Motorcycle Carburetor Tuning and Adjustment

Balancing

The purpose of this page is to illustrate the importance of carburetor tuning on motor cycle engines. For competition, carb tuning is absolutely critical to your machines performance and overall responsiveness. If you have not checked or adjusted valves recently then I would suggest you do that first as it will affect your carb balance.

The first part we need to cover is basic operation and service adjustments, The article will then go on to cover jetting and further adjustments. So as usual allow all pages to load then disconnect from server and read on this will save your browsing time then reconnect when finished.

In comparison to a bike, the car carb is setup and jetted to cruise. Acceleration is mostly a fuel dump affair in comparison to bikes. With a bike carb you are talking a competition carb. It is set up and jetted to accelerate not to cruise. And the majority of circuits and functions on a car are for idle and cruise. A motorcycle carb would be classed as a competition carb on a car.

The good thing about this is that motorcycle carb's are very simple in contrast to other engine carbs. This allows a great deal of maintenance on motorcycle carbs to be easily performed.

A two stroke engine example will only fire at every second stroke with a full open throttle. Any obstruction to the carb throat seriously affects the exhaust scavenging ability and it chokes on its own exhaust. These carbs are set up for wide open operation even more so than street four strokes. Carbs are usually in pairs or multiples on motorcycles and for this we need to have them operating in sequence or exact balance (synchronization). Before we balance the carbs, we need to ensure that they operate correctly as individual units.

If the carbs are not adjusted properly to begin with, balancing will be useless. The first thing we need to check is the float level. There are a couple of ways to do this. The easiest is to measure the float level using a gauge. This is simply a piece of transparent fuel proof tube fitted to the bowl drain and held upright against bowl. When the drain screw is undone, the tube will fill to the same level as the float level. This will indicate the level in the bowl. On bikes with vac operated fuel petcocks, you will need it set to “PRI” (prime) or use a hand vacuum tool with the petcock in the “ON” position. Its best not to try and measure them with the engine running.

The alternate method is to measure the float arm with the float bowl removed. Once the bowl is removed, invert the carbs and let the float rest on the needle, but not depressing the needle. This may require holding the carbs at an angle. Measure from the highest point of one of the two float lobes, to the carb bowl surface with gasket or oring removed. If you don’t have the float spec for a start, the float arm is usually level or parallel with the bowl surface. Once the float is set and you have checked for blockages at needle and seat and or loose jets, the main jet can drop out some times from vibration, so check it for tightness.

A float height adjustment

Now put the bowl cover back on and do the same to all carbs. When that is done, reinstall the carbs. If they are cable pull pairs, or if they have a common throttle shaft or axis to operate the slides or butterflies, we will do a bench balance. Doing this ensures the throttle plates or slides are opening at the same time. Place carbs in a vice at the center bowls with a rag. Just enough to hold and adjust the carb screws, but not too tight or it will change the settings when you release them, or possibly damage them.

A bank of four carb’s

Now you will need a small drill bit about 3 to 5 mm. Then place the drill in the throat of the carb and adjust the idle screw until the drill fits in the gap at the slide or throttle plate at the lower most point in the throat, parallel to the throat. Hold it down with a finger while you adjust the other throats to the same drill bit size. On slide operated carbs, remove the top cover. You will notice a screw or nut in the top. This will set the slide height. What you need to do is get all slides or throttle plates open the same amount at the same time. The drill is your gauge, so set one with the master idle adjustment, and adjust others to match it. This will give you a good balance to start with. You can fine adjust it with a vacuum gauge set after you have reinstalled the carbs to the engine intake manifold. When replacing the top cover, make sure the top O-ring or gasket is in place.

