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Carburetor Syncronizers?

trbob

Freshman Member
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I am looking to buy a syncronizer and would like your opinions about the various options that are available.
 

foxtrapper

Jedi Trainee
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My favorite was always the synch stick. But I always managed to forget and burp the throttle, sucking the mercury out. I got tired of losing the mercury.

Next favorite is the bank of dial gauges with dampers. Used them for many years on god knows how many motorcycles. Works just fine.

Triumphs tend not to have handy manifold port taps for those nice motorcycle synch gauges. So that leaves the rather cheesy ones you hold against the face of the carburetor.

Of those, I prefer vertical ball type. It bounces and drifts, but it's more consistent and repeatable.

I've also got one of those round rotational units with the red tab. Repeatability is not good.
 

Don Elliott

Obi Wan
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I can't understand that a "Sync" can be a good indicator of setting SU carbs when you use one. You remove the air filters, place the tool on one of the carb inlets and it blocks off almost all the air for that carb while you watch the little ball rise in the glass tube. Then you do the same for the 2nd carb. It doesn't make logical sense to me that you block off the air to read how much air is getting into that carb.

I have only ever used an 18" length of garden hose with one end held carefully at the edge of one carb inlet, and the other end is held at the edge of my ear. I listen for the hiss of the air going into the carb. Then I walk back about 15" and do the same for the 2nd carb. I walk from carb to carb because I don't want to swing it through an arc as this may give an inaccurate comparison. If you hear the same hiss from each carb, you know that each carb is sucking the same amount of air. Therefore the carbs are synchronized.
 

rlandrum

Jedi Trainee
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I've never understood the purpose of syncronizing them. To me, if the butterflys are both closed when you tie the linkage between the two carbs together, then they are syncronized. I guess the action on the two individual pistons could be different, thus changing the amount of air and fuel being let into the engine.

If that's the case, then the best way to measure would probably be by using the venturi effect. A small 12 inch long piece of clear plastic tube with a ball bearing or BB Gun BB inside would suffice, I would think. Form a U-shape with the tubing, BB at the bottom of the U, and hold each one in front of the carb inlets. Whichever way the BB tends to move is the side drawing more air. If the BB stays at the bottom, the carbs are synced.

Although, now that I think about it, a BB could cause some damage if sucked in to engine, and something like gasoline or water might be a better measuring gauge.
 

Opa

Jedi Trainee
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I use the same setup you do Don.Mine appears to be the updated version as compared to yours,at least by the "sounds" and look of it :jester:(sold for hearing impaired mechs) :wink: I did go one step further tho, I drilled the insulating block behind the carb and installed a small copper tube which i plug an old vacuum gauge onto. I also uncap these when going thru air test(Smog test),thinking I'd give er a little more air to compensate for the cam at idle test.
 

MGTF1250Dave

Jedi Knight
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Aloha Bob,

I like to use the SU tool Kit because it doesn't require you to remove the air filters and I don't trust my hearing to judge the hissing sounds. The kit also contains a jet wrench and a jet centering tool. This link provides an picture and description of the kit:

https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=37198

I'm sure it is also available from other sources. I bought mine several years ago from JC Whitney. The complete SU tool kit generally cost less than the Uni-syn.

Here is a description of the pieces of the tool kit to use to match carburetor air flow. In the kit you'll find 2 aluminum tubes (one with an oval end and one round) and 2 wires with a different loops on the end. The fatter loop goes into the oval tube end. The other loop goes into the round tube. If it's hard, you're doing it wrong. Take off the damper caps, and fit the end of the tubes opposite the wires into the top of the carbs. Turn the tubes so that the tips of the wires are close to each other, and "tweak" them so that the tips are at the same height. Gently lift each tube, about an inch or so, and let go. Back the fast idle screws out until they are well clear of the fast idle cam. Back the idle speed screws on each carb out until they just touch the tab they press on, then in about 1-1/2 turns each. This should put the throttle butterflies in pretty much the same position.

