Balancing twin carbs

[scotto75]

I have a couple of old Honda twins, which use Keihin CV carbs with diaphragm slides and no holes for vacuum gauges. I’ve been following the procedure in the Honda and Clymer manuals to balance the carbs, which involves testing one side at a time by pulling the plug on the other cylinder and increasing the revs on the carb you’re working on to keep the engine ticking over. This is the method...


1 - Remove plug from right side
2 - screw in throttle stop screw on left carb to get engine to tickover
3 - adjust mixture
4 - adjust idle
5 - repeat on right side
6 - start engine on both cylinders (high rpms now)
7 - adjust idle on both cylinders so that pressure from exhaust is equal

I’m happy with this technique, using a Gunson colortune to assist in getting the mixture right, but would like to know if anyone has a better way of balancing at the final step, other than feeling the pressure at the ends of the silencers?

It feels a little ambiguous and vague. I have a Morgan Carbtune I’ve used on 4cyl Hondas like the CB400F, but these bikes don’t have any vacuum holes on the intake, so other than getting a spare set of intake manifolds and drilling some holes, I require another method. Is feeling the exhaust it?

Thanks!

[redeem ouzzer]

Still not found anything better than kitchen scales for balancing carbs.
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[Triton Thrasher]

If it’s idling at a reasonable rpm while firing on both cylinders, then the idle is balanced well enough for me.

The throttles should open together. Can you see the butterflies moving if you jam the slides up? If the engine pulls evenly at tiny throttle openings without juddering, then again it’s probably ok.
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Rrrrrrrrrrrrrrrrrr

[scotto75]

Yes, the cable’s adjusted so that both throttle levers open at the same time. I usually screw the throttle stop screws out so that there’s a slight gap and then screw back in just so that there touching, and then make simultaneous adjustments on each side from there.

They seem fine. There’s no dip when I open the throttle and they idle pretty steady, I was generally wondering if someone had concocted another method for measuring each carb, but as you say, if it sounds fine and runs fine it probably is fine!

[R1stu]

Is there not screws on the head of engine? My 350 has 2 screws just behind where the inlet manifold bolts to the head. I need to do mine and it's not smooth at all, but need to knock the screw out.
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Current bike 1991 Honda ST1100 Pan European, 1986 Honda cb350sg (Finished)

[Hong Kong Phooey]

You'll be wanting one of these.

https://m.youtube.com/watch?v=Q5OiSE9nq4M
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'81 CG125, '97 FZS600 : '99 CBR600F4, '09 KTM RC8

[bikenut]

Aldi etc. do a relatively in expensive non contact temperature gun.

Equal exhaust header temps ( hot ) as well as carb temps ( cold ) .

A "listening tube" can also be used but takes practice.

There usually blanked off ( by a smallish cross head screw ) take off points on honda's, what models have you got?

The intake flow type could be used, a unit that fits over the carb air intake and measures/indicates intake air flow.......

If you are removing plugs, remember to allow the removed plugs ht passage to earth, or, type dependent, knobble that cylinder LT cb points.
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[scotto75]

[scotto75]

[scotto75]

[Riejufixing]

[bikenut]

cv type with rubber diaphragm dash pot pistons.......if carbs are off the bike, and initial "set" ( after a good clean and all float heights correct etc. ) you can use the ball bearing method or even the drill method, but the ball bearing method is better as you do not "trap" a drill via the sprung loaded throttle butterflies etc..

A said, the carbs are OFF the bike. place 2 ball bearing of the same size down the air takes with the dash pots off. Open the throttle via the mechanism and the idle speed screw, and listen for the balls to drop.

A vice gripped carb holder can be made so the carbs are held secure and level etc. via the holding tool not the carb bodies them selves.

If they do not drop at exactly the same time, the butterflies are not in sink, so adjust via the butterfly balancing adjustment.

Remember that throttle butterfly spindles can and do wear, and can/do leak air in.
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[scotto75]

[scotto75]

[Kawasaki Jimbo]

[Riejufixing]

[scotto75]

[scotto75]

[jjdugen]

I have used fine needles used on injection syringes pushed into the intake rubbers to connect to the carb tune. The tiny hole self seals when the needle is withdrawn. See your local junky or you can buy at a chemists, you might get some funny looks though.
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Try Soi 23 on Amazon for a good read.... Self promotion? Moi?

[bikenut]

If you make the suggested carb holder which would ideally be vice mounted ( the holder not the carbs ) you can when all is level etc. use the float chamber drains to establish equal float/fuel levels.

Some length of suitable clear tubing arranged so they point upwards. Connect test tank and allow float chambers to fill and "shut off" ( ie floats float up due to fuel and then close the float needle valves just like an old fashioned toilet ball valve set up ) .

The fuel levels can then be compared, a bench activity as with the balls, but a good starting point.
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[bikenut]

The video, the synchroniser, mainly used to the crypton ( in my SU days one ), but hay ho.

How much are these on the internet, but something similar could be made, a lightly sprung and damped "flap with pointer".....

As you are matching or balancing the throttle butterflies really, so they are both open the same amount, assuming the engine draws in ( or is that the atmosphere pushing in ) the same amount of air, as long as the "his" heard is the same, then they are in balance.

Tube to one ear, tube in carb mouth just before the dash pot piston ( make a "distance piece ( wrap of tape ? ) so the tube is in the carb the same amount carb to carb, you want them to be "equal, yes? ) with engine at idle or 2k revs ish......

Compare his carb to carb.....if one is hissing more its drawing in more air than tuther......

so its cb250k3 1971 in gold ??
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[bikenut]

cmsnl does not show vacc screws on the head and then times me out for the carbs diagram....there may be take vacc off points on the carbs ??
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[bikenut]

vw porche weber 32$ on e bay........mmmmmmmm, maybe its time, or make on for my bikes.....onlpy duplicating my ears and gauges etc......but it could have its uses.......the crypton unit "dissappeared" many years ago, some bastard "borrowed/knicked" it !

the cmsnl carb exploded view does not seem to show a vacc take off point blanking screw either, but does show the float bowl drain blanking screw and overflow tube, which sadly are not interconnected, so fuel level comparison will be slightly more difficult in so much that to use the drain screw hole, an adapter will be necessary.

Simply find a similar screw, made from brass would be good and drill a suitable hole thru it. Solder on some brass or copper tube to make a stub over which you fit the clear plastic tube. some effort i know but.......
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[scotto75]

Haha! I’ve had several similar thoughts! I had to replace the overflow drain tube on one carb which had snapped off with a small brass m4 nipple used on model aircraft. I did think about drilling and tapping a set of drain plugs to accept one of these, but have also seen people successfully sticking clear tube straight in the hole, after tapering the end and heating a little. The aim is for 2-4mm under the gasket?

I like the idea of making a rack to bench sync and check float levels.

I have a saved search on flea bay and when a suitable, cheap set of intake rubbers turn up I will drill and tap some holes for the Carbtune adapters to fit, but in the meantime, att £30, I’m putting the synchrometer on my Christmas list!
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Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[bikenut]

Expo drills and tools or a good model shop will stock brass tubing, you mode 2 ?

Replace the overflow tube, its there for a reason. Unless of course you have adapted the drain and modified it to become an over flow as well.

Your bike has the, from memory, tin cross over bit to connect the throttles together ?
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