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M/C Mechanics - HOW2:

Balance or Synchronise Carburettors

As usual disclaimer: This is NOT a definitive guide, nor is it a substitute for the PROPER Haynes / Clymar manual for the bike in question. Bike featured here is a 1987 Honda CB125TD-E, But principle pretty much of a much for most bikes with two carburettors.

Syncronising or balencing CarburettorsThe twin-carb Honda CB125T, is pretty simple, as it has just two 'Slide' carburettors and they aren't 'banked' with a complicated linkage to fret about! If/When I get round to it some-time, I may update showing it done on the 750. Anyhow, for now, done on the Super-Dream. Its not so mysterious, and with the right tools, only need take quarter of an hour or so.

Starting with the 'right tools'; you don't need much for this job, but you do need a bit of 'specialist' workshop equipment, a set of vacuum gauges. Otherwise, it can normally be done with some pretty regular screw-drivers. A long thin bladed screw-driver like one in my hand here, is useful to get at the twiddly bits on the carburettor.

And on the CB125, I can do the job without having to lift the petrol tank or anything off, as the carbs and thier adjustment screws are fairly accessible. On other bikes, you may have to remove side-panels and possibly the petrol tank, which may require setting up some sort of 'remote' tank to feed the carbs petrol while you set them up. I have in the past 'improvised', putting the petrol tank removed from the bike on to something higher then the carbs.... a spare wheel on a Black & Decker work-mate or a stak-a-box on an old chair.... and then using a length of long pipe to connect to the carbs.

OK, lets see what Mr Haynes has to say about this job, on the CB125, shall we?

Haynes - 1. Carburettor Synchronisation consists of ensuring that the throttle slides are at exactly the same height at idle speed. that both lift at exactly the same time when the throttle twist grip is rotated and that both are at the same height at any given engine speed. It is necessary to check this to ensure that both cylinders pull together and not against one another.

Well that makes sense. BASICALLY, its matching the carbs to the same throttle setting, so they both get the same amount of charge and make as big a bang as each other. Imagine if the throttle cable to one carb was disconnected, the cylinder that carb fed would be trying to idle, so if you wound the throttle open, the other cylinder would be doing all the work, and more, trying to chivvy the other one up to the same revs. SO syncing the carbs is a good thing. Each cylinder in sync with the next, not held back by it, you will get better throttle response, more power and better fuel economy. ALL good stuff. But looks so COMPLICATED! Do we really need those funny dial things?

Haynes - 3. The Method recommended for synchronisation requires the use of vacuum gauges

Haynes - 4. Vacuum Gauge sets are now available from various suppliers who advertise regularly in the national motorcycle press; while they may be considered two expensive for the average owner, it is well worthwhile considering the purchase of a set if the owner is planning to move on to a larger capacity multi-carburettor machine and is planning to carry out his own maintenance. For those who don't have access to a vacuum gauge set, the machine may be taken to a Honda dealer....

Hmmmm... well Mr Haynes seems to think so, at least. I am certainly inclined to agree that they are well worthwhile, if you intend getting a bigger multi-carb bike. But, in the olden-days, practiced mechanics would synchronise carbs 'by ear', and do a damn fine job of it too. I was once treated to a demonstration by an old master of the art who put a set of gauges on a four cylinder bike, and showed how far 'out' the carbs were. He then removed the gauges, and synchronised the carbs by ear, before putting the gauges back on to show how closely they matched. they were 'spot on'. And for a little twin like this, with such simple little carburettors.... you could probably set them up without gauges 'close enough'... BUT... lets follow the instructions first.

Haynes - 5. Before carrying out syncronisation, check that the contact breaker gaps, ignition timing, tappet clerances, carburettor float bowl levels, jet sizes, pilot jets, pilot mixture screw settings are all correct. also that the air filter elements are clean, that the exhausts are in sound condition, that the spark plugs are correctly gapped and working properly, and that throttle cable is correctly routes=d with no kinks or sharp bends and is properly lubricated and working smoothly. This is because one cannot expect to gain the full benefit of carburettor syncronisation if some faulty or maladjusted component is upsetting the remainder of the engine induction and exhaust system, or the throttle cable is so badly damaged or worn that it cannot lift the slides at the same time regardless of the settings in use

Phew!...Basically, service everything ELSE first!

