Performance carb tuning can be one of the most daunting tasks of tuning a scooter. Where do you start, what does what, and how do you make sure it doesnt blow up?!
In this article were going to be going through the basic components of carb tuning, in particular the PHBG ( a benchmark carb in performance scootering). These principles can also be applied to standard carburetors.
If you’ve gotten yourself a performance carb, the first thing to do is make sure that fits the scooter. For many models Malossi make a specific kit with included fitment parts to fit specific bikes, which is really handy particularly when it comes to chokes, inlet & outlet size, and mounting type. Certainly an easy way to save yourself alot of extra hassle, as many scooters have different fitment methods.
The next obvious step before doing anything else, is to familiarize yourself with the components of the carb, and what they do. Below I’ll be going through a very simplified explanation of the functions of each part of the carb, and some basic examples of tuning it.
All carburetors work off a few basic principles, fortunately for scooter tuners, scooter carbs are some of the most mechanically simple around.
The Engine draws air through the carb, the carbs internal shape creates a venturi effect drawing fuel in with it. The slide controls how much air can be drawn through the carb. The slide is controlled by the throttle cable. When the throttle is twisted, it opens up the slide, allowing more air and fuel to be drawn into the engine, producing power and increased revs.
Attached to the base of the slide is the needle. The needle moves in and out of the fuel feed tube which is where the fuel is fed up from the bowl into the carburetor. At the bottom of the fuel feed tube is the main jet. This is a top to bottom explanation, slide, needle, fuel feed tube, main jet, bowl.
Needles are tapered, like a spike. At the throttle fully closed position the needle is sheathed all the way into the fuel feed tube, effectively closing it off. As throttle is applied and the slide opens, this lifts the needle with the slide, allowing fuel to be drawn up through the main jet and the fuel feed tube. The taper of the needle effects the amount of fuel that can be drawn at once through the fuel feed.
As a specific example, most PHBG carb kits come with a W16 needle. This needle has a heavy taper, meaning its thin and pointy (to put it simply). The heavy taper means that at low to middle throttle, the thinner needle allows more fuel to flow through the fuel feed tube, causing the engine to run richer.
Running a bike with a rich needle commonly has the effect of making the bike feel ‘fluffy’ or ‘spluttery’ unless the bike is on full throttle. This ‘full throttle or nothing’ tuning makes the bike pretty horrible to ride on the road, trying to maintain a constant speed in a 60-80kph zone becomes a mission.
Commonly, a PHBG carb runs well with a W7 or a W9 needle. These needles are less tapered, and thicker. This has the effect of leaning the low to middle throttle range out, and giving the bike a more consistent power delivery through the rev range – where the W16 is far too rich in these areas.
At this point you may be thinking, “ok so what does main jet do then?”. Its fair to say that 90% of your carb tuning lays in getting the main jet and needle settings right – however the Main jet is significantly more important than the needle, and ill explain why.
The main jet is the main method of controlling rate of fuel flow to the engine. The needle controls how much of that fuel the engine gets between idle and full throttle. At idle and full throttle, the needle has no effect. At full throttle the needle is all the way out, its done its job.
For this reason, its important to always centre tuning around the main jet. Get the main jet right, then knit pick the rest of the fuel delivery.
The other major part of carb tuning is the pilot jet. In basic terms the pilot jet is responsible for maintaining a constant level of fuel to the engine while its running. At idle for instance when the needle is either all or almost all the way in, and not allowing the main jet to supply fuel to the engine – it still needs fuel. This is where the pilot jet kicks in.
In most cases, the factory pilot jet can be left alone. The only common instance you may have to fiddle with the size of the pilot jet is if you’re using a high end cylinder. The pilot jet also acts as a sort of reserve of fuel for the engine to have ready for when throttle is rapidly opened. If the pilot jet is too lean, it often manifests as the engine making a “brrrrr” sound from the intake and either cutting out or having a delayed rev response to throttle being suddenly added from idle. This is because there is a short delay in supply when lots of fuel is suddenly required from the main jet, as it needs to travel up the fuel feed, past the needle, ect.. The pilot’s secondary job is to tide the engine over for the half a second it takes for the main jet to catch up with the fuel the engine is demanding.
