Uni-Syn tuning

I am trying to tune my twin (Holley 94s) carb setup (4 cyl Ford Model A), using a new Edelbrock Uni-Syn. The engine idles at 17" Vac. Primary carb at front, with a 3.5 power valve. secondary carb, no idle circuit, power valve blocked. Carbs are setup with progressive linkage.
With the Uni-Syn opened all the way, placing the device on the front carb, causes the engine to die, obviously because of the flow restriction. The float indicator (the little red piece) doesn't move. I've tried two different units, including a brand new unit, same result.
Do I need to drill an additional hole to allow extra flow, or what?
I've used Uni-Syn devices in the past with no problems, not sure what is happening here.
Any suggestions?
Thanks.
Ken

 

When using a progressive set-up there is no need to synchronize the carbs. I take it that is what the device is that you are trying to use?

 

Several of the sources I've been reading, mention that the secondary carb throttle should be set even though it doesn't contribute fuel flow to the idle process. I searched through the tech archives on this forum, finding several different theories.
While I would tend to agree with your ***essment, the issue of the engine stalling when placing the uni-syn on it, tells me that there is some issue with the device.
Thanks for the response.

 

the red float on mine sticks also. have to shake it loose the gently set it on the carb. then screw the big valve down till it starts to rise in the tube. if i recall mine had a ball of metal blocking the p***age between the base and the tube when i first got it. take it apart and check that. not a fan of running those progressive. it will cause uneven distribution in those manifolds and dual 94's are probably 2 much carb for most model a's. also i had to epoxy the flex joint between the base and tube it leaked so bad...piece of junk

 

Two two-barrel carbs with progressive linkage on a 4 cylinder would be a nightmare to tune at lower RPM's. May have to live with a very high idle.

And as posted by Unclee, sychronization would do nothing except at WOT.

Jon.

 

Thanks for the info.
I will try going back to a 1:1 linkage, see if that works better. While the engine idles on one, I can see that there would be fuel distribution issues with this carb/manifold setup. I have an O2 sensor mounted at the end of the header collector, idles at 500 rpm, and a AFR of 11.5:1.
I thought that progressive linkage would work better for this application, doesn't look like that is the case.
Going to a 1:1 linkage makes the use of a synchronizing device a good idea, so I need to fix this unisyn.
Thanks again.
Ken

 

Never mind, I'm not awake yet......

 

With a uni-syn open all the way (big gap or opening), this should not cause a flow restriction at idle. They just don't breathe that much air at idle. So, what is it idleing at?
Second, how are you getting a 11.5 at idle? Is it a wide band O2 sensor and do you have a conversion chart for the voltage, or is this a readout from a meter/gauge?
That's prettty rich, and adding a restriction to the air could cause it to choke out and die. You want ~14.7 in a perfect world, most ususally prefer a bit richer, but anything less than ~12.5 is going to be too rich.
I'd check for vacuum leaks, causing you to have the mix that rich to cover the problem. Also, how big of a cam is it? If it's something with a weak signal at idle, that can cause issues too. I'm more familiar with later engines, but the principle is the same.

 

It is a wide band O2 sensor, reading to a digital display. Innovate Motorsports LM1.
Idles at 500 rpm, 17" vac steady at idle. 2500 rpm 20" steady.
I agree that the AFR is real rich. I don't think I will ever see 14.7, but I would like to see 12.5:1.
Carbs were both rebuilt, new throttle bodies. Rear carb has power valve plugged, front has 3.5" pv. AFR adjustments, rear screwed in all the way, front out 1 turn. Turning the fuel adjusting screws makes a BIG change in AFR at idle. The primary throttle plates have a .050" hole in each throttle plate, made a big improvement in AFR and idle quality. Reluctant to increase the area of this hole until I understand what is happening here.
I've looked for vac leaks, but everything looks great. I checked the carb bodies for warpage also, none found. Throttle response is good, no bog.
Ignition is a Mallory single point firing a MSD 6. Plugs are Champion RJ12YC, gapped at .040". Timing is 8 BTDC idle, total 28 at 2200.
The carbs were in great shape, no signs of internal corrosion, bought rebuild kits from Vintage Speed.
My next step(s) are to (1) remove the primary pv and plug, and (2) install 1:1 linkage.
It's been a long time since I worked with carburetors, so I have to relearn the technology. this forum has been invaluable in that regard.
Ken

 

unisyns do tend to kill the engine, so you just set it on the carb for a second and read the red float real quick. there's a better unit called a synchrometer that doesn't do that. if the linkage is progressive and the secondary carb has an idle circuit, you would need to balance both carbs at idle and adjust idle screws on both carbs. if the secondary carb doesn't at least get an idle airflow that allows the idle jets to function and be adjusted, you'll get a hesitation when it's opened.

 

OK...so you say it idles at 500 with good vac...and has good throttle response with no bog.
So what's the actual "problem" you're trying to solve?
Progressive 2x2 carbs don't need to be "insynch" with each other....the secondary just needs to be closed "tight" and reach WOT at the same time as the primary....just think of it as a 4 barrel.

 

I never thought of a twin 2 bbl as a four barrel carb, but that sure makes sense.
I forgot KISS!
The idle air/fuel ratio is extremely rich, running around 10.5-11:1 at idle, waters the eyes.
Probably just need to keep working with power valves and eventually jets. I'm so used to working with electronic fuel injection, I might be looking for the impossible.
Ken

 

bingo!

 

You don't need 2 carbs on a stock Model A 4 banger.

