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You are here: Home > Lambretta > Lambretta tuning > Tuning options for 200cc engines using the AF Rapido barrel

Tuning options for 200cc engines using the AF Rapido barrel



With the introduction of the Rapido cylinder kit and the controversy surrounding it, things have changed a little.
Firstly it was marketed as a standard bolt on kit to use with standard carbs and exhausts, more recently it has been sold as a tuned kit for use with large carbs and expansions.
Now a new kit has arrived to supply the demand for the standard market.


The kit looks, and is made, the same as the first version of the Rapido.
The main thing that has altered is the exhaust port size.
The piston has been changed from the Asso / Vertex to a standard Indian 3-ring Mahle piston with a standard length inlet skirt, which has reduced the inlet timing to some thing sensible.


The port has been decreased in size to look standard.
The port timings have been reduced to 170 degrees - ideal stage four porting, as per our tuning spec page.
The lower edge of the exhaust port has been raised so the piston isn't exposed at bottom dead centre.
On our engine jig the exhaust port needs lowering by 0.5 – 1mm to put it flush with the piston at BDC.
The exhaust port width is slightly narrow at 42mm.
This will look after the rings of the 3-ring piston used, but this can be increased to 46mm with no ill effect but would increase pulling power.

The down side of the exhaust port is that it has been cast badly WRONG!
From the year dot a standard exhaust port has had a flange that is not equal in shape.
To explain this, as you look at the exhaust port in your hands the bottom exhaust stud has a short distance between the stud and port.
On the upper stud there is a larger distance between the stud and port.
A perfect gasket or exhaust flange matched to the port can be turned around and will still fit but the flange will mask off part of the exhaust port!
This is a common assembly problem people have found over the years and can effect performance of a tuned kit.
We suggest you always check your gasket before fitting.

The Standard Rapido has cast the flange upside down!
You can turn the gasket around but this doesn't cure the problem with a standard design U bend.
To make matters worse the two exhaust studs are offset a little more towards the exhaust.
This is a total nightmare to line up, tune or grind.
The only cure is to cut off the flange and re jig up the exhaust!
Hassle to say the least.
Exhausts used with a slip joint flange (like a Taffspeed) may be turned around and hopefully the whole exhaust system still fits.


The inlet port is the same, but now because of the standard length piston skirt the duration has decreased to 142 degrees, which is more suitable for a standard or mild tuned engine.
This should mean there is little or no spit back out of an open carb.
If the piston is swapped for the Asso piston with the 3mm removed from the inlet then the inlet timing increases to 155 degrees the same as the first Rapido.


The transfers are identical to the first kit, only on our jig the duration has decreased to 126 degrees, more suited to make a Lambretta cylinder work.


The kit we set up had a clearance of .001’’ using a Mahle, if changed to an Asso piston it increased to .0015’’.
Beware that is still tight, we would suggest .003’’ to be safe!


The kit is new and time will tell on how it will run, the porting configuration is very good and will work, we have used this configuration for many years its nice to see our criticisms seem to be listened to.
As for piston to bore clearance air cooled motor bike cylinders run larger than that with more efficient cooling so beware.
The exhaust port is a big error and spoils the improvements.
The piston used is not the best ever made but usually works to a point.
The kit checked, reworked and polished should be an improvement over standard and would possibly have more power that the first kit.
Personally I would still prefer to tune a standard cast cylinder to stage 4 with a Vertex piston and this would prove to be better, but genuine untouched cylinders are hard to find.

May 2001 has seen the introduction of the much publicised and long-awaited AF Rayspeed Rapido 200cc iron / alloy cylinders.

I read the article in Scootering with an open mind.
Given the fact that I knew nothing of the engines set up or indeed the cylinder design or even road testing it I chose to let the information go over my head and waited to see a kit for my self.
On the first day of sale we were asked by a telephone customer "what jets should I fit into my 25mm carb on my Rapido barrel?" Our answer was thanks for phoning but why phone us it's not our kit and we've not even seen one!

The customer did phone back to say what jets he had used in the previously running engine and then to say that he couldn't jet in the engine whatever he did.
With three phone calls in one week asking us about jetting and numerous calls asking how good are they we decided to have a look at the kit.


The first impressions was how heavy it was! With a closer inspection we found out why!

It was difficult to quite understand from the Scootering article how the cylinder was made.
Basically the Rapido cylinder is nothing but an iron cylinder as before, the exception is it has alloy fins.
To explain a little further, the cylinder bore, the area around the stud holes, inlet, exhaust and transfer ports are all one as per a normal cylinder. Then cast around it is the alloy finning.
The idea of this method of manufacturing is to stop leakage between the liner and ports.
With some research we found out that Yamaha had already used this method on a bike in the 60s and on another in the 70s but for some reason phased it out never to use it again!
There are two other methods used, which I would have preferred.
1. The Japanese cast their alloy around a liner that is just the bore and the transfer ports. 2. The repair way is to bore out the existing liner and interference fit a new iron liner with a top hat shape to stop the liner moving when running. MBD have used this method for over 15 years and have done over 300 cylinder repairs. To date we have never known any cylinder leakage using this method! It makes the Rapido method a little over kill!

