Balance of power

Jacob Bronowski: speccy smart-arse, boyhood hero

If you’re a particular age and of a scientific/engineering bent you’ll certainly remember Jacob Bronowski’s much-lauded (and much-mocked) early-70s book and television series The Ascent of Man in which the learned but somewhat theatrical prof charted the development of humankind principally, in its technology.

All of which is a somewhat laboured introduction to what might cheerfully be called The Ascent of Manometer, the next stage in development once you have the technology: namely, a multi-cylinder motorcycle with more than one carb.

I know, I’m wittering. Essentially, if your machine has two or more carburettors, at some point you’ll need to balance ’em – in other words, set them up so they pass equal mixtures to their respective cylinders. How will you know when the carbs require balancing? The bike will have a lumpy and uneven tickover, unresponsive throttle, possibly overheating on one cylinder… in a word it’ll run badly! And if it’s running badly, sync’ing the carbs will improve the situation and provide a complete cure if that’s the core problem.

Spend money or plump for the Blue Peter approach?

To do it you’ll need an expensive vacuum gauge or a cheap(er) device called a manometer (detailed explanation here). Better yet, for a bike with two carbs you can homebrew a manometer  as easy as acquiring a length of plastic tubing and some ATF, brake fluid or similar. And what’s more it’s every bit as accurate.

Yes, really.

Anyone familiar with the carb’d Hinckley Bonnie and variants will know that the Keihin carb is a sensitive instrument which doesn't take kindly to time spent languishing in the garage – during a winter lay-up for example. For a (very) short while I owned a Hinckley Scrambler which, while an undoubtedly handsome bike and with a rorty exhaust note when fitted with TORs to rival any genuine or ‘reissue’ lookalike Brit machine, hated to sit for more than a few days and was, frankly, a big pain in the arse (I traded it for a far superior Harley 48).

Pretty bike Triumph, but no cigar…

The machine needed constant fettling and other than the front brake which was, frankly, shit, the twin Keihins were its weakest point.

Anyway, here’s what you’ll need: between four and six metres of plastic tubing with a 4mm bore and a litre or so of ATF (or your choice of brake fluid, old thinish oil or whatever you happen to have lying around). You might also require some kind of clamping to squeeze the tubes and lower the pressure drop reaching the fluid, but we’ll cross that bridge when we come to it.

Fold the tubing in half without kinking it to form a long loop. Hang it up somewhere with the open end uppermost and, using a funnel, pour in the ATF until you have around 0.75m in the tubing on each ‘side’ of the loop.

Loop the tubing without kinking it

You’ll find that a quantity of the fluid gets trapped at various points in the tube but if you leave it hanging for a while the fluid will settle at the lowest point. Ideally, you should mount the manometer on a rigid backing to facilitate ease-of-use. Mine’s attached to a surplus length of modular shelving bracket which, with cable ties through the brackets slots, is ideal for the job.

The finished instrument here mounted
on a length of shelving bracket

Now arrange your bike so that air can flow around the motor and the exhaust fumes escape (ie stand it outside or at the very least, open the garage/shed door). A large domestic fan pointed at the motor will help keep it cool.

Have a look at the carbs on your bike and familiarise yourself with the location of its vacuum nipples and adjustment screw(s). If there aren’t any nipples you’ll have to remove the balance pipe which joins the carbs and connect your tubing (possibly with some duct tape) instead. Check your workshop manual for details. The Keihins score in the balancing department with accessible nipples and only one balance screw.

I’m being deliberately vague at this point because your experience will almost certainly differ. However the nub of the process is essentially the same for all machines.

At the vacuum nipples, pry off the tightly-fitting rubber caps and attach the open ends of the tubing. Switch on the fan and start the bike. Immediately, you’ll see the fluid in the manometer zoom up one side or other of the loop. If it travels so quickly that it looks like reaching a carburettor, switch off the bike and clamp the tubing such that you reduce the draw on the fluid. You’ll probably find that the tubing must be squeezed until it’s almost flat and it might be a good idea to do this at the outset and release pressure as needed. Now you know why it’s best to have a good length of tube in the loop: it gives you time to react before the fluid reaches the motor.

A clamping arrangement to squeeze the tubes and reduce
the pressure is useful. Mine’s made from one of those
adjustable feet from an office desk and a piece of brass tubing

At idle, turn the balance screw slowly so the fluid begins to rise in one side (thereby falling in the other), reversing the screw where necessary to bring the fluid to roughly equal heights in the tubes (ie within a few mms is fine). Be gentle and turn the screw slowly and by tiny amounts.

When the carbs are balanced, stop the motor, remove the manometer and (very importantly) reinstate the rubber caps on the nipples. That’s it. Quick, dead easy and it cost you at most, a couple of quid.

Get you, just like a real mechanic! Gaffer a few well-thumbed nudes torn from Nuts on your garage wall and you’ll be that man my son.