 

I think the original issue was that the uni-syn was stalling out the engine when placed on the "primary" carb. Make sure the "lid" is wide open on the uni-syn. When you look down at the uni-syn from the top, you will see the inverted dome that has six bumps on it- that's the "lid". Twist it counter-clockwise to raise it up from the funnel. The gap will increase, allowing more air to flow through it. If the red indicator is sitting at the bottom, you can close the "lid" a bit to change the pressure under the uni-syn.

 

After years of screwing with British cars... the way to use the Uni-Syn was described above. Uncrew the middle all the way to let in as much air as possible (you may need to tinker with that middle piece if the red plastic indicator moves to one extreme or the other. Only put it on the carb long enough to get a steady reading... then you blip the throttle to let it settle back down. Go to th rear carb and do the same process... then go back to the front, so on so forth until you can read one carb, blip the throttle, read the other carb and they match.

I know it's sounds dumb, but you need to blip the throttle after each reading/every adjustment. It takes slop out of the linkage and allows the engine to get back to a normal idle.

Tuning with a Uni-Syn is actually fun once you get the hang of it.

-Chris

 

so the new uni-syn's are junk? are they set up for downdraft or can they be set up for sidedraft, i'm searching for an older set for these SU carbs i've got.

 

The clear plastic tube pivots so you can use them for either side draft or down draft carbs.

Unless something drastic has changed... I don't see why the newer ones would be junk. My newest one is about 7 years old, oldest about 30, and they look/feel identical. I'm using the one with the red floating pellet though... not the one with the numbers and "globe" on top.

I'd suggest getting a Uni-Syn from a British car part supplier such as Moss (British car guys live and die by their Uni-Syns!). The last one I got was made in England and is of high quality. I just noticed the one Edelbrock sells is stamped "MADE IN CHINA". That might be why you heard the new ones were ****.

http://mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=37197&SortOrder=1



-Chris

 

I was going by a couple of the comments above.

 

I bought a new edlebrock uni-syn about two years ago. and like Jetmek, it was junk.

leaked vac so bad the ball wouldn't move. Fixed with silicon.

And the funnel thing was so out of line it would not even come close to restricting the flow. But tweaking the casting got it to work.

Such a simple device. How could it go out the door not working?

Easy to fix but where is the quality control? Don't they have a shop vac in China to check these with?

 

Yep... get the one from the Brit site. It shouldn't matter regardless.... they should have quality control, even in CHINA. But in this case go with the one made in England.

 

well they are Hitachi's from '72 but they are identical to the brittish SU's with some improvements. I definitely don't want to sync them with a junk unisyn after all the work that has gone into these. I also have the calibrated rods that go in the tops of the pistons, you are supposed to be able to sync them with those, because it tells you the exact height of the piston while it's running.

 

Well you do whatever floats your boat.

If you've got a good ear and a little experience under your belt, you can synch carbs with a length of hose... or you can even tap your palm on the carb to feel the amount of suction. With some practice you'd be amazed at how close you can get.

Good luck.

-Chris

 

One thing that bothers me about this process is where you have a 3.5 pv (it should be a 6.5) and you say that you are using it as a tuning device for adjusting the idle richness. That thing needs to be closed at idle. And since this is a 4cyl you'll probably need restrictors in the power valve enrichment circuit (about .026 - .028 i expect). Why can't you lean the motor with the idle richness screws? With the lm1 connected and you run the idle screws in, does the motor lean out or just stay the same as if there is fuel entering the engine below the throttle blades like from bad gasket on the pv. I ***ume you are looking down into the carb during this process and there isn't raw fuel coming out the boosters. How do you know that the other carb isn't dumping some fuel in, no idle circuit but there may be other issues, you could just put a plate over where that carbs sits and run it on the primary.
Just some thoughts, good luck, oj

 

I thought the same thing as oj... if you have 17" of vac. at idle, then your 3.5 power valve is 13.5" away from entering the discussion. Unless, of course, it's got a hole in it.

 

Thanks for all the feedback.
Just finished the following:
Removed the primary power valve. Major change in AFR at idle. Now idling at 12.8:1-13:1. Idle vac still steady at 17". No longer waters the eyes.
Changed back to a 1:1 linkage. Opened the secondary carb to idle.
AFR stays around 12.5:1 up to 2500.
When I get back later, I'll tweak on it some more.

Throttle response is diminished from before, but I think that giving a little more timing at idle, and maintaining the 28 total might eliminate the flat spot.

 

Even if the new uni-syn's are junky yellow ****, you still should be able to tell if the dilly-whopper in the center is screwed all the way up. With a hole that size it should flow enough air for any motor you'd be working on at idle.....say at least 300 cfm.....compare that to what is flowing around two ****erflies cracked a mm or two.

 

Where'd ya get the PV? "Common" Holley PV's don't seal properly on 94's without modification....

 

14.7 isn't really a useful goal except for light throttle cruising - it's just a chem-lab number. Idle with anything but the mildest cam needs more fuel, as do starting, acceleration and high speed. OTOH, very light (trailing) throttle can be even leaner, like 15.5.

If you take out the PV you're going to have response problems.
Get the spark sorted out before doing anything else.

 

The PV's were obtained fron Vintage Speed. they have been machined to fit the 94. regular Holley pv won't clear the casting.
I agree that a pv shouldn't have any effect on idle quality, but for some reason, removing it brings everything much closer to harmony. I have 2.5, 3.5, 4.5, 6.5 pv's, tried them all. The only thing I can think of, the intake manifold has a pretty large plenum (in comparison to the intake runners), maybe I'm getting a stronger vac signal from the secondary carb.
Will be working on it tonight.
Ken