The exhaust port looks large but uses the original style flange shape unlike the TS1 cylinder.
The transfer feeds and ports into the cylinder are the same as per a TS1 cylinder, which means they are now bridged unlike a standard cylinder.
The inlet port is now back to the original piston port set up but again is much larger.

The alloy and iron casting is very well made and looks quality.
Close inspection shows some casting lumps around the ports especially inside the transfer feeds into the cylinder, but these are nothing to worry about.

One good advantage with the iron casting method is that inlet and exhaust stud holes are still cast iron.
Vibrating Lambrettas tend to pull on alloy thread holes as most people found out with the TS1 cylinders.
But lets face it 7mm exhaust threads were not strong enough in the first place, modern motors would use 8mm and they still have trouble.
We have heard of a case of part of the cast iron around the exhaust port being pulled away by someone grounding their exhaust at 3mph.


The standard piston that comes with the kit is the 2 ring Asso item, this is also the same as the Vertex piston also available.
This piston has been around since 1984 and is now well proven to be reliable for all round engines except full race engines.
MB Developments use this style of piston in any tuned cylinder set up and now prefer it to a Honda 205 piston because of the over sizes available and reliability of its rings and ring pegs.
The piston has two problems,
1. The inlet skirt has been cut 3mm shorter than standard, therefore effecting inlet port timings and can over tune the inlet port in some situations.
2. The area around the cut outs to feed the transfer ports below the gudgeon pin has sharp triangular edges unlike a modern piston, this is a design fault going back to Innocenti. Long term with a large piston to bore clearance the inlet skirt cracks and fall off!


The inlet port looks a lot larger than a genuine cylinder, it is, and it resembles an Indian inlet port that has been slightly tuned and polished.
The major factor with inlet port design is the opening period or inlet port timing.
Using the piston supplied gives an inlet timing of 155 degrees.
This is around the same as an MB stage 5 or 6-race engine and is ideal at that level. Inlet timing above this WILL spit back a lot of fuel and will not breathe correctly!
This large inlet timing will effect carburation especially on the smaller carbs. This inlet size is suited to 30 – 36mm carbs not 18 – 26mm carbs.
If however a standard length piston skirt was used, then regardless of the inlet ports size the inlet timing comes down too nearer a standard inlet port size and would be ideal for the smaller carbs. This would de tune the cylinder for the better.
Beware - the only piston with a standard length skirt available at the moment is the 3 ring type, long term the rings will break because of the size of exhaust port.


As mentioned the transfer ports look the same as the TS1 cylinder. The feeds at the gasket face are larger than standard more like an old style stage 6 engine.
Ideally your casing would need flowing to help the cylinder work as per TS1 engines. The transfer ports are larger in area this lowers the crankcase compression and slows the pumping effect, but this is nit picking and I don't expect it to affect things too much.
What does effect the Lambretta engine in a big way is the transfer port timing when checked it gives 130 degrees - about the same as I would use on a full race spec group 6 engine! At this duration pumping doesn't work until around 5,000 rpm and can work on to 10,000rpm which is totally out of the range of most working Lambretta engines!
Unfortunately not a lot can be done at the moment here as you can not put back on what has been taken off!
Basically we consider that the transfers are over tuned for a normal road going engine.


Take a look at the exhaust port on an Indian cylinder. The Rapido's exhaust port resembles this shape.
Material has been ground away at the bottom of the exhaust making the port much lower than the piston when it is at bottom dead centre and the port has been raised much higher too.
The Rapido cylinder goes further still.
An Indian cylinder gives a port timing of 168 degrees, ideal for a stage 3 or 4 tune! The Rapido cylinder gives 182 degrees! Which MBD would consider a large stage 5 tune or a road stage six tune.
I have found going over 175 degrees on a cast cylinder loses all the bottom end pulling power that a Lambretta needs.
Some may argue this point and I may agree, but I have tried it many times and always come back too below 175 degrees for normal road going engines. If you want to increase horse - power a larger exhaust port and duration always works but this will lose the drive ability of the engine!


With the cylinder that we set up the bore clearance was .002"; if the cylinder was a traditional iron liner in an alloy cylinder then this clearance is perfect for a Lambretta.
If it was a cast iron cylinder then it is .001" too tight!
This doesn't seem a lot but believe me it’s a massive difference!
So again beware seizing could occur, as the Rapido cylinder is half a cast cylinder and half a linered alloy cylinder.


The port timings are hit and miss, put together in this order the engine will be on and off the power as per Scooterings article, this makes jetting hard to do!
Changing exhausts will control how the engine works but what combination works is just a skirting around a bad design!
If you can ride in the power all the time then you may like the cylinder, if how ever you ride slower, round town or two up then expect the cylinder to be hard work to ride with.

