Tech Answer: Fuel Delivery System and Carb Tuning

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Tech Answer: Fuel Delivery System and Carb Tuning

Postby poopShotgun » Mon May 07, 2007 8:00 pm

Your scooter won't run without fuel, no matter how Mod it is. Even if you do have fuel, you still have to mix it correctly with air to facilitate the two-stroke process. Add onto that engine lubricants and you have a recipe for hair-pulling.

When should this be done?
Carb tuning is often done when the scooter starts to feel sluggish, bogs down at higher revs, won't idle, idles too fast, won't come down from higher revs, or revs out and stays. If the scooter is already running at reasonable performance, a little tuning can often eek out that extra bit of power you seek from the engine. Anytime you add or modify a cylinder/piston kit, you will have to re-tune the carb. You may also want to tune the bike a little differently when seasons change (since higher atmospheric moisture content will affect performance) or when changing altitude (thinner or thicker air will do the same). Lastly, before going on a long touring trip, you may want to upjet to help cool the engine.

Note: The following is based off my experiences with a Dell'Orto SI 24/24E carb found on a vintage Rally 200, but for most vintage bikes you should be able to use the instructions herein. For side-draft and reed-valve setups, you can find many guides for tuning on the web; There is a wealth of information for such setups, but very little for vintage carbs in little boxes.

The Carb Box ->
The carburetor box holds your carb, protects it from road dust, and makes repairs and troubleshooting a pain in the ass. The carb box has three screws on top; Two hold the top of the box in place, the third is the idle adjustment screw. The idle adjustment screw fits through a hole in the top of the box and you'll easily be able to tell which one it is because it usually has a rubber seal around it. Remove the two screws on the side of the box and finagle the cover off the box.

The Carburetor ->
Now that the carb box is off, you'll notice the collection of cylinders that is your carburetor hiding under the air filter. Undo the two screws that hold on the filter to gain access to the carb. Under the filter is the intake, on the sides of the intake are two strange looking screws sitting next to each other. The small one is your idle jet, the large one is your main jet. You can unscrew and remove them from their housing to inspect and clean them. Make sure there are no deposits on or in the jets. You can blow them out with compressed air and soak them in carb cleaner to remove deposits.

The carb itself is bolted onto the engine on either side. Removing these bolts should be done only after unscrewing the fuel line, which is inboard towards the frame. It has a large screw, rubber hose, and a hose clamp and is easily recognized. Also detach the throttle and choke cables from the carb. You can use a screwdriver to pry the choke lever forward enough to remove the cable, and the throttle cable is held on either by a nub at the end of a cable or a pinch bolt. Pull the slide towards the front of the bike to detach the cable.

If you choose to remove the carb for cleaning, you must replace the bottom gasket. Ensure you slick both sides of the gasket with non-detergent 30W oil before you put the carb back on. Cleaning the carb is a relatively simple affair and you should be able to ask help from your fellow club members. Simply take it apart and soak the parts in carb cleaner for a good half hour or so. You should also blow it out with compressed air if possible. Let it dry thoroughly before reassembly.

Basic Tuning ->
There are two screws on the carb itself. The idle adjustment screw we've already gone over, and the fuel/air mixture adjustment screw. The mixture screw is situated on the back of the carb over your transmission. You can access it from a hole in the back of the carb box. You also have two jets, the idle jet and the main jet. The idle jet, as the name suggests, introduces fuel at low revs. The main jet introduces fuel at higher revs (anything over 1/8 throttle, I think). You usually do not need to mess with the idle jet.

This leaves three possible areas for tuning, and only two of them really affect your engine. The third is used when the engine is running well.

The Main Jet ->
Your jets affect how much fuel can enter the engine via the carb. A larger jet lets more fuel through, a smaller jet allows less. With that in mind, it is often best to know what main jet your bike came with when it was stock. Since jet size depends on the carb intake size, intake timings, number and size of ports, cubic capacity, and crankcase volume, it is always best to start with the stock jet unless you have tuned your engine. If, even after mixture adjustment, you are still not getting enough fuel, increase the size of the jet and start tuning over. The reverse applies to too much fuel.