Tuning process and order
Now that we’ve been through the function of the different elements of the carburetor, its time to talk tuning specifics.
It can be intensely frustrating when you’re trying to tune a scooter, and there’s a problem. Maybe its cutting out as soon as you give it throttle, maybe its spluttering terribly, maybe it just isn’t making much power?
The first and biggest key is to ONLY change ONE thing at a time. Panicking and changing 5 things to try to fix a tuning issue often leaves you worse off than you were originally, and you have no idea what change did what. Maybe one of the changes was right, but the other four changes were wrong and hid the benefit made by the right one?
To reiterate, ONE thing at a time!! Its that important.
Tuning is alot of trial and error. Changing one part at a time is the only way to distinguish error from correction.
First Thing Is First.
1. Main Jet.
As stated above, the main jet is the most important tuning aspect to get correct. If the main is too lean, you run the risk of seizing the piston. If its too rich, the plug will foul and the bike will make alot of smoke and not go far.
In terms of specific jet sizes, there’s a few basic rules we’ve established over the years. Unfortunately there isn’t a set size, there are a couple reasons for this.
As an example of why – a standard factory Piaggio hiper 2 motor (found in Zip 50, Typhoon 50, Aprilia SRMT ect..) has a Dellorto 17.5mm carb with a 53 jet. A standard factory SYM Jet4R however has a 12.5mm carb with a 70 jet. Theres a reason for this huge difference in jet, you guessed it too – its carb size. Bigger carbs require a smaller jet, as a general rule.
Back to choosing a jet size for your carburetor, if you’re installing a performance carb in your scooter, there’s a good chance you’ll end up going down in jet size compared to the jet size in your standard carburetor. This is normal. For this reason, the best port of call generally is to start with the standard jet included in the Malossi Carb kit. Its often pretty close. That does NOT mean you can bolt the carb in and expect it to ride well without further tuning, but as a starting point its a solid start.
2 Stroke engines are in many ways fairly easy to tune because they give really good feedback as to how they’re running. Spluttery-ness is generally too rich, and the ‘Brrrr’ noise when throttle is applied is generally a marker of an engine running too lean. The other big sign is the burn colour on the spark plug. Its a good idea to either put a fresh plug in your bike when you’re tuning the carb, so you can an accurate idea of the heat of the burn.
Start the bike up, rev it on the stand to see how it sounds. Make sure its revving (keeping in mind the choke will effect things in the first couple minutes of running). If you’re happy, take it for a short 50m ride up and down the street, again seeing how it feels. Is its making a ‘Brrr’ noise and bogging down when you apply throttle suddenly, richen the main jet and try it again. If its spluttering and not going anywhere fast, trying going down in the main jet.
Once you’re confident the bike feels good riding up and down the street, take it for a 1-2km ride. Try to mainly ride on short bursts of full throttle – right now you’re only trying to tune the main jet, and riding around on less than full throttle is riding on the needle more than the main jet in terms of getting a guage for how close your settings are to right. When you get back, check the plug against the above chart. The plug check is only really to confirm what you already feel, the way the bike feels under acceleration is your main guide. Repeat this process until you’re satisfied the power and responsiveness, then address the Needle in essentially the same manner.
The correct way to go about changes in jets and needle is ONE setting at time – by this I mean if you ride the bike with a 70 jet and it feels a little lean, go up to a 72 or a 74. Dont jump to an 80, baby steps. The bike for instance may only be slightly lean on a 70 jet, but an 80 is far too rich. By making that big jump, you’re likely going to confuse yourself.
Adjusting the needle clip is where you’ll find alot of the smoothness around the mid throttle range. The needle has a small circlip at its flat end (the non pointed end) which acts as a stopper, as the needle slides through its hole int he end of the slide. The needle has a number clip settings at its flat end, to which the circlip can be moved to change its setting.
