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MARK BROADHURSTS CARBURATION GUIDE FOR TUNED SCOOTERS
Last issue Mark was foolish enough to print a list of carbs and carb spares that he stocked and sold from his tuning emporium. The result ? Hundreds of calls from people (many of whom had engines tuned by other people) who had queries about carburation.
In an attempt to get some peace from the dreaded telephone, Mark offered us his guide to running in and setting up tuned engines. It is designed as a simple guide to getting the most from a performance motor. Be warned however that perfect jetting is very much harder to obtain than useable jetting, to get it spot on may require the buying of different slides, needles, atomisers and pilot jets as well as a range of main jets (which is all most people buy) and these parts don't often come cheap. If you are going to go into jetting to that extent it may be worth consulting the following books:
Delorto - A Guide Performance Tuning Two Strokes Theory And Practice by A. Graham Bell
Tuning Vespas by Norrie Kerr
Lambretta Tuning Manual by Dave Webster
Spark Plugs
Choose the corect spark plug for your engine and for the use of the engine. Manufacturers recommend different grades of plugs per engine. Basically, you will require a hot plug for stopping and starting, running in and around town work. A medium grade plug will be required for all round work, ie a bit of town, dual carriageway or motorway. A cold plug will be required for fast use, ie motorway touring.
There are various plug types available, some good, some bad. Usually NGK, Nipon Denso or Champion are employed, each has different numbers and letter, but all basically are equal to each other's heat range.
| NGK | Nipon Denso | Champion | ||
| Hot | B7 | W22 | N5 | Town |
| B8 | W24 | N4 | ||
| B9 | W27 | N3 | ||
| Cold | B10 | W31 | N2 | Motorway/Racing |
Running an engine on a hot plug used flat out on a motorway would soon overheat, pink and finally bum out. Running an engine around town on a cold racing plug would soon foul up, causing misfires and again, finally pack up! So, it does help to keep an eye on plug grades.
Plug gaps should be correctly set. This varies from engine to engine. Gaps vary from .016" to.022" (.35 mm -.55 mm), usually a choice of.018" -.020" is used.
Now, providing that your ignition system is in perfect working order, then you shouldn't have a problem with spark plugs.
Two Stroke Oil
There are plenty of 2-stroke oils out there to choose from. I find Lambrettas and Vespas are one of the most undercooled engines that I have come across, so I would therefore advise more oil than is recommended. I prefer fully synthetic racing oils, and have found these to be very good. Castrol TTS mixed at 25:1, Putolene MXS mixed at 30:1,
Silkolene Pro two mixed at 30:1. There are other oils which are also very good, it's personal preference and availability. A word of warning! Try to use the same make each time. I don't like to mix them.
Fully synthetic oils are expensive and sometimes hard to get, so any old mineral oil will do, but mix at 20:1/18:1 and don't swap with synthetic oils. It can be an advantage to mix more oil when running in or for fast road use. Around town your engine may smoke so a weaker mixture may do. A warning! More oil can foul plugs, beware! Engines run on mineral oils will require decoking more often than synthetic oils.
Fuel
When adding oil to the tank, turn off your petrol tap, add your oil based on what amount you require, fill with petrol, therefore mixing the oil, use some form of measure to get the quantities correct and re-top up with oil when you know the exact amount of fuel used.
I have found 4-star or Super unleaded to work OK, I do not recommend lead replacement petrol. On tuned engines there should be no need for additives.
To prevent fuel starvation at high rpm it is advisable to fit a fast flow fuel tap or have yours modified. A tap should flow at least a litre of fuel in a minute.
Initial Starting
To start, turn on your petrol tap, choke lever and ignition switch.
Within reason, regardless of carb jetting an engine should start after only a few kicks, including full race spec motors.
Firm kicks are required on or off the stand, as you kick, slowly open the throttle up to 1/4 throttle.
This is when the choke works correctly the engine should fire, or fire up immediately.
Don't worry if it doesn't, some new engines take a little longer until bedded in. If it doesn't fire after a number of kicks check your spark and operating procedure.
Once the engine starts, turn off the choke after a few moments, then leave it on a fast tick over for approximately 30 minutes to bed it in. Slow down your tick-over to even beats, then let it cool down. If all's fine, then you can start the running procedure. before flying off down the road.
Jetting and Carburation
Jetting is important. Jetting is very misunderstood and can take years of experience to set a Garb correctly. I will try to pass on my knowledge to help make your job much easier without complicating the issue.
Carb Operation
Lambrettas and Vespas usually use Amal and Delorto carburettors with sizes ranging from 22 mm to 38 mm in tuned engines. Both carbs work in a similar fashion. Various parts of the carb work like this:
Airscrew and Idle Jet Adustment
The air screw should be set correctly.
