Idle mixture screws have no affect on synchometer calibration

[RandyMettler]

I have dual 94's. My plastic snail synchometer vacuum readings are high. One is higher then the other. When adjusting the idle mixture screws, the snails readings do not change much, if at all. I pulled all 4 screws to check the condition of the end surfaces and they appear to be in good condition.

I also pulled the 94 bodies and set the throttle plates to close at the same time by adjusting the non-progressive linkage.

Is this a common problem with mixture screws?

Regards

[cas3]

do the throttle shafts wiggle around? usually they are worn out, leaking air, and need to be rebushed to control the idle air

[RandyMettler]

I could test if the shafts are worn by using propane at the shafts? Will the idle increases if they are worn? The shafts don't seem to wiggle up and down or side to side but do have very slight left to right play.

[Tinker]

It seems like deja vu.

[Terry,OH]

Adjust the idle RPM set screws not the idle mixture screws. Disconnect any mechanical throttle linkage between the two carbs first, so an adjustment on one carb does not effect the other.

[scicala]

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Listen to Terry,OH. Idle mixture screws do not control the amount of air going by the throttle plates. That's what a unisink measures.


Sal

[RandyMettler]

Tinker, I can see why you would say that,

I didn't think the problem was the throttle shafts when I started this tread, but simple the idle mixture screws. They have no affect when trying to sink the carbs. I am trying one last time to work through my carb problems before shipping them to Charlie. The idle screws really don't seem to affect anything unless they are turned all the way in. From research they should. I don't know if the shafts could be letting in enough air to cause good idle screws to have little to no effect on the vacuum?

[Tinker]

If the idle screws do nothing, you have a air/vacuum leak somewhere.

[RandyMettler]

Okay, The idle RMP set screws don't have any affect either. That is why I started the thread. No adjustments change the vacuum readings. I guess that indicates throttle shafts need to be replaced/reemed or as Tinker states a vacuum leak elsewhere at both 94's. I can get the car to idle smooth and reeve smooth, but I know they have to sink to preform properly. The frustrating thing is propane test indicate no vacuum leaks. What is a good vacuum reading for the 94's

[Tinker]

That would be best Randy. Second pair of eyes on it.

Have you done a load test yet? Meaning drive the vehicle underload. Not to sidetrack. It can sit in a garage idle and rev, but fall flat underload. All in good time.


.

[RandyMettler]

No load test yet. The front end is still in pieces. Not street legal. I was hoping to iron out my engine problems before reassembling the front end. It is just easier to work on the car when there is nothing between the motor and you. I have assembled and taken apart the car a couple of times now and was hoping this time would be the last, at least for a while.

[Tinker]

You'll get it figured out Randy. Look forward to your updates.

[JSeery]

I don't see any way adjusting the idle screw would not affect the flow. That would be similar to saying opening the throttle doesn't increase the flow, because that is what the idle screw is doing. If the idle screw is not moving the throttle plate I would think something is not assembled correctly.

[RandyMettler]

You are correct. The idle screw is changing the idle and fuel mixture. My problem is I can not get the snail to read less than pegged. I cannot get it to change for the positive, or should I say negative,. Like Tinker suggested, I may have serious vacuum leaks else ware.

[JSeery]

Are you sure the throttle plates are centered in the bores and capable of closing completely? Sounds to me like something is not assembled correctly with the throttle plates. You should be able to almost totally stop the air flow through the throttle plates.

[Terry,OH]

If you have a unisyn are you aware of the initial adjustment to get the ball to float within the range of the view tube, by screwing the unisyn center disk up or down? Yes the idle mixture will effect the vacuum each carb draws but the unisyn is a comparative device comparing the two carbs so each can be made to contribute equally at idle. To adjust idle mixture use a vacuum gauge on the manifold vacuum. If you screw the idle mixture screw all the way in and the engine runs rough or stops It appears the screws are working.

[scicala]

This is getting a little too involved and out of hand. Do as Terry, OH said in post #5 and do not pay attention to manifold vacuum. Pay attention to making the unisync the same on both carbs to get the idle RPM you desire. Then you can adjust the mixture screws for best idle (highest vacuum). All this with the throttle linkage disconnected from the carbs.