Start it the engine and take the RPMs up to about 1500 - 2000 RPM by adjusting the idle speed screws the same amount, 1/4 turn at a time. Observe the ends of the wires, are they still at the same height? (They'll be vibrating a bit, but should be buzzing at the same height.) If they aren't at the same height, turn in the idle screw on the carb with the lower wire until they are. The carbs are now flowing the same amount of air.

I edited this description from an article written by Chris Kotting. The article is titled Tuning 101 and is an excellent reference and very descriptive. I found using goggle and a search for SU carb synchronizer.
 

Don Elliott

Obi Wan
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R Landrum - After you have them synchronized (by whatever means - or if you haven't done it), you take off the air filters and blip the throttle linkage while looking with a far-away stare between the two carb inlets. This way you can see the inlets for both at the same time. As you blip the throttle, the pistons will rise and if the damper oil is the same in both carbs and if the pistons and bores are all clean, both should rise at the same time and the same speed. That way you know you have the same vacuum in both carbs, therefore both carbs should give equal output (horsepower) from the carbs that each is feeding.
 

PeterK

Yoda
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I use the Synchrometer STE (aka snail) and have always had good luck with it. My old UniSyn (the one with the red ball in a plastic tube) finally cracked but was never much use anyway.

The Synchronometer doesn't close off the carb inlet but measures the air flow.

My TR3A has a nice even smooth idle @500 RPM.
 

poolboy

Yoda
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I like a cheap stethoscope with the amplifyer removed and the tube placed about 1/4 inch into the carb, keeping my hand far enough away so as not to block the air.
OPA, that looks interesting. I'll have to remember that one.
 

TR3driver

Great Pumpkin - R.I.P
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My personal favorite for SU carbs has always been what comes with the "SU toolkit". Basically two rods that fit into the dashpots, with two wires that you insert and bend so they point at each other. The big advantage IMO is that you can check for synchronization both on the idle stop and just off idle ... which can easily be different. In fact, with the TR2-4 linkage, it's always different, because the pedal connects to the front of the front carb, but there is a return spring on the rear carb. The return spring forces the linkage between the carbs to flex slightly and so the rear carb always opens slower than the front.

The SU kit makes it easy to check for synchronization with the throttle open slightly (meaning the linkage between carbs is torqued); and then you can set it again with the idle stops (keeping the linkage torqued).

How much difference does it make ? Not much ... but it did seem like my engine pulled a little smoother under light throttle after making the adjustment.

Unfortunately, the rods don't fit ZS carbs ... I'm considering making my own to fit as I agree the 'Unisyn' is way too cheesy and there are no ports for "carb sticks".

FWIW I recently bought a Unisyn (the Stags have ZS carbs) and found that I had to do some re-engineering to get it to work at all. Casting flash on the ball was making it hang up in the tube. I also discovered that I had been shown how to use it wrong ... the tube is not supposed to be vertical but instead nearly horizontal so the ball moves easier. That way, you can keep the opening wide enough to have minimal effect on the airflow, instead of having to choke it down to pull the ball straight up.
 

foxtrapper

Jedi Trainee
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rlandrum said:
I've never understood the purpose of syncronizing them.
On these engines, with their large cross over tube in the intake manifold, it doesn't matter all that much.

But, get into dedicated individual runners, and it makes a tremendous difference at sluffing speeds. Things like maintaining speed on level ground or a slight downhill grade. Then you get into all kinds of surging and splutterings with one cylinder pushing, another pulling, etc.

The big balance tube on the SU manifolds Triumph uses negate almost all of that problem.
 

Roger

Luke Skywalker
Bronze
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I used to do SUs with a bit of hose and two bits of bent welding wire. Use as described above.
 

jeffyfarley

Freshman Member
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Does anyone actually disconnect the throttle linkage before syncing their SU carbs? The manuals all have that instruction but from what I have read and seen on Youtube the carbs can be synched without disconnecting the linkage. I've been working on my tri-carb BT7 without disconnecting but would like to know if there is an advantage to doing so.
 

