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Carb Cleaning And Setup

62K views 20 replies 11 participants last post by  Remy 
#1 ·
CARB CLEANING

Download the manual, read it. Stripping and cleaning carb is one of the easiest jobs to do on a 2stroke bike.

Grey residue is likely result of water mixed with 2T oil saturating / flooding your spark plug as you have water in the float chamber that is submerging the mainjet (mainjet is in base of float bowl, and petrol floats on water), so when you open throttle, bike gets a lung full of water rather than fuel.

Flip up tank. Turn fuel off. Look down at carb. Either side of it you'll see 2x jubilee clips clamped over rubbers. Undo those. Grab carb... move it backwards and forwards and work it out from between the rubbers. Keeping it the right way up, undo big brass bolt in bottom. Bottom will then come off. This bottom part is the float bowl. Ideally, open the bolt over a glass bowl so that you catch the spilt fuel and can inspect it for signs of water or contamination.

If no signs of globules of liquid suspended in fuel, inspect the bottom of the carb that you removed for signs of contamination - dust, rust, dirt etc. Rinse it out.

The jets are small brass things with a hole in the middle and a flat-blade screwdriver cut - you'll see em. Take them out one at a time, blow thru them, rinse in boiling water. Put em back in again. Tighten em til they nip gently.

Put float bowl back on. Do up the big bolt to secure it in place. (Note, this big bolt has the mainjet fitted in the opposite end of it).

Fit carb back inbetween the rubbers. Put the jubilee clips back on. Turn on fuel tap.

See if it starts. See if problem still exists.

Meccano is harder to get to grips with than stripping and cleaning a carb. Don't let it scare you - it really is easy to do... otherwise, I'd suggest selling the bike and getting a 4T, as any regular maintenance that is a necessity for a 2stroke bike is gonna scare you, and you're gonna end up forking out hundreds in labour hours to the local bikeshop for tasks that really you should learn to do yourself.

CARB SETUP

Set it to rev at 2000 with the idle screw, then twiddle the air screw just to the left of the idle screw... turn it all the way in, then back out 1.5 to 2.5 turns, listening to engine. As you turn it, revs should increase slightly. Set it to a point that revs are at highest and smoothest. Now use the idle screw to tune it back down to around 1100rpm ( i normally set mine at 1500rpm )... that should be the carb as set as it can be.

i think its
in = increase
out = decrease revs
some carbs maybe different but you will know when you do it ok
 
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#2 ·
Also, with the PHBH28 BD carb, the mixture screw is to the RIGHT of the idle screw (not an air screw like the RD)

Turning in = Leaning the mixture out
Turning Out= Richening the mixture

If you find yourself grabbing a handfull of throttle and the bike bogs down, you may need to adjust your mixture screw the richen the mixture up. It should be around 1 1/2 to 2 turns out, IIRC mine is at 2turns out.

You may also like to adjust the needle position to richen or lean out your main jet, the standard setting for the needle clip on a BD is 2nd notch from the top.

Lowering the needle clip = Richening the main jet
Raising the needle clip = Leaning out the main jet

This may be a good thing to know for when the weather changes as winter weather = thicker air so richer mixture
and summer = thinner air so leaner mixture

Remember 2nd from top is standard so it's likely the safest option!
 
#3 ·
Also, with the PHBH28 BD carb, the mixture screw is to the RIGHT of the idle screw (not an air screw like the RD)
the best way to say it is if the small screw is closest to the engine or the airbox,if the screw is closer to the engine....its a MIXTURE SCREW. if its closest to the airbox...its an AIR SCREW. becuase some carbs have the adjustment screws on the left of the carb
.
Turning in = Leaning the mixture out and visa versa if it is an air screw. turning it OUT to lean it.
Turning Out= Richening the mixture

If you find yourself grabbing a handfull of throttle and the bike bogs down, you may need to adjust your mixture screw the richen the mixture up. It should be around 1 1/2 to 2 turns out, IIRC mine is at 2turns out.

You may also like to adjust the needle position to richen or lean out your mid range, the standard setting for the needle clip on a BD is 2nd notch from the top.

Lowering the needle clip = Richening the main jet
Raising the needle clip = Leaning out the main jet

This may be a good thing to know for when the weather changes as winter weather = thinner air so leaner mixture
and summer =thicker air so richer mixture

Remember 2nd from top is standard so it's likely the safest option!
 
#4 ·
lol thing is how do you know your mid range is rich or lean because unless your on a dyno and can get the fuel/air ratio of your carb at all throttle positions its rather hard for us lot to judge it.. unless do loads of tedious plug chops at different throttle positions??

Cheers
Tadman
 
#5 ·
plug chops yup, and by feel
 
#7 ·
Jetting Shouldn't Be Scary!