Sal

[Flathead Fever]

The unisync is not a vacuum gauge its a flowmeter. You say it is "pegging" the gauge. That means the throttle plates are open too much. The idle mixture screws will have no effect because your plates are open too far. The carburetor is no longer feeding fuel through the idle circuit. Gently turn the air fuel mixture screws in and back them off 3/4 turn and forget all about them for now. They are not your problem.

If the shafts were leaking that would "not" make it pull more air through the top of the carb. A vacuum leak would lower the volume of air going through the carb because air is bypassing the top and going in through the shafts. Your Unisync would not be "pegged" because of a vacuum leak.

Like everybody keeps saying. You need to look at those throttle plates and make sure they are closing all the way. Make sure the choke fast idle screw is not preventing the plate from closing all the way. You cannot adjust anything until you make sure those plates are capable of closing all the way. Disconnect all the throttle linkage from the gas pedal and between the carbs. Your not hooking it back up until you are completely done adjusting the carburetors. Make sure you have return springs built into the carbs or auxiliary return springs hooked to the carbs throttle linkage. Otherwise the second you start the engine the vacuum can open the throttle plate wide open and your engine will go from 0 to 5,000 rpm in a split second. Must have returns spring on carbs to start them with the linkage disconnected.

How fast is this engine idling?

[scicala]

"Make sure you have return springs built into the carbs or auxiliary return springs hooked to the carbs throttle linkage. Otherwise the second you start the engine the vacuum can open the throttle plate wide open and your engine will go from 0 to 5,000 rpm in a split second. Must have returns spring on carbs to start them with the linkage disconnected."


I agree a return spring should be connected to do all of this to make sure the throttle plates are at a position where they would be when the linkage is re-installed. However, without a return spring, the throttle should not open by itself with the engine taking off to high RPM. That's the only reason why throttle shafts are offset in the throttle bores (not in the center of the bore), so the manifold vacuum holds the throttle closed. A safety thing incase a return spring breaks or comes off.


Sal

[Flathead Fever]

I had it happen, that is why I mentioned it. It was not on a flathead, its was on a '56 324 Olds I have in a Model A Pickup. It has a two-carb Edmunds intake on it. The carbs were on it when I got the car. They are 4-bolt Stromberg "Aerotype" two barrel carburetors. I have never figured out exactly what they are off of? Somebody told me they were Buick straight eight carbs but I don't know that for sure. Nothing showed up searching the carb number. No carb kits, nothing.

I had the linkage disconnected and no return springs on the carbs. I started it with a remote starter switch and those throttles flew right open. It took me only a split second to get to the ignition switch but that engine was already way past its redline and in Nascar territory.

[Ggmac]

Switch carbs front to rear rear to front , recheck . This will not help but could possibly eliminate a leak at other than the carb butterflies or shafts . I once had a TR-6 that I just installed a manifold and Webers on . No matter what I did I could not sync carbs . Working at jaguar at the time so it was done every day . Narrowed it down to a casting flaw causing some kind of unequal airflow .
Even if you send your carbs out , chances are your going to need to get the basics down on synchronized carb set up procedures.
A carb butterfly could be bent , hole drilled in it for hi , very hi lift camshaft , or a few other possibilities. If you remove your sucking carb and make a rubber adapter to attach to each barrels base and attach a vacuum cleaner while having a small hose on its top side listening for air flow , now do the other barrel , it should sound the same . Adjust the idle screws shut while doing this or before . Make sure your choke linkage isn't holding it on hi idle .
If your up to it now do the other carb , each barrel by itself and compare the sucking sound to each other .
Just a suggestion, much easier for me to show than trying to explain .

[flatjack9]

I tune my cars initially by listening at the mouth of each carb with a length of hose. You can get pretty darn close by adjusting to get the same sound. Then to fine tune, use the meter.

[scicala]

Originally Posted by Flathead Fever

I had it happen, that is why I mentioned it. It was not on a flathead, its was on a '56 324 Olds I have in a Model A Pickup. It has a two-carb Edmunds intake on it. The carbs were on it when I got the car. They are 4-bolt Stromberg "Aerotype" two barrel carburetors. I have never figured out exactly what they are off of? Somebody told me they were Buick straight eight carbs but I don't know that for sure. Nothing showed up searching the carb number. No carb kits, nothing.