The Mixture Screw ->
This tells your carb how much of the incoming fuel to mix with the incoming air. To start, screw this all the way in, then one and one half turns out. Take the bike out and do a plug chop (discussed later). Depending on the plug condition, you'll either want to add more fuel (loosen the screw) or restrict the fuel (tighten the screw). When adjusting this screw do so by no more that one quarter turn and always do a plug chop or two after adjustment.

The Idle Screw ->
This tells the carb how much fuel to allow at idle, which affects how fast the engine idles. To start, turn this all the way in, then two turns out. To speed up the idle, tighten the screw. To slow the idle, loosen the screw. Your idle speed should be fairly slow; A little bit past the minimum speed of the engine. You can safely play with this setting to some extent once your mixture and jets are correct.

The Spark Plug ->
Your spark plug not only ignites compressed fuel/air within the engine, it is an indicator of how your engine is running. The plug ceramic and metal that is exposed to the combustion chamber will change as more or less fuel is introduced, and can even indicate overheating problems. The ideal color for your plug is a light chocolate brown. If the plug is lighter, you will need to add additional fuel to the engine via carb adjustments. If the plug is blackened or wet, you will need to reduce the amount of fuel. If the plug ceramic is cracked and there are white deposits on the metal, you have a problem with overheating. If either of the plug electrodes are burnt away, you either have a problem with pre-detonation or the plug heat value is too high.

Something else to mention here is plug gap. Your spark plug electrodes need to have a correct gap to fire properly. You will need some feeler guages to gap the plug properly and can get a suitable set at any auto parts store for a relatively low price. You will also need a plug gapping tool. Most feeler guage sets come with this, but you can substitute a hard surface in a pinch. Press down on the outer electrode making sure not to let the two electrodes touch. To gap the plug, start with a small feeler and work your way up to the correct size. Vintage Vespas should run with a plug gap of .02 to .025 inches.

Plug Chops ->
You can perform a plug chop by taking the bike up to normal running speed (I ride high on the throttle in third gear during my normal commute, so that is how I check it) and normal operating temperature (a few minutes of riding), then engaging the clutch and immediately cutting the engine power. Shift into neutral and coast to a stop. Take the plug out and check the color. The idea is to get a snapshot of the plug at speed. You must decide when to do the chop because you are tuning the bike for normal operation; Ensure you ride like you normally do.

Feeling Out The Engine ->
Aside from plug chops, you'll also want to pay attention to how the engine feels when you ride. If the engine reaches higher revs very fast and comes down from revs slowly, you have too little fuel in the mixture. If the engine feels "rumbly" at higher revs and bogs out a bit, you have too much fuel in the mixture. Ideally the engine should be responsive, not feel boggy, and come down from revs fairly fast.

Special Situations ->
If the plug indicates you are overheating or having problems with predetonation, try switching to a higher octane fuel. If you are already using premium, try using a larger jet as fuel does double duty in a two-stroke; Not only does it make the bike go, it helps cool the engine. You should also inspect the engine's cooling fins and the flywheel fan for problems.

If the engine winds out by itself and either falls back down from revs too slow or not at all, you most likely have an air leak. Air leaks can happen in several places in the engine; It is best to inspect all gaskets, seals, and mating surfaces on the engine.

If you have an offset compression chamber like I do, you may experience fouling on one side of the plug. This comes about because the plug is nearer to the side of the cylinder and oil may be scraped off the sides by the rings and flung at the plug, especially when the engine is cold. Do not be disparaged, instead pay attention to the non-fouled side of the plug which should still be a reliable indicator of engine operation (so far it has tended to be so with my bike).