Placing the circlip closer to the pointed tip of the needle will richen the needle. Placing the circlip closer to the flat end of the needle will make the needle leaner. This works by dropping or raising the overall position of the needle. Different needles have different numbers of clip positions, for instance Dellorto A needles (for PHVA) have 5 clip positions, where Dellorto W needles (for PHBG) have 4.
Like the jet, the best way to tune needle position is to move one clip at a time, and test ride. It is fairly normal to do many test rides to get the tuning right on a new carb. Regular checks of the burn colour on the spark plug are also essential.
3. Pilot jet, and troubleshooting.
Lets say you’ve followed these steps above, but something still isn’t right. Scooters are fairly simple as far as engines go, but theres still many, many parts that must work in conjunction for them to function properly. So I’m going to address some of the issues that we’ve encountered over the years. many of them are specific fixes for specific bikes, some are more general.
Maybe you’ve been through a bunch of main jet sizes but you just cant get the bike to ride right. Typical symptoms/fixes of a pilot jet issue are:
- The bike bogs at take off. Maybe you’ve had to go up to a huge main jet to fix the bog, but now it splutters terribly. – the Pilot jet is too lean. By increasing the main jet you’re compensating at low throttle, but running far too rich once the bike has started to rev. Richen the Pilot jet and return the main jet to its original size, this will allow you to tune the main jet properly
- The bike takes off well, but seems to bog a little on full throttle past about 20kph. The plug burn might also be really lean! – The Pilot jet is too rich. By leaning the main jet off you’re compensating at low throttle but running lean on full throttle. Lean off the Pilot jet and return the main jet to its original size, this will allow you to tune the main jet properly.
Many bikes have rev limiters. Some of the most classic examples of this are the TGB 101S and the Kymco Agility 50cc. With both of these scooters there are some year models where the original CDI was able to be modified to have the limiter removed – However it isnt something we recommend, because its an easy way to destroy your CDI. With both of these examples, the CDI needs to be replaced.
CDI restriction is fairly easy to diagnose. It manifests as a the bike getting to a set speed and revving no further. With the TGB, it makes a very particular limiting sound, like the engine is stuttering. The easiest way to diagnose is to put the bike on the main stand, and (while steadying the bike to ensure it doesn’t fall off the main stand) rev the bike up to its max rpm. With no resistance it’ll rev to its max revs very quickly, and a rev limiter should be easy to hear.
If you’re installing a race cylinder in a Piaggio Hiper 2 motor, for instance a single ring MHR cylinder in an Aprilia SR50R – You will need to purchase a Malossi TC unit, as the standard CDI has a rev limiter around 10k RPM, which most race cylinders will hit.
When installing a performance carb, its always a good idea to check your reed petals. Reeds are something that are easily overlooked, but understanding that for every rev of the engine the petals are flexing open and then flapping shut can give you a better respect for how hard they work. Over time they can develop cracks, pieces can even break off.
The job of a Reed Valve is essentially a one way valve, air and fuel can get into the engine but not out. Cracks and broken pieces cause an air leak, which causes all manner of problems. It can make the bike virtually un-tune-able, not idle, and be an absolute pig to ride.
It sounds unlikely, however on some models in our experience over the years (Particularly Yamaha/Minarelli engines) that using rollers which are too heavy can cause a splutter on acceleration.
This can be quite puzzling to the unknowing, as engine spluttering is in every other case a carburation issue. On these particular bikes, lightening the rollers instantaneously resolved the issue.
In general, often installing a larger carburetor will require lighter rollers as the bike may need to rev harder to make torque. Installing a larger carb you’re in most cases sacrificing a bit of low down torque for high end power. In this case the engine will need to rev harder to produce the same amount of torque, which is whats going to get you moving along. If you’re tuning a new carb, but just cant get the bike to rev – The first place to start is lightening the rollers.