An idle mixture screw

Next is to set the idle mixture screws starting by screwing in two of the screws and count the turns then average them out IE one screw 1 turn the other 2 then all screws set at 1-1/2 turns out and remember don’t screw them in tight just to the seat and back out again. If you have a problem with idle and you cant seem to get any adjustment at the screws you should remove them and check the tips of them, the screws get wound in too tight sometimes and the tip breaks off in the seat not generally in the carb illustrated above but in screws mounted under the throat such as a set of Mikuni's, if you have a light in the bore you will see the screw hole seat blocked, its very small and the tip of the idle adjust screw will be broken off just at the end also very small. If this is the case I am sorry the carb’s are bin fodder. Or permanently shagged.

When you have done the mixture screw presets as above then fit the carb’s to the bike, you will need to ensure the manifolds are in the correct position a vacuum leak on a bike is terminal so make sure the manifolds are not split or cracked and that the carb’s firmly " lock " in place, there is usually a rib inside the manifold and on the carb and head spigots that locate the carb’s, make sure they are also not twisted out of place you can line up the rubber cast line on the manifold with a common point on the head some have a locater. Once the carb’s are in place clamp them up manifold to head clamps first then manifold to carb’s.

A vacuum leak at the manifolds will usually create a surging affect or an inconsistent idle speed.

The next thing to check once the cables are hooked up is free play this must be done with steering turned to the tightest throttle cable angle if you set the free play and steer the bike from side to side you will notice the free play or cable slack change. Also ensure the choke cable has plenty of slack as it does not take much to pull it on a little and this will upset the mixtures.

Whilst we are on the subject of the choke a butterfly or throttle plate choke is fairly simple it covers the throat over or it doesn’t just make sure it works through its entire range on and off. The other type of choke is a plunger it is pulled open and allows fuel to flood the main bore on four carb’s there are usually four on twins you will find a balance choke tube and one plunger either way if any plunger does not return or its O-ring is broken or perished one carb will run very badly or very rich so check that the chokes turn off and remove and inspect them if you have an odd mixture problem with one carb. IE too rich.

A plunger type choke

Now you have the cables and controls adjusted and pre-settings on carb’s done start the bike, start with choke so you can make sure its working properly then once the choke is off you can back off the main idle adjustment or bring it up to proper idle speed once this is set turn the engine off.

Ok you are now coming to the tricky bit, to balance the carb’s running if it is a four stroke with manifold vacuum access holes or tubes we will use gauges if not and it is a multi pull cable I mean one cable per carb skip this. ***Note some engines have an induction balance tubing either at the carb’s or manifolds or inside the head these balance circuits must be isolated or blanked from each other during carb balancing of all types including two stroke twins or you will be adjusting nothing. This also includes Air injection or any smog pipes isolate it back to raw machinery for the balancing only. IE isolate each cylinder from each other so that each carb will only affect the cylinder it is on and the exhaust note given from that cylinder for the test.

Firstly we need access to the top of the carb’s of which you will notice are neatly tucked up under the tank, problem is to run with the tank off the bike and supply fuel to the carb’s, to balance whilst the engine is running. I use an old lawn mower fuel tank on a trolley but I have done it with a friend holding the tank and some bungee straps and rag. I refuse to describe the position you need to hold the tank but get a friend and/or don’t drop it on the ground. That part I will leave up to you but if you use a remote tank you need to block the vac line for the tap, if it goes to a single manifold don't bother we will plug the gauge in there anyway.

The next part is the gauges you will note access screws or rubber tips at each manifold or carb throat on the vacuum side of the slides, these are for balancing remove all four if they have a spigot then you only need four pieces of vac hose one to run from each to you and the gauge set if they are screw hole types you need to get four screws the same thread but longer and drill them hollow then cut off the heads and screw them in finger tight. I like to use a plain internal hex bolt drilled in the centre and fit a vacuum hose to each of them, with the hose off you can use an Allen key to undo them.

Now we have the four hoses we need a vacuum gauge it can be any type don’t get too fussy you are only comparing one carb to another not measuring the actual vacuum level. Now what you need is a control the best yet is a fish tank air pump four tap block with manifold into one, they are cheap and plastic with this you can switch on the vac from one carb / manifold at a time to get its vacuum reading. Don’t use the clear plastic tube it will melt to the motor use black rubberized tube vacuum tube. I personally have bought a four gauge set but you still need four taps or those hospital I.V. drip hose pinch rollers are handy if you can get a few and fit them to a section of clear vac pipe that up at the gauges as dampers.

