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MG MGB Technical - Balancing Carbs
|Thanks all for your help with my 1970 GT so far, the car is now running OK - with all cylinders firing.|
I have a quick question about balancing the carbs - is there a nice simple method?, or perhaps a good guide for doing this. The car sounds "breathy".... is that word?..... and I'm thinking that the carbs are allowing too much air in?
|G'day Mike. There are numerous ways ranging from cheap and simple to expensive and simple.|
Cheap way is to use a hose and listen to the noise at the inlet to the carbs. Another way is to put a tube into each dashpot where the damper goes and measure the height. This was actually part of an SU tuning kit supplied by the manufacturer at one time. The most expensive is to purchase a purpose made balancing device. Obviously the idea is to get both carbs drawing the same amount of air.
I have used the pipe in the throat and the tubes in the dashpot methods with good results.
|Here, have a look at this video. Explains it all.|
|You might like to have a look at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'SU Carbs' and 'Setting-up'. |
|Paul Hunt 2|
|Awesome video. Gotta love YouTube...|
My understanding though was that you should really adjust the mixture with the air filters installed - the video had them off. The reason for this is that when you then go and put the air cleaners back on, they're going to restrict the air flow ever so slightly that it'll tend to choke the carb and make it run a little more rich.
|It's what I do too, although if they are going to offer an amount of restriction that will change the mixture at idle, just think what the restriction will be like at full throttle. OTOH I know K&N filters offer less restiction at higher throttle openings than the standard filters, which is when one has to start experimenting with 'richer' needles.|
|Paul Hunt 2|
|Get a balance meter.....well worth it for the novice. I've seen them for as low as $10. |
Also, get a dwell/tach meter. These go for around $40. Again, for the novice (like me) it made all the difference in the world.
When you raise the piston you can see the needle jump on the tach, indicating perhaps a couple hundred rpm increase. I could not tell this by listening. Having the meter allowed me to dial the mixture right in quickly and easily......allowed me to pretend I was a pro!
Before you do anything, make sure you disconnect the two carbs from one another at the linkage so they are working independently. Balance the carbs and set the mixture on both, then be careful reconnecting the linkage to be sure that when the throttle is pushed, the carbs open at exactly the same time.
73 MGB SC
69 MGC GT
70 E-Type 2+2
|"the carbs open at exactly the same time"|
The best way to check this is by desensitising the balance meter and checking air-flow off-idle.
|Paul Hunt 2|
|Years ago, I tried the 'ol "hose-in-the-ear" trick|
(with the other end in the carb throat) to try
to balance my SUs.
I must be tone-deaf because when I plopped
on my Uni-Syn carb balancer - I was really
off the mark!
I highly recommend getting a Uni-Syn ...but
get the original Edelbrock version. Those
Made In China repros sometimes have floats
that stick in the flow tube.
|I agree with Daniel on the Uni-Syn. That inexpensive tool is a good investment. I have had mine for 30 years and it has paid for itself many times over.|
Balance the carbs and adjust the mixture and idle with the air filters off, then install the filters and do a final adjustment of mixture and idle before tightening the linkage.
|As a medical doctor, I have to admit to using my stethoscope...good enough for hearts, more than good enough for carbs!|
Oh, and the carbs are adjusted properly without the air filters, as they're set a little rich. Then once the air filters are on, the small amount of restriction actually means the mix should be bang on.
|Balancing the carbs at idle does just that; it has no effect whatsoever as soon as the throttles are opened. It's nice to have a smooth idle, but its much more important to make sure that the throttles are synchronised. The SU Manual says to check synchronisation by re-checking balance at 1500rpm.|
Unless you use a listening tube, you'll need the air filters off to balance at idle using a flow balancer. You could put the filters on I suppose to set the mixture, but the SU workshop manuals say otherwise. And if you do put the filters back on to set the mixture, you'll have to take them off again to reset the balance because that'll change. And you'll then want to check the balance at 1500rpm too. And finally, a little-bit-rich is way better than a little-bit-weak!
I've got a gunsons 'carbalancer', which is not the best tool in the box... First of all you have to find somewhere to hang it where it DEFINITELY will not move when you change between carbs as a slight change of angle will affect the reading. Then it's difficult to get a good (ie consistent) seal on the carburrettor. I've seen other meters in use that look much better, although much more expensive too!
|Get one of the SU toolkits from one of the usual suspect (it consists of tubes that go in the dampers and wires that go in the tubes -- you compare the height of the wires against each other to determine balance) and you can easily and quickly check the balance at any rpm from idle to redline, without removing the air cleaners, and without any cumbersome tools like a Unisyn. It also gives you a nice visual reference for checking the mixture via the lift-pin method -- just lift the piston about a wire's-width.|
I own a Unisyn, but I haven't seen it in years. It's rattling around in the bottom of one of my tool drawers somewhere. On the other hand, I have an SU toolkit in each car....
|Here's a photo of the kit I'm talking about (photo is from eBay, but the kits are available from all the LBC vendors).
This thread was discussed between 25/09/2007 and 28/09/2007
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