Haynes - 6. Start the engine & warm to normal operating temperature. Stop the engine, remove the side panels, unlock and raise the seat, propping it with a piece of wood, and release the petrol tank rear mounting, raise the rear of the tank as far as possible without straining the petrol pipe, then prop the tank in that position with a piece of wood. Check there is adequate fuel in the tank and switch the tap to 'res' position. Slide up the rubber sleeve fitted over each throttle adjustor in each carburettor top and screw fully in the adjustors. Slacken fully the locknut fitted to the throttle cable twist grip adjuster, then screw in the adjusting nut to gain the maximum free play in the cable.

Right..... Warm engine, slacken throttle cable.

Haynes - 7. Using an impact screwdriver, slacken and remove the vacuum gauge adaptor blanking plug from each of the cast inlet stubs. These are 5mm screws; Replace them by screwing in the vacuum gauge adaptors. Attach the hose from each gauge to one of the adaptors, then start the engine.

Right..... OK, so that blanking screw will be the one I have put in the red circle marked '1' in pic on left, beneath. And in the picture centre, the brass tube thingies screwed in its place will be the vacuum gauge adaptor. And pic right.... hose to go to the guages. Good, good. That looks simple enough.

Syncronising or balencing Carburettors Syncronising or balencing Carburettors Syncronising or balencing Carburettors

Just make sure that the hose is attached at the other end to one of the gauges. Unions arrowed in pic below.

Syncronising or balencing Carburettors

This is of course a four-way set of gauges, so up to four carbs can be balanced at the same time. Convenient for motorbikes that have four cylinders and four carbs. Of course I'm doing a twin, that only has tow carbs, so only have to use two of the gauges. These are also dial type vacuum gauges. There are also 'manometer' or 'tube' type gauges, which are more accurate, but not as easy or convenient to use. However, they are fairly easy to make from clear PVC tube and some hard-board, and if you are a real make and do freak, have a google, there are plenty of 'how-to's on how to make your own. You also don't need to have one gauge for every carburettor. You can synchronise any number of carb's with a simple balance 'pair' of gauges; you simply balance two carbs together, then you pair one you have balanced with the next one you haven't and leaving the 'done' one alone, balence the one that's not done against it, then again for however many carbs you have. It's just more fiddly and time consuming than having a gauge for each carb. Anyway, lets get on with the job.... what was Mr Haynes saying?

Haynes - If the gauges are fitted with adjustable damping valves, adjust the degree of damping until a steady reading is gained. but be careful to check that the instruments can respond quickly to throttle changes. Note the readings obtained, disconnect the hoses and attach each hose to the opposite adaptor. The readings should be the same; if a discrepancy is found in one particular gauge it must be noted and compensated for.

OK... so all connected, engine running, and the damping valve on my gauges is the white 'dot' in the hose between adaptor and gauge. Little plastic 'tap' basically. Screw it out and the needles go crazy wavering around some difficult to descern mid-point, more you wind in the screw, less the needles wobble! Get them fairly steady, and if you can see behind, they are both showing fairly similar numbers, these carbs (set 'roughly' by ear!) were not far out. Swapping the pipes over; readings swap dials but remain the same, so my gauges are pretty well calibrated, but if needs be I can do that 'compensation' thing, by unscrewing teh cover off the front and adjusting the calibration screw, that drags the needle's start point round in relation to the scale behind.

Syncronising or balencing Carburettors Syncronising or balencing Carburettors

Haynes - 8. With all preliminary checks and preparations carried out the task of synchronising can begin. With the engine idling smoothly, note the readings shown by the gauges, rotate the throttle stop screws until the two readings are consistently identical; some careful work will be necessary to achieve this result. When both gauges show an identical reading, check the idle speed shown by the tachometer is within the limits of recommended idle speed, given in the specifications section of this chapter. If not, rotate both throttle stop screws by exactly the same amount to achieve the recommended speed. Re-Check the vacuum gauge readings and modify the screw settings until the readings are once more identical.

OK, well checking the specifications, suggests that for the CB125, idle speed ought to be around 1,200 rpm or so. WHY cant they just print the ruddy number, rather than making you dart around!

But onwards; 'throttle stop screw'? Ah... more flipping and we find another graining black and white picture, under a different section! There it is, on picture showing the right hand side carb, in circle marked 'B'. Of course the carbs are identical, so on the right hand carb, its idle screw is in the same place, on the left hand side, but of course that's hidden between the two carburettors, so you cant get at it very easily! This is where the long thin electricians screw-driver comes in handy, so you can get to both idle set screws from the same side of the bike.