Carefully screw in the air screw until it stops then screw it out 1 1/2 turns for an Amal, or 2 1/2 turns for a Delorto.
With the Amal screwing in richens up the tick-over mixture.
With the Delorto screwing out richens up the mixture. With Dellorto flat side type carbs screwing in richens the mixture.
When finely tuning in, 1/4 of a turn either way can make a big difference if you turn either way more than 1/2 turn, then a larger or smaller idle jet is probably required.
The air screw can drastically affect jetting up to 1/2 throttle. This setting can be done on the stand when the engine is warm. This is easy to set.
If your motor ticks over nicely, with regular beats, then you are something like right. If your motor revs up and down erratically on its own, then the idle/air screw is weak.
If your motor beats irregularly and lumpily, it will sometimes stall or flood, or spit out a lot from the carb, then it's too rich.
A nicely set idle/airscrew is when you rev up your engine and shut off. The motor should come down to its tick-over position, quite quickly.
If the motor takes a long time to come down, then it's weak. If it shuts down straight away, it's too rich.
Once you are happy with the idle/airscrew setting, finally adjust your tick-over screw to give an even beat, as slowly as the motor will run without stalling. This could be from 1,000 rpm with a heavy flywheel to 2,000 rpm with a lightened flywheel.
Setting Off and Slide Adjustment
It is now time for a road test, providing your clutch works correctly. Knock it into gear to set off, slowly; open the throttle as you let out your clutch and your motor should bite, pick up and set off to a smooth even flow.
If, however, the motor stalls, providing you had enough throttle on, then your slide could be too weak.
An overweak slide would mean you would have to set off with the carb nearly on the main jet.
If, however, when you set of the engine doesn't clean up and splutters, ie running as if the choke is on (called 4stroking - this sounds like machine gun fire), and you go through each gear and still doesn't clear, then your slide is too rich.
If, once run in, you shut off from a high speed and the engine seems woolly and light, or thinly jetted, then it is possible that your slide is too weak. This is very important to get right for cruising at a later date.
A good idle/airscrew/slide setup would splutter (4stroke) a little when setting off from cold, but would soon clear once 3rd and 4th gear is reached.
Running In Procedure
For the first part of the running in procedure you should probably only use these jets and they should take you up to around 50 mph.
All engines should require some running in regardless of engine type. A well run in engine usually lasts a lot longer than a poorly run in engine.
For the first 100 miles take it very steady. Only drive around back streets. Stop and start using all gears. Let the motor cool and start again.
For 100 - 500 miles drive around town/villages. Avoid motorways and dual carriageways. Vary your speed and alter your idle, slide jetting as you get used to your motor. Don't stick at constant speeds. Avoid driving two-up and making the motor pull hard into winds and up hills.
Occasionally increase your speed, giving it a quick burst. This will clear out the motor as a tuned motor can tend to choke up a little at slow speeds.
Needle and Needle jet Adjustment Whist Running In
At this time, it is advisable to check out your needle jet (atomiser) and needle settings.
This could be the most difficult of jets to get perfect.
If you open up the carb under normal acceleration and there seems to be a flat spot on the mid range (not to be confused with a peaky expansion chamber) ie doesn't really start to pull until the carb is fully open, then it is likely that the needle/needle jet is too weak.
If, however, you open up and the motor suddenly splutters, then suddenly clears as the main jet comes in then you are too rich.
The motor should pick up cleanly through all jets up to the main jet.
Adjust your needle clip downwards to raise the needle and richen up the mixture or raise the clip to lower the needle and weaken it off.
A basic guide is if you get to the top clip position you may require a smaller needle jet. If you get to the bottom clip position then a larger needle jet is required. Then further testing will be required.
Amal supply a number of needle jets, but only one needle per size of carb which makes Amals the easiest to set up! but seems very crude compared to Delortos.
Delortos supply a number of different atomisers (needle jet) coming in various sizes and shapes. With this they offer a lot of needles which alter in thickness, tapers and size, making Delortos harder to jet in. For this reason you can really finely tune in your carb, given the time and money. There are a number of atomiser/needle combinations that work. This can be a hit and miss affair to start off with.
A correct idle, slide needle and needle jet setting would give a responsive action as you open up the carb and the motor may well start to really pick up as you reach the power band.
If you have reached this part of jetting then do another 300 miles as part of your running procedure, occasionally increase your speed and do some hard accelerating for short periods. Still avoid motorways and dual carriageways and constantly vary your speed.
