Balancing HIF4's

What is the best way to balance SU's without buying the expensive little tool?

Tha Haynes manual say to use "the tube to the ear" method. How does that work?

Any other methods?

Thanks,

Bryan

Buy the tool (Meter).

Bryan,

The "the tube to the ear" method can be used to get close but not as precise as a proper tool. What you are doing is balancing by the sound of the suction which can get you close if you have a good ear for it.

Cheers,

Rob

Listening to the "hiss" isn't too precise.

It's a good way to find out how noisy a running B engine really is.

The tube to the ear method is just how it sounds. Stick one end of a bit of tube, fuel hose, etc into the throat of one carb and the other end of the tube into your ear.

Do the same for the other carb. Fiddle with things till you think the "hiss" is balanced between both carbs.

Really better off to purchase the tool.

Probably not too hard to find a used one on this site or Ebay.

Twist has a video on this.on youtube. go look (no link)

He uses the tool. He could use a hose.

buy the tool. You likely don't have the experience to properly use the hose <G> (I can, sometimes, but find comfort in the "precison" of the Unsyn instrument.)
There are others.

http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=41211

is one.

http://www.minimania.com/web/AddedFrom/Suggest_Cart/Item/UNI-SYN/InvDetail.cfm

is the original

A much (MUCH!) clumsier thing is

http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=31163

(a couple pieces of 3/8 dowel and some cleverness can substitute and be reasonably precise, bit nowhere as convenient.)


The unisyn is a solid tool. I've had mine for 40 years. You can foind thenm for about $30 on ebay.

If you are going to play the game, you have to have the tools. And this is one of the basics. Along with dwell tach and timing light.

> The Haynes manual say to use "the tube to the ear" method. How does that work?

The pits (as everyone else has said).

Unisyns are cheap. As an exercise, set it up as precisely as you can with your ear, a hose or any other trick you can think of. Put a Unisyn on each carb and you will be astounded at how badly you did without the meter!

I usually check the balance at several RPMs - not just idle. In theory if the linkages are perfectly aligned you don't need to do this but I've found that the Unisyn is a valuable check that your alignment is correct.

Here's the John Twist Link. Great Stuff!

http://www.youtube.com/watch?v=ASeMfXfjNpw

Buy a Uni-Syn. Pays for itself in one tune up.

I have uni-syn and have hard time getting to operate because you must open and close middle section to increase or restrict air. I watched an SU timing video and they had some difficulty also. However, many learn to use properly.

I use a $30 carb sync air guage from JC Whitney and like that.

Pat Chambers 1973 MGB
Peoria, IL

Expensive tool? The Crypton meter and others similar to it could be considered expensive but a Unisyn is pretty cheap and will last for years and years.

Other methods, some better than others, include "bent wires down in the dampers", "sucking sound through a hose" and the best of all "I just can tell when they are right".

Probably what most folks miss is that the carbs must be "disconnected" from each other and the linkage set correctly.

Loosen all four cinch nuts and make sure the links are free to move on their shafts. Back the choke stop screws off a bit and go about your business setting the mix and idle on each carb in turn using your flow detection device.

Then loosen the throttle cable stop and pull the cable down through the stop with a pair of pliers until you have removed all of the slack back to the pedal. At the same time pull down on the stop itself to rotate the link bar until the other end of the lever contacts the choke link bar.

At this point secure the stop to the cable. A Whitworth open end wrench is usually thin enough to fit the inner nut but if you don't have one sacrifice a 7/16" standard by grinding the sides down.

Now here is what is most often overlooked. Go back to the adjusting screws on the throttle links that are supposed to contact the choke cams and very carefully screw them in so that they are just clear of the link cam base circle. What you want here is to have those screws NOT sitting there holding the throttle shaft open but close enough to begin to make contact with the cams when they are first moved by the first bit of a pull on the choke cable. Then as the choke cable is pulled the screws riding on the cams will slightly open the throttle before beginning to operate the rest of the choke linkages that actually drop the jets for fuel enrichment.

