal of time and money into the TMX carb.
MIKUNI TMX 35MM CARBS
The Mikuni TMX 35mm carb is nothing new, it has been used on Japanese motor cross style engines for years, they were designed to give instant throttle response at all throttle openings.
They are physically smaller than the VHSA 34mm yet still easy to fit, they use the same carb rubber as all Amal 28 – 34mm and the Dellorto VHSB 30 – 32mm carbs which means there is no need to change your inlet manifold or carb rubber.
This carb unlike any others around doesn't use needle jets, all they have is a number of needles to set up the mid range making them VERY simple to set up!
The TMX can be fitted to a fast stage 5 cast 200 tune or any TS1 tune.
Because of the way they respond to throttle opening and more importantly flow air and fuel in flat out circumstances we believe this carb to be the best available at this time.
CARBURETTOR SIZES TO SUIT LEVELS OF TUNING
Carb sizes vary for different applications.
M B Developments usually set up a tuned cylinder to suit a carb size and exhaust being used, basically the larger the carb the quicker your top end power, at the expense of losing some low down power!
IDENTIFING A TWO STROKE CARBURETTOR
If you have already an existing carb kit and is not new then you must identify that carb to make sure it is OK for your engines application.
All information given was presuming the carb was a two-stroke type carb.
To identify a two-stroke carb look down the main carb hole.
Where the needle comes out as the slide is raised, a brass spray tube should stick up protruding into the main carb hole, if the brass spray tube is flat to the carb casting then the carb is a four stroke type.
Stay clear of this type of carb, it is possible to jet them in but would require a total different set of jets.
A word of warning ! MB Developments only recommend rubber mounted carbs solid types can cause varying jetting changes due from fuel frothing in the float bowl at various rev ranges.
A word of warning ! Chrome plated carbs cause jetting problems M B DEVELOPMENTS do not recommend their use.
Presuming that all carbs used are going to be rubber mounted then some care should be taken to use the correct rubber.
Please E-mail us with any questions or comments.