Other Parts Of The Fuel Delivery System
Fuel Tap ->

This part opens or closes a valve allowing fuel to flow from the tank to the carburetor. There are two things of note here. The first is the valve seal, which can go bad occasionally and cause excessive flooding. When you replace it, make sure the outer metal seal is completely flat, otherwise you have screwed it on with too much pressure. Too little pressure and it will leak into the bike. Also note that the rubber seal is rounded on the outer surface; Make sure you install it correctly. Second, inside the tank a filter surrounds the fuel intakes. Inspect this when you refill a tank from empty.

Fuel Line ->
This is a rubber hose that runs from the tap to the carb. If the hose stretches with little resistance, replace it because it is degrading. If either end is hashed from the hose clamps, either cut off the offending section if it is long enough or replace the hose entirely. The hose should be long enough that you can access the clamp screw on the fuel tap when you pull the tank out of the frame, but no longer or you could experience fuel starvation.

The fuel line should be 24" in length. One inch either way shouldn't make that much of a difference, but the line acts as a reservoir of sorts that supplements the one in your carb. Too short and you'll get fuel starvation. Too long and the fuel may never reach the carb. Most Vespas with stock carbs should use 5/16" fuel hose although this may vary by model, so check your old line before getting new line.
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Postby Tuna Cowboy » Mon Nov 23, 2009 7:25 pm

This is a great link for how to tune Dellorto's, I'll try and find the mikuni tuning guide and post it too.
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Re: Tech Answer: Fuel Delivery System and Carb Tuning

Postby spiderwebb » Fri Jan 20, 2012 5:01 pm

Hot Rod Al carb tuning info...!!

Spaco 20/20mm or Del 24/24 mm Crab.
pics here


A carburettor is simply a device for mixing fuel and air and delivering it to the engine. The Spaco 20/20 or 24/24 mm Carb or dellorto. is a very simple carb. however it does differ to other carbs used in other scooters and motorcycles.

thats the easy bit........

There are two 'circuits'. The idle or slow speed circuit and the main jet circuit. Both these two circuits overlap more so than many other carbs, which usually have three circuits ; an idle circuit, a needle jet for mid range and the main jet circuit.

Both the idle jet and the main jet on the Si feature an upper section that lets air in at a fixed rate, a lower section that lets fuel in and both this fuel and air mix together in the central mixer/atomiser tube.

The idle jet is a fixed jet. It either has to be swopped for a richer or leaner idle jet OR the mixture screw on the back of the carb. can be adjusted to control the flow of the idle mixture into the venturi.

The main jet stack can be altered in three ways, seperate air correctors, mixer/atomiser tubes and main jets are available.

The idle circuit works from idle right through to full throttle but works mainly in the 0 to 1/2 throttle range, tapering off between 1/2 throttle to full throttle but never actually stopping delivery of mixture! It's influence on the mixture between half and full throttle is not that great however. When you come off the throttle at high speed/revs, the only fuel and oil your engine is then getting will be from the idle circuit.

The main jet circuit works in the 1/3 to full throttle range. As you can see, there is significant overlap of both circuits.

The air corrector on the main jet stack and the top opening in the idle jet determine the flow of air to be mixed.
The main jet and the bottom part of the idle jet determine the amount of fuel to be mixed with the incoming air.
The mixer tube/atomiser and the middle part of the idle jet determine the amount of mixed fuel and air to go into the venturi of the carb to be burned. In the case of the idle jet, the mixture screw determines the amount of mixture from the idle jet to go to the venturi.
There is significant overlap in where in the throttle position the two mixture sources operate.
The first section describes how to adjust the slow speed or idle circuit and is followed by adjustment of the main jet circuit.

Neither should be adjusted in isolation because of their overlap.

Tuning the standard Si 24 Carb. Setting the mixture screw and idle
Tools required : 7 mm small spanner, or the specially modified NH spanner (see below) or in the case of a Vortex, a flathead screwdriver.