Our view to improve the cylinder would be to increase the exhaust port timing between 6 and 10 degrees, use a 30 - 34mm carb with a high revving exhaust. This when done correctly could hit around 28 HP! The Asso or Vertex piston wouldn’t quite stand that for long!
BUT this would be far from any decent stage 4 touring engine expected from a bolt on cheapish after market cylinder kit.
It’s a full race spec group six barrel and something that people have never really wanted except for racing!


1. MBD can offer Japanese piston conversions BUT this will then get expensive going away from what the cylinder seems to be designed for, a cheap bolt on alternative to the TS1 cylinder kit.

2. Changing the con rod to either the Ape or Yamaha 110mm long rod would allow use of Japanese pistons. This gives more scope by machining the cylinder then using a combination of packing plates and thicker head gaskets to set the port timings, making the cylinder more rideable, driveable and reliable. This article is not done to slag off the Rapido cylinder, it may prove us wrong, work and be very reliable!
What we are saying is we think the kit is not quite perfect and things can be done to improve it.

Picture section

A Rapido cylinder came to us after a nasty accident under the exhaust.
In the big scheme of things it wasn't a bad smash, just a quick knock of the exhaust.
The outcome of this is that the exhaust u-bend ripped out the part of the barrel where the exhaust studs go.
It came for us to repair the two holes.
There is no easy way to repair the damage so the cylinder was donated to the MB scrap barrel collection.
It was no good for us to use for anything so we had a bit of fun with the band saw and cut it up to have a proper look.

dsc1706.jpg The cylinder cut down the middle shows a solid iron cylinder including full transfers with an alloy casting around it for cooling.
Note that the cylinder had seized up before the exhaust accident.
dsc01707.jpg A cut through the inlet port to show how easy it would be to grind or polish straight into the cylinder stud hole when tuning.
This is a fairly standard inlet port.
dsc01708.jpg The exhaust side of the cylinder, note the seize marks.
We have heard of Rapidos seizing in 2nd gear from new and in 3rd gear at 30mph.
This cylinder we have cut up is the early type Rapido, first classed as a standard bolt on kit but later sold as a tuned kit (when the Rapido "classic" was launched with it's standard exhaust port.
The exhaust port here is as it came from the factory, VERY large.
dsc01709.jpg Through every cross section none of the cylinder is aluminium apart from the fins.
In terms of manufacture the quality is very good with the iron and aluminium casting and this one had very few flaws.
dsc01710.jpg The external view of the damaged exhaust port.
The rest of the cylinder has been well manufactured, possibly over built, but around the exhaust port, where it needs to be strong, it turns out to be the weakest area.
One slight knock on a kerb or speed bump just rips out the small amount of cast iron and aluminium.
dsc01711.jpg The internal view showing the lack of cast iron.
In this situation cast iron is stronger then the aluminium, but there is a lack of either.
Possibly the only way to repair this damage would be to grind away all the cast iron then alloy weld up the holes and drill and tap some new holes.
We only worked out this repair after cutting up the barrel.
We are capable of doing this job but we don't want to do it, it will take an good hour and a half, time we haven't got.
Get a new cylinder!
dsc01688.jpg On the left is a Rapido classic transfer port.
Compare the size with the standard 200cc cast cylinder on the right.
dsc01689.jpg Left Rapido, right TS1.
At a glance they look the same but when you are scribing the casings to match the transfers they are a little different.
dsc01690.jpg A Rapido classic exhaust port, much smaller than the original Rapido.
The port timings are a lot more conservative with a good duration of around 167-168 degrees.
dsc01691.jpg An MB stage 4 tuned exhaust port on a standard cast iron 200 cylinder.
Note it is a similar shape to the Rapido classic.
However the MB tune uses a wider and slightly higher port.
The outlet side is matched to an MB exhaust flange and big bore gasket.
dsc01692.jpg The MB 200 stage 4 inlet port matched and flowed with an inlet duration of around 148-150 degrees.
Ideal for all road-going engines - road/race/touring motors with little spit back from the carb.
dsc01693.jpg The Rapido inlet port.
Both the classic and tuned Rapido use the same inlet port, piston skirt length and port timings of around 155 degrees (using a 2 ring piston).
This is usually too high for most road engines.
You will loose power low down and will spit back petrol.
You can cure this problem with the port timings by using a 3 ring piston with a standard skirt. This will reduce the timing to a good road spec.
Alternatively, the new MB UK made forged pistons will be ideal for use.
dsc01694.jpg The standard 200 casing showing the transfer feeds.
dsc01695.jpg As above.
dsc01698.jpg The scribe lines show how much metal needs to be removed to suit a Rapido or TS1 transfers.
Note they are slightly different.
dsc01699.jpg Transfers opened and flowed to suit the Rapido cylinder.
dsc01700.jpg Doing this has proved to totally alter the engines performance, but we wouldn't strip an engine down especially to do it, we'd only do it if we were starting from bare casings.
dsc01704.jpg The Rapido classic has the exhaust flange machined upside down compared to standard.
This may throw out the exhaust alignment on some bikes.
dsc01705.jpg Even when the flange is turned round the opposite way the port is still not quite right and requires some extra tuning work.

Please E-mail us with any questions or comments.