Carbie balance and tune

The purpose of this page is to illustrate the importance of carburettor tuning on motor cycle engines. For competition, carbie tuning is absolutely critical to your machines performance and overall responsiveness.

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 carbie 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 carbie, it is set up and jetted to accelerate not to cruise, the majority of circuits and functions on a car are for idle and cruise. A motorcycle carb would be classed as a competition carbie on a car.

The good thing about this is that motor cycle carbie's are very simple in contrast to other engine carbie's this allows a great deal of maintenance on motorcycle carbie's to be easily performed by the owner / operator .

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

If the carbie's are not right balancing will be useless. The first thing we need to check is the float level, to do this there are couple of ways 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 tap you will need it switched on direct or another container or tank to fill the carbie's, its best not to try and measure them engine running.

The alternate method is to measure the float arm with the float bowl removed. Once the bowl is removed invert the carbie and let the arm and float sit closing the needle then measure the arm to the carb bowl surface. If you dont 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.

A float height adjustment

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

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 carbie 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 carbie's are bin fodder. Or permanently shagged.

When you have done the mixture screw presets as above then fit the carbie'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 carbie's firmly " lock " in place, there is usually a rib inside the manifold and on the carbie and head spigots that locate the carbie'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 carbie's are in place clamp them up manifold to head clamps first then manifold to carbie'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 carbie'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 carbie 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 carbie. IE too rich.

A plunger type choke

Now you have the cables and controls adjusted and pre-settings on carbie'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 carbie'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 carbie skip this. ***Note some engines have an induction balance tubing either at the carbie's or manifolds or inside the head these balance circuits must be isolated or blanked from each other during carbie 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 carbie 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 carbie'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 carbie'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 bungy 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 carbie 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 carbie 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 carbie / manifold at a time to get its vacuum reading. Don’t use the clear plastic tube it will melt to the motor use black rubberised 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 carbie 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 carbie'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 carbie 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 carbie gauge or doing it singularly note the cylinder with the lowest vacuum reading, this is the carbie 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 carbie 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 carbie as some twins have you must balance all the other carbie's to this one.

Now the carbie'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 carbie'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 carbie setups are independently mounted and so usually have individual controls for each carbie's operation save the case of a choke where there is a balance tube to supply choke to other carbie'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 carbie tops unless you run out of slack. Then undo both idle screws and count the turns out if you have access to the carbie 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 carbie'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 carbie's in between adjustments. Now the idle of the carbie'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 carbie 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 carbie'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 carbie'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.