When you have the gauges set up or a gauge and tap set as described you will save megabucks on tuning as the carb balance is about the only thing you could not or perhaps would not normally perform in the way of tuning on your own bike and is absolutely necessary for a good tune. SO if you can balance and tune the carb’s yourself all you need is a little valve adjusting skill which I will cover in another page and you are doing all your own servicing / tuning work. There are other gauges you can make like mercury or water in tubes but I think there a toss.

To cut it short you will save a lot more than the cost of the gauge setup.

Now if you are ready to balance and the fuel is setup remove the carb tops to access the slide balance screws they also have a locknut on them ( usually painted ) then set the idle slightly higher and start the bike, no choke. With the engine running check the gauges you will notice they pulsate this is what you need vacuum taps for you will need to screw the vacuum tap in to damper the vacuum to the gauge in until the gauge all but stops to get the closest reading the needle should pulsate just a bit so you know it is still reading but not so much you cant get a single reading then noting the vacuum at each carb gauge or doing it singularly note the cylinder with the lowest vacuum reading, this is the carb with the highest slide or more opening.

What we will do is adjust the other slides until the vacuum in those drop and the first reading comes up at the same time do not adjust any of the other slides so much that it drops below the lowest value bring them all down gradually starting from the one most out of sync. You will notice the revs rise and you will need to back off the main idle a bit perhaps and adjusting one slide will affect all the others equally.

You will note it is tricky but take your time it will be worth it but make sure you don’t rush it if its longer than ten minutes switch off and cool the bike for a 20 minutes don’t let it idle too long, this is important. Undo the locknut on the slide top screw, adjust the screw slightly and tighten the lock nut, rev it slightly to settle the slides and read the vacuum gauges again this is the procedure you must repeat with each carb until they are all at the same vacuum reading then you can adjust the main idle adjustment screw to correct idle

If the main idle screw does not have its stop on the throttle shaft next to the cable and it is on one carb as some twins have you must balance all the other carb’s to this one.

Now the carb’s are balanced to perfection we can clean up and replace covers next is to adjust idle mixtures and this will need to be done, bike hot. Once it is warmed up you adjust the idle mixture screws by turning each in one eighth of a turn together and again until a drop in rpm is noted or a faltering in the idle then rotate all screws back out up to 1/2 turn this should give you good off the mark throttle if you note a bit of flatness you can open them another 1/4 turn but that is about all you should need if you have more than three and a half turns out on the screws or the screws have to be screwed in all the way to get a smooth idle or snap throttle you probably need to do a little jetting, if unsure check your manual some are unusual also screws differ some are in for lean and some are out for lean.

For balancing multi pull cable type carb’s on many two strokes and some fours go to next section skip to next page three for jetting and other stuff.

On a two stroke bike or a European or racing four stroke you will often find multi carb setups are independently mounted and so usually have individual controls for each carb’s operation save the case of a choke where there is a balance tube to supply choke to other carb’s.

For this case we will balance using exhaust note to determine the difference at idle if you have manifold access you may refer to the above vacuum gauge setup and use but this part covers cable and idle screw balance so continue anyway if they are independent mount or multi pull cables.

For the first part of the exercise you set the idle screws so we first must back off the cable at the throttle lever or single section of the cable not at the carb tops unless you run out of slack. Then undo both idle screws and count the turns out if you have access to the carb intake throats then look down them and back off idle until slides are at base and shut. If you don't have access to the intake because of the air box the trick is to operate the throttle on and off idle and as you back off the screw you will feel the slide contacting the screw through the screwdriver, you must feel for it then when the slide zero's it will stop contacting the screw as you open and shut the slides with the twist grip. Do it to both carb’s then screw both in the average of what the came out IE #1 came 1 turn out #2 came 2 turns out then screw both in 1/2 turns to set slides to same height at idle, that is the first part now set the idle mixture screws similarly as described above but for the idle mixtures screw them in until they bottom lightly in the thread and then back out the average as above then start the bike.