Jetting is the process of making adjustments to the air and fuel jet sizes in order to fine tune the carburetion to suit the load demands on the engine and make the power delivery consistent and optimum. Too much anxiety is placed on jetting. Most people just want to call me on the phone and ask what jets they should put in their carb. That's an impossible question because that the big dirt bike magazines attempt to answer just to increase readership. People get confused because they read jetting specs in a magazine, put those jets in their bike and seize the engine. Any quoted jetting in this book is just a baseline. Most magazines don't list parameters for their jetting specs like; Brand new bike running with VP C-12 fuel with Silkolene oil mixed at 30:1 and a NGK 8 spark plug, ridden by a really slow lard-ass editor twisting the throttle on a hard-packed track.
Some part numbers and jet sizes are given in the Tuning Tips section for models that definitely need certain jets in order to get the bike near the baseline. There is an old saying that says you can fish for a man and feed him for a day or teach him to fish and enable him to feed himself for life. Here is a quick lesson on how to jet your dirt bike.
Ride and Feel Method

The most basic method of determining correct carburetor jetting is "ride and feel." This method requires you to determine if the carburetor tuning is too rich or too lean by the sound and feel of the engine. The first step is to mark the throttle body in 1/4-throttle increments, from closed to full open. Then, this method requires that you ride the motorcycle on a flat, circular course. To check the carb jetting for throttle positions up to 1/2 throttle, ride the motorcycle in second or third gear. Roll on the throttle slowly from 1/4 to 1/2 open. If the engine is slow to respond and bogs (engine makes a booooowah sound) then the carb jetting is too lean. You can verify lean jetting by engaging the carb's choke to the halfway position. This will make the air-fuel mixture richer and the engine should respond better. If the carb jetting is too rich, then the engine will make a crackling sound. The exhaust smoke will be excessive and the engine will run as if the choke is engaged. Careful engagement of the choke can help you determine if the jetting is rich or lean. Another important tip is to just change the jets one increment at a time, either rich or lean, until the engine runs better. Most people are afraid to change a jet because they think that the engine will be in danger of seizing. Believe me, one jet size won't make your engine seize but it could be the difference between running bad and running acceptable.

To check the jetting for throttle positions from 1/2 to full open, ride the motorcycle in third and fourth gear. (You may need to increase the diameter of the circular riding course for riding in the higher gears.) Check the jetting in the same manner as listed above. The carb jets that affect the jetting from 1/2 to full throttle are the jet-needle, main jet, power jet (electronic carbs) and the air jet (on four-strokes).

If you want to take this technique out to the racetrack, you can test the pilot/slow jet when accelerating out of tight hairpin turns, the needle clip position on sweeper turns and short straits, and test the main jet on the big uphill or long straits. Of course be careful if you try to use the choke technique because you could lose control when riding one handed.

Jetting for Riding Techniques

Certain types of riders require jetting to compliment their technique. For example beginner minibike riders will need slightly richer jetting on the pilot/slow jet and the needle clip position to mellow the powerband and make it easier to ride. Conversely desert racers who hold the throttle wide open for long periods of time need rich main jets to compensate for the high load.

The Weather Makes The Biggest Difference!

The weather can have a profound affect on the carb jetting because of the changes in air density. When the air density increases, you will need to richen the air-fuel mixture to compensate. When the air density decreases, you must lean-out the air-fuel mixture leaner to compensate. Use the following as a guide to correcting your jetting when the weather changes:

Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must select jet sizes with a lower number to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.

Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should change to smaller jets.

Altitude: In general, the higher the altitude, the lower the air density. When riding at racetracks that are at high altitude, you should change to smaller jets and increase the engine's compression ratio to compensate for the lower air density.

Track Conditions and Load

The conditions of the terrain and the soil have a great affect on jetting because of the load on the engine. Obstacles like big hills, sand, and mud place a greater load on the engine that requires more fuel and typically richer jetting. In motocross, track conditions tend to change over the course of the day. Typically in the morning the air temperature is cooler and the soil wetter requiring richer jetting. In the afternoon when the temperature rises and the track dries out, leaner jetting is needed in order to keep the engine running at peak performance. Other changes for mud and sand riding might include changing to a lower final-drive ratio (rear sprocket with more teeth) to reduce the load on the engine and help prevent it from overheating. Advancing the ignition timing will make the engine more responsive at low to middle rpm.

Fuel and Oil Mixture Ratios

When we talk about the "fuel" in the air-fuel mixture for a two-stroke engine, we are really talking about a mixture of fuel and oil. If you richen the pre-mix ratio (20:1 as opposed to 30:1) there is more oil and less fuel in the same volume of liquid, which effectively leans the air-fuel ratio. And this fact gives the clever tuner one more tool to use when the correct jet is not available or when none of the standard jets are exactly right. You can richen the jetting by slightly reducing the pre-mix ratio (less oil). You can lean the jetting by increasing the pre-mix ratio (more oil). The best part is that changes in the pre-mix ratio affect the jetting over the entire throttle-opening range, but the changes in ratio must be small to prevent excess wear from lack of lubricating oil or fouled plugs from too much oil.

