Carb balancing with vac gauges

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Greenbat
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Carb balancing with vac gauges

Unread post by Greenbat » Thu Aug 20, 2009 12:47 am

Right, carbs.

As you may be aware, some bikes have more than one cylinder. Some of these have more than one carburettor, meaning they need balancing.
The basic idea is to ensure that as the throttle is opened and shut, all the carbs open and shut the same amount. Vac gauges do this by measuring the vacuum/pressure in the manifolds between the carburettor and cylinder-basically, how much each cylinder is sucking. This is probably the best method, as it isn't affected by worn bits. If the cylinders aren't sucking the same, then the engine runs lumpy and rattly, which really does suck.

So, first off, count the carbs. If you can only see one, go and look at another thread :) For various bizarre reasons, some singles have 2 carbs and some multis only one. For this reason, I will talk of carbs, not cylinders.

I'm demonstrating with a 76 honda CB400f, which are notorious for needing this done. It is easy to see why, the throttle linkage looks like a mecanno set gone horribly wrong.

You will need a set of vacuum gauges, ideally one per carb. I got a nice set of 4 off fleabay for about £30. There is a technique to check they are all reading right, which we shall come to later.

You also need to check if your bike has gauge probe holes. On the 400, these are just M5 holes with blanking screws.

Important-ensure these screws are in place, tight and sealed during normal use. Never start up with the holes open, you could cause damage

If you do not have these, I advise making them. Get an M5 tap, take the carbs off and put a rag down the manifold to catch the swarf. Fit blanking screws with soft washers (e.g. ally, copper, red felt)
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Probe point.jpg

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Greenbat
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Re: Carb balancing with vac gauges

Unread post by Greenbat » Thu Aug 20, 2009 12:51 am

First job, warm the engine up. Go for a run of a couple of miles, you want the engine balanced whilst riding not warming up.

Once you get back, put bike on centre stand and remove the fuel tank. You might be able to sit it on the frame still connected, otherwise you'll need an external fuel supply. The 400 tank should balance OK as shown, but make sure it is steady.
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tank.jpg

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Greenbat
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Re: Carb balancing with vac gauges

Unread post by Greenbat » Thu Aug 20, 2009 12:55 am

My set of gauges can be bungeed to the bars as shown. Ideally, you want them out the way of the ignition switch and electric start/kill switches. Consult your kit instructions on how to connect the probes. On mine, you can see in the previous picture how there are 4 brass tubes (2 shown). The long ones go on the inner carbs, the short ones on the outside.
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gauges.jpg

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Greenbat
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Re: Carb balancing with vac gauges

Unread post by Greenbat » Thu Aug 20, 2009 1:01 am

With fuel and gauges connected, you can now fire her up. You will immediately spot that the needles flutter all over the place. To stop this are dampers, which are basically valves that restrict the airflow. with the engine on tickover, screw them in until the needle just stops fluttering, so you can take a reading. Bring the revs up a bit, the needles should all move in the same direction. If you damp it too much you won't get a useful reading. The actual reading is not important.
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dampers.jpg

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Greenbat
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Re: Carb balancing with vac gauges

Unread post by Greenbat » Thu Aug 20, 2009 1:05 am

Now the balancing begins.

Another important note-do not excessively rev the engine or run it for very long. There is no air flow to cool it

You will have to consult your manual for how to adjust the throttle opening on each carb. On the 400, it is a slot screw with a lock nut, as shown below
Attachments
adjuster.jpg

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Greenbat
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Re: Carb balancing with vac gauges

Unread post by Greenbat » Thu Aug 20, 2009 1:14 am

To balance I run at a fast tickover-2k rpm on the 400. I use the idle speed adjust screw to bring the speed up, the screw is being pointed at by a screwdriver. The 400 one is on the right above No.4 carb. Blip the throttle a bit then let the engine settle-you will notice the gauges flap about, then settle down. Using the adjuster, adjust each carb until they are all equal. My gauges read vacuum inches, and I aim for them all to be within 1-2 of those. I usually leave No.1 carb, and adjust the rest to it. The actual reading doesn't matter, as long as they are the same. Adjust a bit at a time, and when you are happy with each carb raise and lower the revs to check if they are still balanced. When you are happy, nip up the lock nut. Make damn sure the screw doesn't move and cock up your settings!
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DSCN2163.JPG

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johnr
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Re: Carb balancing with vac gauges

Unread post by johnr » Thu Aug 20, 2009 11:00 am

most jap cv carbs have 3 adjusters for the 4 carbs. what you do is adjust the carbs in pairs. my suzuki hasnt got an adjuster on number 2 carb. so, you adjust number one to match number 2. you then adjust number 4 to match number 3. this gives you 2 pairs of carbs, and finally, you adjust the centre adjuster to match the two pairs up. its also important to remember that carb balance is the last thing you do. so do the plugs and points first, valve clearances and oil/filter, finally adjust the carb balance.

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Greenbat
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Re: Carb balancing with vac gauges

Unread post by Greenbat » Fri Aug 21, 2009 12:41 am

Thanks John, useful stuff.

Right, that hint about checking gauges. Dead easy, balance the carbs, then swap the pipes round. If the carbs still appear balanced, your gauges are OK.

Think that is it. as I say, the 400f is notorious for going out of balance, I think the generally accepted figure is 500 miles.

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FastFrank
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Re: Carb balancing with vac gauges

Unread post by FastFrank » Fri Aug 21, 2009 8:35 am

Wow... 500 miles... I dunno if I could live with that, but great thread Greeny.
If iver tha does owt for nowt alus duit for thissen!

TomC
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Re: Carb balancing with vac gauges

Unread post by TomC » Fri Aug 21, 2009 1:47 pm

FastFrank wrote:Wow... 500 miles... I dunno if I could live with that, but great thread Greeny.

Hi FastFrank
My CB400F1 was bench balanced when I got it running 3,500 miles ago. Later I bought a set of carburetor balancers. Some day I will try them out.
Hi Greenbat
Why do feel the need to balance your carburetors so often?
TomC in Ohio

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