It is too simplistic to say "Set the mixture screw on the back of the carb. to 1.5 turns from closed." In practice it depends on a whole host of factors which include the weather, state of tune etc as to what the settings will be.

The Si E 24mm carb is found on all Vespa P 200s, save for a few models in the USA where the 20mm was used for a short while. The Si G version is found on the T5. The Si G version has different jetting, a different slide profile (larger cut aways allow more air in at idle and hence it can be set for more fuel, for a 'crisper takeoff').

It has a shorter body height that allows a bigger air filter and hence a quicker throttle response! Newer versions are advertised as 26mm, they actually have a 25.5mm venturi. You'll need to match the lower carb. box that sits on the cases to take advantage of the larger venturi! A test a few years ago, Si G 25.5mm V's Si G 24mm in Scootering Magazine by Taffspeed, showed no performance enhancement on a T5. The article was biased in that:

1. The carb box lower wasn't opened out for the 25.5mm carb!

2. The jetting wasn't altered!

3. Had the 25.5mm carb. been compared to the Si E 24mm on the PX200 and not the Si G on the T5 then there would have been a difference I believe!

The idle speed screw is located to the right of the main jet stack in the above photo. You can't miss it, it's very tall and on a standard PX sticks through the top carb box and can be adjusted without taking the carb box top off!

The mixture screw on the back of the carb determines the amount of mixture from the idle jet to go to the venturi. The idle jet supplies fuel 100% of the time the engine is running, however it's contribution to the overall mixture between half and full throttle is small.

Many people have modified this mixture screw that is plainly too long to adjust easily, by cutting it in half and adding a flathead screwdriver grooved top. The Vortex Carb does just that on the left!

The other alternative is to modify the adjusting spanner!

Take a cheap, forged 7mm spanner, heat it up on either a gas stove flame or BBQ flame, holding it with a pair of pliers. When it gets hot, simply use another pair of pliers to bend it into shape, as shown below. Thanks to Nick for the NH Si Carb Adjusting Spanner!

To adjust the mixture screw hold the open end, and use the closed end coming from above, with the downwards section of the spanner pointing from above onto the fuel screw, to adjust. You will find you can adjust the fuel screw 1/4 of a turn at a time. It's much easier than a standard 7mm spanner!!!

The Vortex fuel screw is easy to adjust, it's shorter, has a flathead screwdriver thread and is easy to get to. Use a screwdriver rather than an 7 mm spanner. Also, make sure that you still use the rubber bush on the carb box where the fuel screw slots through the lower carb. box. This rubber bush is not designed to keep air or weather out, it's job is to stop the fuel screw vibrating loose! Make sure it's on!

To set the mixture screw correctly, here's what you need to do :

Start the scooter and go for a 1-2km run to warm the engine up.

1. With the engine running on the stand, take the engine side panel off. Turn the idle speed right up, the long screw with the flathead screwdriver fitting that pokes out of the carb box top, turn clockwise in.

The engine will be racing now!

2. Immediately turn the mixture screw on the back of the carb all the way in, the engine will get choppy and the idle will drop. On PX200 models it will require an 7mm spanner, there's not much room in there.

3. Immediately then turn out the mixture screw from closed in 1/4 turn increments, the idle will increase and the engine will start to smooth out. Take a few seconds wait between each 1/4 turn out. Count the number of turns as you open the fuel screw out.

4. You'll get to a point where the the engine will have smoothed out and the idle stops increasing when you turn out the mixture screw. This is close to where it should be set.

5. Adjust the idle speed back down to an acceptable running level. Then listen to your engine when you blip the throttle.

6. If the engine 'bogs' and feels flat when you blip the throttle it is probably set too rich. If the engine 'hunts' and takes more than 2 seconds to come back to a steady idle after blipping the throttle, it is probably too lean. A lean idle that 'hunts' the revs will make a 'pip, pip, pip sound.'