Pre-mix oils are formulated for a fairly narrow range of pre-mix ratios. You should examine the oil bottle for the oil manufacturer's suggestion on the pre-mix ratio. All production two-stroke dirt bikes have a sticker on the rear fender suggesting that you set the pre-mix ratio to 20:1 That sticker is put there for legal purposes. Always refer to the oil manufacturer's suggestion on pre-mix ratios. In general, small-displacement engines require a richer pre-mix ratio than do large-displacement engines because smaller engines have a higher peak rpm than larger engines. The higher the engine revs, the more lubrication it requires.

shaun
 
#8 ·
This is a new plug from my mito @ 400 miles,it has been worked on But I don't know how much bikes done 7,000 miles,I'm guessing rings.just thought I'd chuck a pic up Looks on the edge of running OK ? has a bit of fuel weeping from the carb bowel have to check it out don't think it's float related,Followed the step by step here running good,It may be a pain sometimes but the info is a great help.
 
#9 ·
I added this many moons ago, May help someone.


Mikuni Motorcycle Carburetor Theory 101Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow. Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalize The moving air going through the carburetor will pick up fuel and mix with the air. Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.

Most motorcycle carburetor circuits are governed by throttle positionand not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:* pilot circuit* throttle valve* needle jet and jet needle* main jet* choke circuit The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how muchairenters the circuit. If the screw is turned in, it reduces the amount of air and richensthe mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a leanmixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle. The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer. The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet

The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the main jet, the more fuel that can flow through it and the richer the mixture.

The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed. The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6shows the actual air/fuel ratio for any given throttle opening.

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.


While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting. Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color (for more info on reading spark plugs click here). If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run. After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture. The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs. Altitude, Humidy, and Air TemperatureOnce the jetting is set and the bike is running good, there are many factors that will change the performane of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32deg Fahrenheit may run poorly when the temperature reaches 90deg Fahrenheit. Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air. Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases. Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correctin factor is found. Using fig 8as an example, the air temperature is 95deg Fahrenheit and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.

orrection factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9and determine the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.
Correction factor 1.04 or above 1.04-1.00 1.00-0.960. 96-0.92 0.92 or below
Needle jet Two sizes larger One size larger Same size One size smaller Two sizes smaller
Jet needle setting Lower clip position Same Same Same Raise clip one position
Air screw opening One turn in 1/2 turn in Same 1/2 turn out One turn out
 
#12 ·
might have been sealed in with sealant , to stop leaks, try running a blade between them, be carful not to cut into the pipes.
 
#14 ·
cool
 
#15 ·
does anyone know where to take the messurment from to find out the correct hight of the floats in the PHBH dell carb??

cant find any info in the w.s.m......

thanks
 
#16 · (Edited by Moderator)
Setting the float levels in DellOrto carbs

To set the correct float level position:

for connected floats, hold the Carburetor body in the position shown in fig. 13 and check that the float is at the correct distance from the Carburetor body face as specified in the table.

for the floats with independent parts, hold the Carburetor upside down (fig. 14) and check that the float arm is parallel to the Carburetor face.

Whenever the float or float-arm position does not correspond to the proper specified level setting or is not parallel to the float chamber face, bend the float arms carefully to set the correct position.

Carburetor
float position mm
PHBG 16.5 + 15.5
PHBL 24.5 + 23.5
PHBH 24.5 + 23.5
PHBE 18.5 + 17.5
PHF 18.5 + 17.5
PHM 18.5 + 17.5

 
#17 ·
plug chops are done at closed(idel) 1/2 and full throttle mate. not rpm. inbetween can be done by feel once you get more firmiliar with how your bike behaves, or chopped at 1/4 ,3/4ect but its hard to get accurate readings between diff circet/ jets kicking in. can be faulse readings.
 
#18 ·
Hi there, forgive me if i am not posting in the correct place, i am new !!!
I "desperately" need some advice ! Its a Cagiva Mito 125 1996. Me and my mechanic have been at it for "4" days now, and i still cant get it running properly. (all day all night in a freezing garage).

Got it started for a few minutes then it dies, then wont fire again! Looks to have good compression (back tire locks when push starting).
Stripped carby completely, cleaned everything. Floats springs back ok. Drained fuel and 2T oil, spark plug, replaced with new. Reed valves looks fine. Has good spark. Cant get it to fire at all now. "It keeps flooding the cylinder and airbox"??? Sometimes fuel poors out of two clear hoses from carby ? I downloaded manual, but i cant make much sense of it. I put a funnel of fuel on fuel hose and held it up, it doesn't even siphon in carby, even when throttle is open??? I think i have cylinder full of fuel now! I dont know how to drain that fuel out now ? ANY HELP ASAP would be greatly appreciated !!! I would even buy a call card to call anyone for advice, im in Australia ! Cheers guys...
 