Once its running set both idle screw exactly the same amount to get a good idle and set the mixtures the same until you get the best idle you may need to readjust the idle a bit after the mixtures are set but so long as you move the screw sets the same amount per adjustment with a short rev to settle the carb’s in between adjustments. Now the idle of the carb’s is set and balanced we need to balance the opening of the slides it is not enough to have the idle balanced there are two, three or four cables on some and they must pull the slides open together right on the mark so adjusting the slack after idle will not set them correctly.

Allow the bike to cool and restart then adjust the twist grip end of the cable up until the rpm rises slightly and then back off idle screws one turn exactly so idle is maintained by the cable tension and not the set screws then listen to the exhaust note for balance of firing noise or check vacuum gauges then adjust one of the cables at the top of the carb until you have a good idling balance, one thing to ensure is you don’t move the handle bars during as the idle is set by the cable tension and it will affect idle speed by turning steering. Once you have the engine idling balanced you can wind the idle set screws back in one turn to there original position and adjust the twist grip or upper section of the cable to give overall cable free play.

Now the carb’s should be at correct idle set balance and should crack open exactly together you may need to fine tune idle speed or idle mixtures after balance if so maintain the balance in the screw adjustments. Once the carb’s are set on a two stroke you should recheck oil pump settings if fitted I will cover that in the servicing page coming soon. For jetting and adjustment above what is covered so far go to next page.

Jetting and other

This page will cover further settings to the carb if the previous page has not covered tuning sufficiently or you would like to check and perform some fine tuning or major jetting. You will note the carb pictured alongside is a fairly simple unit. For our purposes we can assume that most are simple variations of this carb in sets of four, sets of two, or singularly. You will find minor variations including choke mechanisms and flat slides (rectangular section) and possibly a second pilot or compensation circuit, but to set-up jetting you will only need to base it on the jets shown in the diagram. These cover the basic operations in all the carbs including CV's (constant vacuum) which have a rubber diaphragm on top of the slide to lift it and a throttle plate in the main bore.

Noting the parts that are adjustable starting with the needle, clip and the slide and next the main jet and its housing which is the nozzle or emulsifier tube next to that the pilot jet and pilot mixture screw and in the main throat you will find a main and or a pilot air compensation jet as pictured alongside.