You should be able to blip the throttle, the engine should rev. clean and it shouldn't either bog, or 'hunt out the setting'. It should rev. and return to a good idle within 1-2 seconds.

7. Make a small adjustment here if neccessary. Then adjust the idle speed slightly.

8. On tuned Vespas, if it takes more than 4 complete turns, then pop in a richer idle jet, and repeat. This is the smaller jet on the left. The PX200 runs a standard 160/55 idle jet. The T5 runs a richer 100/50. The richness of the idle jet is the ratio of the two numbers, the lower the number, the richer the idle jet. The 100/50 is 2.0, the 160/55 is 2.9. The 100/50 is therefore richer.

If it takes less than 2 turns on a tuned Vespa, consider popping in a leaner idle jet and repeat.

On a standard PX200, with an Si E Carb. there should be no reason to change the idle jet of 160/55 unless you have done some tuning work. This could include fitting a genuine expansion chamber system where the 160/55 idle jet may be too lean. Concentrating solely on the main jet may not be good enough because of the huge overlap with the two circuits.

In colder climates you will find that the mixture screw needs to be set at more turns out, ie richer to cope with the dense cold and greater oxygenated air entering the carb. In warmer weather it will need to be set less rich as the air will be less oxygenated.

You may find that a standard PX200 will only need to be set at 1.25 turns from out in Queensland all year round but in a Tasmanian winter it may be at 1.5-1.75 turns from out. On tuned PX200s the settings will be very different and could probably be in the range of 2 to 3 turns from out, depending on the tune and conditions!

The Si 24mm carb. has two types of throttle slide. The Si E found on the PX200 has smaller cutouts compared to the slide on the Si G carb. found on the T5. The throttle slide cutouts allow air to pass through and mix with the fuel at idle. The T5 G carb. with it's larger cutaways allows more air in at idle compared to the Si E cutaway. This will also have some effect on takeoff as it will enable more air in and hence with the jetting, more fuel. This is the reason why the Si G carb has a richer idle jet than the Si E carb.

Setting the Mid Range and Main Jet
Firstly, as was stated earlier in the setting the fuel screw and idle section, there is a huge overlap between the components on the Si carb.

Main Jet Stack:

Air Corrector

This is the hole that a set amount of air comes in to mix with the fuel coming in. The larger the number, the more air is delivered. A 160 air corrector is therefore 'leaner' than a 140 because it has a larger air hole and lets more air in. Changing the air corrector will affect the mixture throughout the rev range from 1/3 throttle to full throttle!

Atomiser/Mixer Tube

This is where air from the air corrector and fuel from the main jet mix before going to the venturi. Moving from a BE3 to a BE4 mixer tube reduces the amount of mixture from the main jet stack to be introduced into the venturi to mix with straight air from the inlet. This reduction is caused by the BE4 simply having fewer holes for the mixture to pass through. So a smaller amount of mixture goes to the venturi where it is mixed with the normal amount of air coming through the carb throat, thus producing a leaner running condition. Therefore, the BE4 is 'leaner' than a BE3.

Main Jet

This allows a set amount of fuel into the atomiser/mixer tube to exit the carb and out the spray bar into the venturi then into the case inlet. On the Si Carb. the fuel exit on the main jet is a metric size. Eg a 116 main jet has an exit hole of 1.16mm. The larger the number, the 'richer' the main jet. A main jet of 118 is therefore richer than a 116. The main jet stack has its effect from about 1/3 throttle to full open. This is a much wider range than most other carbs that have three jet circuits and not two, like the Si.


As you can see, there is a huge overlap between the two circuits that supply fuel/air to the venturi!!! Many people merely concentrate on the main jet when tuning but fail to look at the idle circuit, which overlaps. An example would be someone putting an expansion chamber on a PX200. They usually up the main jet by 2 or 3 points but fail to increase the richness of the idle circuit. Many engine seizures happen in the low to mid throttle range, I'll leave you to draw your own conclusions..........