Avoid constant speeds for miles because you only use one jet. If that jet is too weak then you could seize or blow a hole in your piston. If you vary your speed at least different jets are being employed and there is less chance of problems.
Problems That May Occur (Pinking and Seizure)
If you encounter a problem throughout jetting, or after jetting, and a seizure occurs, then stop the bike, let it cool, then check the plug colour.
If it's white or light grey to brown, then you are running weak. Keep a mental note of the carb position and adjust the setting accordingly.
If, however, you are encountering pinking problems (pinking sounds like a can of marbles in your cylinder head), and cannot be improved by upjetting (this in turn may richen up the carb in other areas, so adjust accordingly), then it is likely that your ignition timing is out, or a too hot plug is used, or you have a faulty exhaust which can be a common problem.
Plug Checks
The plug colour should be a nice chocolate brown when perfectly jetted, regardless of throttle position employed.
If, however, it is a dark brown to black, then at some part of the throttle opening you are too rich. It's always an idea to run slightly rich when running in to be on the safe side, then weaken off when fully run in.
Checking plug colours should be done when a constant type of riding has been done, ie flat out work, cruising work or town work. Don't be fooled into fast
motorway work then ride around town, and check the plug as town work usually runs richer than when the carb is fully open, so a false reading may be read.
Plug colour checks should be done after some miles have been clocked. It takes time for a good colour to appear. Standard mineral oil will show a colour long before a racing synthetic oil does (they are designed to burn clean, therefore preventing carbon build up!)
Fine Tuning of the Carb When Run In
Once the motor has been run in, then fine tuning of the carb can take place. If you have set the motor up as described then your carburation may be perfect and clean around town, but on a run your jetting may seem to alter.
It does! and it will! Jetting also alters with the weather and altitude, unless it's a full race bike then don't let this worry you!
But keep it in mind if you seized on the first hot day of the year for instance.
Jetting can become a compromise, especially if you vary your engine's usage. It's usually safe to set the motor for fast road work which will in turn make the motor run a little rich around town.
First Touring or Major Run
When a 1,000 miles plus has been achieved and you are happy with its set up, then normal running can take place for rally work. It is an idea to do some miles steadily at 50 mph, then slowly increase your speed to 55, 60, 65, 70 mph, etc, to make sure all parts of the carb are jetted for the work the engine is required to do. If a problem occurs modify the jet being used.
Boy Racers Beware!
If you tend to ride flat out everywhere and set the main jet perfect, do be careful if you suddenly slow down, say to let your mates catch you up, if you haven't adjusted your needle/slide, then when cruising, a problem may occur.
Be careful when testing or using the main jet. To slow down, slowly shut off your throttle and use your rear brake to slow the engine. If you don't, and suddenly shut off when flat out, your engine is only fed with a tick-over mixture a seizure can occur.
The curse of a 2-stroke engine is over-heating. Providing that the correct compression ratio, exhaust and ignition timing are employed then it's usually incorrectly jetted motors that go wrong, so beware!
Spend the time! Go up rather than down. If you are not sure, an over-rich carb is better than an under-weak carb. Over-rich jets can usually spit back out of the carb bell mouth.
A quick look down when riding at that position could confirm this. Sometimes a motor may seem to hold back and cease to rev.
When opened fully it may miss a beat slightly (it sounds like an electrical problem), and it may also seem to cut out. This indicates an over-rich main jet.
A weak main jet could be indicated by a backfire as there is a lack of fuel being delivered. Most people only change the main jet, not understanding the rest of the jets. This becomes a common problem.
Because the main fuel system is fed through the main jet hole, then alteration can affect running, right down to the slide, so most people get away with it!
If you can collate this information then you should have trouble-free miles ahead of you, all being well!
Carb Choice for Tunes Being Used
Carb sizes vary for different applications. I usually setup a tuned cylinder to suit a carb size and exhaust being used:
Lambretta:
Stage 3 tune will require from 20 mm - 26 mm carb
Stage 4 tune will require from 26 mm - 30 mm carb
Stage 5 tune will require from 30 mm -34 mm carb
Stage 6 tune will require from 34 mm - 38 mm carb
TSI stage 1 - 5 tune will require from 30 mm - 38 mm carb.
Vespa:
Carb sizes are quite restricted. I don't relate Vespa tuning in stages as with Lambrettas. So most road tunes work fine with standard carbs up to 24 mm. The next stage is to jump to 28 mm to 30 mm and this is quite adequate, but it is not uncommon to use carbs up to 34 mm, although these are not really needed.
This is not a hard and fast rule. It is my opinion based on hundreds of tunes and engines successfully tuned by Mark Broadhurst. Good luck and happy motoring.