To perform any kind of jetting or jet changes you need to know what jets affect what operations in the carb and then you need to gauge what the engine needs this will be done by plug reading and seat of the pants unless of course you have access to a reliable rider and circuit to check lap times or a dynamometer.
If you are a normal human and only have your fists, brains and toolkit you can use the first method described. Why fists, you'll need those to hang on to the handle bars. A second and most important factor is choose a day where the temperature and humidity most suit your average riding , If it is for racing simply log the ambient temp, track temp if you like for your tires and humidity then you can work your way to better tuning every meeting at that track.
The first thing to do is balance the carb’s so if you thought you had fortunately missed the first page you have not and need to go back and do it. If that is done then you are ready to do some jetting and the first thing you will need to do is fit a new set of plugs, we need to do this to do a plug run if you cannot get a straight section to open it up on and have to ride there take the plugs with you and a spanner for plugs, you can take the main jets and tools needed to change them and don't forget a rag to soak fuel when you drain bowls to change main jets.
Once plugs are fitted we need to ride it and I mean ride it, full throttle all gears (well from second or third at least ) to red line going up gears to the highest gear you have the opportunity to go to, though not essential. At this point you must cut the kill switch with throttle still open and pull the clutch together effectively killing the motor and stopping it dead at full revs then braking slowly to a stop and I mean slowly the bike has no engine braking or gyroscopic engine stability only the wheels don't smack on the brakes and don't release the clutch and run the motor it will wash the plugs and you must read them at the point the where when you hit the switch at full speed.
Above all be careful when doing a plug run remember you are riding and tuning so don't forget the riding part and don't go in your overalls, suit up. If restricted go to third or fourth gear and kill it, this will still give a fair reading at the plugs.
Once the bike has come to a stop remove a plug if and check the color. What you need to look at is the white centre nose cone insulator, inspect it for the color in comparison to its original white if it is pure white and there is light Gray body it indicates lean condition but it is not too far off the mark on a bike plug you only need to look for a small quantity of the fuel residue color if it is leaded then you will note a light tan on a road bike and for racing more like a bone color, If you are using an unleaded then you will have a grey dusting affect this also applies to avgas although it depends some blend super and avgas with methyl benzene and synthetic oils so this gives different colors again. I merely say check for the amount of burnt fuel residue left on the plug, whatever fuel you use it will leave a small amount if it is burning correctly, even the most efficient engine and plugs wont burn it all.
If it is rich it will just be heavily colored if it is lean it will be barren and white so you need to achieve a small amount you must also have the correct temp range plug if the nose cone is shining or polished it is too hot it must always appear a dull matt finish on the nose cone even when lean it will just be ivory white if you see grey spots or flakes appearing on the outer metal edge of the plug. Danger Will Robinson that is piston material, TOO hot.
Anything that is not meant to be there, or there is excess amounts of in the firing mixture will condense against the cool edge of the plug casing and so oil or lead will build up there fast but in your new plugs for the jetting job we are looking for a single fuel reading so this test will not show up much more than lean or rich unless the motor is really dead you should check all the plugs first time round to make sure all is OK engine wise and that you haven't got a wonky carb, float or choke if you feel that you can confidently remember the plug colors then move on if not fit another set and you can compare them you can keep them for your next set of plugs anyway once you have read the plugs and or noted output on dynamometer or backside.
You need to decide whether it needs more fuel if its (the plug) dark go lean if it is distinctly light go richer you do this at the main jet this is located in the base of the nozzle in the bowl chambers you need to do all the carb’s main jets. You need to remove the bowls or if you have access caps remove them and refit a smaller or larger main jet it simply screws in and out then refit caps or bowls , turn on fuel, start and check for fuel leaks then repeat plug reading test as above by riding the bike as described. Note for a change in power particularly in late revs and then chop it and stop remove the plug of your choice and check it. Note if you feel a drop in power stop straight away and go up one richer do not persist with a lean condition you may cause damaged if you ride it so if you have set up camp some where idle back there to change the jets or do it on the spot of so equipped. ( motor cyclists often are )
Repeat this until you are satisfied with color and peak power IE maximum next we will determine whether the bottom end is ok or the roll on of the throttle.
For a high majority of carb’s on unmodified bikes you will not need to do more then a main jet and mixture screws it may only involve a needle clip changed one spot in extremes such as major exhaust or air box changes. This document assumes you wish to jet the lot so read what you need but don’t leave too much out.
After you have done the main jet settings we move down in terms of jetting to the part that affects lower end power, you may imagine we move to the main air jet which affects the main jet range but as the bottom end is where the engine starts in Revs the result of its mixtures may be overlapped into the upper operating range so due to this we move to the pilot system also the main air jet is not generally adjusted unless there is a radical altitude variation or a change to intake volume and speed due to engine mods.