Syncronising or balencing Carburettors

And Mr Haynes simply suggests we simply 'fiddle' with the settings until we get both vacuum gauges showing the same thing, and the tacho needle showing the right revs, potting the whole process down to 'Some Careful work will be necessary', and offering absolutely NO useful advice how careful or how it should work! OK! Time for me to walk you through it.

Syncronising or balencing Carburettors

Pic of the gauges again. These read 'backwards' As vacuum pressure increases, the needles swing anti-clockwise around the dial. Its a perverse following of convention, as we are measuring negative pressure, ie less that atmospheric, so the needle is going the other way to the way it would on a compression tester or tyre pressure gauge that reads positive pressure above atmospheric. Looks scary and complicated, with numbers round the outside, the inside and some sort of segmented guide between them. Its pretty straight forward though. The large white segment marked '0' is the rest position, atmospheric pressure, and where the needle ought to be before you connect pipes and turn engine on. Clock-wise from that, marked 1,2,3,4, 'Normal' 8,9; is positive pressure, which we can ignore, we are going to measure 'vacuum'. So we are looking for the needle moving anti-clockwise, and this gauge is showing about 200mm of vacuum, in a red segment of the guide, which looks worrying. Its actually about 7.5 negative psi or half normal atmospheric pressure, so actually its not that bad. Needle probably wont fall anywhere close to the green bit marked 'Normal', which is around 20 psi vacuum less than one atmosphere beneath atmospheric pressure. Will go down if you rev the engine up, but I'm not sure how far it might go, I have never been brave enough to try it!

OK.. we are familiar with the dial, and we know which way the needle is going to swing, and all we have to do is leave one idle set screw alone, and twiddle the other one, and watch what happens to the needle. If we screw the idle screw in, and the needle drops, then we look at the other gauge and decide if we want the needle to go up or down turn the idle set screw the corresponding way to get the two to balance. Simple, right?

Well not quite. As you bring the carbs into closer synchronisation, the idle speed will tend to increase; this is because the 'eager' carb, giving once cylinder more charge, is either being closed up, to give as little charge as the lazy carb, or the lazy carb is being opened up to give as much as the eager one, but in either instance, the 'eager' cylinder isn't having to 'drive' the lazy one so much, so less power is being sapped, and that is going to make the engine accelerate a bit, and the revs will go up.

Balancing by Ear - This is how old-time mechanics could balance carbs 'by ear'. And its entirely possible, on a simple little bike like this, if you don't have any gauges.

Leave one idle screw alone, and simply twiddle the other idle screw until the revs start to rise. Keep turning that idle screw which ever way makes the revs lift, until they start going down again, then back a tad the other way until you find that 'peak' idle speed from setting THAT carb.

Now you can ponder the idle speed, and if its a bit high, you wind out the idle screw on the carb you left 'set', perhaps half a turn. Remember as you go out of sinc the idle speed will drop by more than what it will be when synced, so you let the revs down to something a tad less that you would like on the 'set' carb, then you bring the other down by half a turn, and as you do so, the revs should rise again, as the carbs come back into sync. And again, quarter turn either side to find the 'peak' balance point.... and back to pondering the idle speed. Repeating the process, adjusting the base idle on the one carb, then finding the balance point on the other, until you have the best balance at the idle speed you want.

Its THAT simple?...... so why all these ruddy gauges and stuff!?! Yes... well, we do have a tendency for over complication some-times.

Balancing By Gauge - The gauges, though make it a little easier and can get the synchronisation a little more accurate; its the same process you follow, but instead of just listening to the engine note or looking at a probably wobbling rev-counter needle, you can look at the gauges, and when both needles are reading the same, the carbs are in sync.

Only thing you really need to be aware of, is that the reading in the gauge for the 'set' carburettor will change with the idle speed. As you come closer to sync, the idle speed ought to rise, and that will make the reading on the 'set' carb gauge drop. So as you turn the idle screw on the other carb, its a matter of watching both needles, and 'catching' them as you find the balance point. And its a lot easier if you adjust the idle screw very slowly, so that you don't get any 'lag' in the readings, or you make a small adjustment, then let the engine settle and watch the gauge readings for a moment, before re-adjusting.

