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Balancing Carbs

Guest WalneyFrankie

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Carburettor Balancing (full article with Pics)

also see video

http://www.themotorbikeforum.co.uk/view ... 10&start=0


Bikes need their carburettors balancing periodically, or after a piece of work that could affect intake pressures, for example, adjusting valve clearances. The balance won’t change much otherwise so isn’t something that needs doing often. Carbs in balance means that the cylinders should fire evenly and the engine run more smoothly, this is unsurprisingly more important with twins than fours. Balancing involves using vacuum gauges to adjust the settings on each carburettor so the vacuum in each intake (between the venturi and slide) are equal.


Apart from a screwdriver, all you really need is a good set of vacuum gauges. I personally use and recommend Davida gauges, a set of which I’ve had for many years. Many people use Carbtune steel rods (the old mercury ones are no longer around); I have no experience of these but everyone seems happy with them so they can’t all be wrong. Avoid cheap ones as they are not worth the money. Don’t even think about anything that attempts to measure slide height, these are too crude and, as you will see, pointless for CV carbs.

Ideally you want one gauge per cylinder, but as long as you have a pair then you can balance anything, it’s just a bit more fiddling. If you can only afford a pair then this is worth knowing!

The Carbs

Your bike will almost certainly fall into one of two categories for this purpose: slide or CV. CV carbs are the most common these days as they provide smooth action over a variety of engine speeds and throttle settings. Slide types are older or used on race bikes where the slow slide lift (relatively) of CV carbs is undesirable, and no-one is going to ride it round town anyway.

CV carbs have the throttle cable connected to a butterfly valve downstream of the slide. As the throttle is opened, the air speed increases through the venturi, and the pressure drops correspondingly. The air in the chamber above the diaphragm is sucked out through a small hole in the bottom of the slide, so atmospheric-pressure air under the diaphragm (supplied from the airbox) pushes it up. The slide rises, unblocking the venturi, so the air sucks more air/fuel in and produces more power.

In a multi-cylinder bike, all the slides rise at the same rate. Balancing is done by adjusting the positions of the butterflies at tickover so they all start from the same rest position, this they will all be in the same place at the same time. This is why sometimes with out of balance carbs you can part-close the throttle and open it again, and it’s smoother, as the slides have levelled themselves out. The throttle stop screw (to adjust tickover speed) prevents the butterflies from fully closing.

Slide carbs have a cable attached to the slide of each carb. As the throttle is opened, the cable pulls the slide up, unblocking the venturi and so the engine can suck more. If you whack it open there’s nothing to damp out the overcarburetion at low speeds. The throttle stop screws, one in the side of each slide tube, bear on an angled portion of the slide, so screwing them in pushes the slide up (same action as opening the throttle) - see below.

The cable comes in through the carb top, usually screwed onto the top of the slide tube, via a cable adjuster. Balancing is done at tickover using the throttle stop screws, and at 4000-5000rpm using the cable adjusters.

Get Started

Before you start, the bike needs to be at normal operating temperature. If you’re servicing it, do the air filter/plug replacement and valve clearance checks first, and fix any exhaust holes etc.

Attach the gauges. Most bikes have either a blanking screw in each of the inlet stubs (on the head), or a capped-off spigot. Where there is a spigot in use on one to operate the fuel tap/fuel pump, I usually tee this in. Sometimes you have to take the airbox off to gain access to the balancing adjusters, but it’s better if you don’t as the gauges flutter more. Slacken any locknuts. If you have two gauges and more than two carbs, start with the first two cylinders. More on this later.

Start ‘er up! The gauges will flutter a little and wander a little. I find you can get a good reading on overrun, i.e. rev it a little and watch the vacuum as it drops (can’t use this for slide carbs’ off-idle adjustment). Now you decide if it needs fiddling with.

CV carbs The adjuster screw alters the relative positions of the shafts that hold the butterflies. For multi-cylinder bikes, there is often one screw to split left and right pairs of cylinders, then another each side to balance each pair against each other. Twiddle it, blip the throttle a few times, check it. Once you’re happy, knock it off and tighten the locknuts, job done. If you’re doing a four with two gauges, either balance the carbs in pairs (1 and 2, then 3 and 4) then against each other using the centre adjuster (1&2 vs. 3&4), or keep one gauge on no.1 and move the other, but if you do this then pay attention to which one you do next.

Slide carbs, more of a fiddle. For a four it is a lot easier to use four gauges than two compared to CV carbs, but still doable. At tickover, adjust the throttle stop screws till all vacuums are equal (throttle stop screw circled below).

Adjust tickover speed if required, keeping vacuums equal. Off-idle tickover is adjusted by the cable adjusters (see next photo).

Open the throttle a little, usually takes the bike to 4000-5000rpm, and check the vacuums. Balance them out using the cable adjusters. Check you’re happy, knock it off and do up the locknuts.

That’s basically it. With twins you’ll probably find that a slightly higher vacuum in one carb is okay, but in the other is lumpy. Twins don’t fire evenly (except boxers), so there is a longer gap between bangs followed by a shorter gap.


Edited by Anonymous
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So, what do the vacume guages actually tell you? how fast that cylinder is running?

the speed of a cylinder is measured by the rev counter, revolutions / minute

vacuum guages measure the vacuum created by the carbs, the theory being that each carb has to deliver exactly the same amount of fuel/air mixture as its neighbour otherwise the engine can’t run evenly or efficiently.

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