Hope you all understood that more Chews ????


Vortex Carb.
When tuning a Vespa, many people simply slap on a bigger carb. as well as an exhaust and upjet.

Some people slap on a 28 or 30 mm side draught carb, with a view that more is better. Then at somewhere between 80% throttle and full throttle in fourth gear, the same scooter, if well tuned may suffer from fuel starvation and require a fuel pump to be fitted! Your fuel economy will go down considerably, making it difficult on runs to travel from one town to another without worrying where the service stations are!

At the same time you also lose the oil injection capability of the standard Si E/G Dellorto carb! You then have to carry in your toolbox 2 x 1 litres of two stroke oil! Valuable space I say, space better occupied by either tools or 750ml bottles of naturally conditioned beers!

So what is the answer? Can I forget the side draught carb. with it's fuel guzzling antics and free up some space in the toolbox for some better products?

Well, yes you can! The answer is the Vortex 28 mm Carb from Hot Rod Scooters in the USA.

The HRS Vortex Carb. utilises a velocity stack (bell mouth) that increases airflow into the carb. The theory is, the more air you can get in, the more you can increase the fuel in and then perform!

This system has been used in Kart Racing circles for a few years now.

The Vortex Carb. is a standard 25.5mm (earlier versions were a 24mm) Si G modified carb. The Vortex allows you to run your standard PX200 set up with auto lube. The Vortex comes in 24, 28 and 32 mm sizes, all through the 25.5mm venturi. For a PX200 the 28 and 32 mm models are neccessary.

The Vortex carb. runs the Si G slide. Takeoff is crisper and quicker due to the large air cut outs on the slide compared to the standard Si E slide on the PX200. This will mean you may have to run the richer idle jet from the T5, the 100/50, supplied with the Vortex. It's definately also worth ordering the slightly leaner 120/50 and 140/55 to give further options. The fuel screw may well need to be turned out slightly to allow more of the atomised mixture from the idle jet circuit into the venturi. (see earlier section)

Because of the modified air box on the Vortex, you will need to also up the main jet by 2-3 points. Pop in a rich main jet and work down, doing 3/4 throttle plug chops in third gear.

The Vortex comes with foam that has been glued into the trumpet shape on the air box. Although it has a support section beneath it, there is a risk it could come lose. Automotive foam and a 60mm hose clamp make for a better structure.

hope that clears up a few things Al..
currently on Saab tuning project. and getting confidence to get back on the scoot.

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Re: Tech Answer: Fuel Delivery System and Carb Tuning

Postby spiderwebb » Fri Jun 15, 2012 9:28 pm

nice info...!!
& do click on the bottom pic
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Re: Tech Answer: Fuel Delivery System and Carb Tuning

Postby spiderwebb » Mon Sep 16, 2013 10:56 pm

from oopsclunkthud:

& from Garagerocker:
45/140 = 3.11 < Leaner
55/160 = 2.91 < Richer

Jet sizes
45/160 =3.56
42/140 =3.33
48/160 =3.33
50/160 =3.20
38/120 =3.16
45/140 =3.11
40/120 =3.00
55/160 =2.91
50/140 =2.80
52/140 =2.69
45/120 =2.67
50/120 =2.40
50/100 =2.00
55/100 =1.82

The older solid slow running jets without the hole was mounted in carbs that had an air intake hole in the carb body. These carbs could be upgraded with the new kind of jets by blocking the hole with a lead shot(see pic) When doing this operation it was advised to also adjust the idle air/fuel mixture adjusting screw by screwing it a ½ turn out.

Reason for changing this was to prevent the carb from flooding when engine was shut off while parking
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Re: Tech Answer: Fuel Delivery System and Carb Tuning

Postby spiderwebb » Thu Aug 28, 2014 10:44 pm

PWK KEIHIN carb / jetting / needle app
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