CJD

Yoda
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I do not, so long as I am able to set the front carb without the linkage interfering.
 

Geo Hahn

Yoda
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I suppose if the carbs are very close to being in sync then they could be tweaked a bit w/o undoing the linkage -- but to really get them the same it seems to me they have to be able to move independently.

The good news is that (in my opinion) it is not super-critical. Two carbs for 4 cylinders and a pretty big balance tube connecting each side of the intake manifold allows things to run pretty well even if the are not right in sync.

Edit: Okay, thinking about, it I can see the real purpose of syncing.

In my usual tune-up sequence:

Plugs out and cleaned
Valves set
Points set
Static timing set
Plugs back in
Engine started
Carbs synced
Mixture adjusted

Syncing the carbs gives you something to do while you wait for the engine to warm up enough to set the mixture.

The engine does not have to be warm to sync the carbs - indeed if you were able to spin the crank with a big motor you could sync the carbs as all you are working with is the the vacuum created by the pistons and open intake valves.
 

dklawson

Yoda
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Does anyone actually disconnect the throttle linkage before syncing their SU carbs?

Yes. I unhook both the choke and throttle linkages while tuning dual carbs... ZS or SU. Since at idle you are using the idle screws to set the airflow and idle RPM it can be very easy for one carb's butterfly to move when you adjust the other if the linkages are connected. Further to that, on SUs once I have set the idle airflow equal, I open the throttle using the linkage to achieve about 2000 RPM. Then I will repeat the balance adjustment NOT with the idle screws but by moving one linkage end relative to the other such that the carbs are again flowing the same amount of air.

I have used the tuning wires but prefer to use a Synchrometer. (I was given a Unisyn but have never used it). Regarding comments above that you are blocking one carb and the air flow changes... that is correct but you are making a flow measurement for each carb individually under the same conditions. Therefore you are getting an indication of how much air each carb is flowing for a given set of conditions.

Regardless, carb adjustments and tuning are one of my least favorite things to do.

 

poolboy

Yoda
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The throttle shafts should be independent during synchronization; otherwise what you do to one throttle disc opening will be done to the other.
Once they are synchronized and if you wish to alter the idle speed, it's fine to have the throttle shafts connected; just do to one adjustment screw as you do to the other's.
The one of the objections that I have with the Unisyn and 175 CD2SE and earlier carbs is that it covers the idle speed float chamber vent.
 

Sarastro

Obi Wan
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Back in the Dark Ages, when I had my original TR4A, I tried syncing the carbs with the "listen for the hiss" method and checked the results with the Uni-syn. It wasn't even close. For that reason, I've never tried the hiss method again. The greatest problem with it, in my view, is that you really have no way to know how accurate it is.

The Uni-syns sold today are pretty junky compared to the ones made in the 60s. The old ones were sensitive enough that you didn't need to restrict airflow to any significant degree. As long as you don't experience any change in engine speed when you use one, you're not restricting the air flow significantly; and, to the degree that you are, it's the same at both carbs, so it shouldn't matter. The best thing about using a sync tool is that you are making a direct measurement of air flow, which is what you want to equalize. Any indirect method leaves an uncertainty about whether you are really measuring air flow equally at both carbs.

Going by the damper height is an indirect method, and it depends on the dampers' characteristics being identical. Are they? Not sure how you would determine that for sure.

I use one of the "snail" synchro tools on my Porsche, but I don't know if it works in a horizontal position; maybe, when my current TR4A is ready, I'll look into it. For downdrafts, it is the preferred tool, and everyone uses it.

https://vwparts.aircooled.net/Snail-Type-Syncrometer-Carb-Sync-Tool-GERMAN-SK-p/ste-sk.htm
 

poolboy

Yoda
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Balancing Carbs.jpg
Personally..if I found a difference between the hiss and the Unisyn... I'd trust the hiss.
For the sake of consistency I use this thread hole in the ZS carb as a listening station.CarbSyncing.jpg
 
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