So, when you come to it, the gauges aren't 'essential', and on these little twins, I will as often as not balance the barbs without gauges, entirely by ear, and they will, as pic, be within a few mm of mercury vacuum of each other; but when 'setting up' a freshly built or serviced engine, or after major works like a carburettor clean, I'll use the gauges to help me get them 'close' to base settings, with less 'faff' and get the sync closer when finally set. And as a bit of a confidence check, as well. But, I'll be working a BIT by ear and intuition and a bit by the gauges.

Worth also, remembering the comment Mr Haynes made at the beginning; "Carburettor Synchronisation consists of ensuring that the throttle slides are at exactly the same height at idle speed." Its not very easy or so practicable, BUT, another trick of the old-time mechanic, particularly on things like the big old Brit-Twins like the Bonneville, was that they would 'balance' the carbs, literally by measuring the gap between the bottom of the slide and the carburettor throat! Big-Brit-Bike, with a pair of 1 1/3" 'Open' Amal slide carbs, this was not so difficult; little CB25 with a pair of tiny 24mm carbs, tucked in a tight space between the frame rails, and plumbed into air-boxes, its not that easy. BUT; if the carburettors are off the engine, its possible to use this method to set a fairly close 'base' setting to work from, before fitting them; winding the idle adjustment screws all the way out, then putting the slides into the carb-bodies, without the cables attached, so that the 'bottom out', then adjusting the idle screws so that they 'just' lift the slides, and then checking that the heights are 'even' by eye, before fitting, as a reasonably good place to start before balancing by ear, with the air-boxes all plumbed in.

OK, so that is the carburettors synchronised. Well, at least at 'idle'. Other thing Mr Haynes said at the beginning; "that both lift at exactly the same time when the throttle twist grip is rotated and that both are at the same height at any given engine speed"

Haynes - 9. When the engine is idling smoothly and at the correct speed with both vacuum gauges showing the same reading, fully open and close the throttle once or twice. The engine should return to the same idle speed and vacuum readings; if not stop the engine & examine closely the throttle cables. If any tight spots or other damage is causing the cables to lift the slides unevenly the cables should be renewed. If all is well enlist the aid of an assistant for the next step and stop the engine.

OK, in short, give the engine a rev and make sure it comes back to the idle you have set. If not, check the cables.

Haynes - 10. With one person operating the twist grip and watching the tachometer reading, the other can watch the vacuum gauges and adjust the carburettors. Start the next step by rotating the adjusters in the carburettor tops until all but the slightest trace of free play from the junction box / carburettor top sections of the throttle cable is eliminated. Start the engine and recheck the idle speed and vacuum gauge readings; these should not have altered. Increase engine speed to 4000rpm and note the new readings as soon as they have stabilised. A tolerance of up to 4cm Hg is permissible at this stage, but it is still better to have the two readings identical. adjustment being made by rotating the throttle cable adjuster in the carburettor tops. This requires care if it is to be carried out accurately. Avoid risk of engine damage through overheating by minimising the amount of time the engine spends at 4000rpm; the correct adjustment takes time. Stop the engine and allow to cool down before continuing.

Syncronising or balencing Carburettors

So... basically this is the same deal over... only difference is we are going to balence the carbs on the cable adjustors, 'A' in the photo, rather than the idle speed screw.... and we are going to balance them at some higher engine speed.... hayned suggests 4ooo rpm.

And what we are trying to do, is get the carb slides at the same height, alowing in the same amount of air.

Having done that at idle, really if the cables are the same length and picking up the slides at the same time... ought to balance anyway, if we have done job right at the beginning.

OK... well, right at the beginning, Mr Haynes told us to slacken off the throttle cable at the twist grip, so there was no chance that the slides were being held off the idle set screws by the cable. Now he says we need a second person to hold the twist-grip and watch the revs......

What I do.... Adjust up the twist-grip cable adjuster, until the idle speed starts to lift. Almost guaranteed one of the individual carb cables will be longer than the other, so when I have adjusted up at the twist-grip end and the revs start to rise, ONE slide will be being lifted off its rest. Other one probably isn't.... so I fiddle with the cable to find which one is loose.... and adjust out the slack.

Then, like doing the idle-balance, leaving that one alone, come onto the other one, and do a 'swing' by perhaps 1/4 turn either way and listen for a revs increase and find a peak balance point.