Likewise the slide cutaway angle which faces to the outside of the carb affects the point in air speed at when main bore fuel flow is started and the mixture then being controlled by the needle taper in the bore of the main jet nozzle, adjustment of the cutaway is by changing the angle or slide number this as with air jets is only required for considerable air flow changes. To control the point at which fuel is controlled from the main bore is done by lowering and raising the needle in the slide, you will note it has a clip and notches in the needle.
If the clip is fitted to a lower position on the needle it will open the needle earlier when the slide is lifted thus allowing more fuel at the same slide position then further opening to full throttle is controlled by the needle taper another adjustable. The needle comes with a number stamped on it. When the needle is fully with drawn you are operating on the main fuel jet and air jet. There is also adjustment values for the main jet nozzle or tube, you will note it has small holes on the side of the tube, this is where the air enters to mix with fuel rising from the main jet and then out into the air stream the nozzle like the air jets and cutaway are largely set and forget and will control the range of affect of the system they operate on. In the case of fuel jets it will control the actual mixtures through that range IE if you need more for longer over a power curve due to air speed or port / cam timing mods you will need to change air jets to lengthen the effect of the main or the mid range needle taper and not so much but even the duration of the pilot jets effect with the pilot air jet which I might add is usually a fixed jet.
If you need more fuel at the curve you would change the fuel jet or needle taper or the mixture screw. So if you have a broken up or lumpy response by that I mean good and bad acceleration spots try adjusting the needle positions and this will indicate the direction you need to take with air jets once you have a more uniform acceleration you can adjust fuel mixtures by changing pilot fuel and screw settings then the needle position and taper and if necessary main jets again to fine tune. As I said start at the balance then do main jetting then move to pilot and needle positions these will indicate your deficiencies.
If you adjust the idle mixture screws in and out counting the turns and test of the mark from idle and starting also blip it in neutral and clean out flat response with mixture screws then riding off the mark and slow cruising at walk pace to see if it falters you can determine by then counting the turns on the mixture screws if it likes a lot you might want to move up a pilot fuel jet size then try it again until it feels right and the mixture screws are close to original specs which you will note in the manual. The idle screws settings are expressed in turns out from base.
Next we will move to the next range up and test this by cruising at walk page and accelerating at varying rates of acceleration try of the mark using the clutch from stop also. Then try short shifting at low revs into second and accelerate with a bit more load doing this and in between lowering or raising the needle or vice versa for the clip position until you are satisfied, it must be a smooth transition from idle and cruise at all throttle speeds quick and slow, then noting the needle position if it is high it needs more fuel earlier and if the pilot does not correct it you will need to change the needle or cutaway but as I said the cutaway is virtually pre-settable if you put a bigger carb on a small bike you need a larger cutaway to have the air speed at the main nozzle at throttle opening.
The cutaway adjustment is in line with air speed changes so move to needle tapers for further adjustment, if it is a serious shortage or excess at mid range roll on you will have to select a different nozzle this is similar to an air jet adjustment and will affect the point in air speed at which the main bore operates.
You may end up needing to decrease the main air jet size to increase the spread of the main jet operating curve but remember you can spend a long time jetting and it will only be spot on if you do the adjustments one by one and read the results then be prepared to go back over your tracks to reset other settings.
It is a juggling act but most of the carb’s can be set such as nozzle, needle, slide cutaway and pilot jets then out at the track or next tune you only need the mains and the main air jets can be used to play with top end curve if you like and mixture screws to set easily then if you have multi cable carb’s needle heights are a snap. Also take note that when you have done jetting and tuning you need to have the mixture screws at or close to preset turns and needle height at centre position, if the carb operates properly and your jetting is correct you have some quick adjustment range available at your screws or needles for fine tuning after you have set your main jets for the session/day/race/season whatever you like or how ever many times you can be bothered tuning. If you leave the screws in or the needle all the way up to get it right, when you need more you wont have any more adjustment and you will have to do the rest of the jetting then so do it now.
	In a CV carb the same applies as with a slide controlled carb in respect of jet placement and adjustment but do not adjust the slide cutaway, it can be done but don't mess with it as it is crucial to the slide operation. And if you note it is hardly noticeable in comparison the carb at the top of this page.

If you intend to use a carb on a different machine to original then you need to consider whether it is operating in a higher or lower air speed, I suspect a lower air speed so you will need to adjust the size of air jets including slide cutaway and nozzle size plus the fuel jets to cater for lower air speed if you only do the fuel jets you will have a very lumpy carb or in power a very unpredictable power curve at throttle on, ie; all or nothing. If you are prepared to do the work you would be surprised at how big a carb you can run retaining a good bottom end throttle response. If you intend to make a radical change requiring more fuel you may need a larger fuel needle seat inlet size also.