BTW: When you go back to cinching the links down on the throttle shafts try to get the tangs set DOWN and against the inside of the openings. This will effectively take the rest of the slop out of the system as you begin to depress the accelerator pedal. Doing the same for the choke shaft links will also increase the efficiency of the choke action too.

Jack

IMHO...

The rotating of the center disc of the Unisyn shouldn't be a bother. All you have to do is work it a little IN THE BEGINNING to get the flow close enough to allow the poppet to be within a workable range. It is there only to allow the tool to be used on a wide range of carb/engine applications, like the difference in flow between an HS4 on a B and an HS6 on a six cylinder Jag motor.

After that you shouldn't mess with it, as the "balance" process will be VIA adjustment of the stop screws within said range of the tool.

Jack

The last new uni-syn I purchased was junk, nothing like the one I bought 45 years ago and sold 20 years ago because I wasn't planning to own another car with twin SU carbs. I found a Gunson's Carbalancer on eBay that works much better. You can purchase a new Gunson's Carbbalancer here.
http://www.rdent.com/pages/tools.html
I have never purchased from them and cannot comment on their service.

Clifton

I agree the unisyn will get them the same. I tried the tube then used my unisyn and they were close but not right. But when I use my unisyn to check the balance if it is on for more than a few seconds the engine tries to die. I have it adjusted so they are both in the middle when I first slap it on. Am I doing somethig wrong? Should the motor fall off while being checked?

Greg

mac townsend Wrote:

gcan Wrote:

I've got the tool lit and if I am using it correctly it is to align the rods to be sure the lift is the same. I did this as well, don't get me wrong my 74B is running good I am just watching this and asking questions to be sure I haven't missed anything.

To use the toolkit to sync the carbs, you install the rods and wire in the dashpots and align the wires with the engine off. Then you start the engine and adjust the throttles (after slackening the interconnect between the carbs so that they operate independently) so that the wires are aligned at idle. When the wires are aligned, that means the pistons have risen the same amount. For them to have risen the same amount, the same amount of vacuum must exist in each carb*, and for the vacuum to be the same, the carbs must be in sync. You then tighten the interconnect linkage to keep them in sync. You can then open the throttle(s) varying amounts and verify that the carbs remain in sync throughout the entire rev range.

HTH!

*Assuming that the carbs are matched and in good repair. If not, they must be repaired before they can be tuned. No tool will allow you to tune worn-out carbs!

Here's a little mod for the "Anal" among us.

You're setting your carbs, twisting screws, blipping the throttle, checking airflow...pretty busy project.

Everytime you use your Uni-Syn, are you centering it over the opening of each carb?

I took my trusty tool years ago and installed two small screws that protude out of the back of the unit. These screws are set to the pattern of the holes that the air cleaner cannister bolts to the carb body. They are smaller then the diameter of the holes.

So when I place the Uni-Syn over the carb, the screws drop into the holes in the carb body, ensuring a perfect alignment every time.

I try to align it correctly but this is a great idea so you wil be sure

Thanks Robert!

Or you could drill and tap 2 holes in the exhaust manifold and install 2 air/fuel ratio meters. Right on the money then. Remove the meters when carbs are correct and install bolts in the bung holes you just made.

I used tube in ear or stethoscope for years until unisyn came out. It is much easier.

I have used a 2ft length of garden hose for years - works for me!

One comment on Jack's instructions - place a thin piece of card (a business card is ideal) between the cable "stop arm" and the choke linkage when you tighten the throttle arms to the rod. This gives a tiny amount of play between the throttle linkage and the throttles as required in the tuning instructions. It allows for slight changes in geometry due to temperature etc. and makes sure the throttles always go back onto their stops.

Eric, that's a good idea. I have the devil's own time getting that gap right since it's hard to get a feeler guage to fit properly in that spot. (Plus, it gets too bloody hot in there to spend too much time putzing around.)

R.

Rick - I was taught that one on my first MGB, back in 1973 (it was a '64), by someone who knew a lot more about tuning SU's than I did back then!

Actually, the "spacer" used to be a piece of a cigarette pack, but that is not "PC" these days (nor so readily available)!