Cables now 'closely' balanced and lifted just off the stops; I can wind the revs up to about 3-4ooo rpm on the twist-grip end adjuster; so the slides are well clear of the stops and the cables are under tension.

Bit of luck, the gauges will tell me they are in balance still, but if not, watching the gauges, and listening to the engine note, I can swing one adjuster 1/4 or 1/2 turn in either direction.... slowly..... to find a balance point. The actual revs now do not matter. But they will go down as you go out of sync and up as you find the balance point, and you ought to be able to tell by ear, and not have to look at the rev-counter or have some-one telling you what its doing! Ought not change much any-way... 500 rpm either way max, and your ear is far more sensitive than the rev-counter anyway! Especially as its probably only six inches away from the pistons!

OK. New mid revs balance point found. At higher revs, the vacuum reading ought to be higher, by the way.

So, now, you can increase the revs a little, maybe 5-6oo rpm, on the twist-grip, and check the gauge readings, which ought to still be pretty close. If not, adjust in the twist grip adjuster to hold the higher revs, and do another balance.

When happy, wind the twist-grip adjuster out, and let the revs fall back to idle. Then set the free-play on the twist grip so that at idle the tension is off the cable, and the throttle slides are resting on the idle stop screw. You ought to have perhaps 1/4" or 3-4mm of movement in the twist-grip before it starts doing anything.

More, you may not get full throttle. Less you are likely to have the carb-slides hanging on the cable, not fully closing, and likely that as you turn the handlebars from lock to lock, any change in curve in the cable, from being pulled tighter or wider as the twist grip moves in relation to the engine, through cable routing, the engine revs will change, the engine most likely racing as you turn the bars to the right, pulling the cable into a tighter curve, lifting the slides.

Swing the handle-bars from lock to lock, slowly, and make sure that you have enough free-play at the twist grip.

And THAT is about it. Last check, before disconnecting the gauges is to check the balance at idle, and up the rev-range a bit, accelerating the motor slowly, and letting it 'settle' at a steady engine speed before worrying that the gauge needles don't line up or are going all over the place. As you open the throttle under acceleration, vacuum drops, as does air-speed through the carburettor, until the engine starts accelerating, then it takes a little time for the vacuum to stabilise, once you have found and held a new engine speed.

Don't go nuts and rev the bearings off the thing, but go through a few engine speeds, and see that the needles stay within that 4cm of mercury tolerance, and then pack up. Job done.

Oh... Hold on.... Mr Haynes has some more to say on the matter.

Haynes - 11. When the vacuum gauges both show a reading that us consistently identical at 4ooorpm, with a difference of up to 4cm Hg being acceptable at this engine speed, the carburettors can be considered synchronised. Close the throttle and allow the engine to retuirn to its idle speed which should not have altered, nor the vacuum gauge readings at this speed. Stop the engine and replace the rubber sleeve which protects each carburettor top cable adjuster taking care not to disturb the adjuster setting. Using the adjuster underneath the twist grip, adjust the throttle cable so the twis tgrip can be rotated 10-15 degrees before the slides begin to lift. Tighten securely the adjuster locknut then fully open and close the throttle once or twice to settle the throttle cable and start the engine. Check the vacuum gauge readings are still identical at idle speed and within set limits at 4ooo rpm. If all is not well, the fault is likely to lie within the throttle cable.

Yeah... Ok.. more or less what I said.

Haynes - 12. When the carburettors are accurately syncronised,disconnect the hose from the adaptors, remove the adaptors, then re-fit and tighten securely the blanking plugs. Return the petrol tank to its correct position and secure the tank rear mounting, then lower the seat and refit the side panels.

That's the elaborate way of saying 'Then Pack up' then!

SO, job done. Carbs balanced or synchronised or synced, whichever you prefer. Basically the slides in each carb set up so that at idle and when lifted by the cables, they are at the same height in the carb body, or at least flowing the same amount of air so that the bang in each cylinder is the same strength and you don't have one eager cylinder and one lazy one, the lazy one holding back the eager one, robbing you of potential engine power or economy, and the engine ought to respond more eagerly, give better mpg, and more power. This is 'Tuning'... harmonising everything in the engine to pull together, like a conductor tuning an orchestra to pull together and make sweet music and it's all good stuff, and where small simple changes, can make a big difference.


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