#19 · (Edited by Moderator)
Borrowed this info ..

Pilot jet/mixture screw:

If you can start it cold with no choke, the pilot circuit is too rich.
If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.

The best way to set a pilot circuit is with an RPM guage. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again.

On a 2-stroke - You should be able to ride in 3rd gear, throtlle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean.

Needle Clip:

On a 2-stroke - Riding in 3rd gear, with a warm engine and the throttle BARELY cracked open, roll the throttle to 1/2. If the bike sputters and crackles, and you feel like you have to keep rolling on the throttle to smooth it out, the needle is too rich. If, on the otherhand, you get the dreaded 'buhhhhhhwaaaaa', the needle is too lean.

Main jet:

On a 2-stroke - Riding in 3rd, with the throttle BARELY cracked open and cruising along, whack the throttle wide open. If you end up with a set of handlebars impacting your nose, or you loop out, the main is perfect! If it crackles, smokes, and won't get 'on the pipe' quickly, then the main is too rich. If it gives a 'buuuuhhhhwwaaa' sound and feels like it's sucking for air, then the main is too lean.

Advanced Topics:

I will continue to add to this FAQ as time allows. I'll start with the needle because that's the circuit that is 'in play' the most.

The needle regulates the mixture from around 1/4 - 3/4 throttle. Most people are familiar with the clip position, as it's the most common adjustment, but there's much more to the needle. The jet needle is a long rod that fits into the needle jet. On most carbs, both are replaceable with different sized components. As the throttle is opened, the jet needle is retracted from the needle jet and this creates space between the two for gas to flow through. The more you open the throttle, the more the jet needle is pulled out of the needle jet, and consequently the more gas can pass through the increasing space between them. Below I'll outline the various parts of the jet needle.

Length - The relative length of the needle is adjustable by raising or lowering the clip. If you lower the needle (by raising the clip), then the needle sits deeper in the needle jet. This leans out the mixture across the range of the needle. Conversly, if you raise the needle (by lowering the clip), then the needle is further retracted from the needle jet, and this richens the mixture across the needle's range. Needles are offered in various lenghths. If you have a needle which is still too rich, even though it's in clip position 1, then you need to order a longer needle. For example, needle 'A' in clip position 1 is the exact same relative length as needle 'B' in clip position 3. If you had needle A in your bike, and it was still rich - even though you had the clip in position 1, then you could change to needle 'B' and lean things out by going to clip position 2.

Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the otherhand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).

Needle taper - Needles taper from top to bottom. As with all principles regarding the needle, the taper is relative to the diameter of the hole in the needle jet. Tapers are rarely changed, but here's a condition which warrants a taper change. Let's say the jetting is perfect at 1/4 throttle, but becomes increasingly leaner as you approach 3/4 throttle. In that case, you would want a needle with a shallower taper. Conversly, if the mixture is great at 1/4 throttle, but getting richer and richer as you approach 3/4 throttle, then the needle taper needs to be steeper. In my experience, needle taper only needs to be changed when the factory mis-spec'd it to begin with. Under very rare circumstances, big modifications to the motor - such as an overbore kit - will require a change in needle taper.

Remember that jetting needs to be adjusted for every 2000' elevation change and every 15 degree temperature change. If it was jetted right this summer, it's sure to be too lean during the winter. If you rejet it now, when it's cold out, make sure to lean it out a bit in the spring.

Good luck!

Read more: http://forum.dirtrider.com/discussion/7288955/jetting-faqs/p1#ixzz29OszZo00
 
#20 ·
dellorto VHSB 34 LD jetting

this is the base jetting for the only cagiva 125 that came whit dell 34 (cant remember whic one)

High speed jet 152
Starter jet 60
Low speed jet 40
Throttle piston 50
Metering pin / Needle K43
Clip 1st position
Main nozzle 266 DQ
Float n^2 3.5g
Idle screw 1&1/2

and these I found on mitoclub.it

Getto massimo 168 -High speed jet max jet
Getto minimo 55 - Low speed jet
Getto avviamento 65 - Starter jet
Valvola gas40 - Throttle piston
Spillo conico K49-2a tacca - Metering pin / Needle - needle clip position 2nd
Polverizzatore 266 DP - Main nozle
Galleggiante gr 9 - float
 
#21 ·
here is a "table" whit few needles for dellorto VHSB 34LD

if you want any to be added just as on pm or here, and Ill re-upload it.

there are layers in a PDF that you can turn on/off to leave only two needles to compare them
View attachment needle diference-Model.pdf
 

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