I had the linkage disconnected and no return springs on the carbs. I started it with a remote starter switch and those throttles flew right open. It took me only a split second to get to the ignition switch but that engine was already way past its redline and in Nascar territory.

The Stromberg 97 carbs seem to have the throttle shaft pretty much in the center of the bore. Anyway, a lot closer to center than others. If the Stromberg two barrels you used were made the same, I could picture the throttle opening by itself.


The Holley 94's and all others I can think of are offset from center.


Anyway, that must have been a scary moment with your car !


Sal

[RandyMettler]

With all your advice, the process is logical now. The problem with only one carb sucking and pegging my snail synchometer at 30 kg/h easily swiched to the other 94 when taking off the linkage and adjusting the rpm screws. This indicates that the carb is not the problem, it was my previous adjustment procedures. I was using the linkage to adjust the throttle plates.

I know my throttle plates have the ability to fully close and seal because I have had the 94s apart a few times and replaced the accelerator plunger, gaskets, power valves, jets, etc. The throttle shafts appear to be tight.

I have since been able to sync the carbs both at 18kg/h. Anything lower and the car wants to stall. I ran out of gas so I will fine tune tomorrow. The car does like its gas. It goes through a gallon of gas in about 15 minutes of idling, even with my new fuel pressure regulator set to 3 pounds.

My new Charlie NY fuel pump did stop the fuel to oil contamination. Thanks to Charlie!

If you read the"My dipstick is lying to me", I have since scored my dipstick at the 4 quarts full level. Again, thanks to all the senior members that helped me out.


And thanks Flathead Fever to for the heads up for the need for throttle plate tension during the syc process!

Its good to be fixing all the annoying problems. I am looking forward to getting the thing out on the road.

[scicala]

Great to hear !


Sal

[scicala]

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"They are 4-bolt Stromberg "Aerotype" two barrel carburetors. I have never figured out exactly what they are off of? Somebody told me they were Buick straight eight carbs but I don't know that for sure. Nothing showed up searching the carb number. No carb kits, nothing."


Buick used a Stromberg two barrel from the late 30's through about 1955 (model AAV). In '56 it was replaced by the Stromberg WW two barrel. I believe other makes of cars used the AAV carb too.


Sal

[RandyMettler]

With all your advice, the process is logical now. The problem with only one carb sucking and pegging my snail synchometer at 30 kg/h easily swiched to the other 94 when taking off the linkage and adjusting the rpm screws. This indicates that the carb is not the problem, it was my previous adjustment procedures. I was using the linkage to adjust the throttle plates.

I know my throttle plates have the ability to fully close and seal because I have had the 94s apart a few times and replaced the accelerator plunger, gaskets, power valves, jets, etc. The throttle shafts appear to be tight. I have since been able to sync the carbs both at 18kg/h. Anything lower and the car wants to stall. I ran out of gas so I will fine tune tomorrow. The car does like its gas. It goes through a gallon of gas in about 15 minutes of idling, even with my new fuel pressure regulator set to 3 pounds.

My new Charlie NY fuel pump did stop the fuel to oil contamination. Thanks to Charlie!

If you read the"My dipstick is lying to me", I have since scored my dipstick at the 4 quarts full level. Again, thanks to all the senior members that helped me out.

Its good to be fixing all the annoying problems. I am looking forward to getting the thing out on the road.

[Bob C]

[QUOTE=scicala;1727323]"They are 4-bolt Stromberg "Aerotype" two barrel carburetors. I have never figured out exactly what they are off of? Somebody told me they were Buick straight eight carbs but I don't know that for sure. Nothing showed up searching the carb number. No carb kits, nothing."


Is this what they look like?

[Flathead Fever]

That sure looks like them. I went out and looked. It little hard to get to the car.

The poor car. The last time it was started was for the 60th Hot Rod Magazine Reunion, for cars featured in the magazine. They called me up recently for the 70th reunion and I declined. It still has the same fuel it in from the 60th reunion. I'd would have had to clean it for the show and it just starting to get that nice barn find look. I've had five back and shoulder surgeries since that 60th reunion so productivity has gone down around here.

This is a '28 Model A Roadster "Car" converted into a pickup cab. It was a '25 Model T bed on it. Built during the War it was originally red with a flathead and then repainted black and a 303 Olds installed in 1950. It now has a 324 Olds. It took second place in it class at the first 1950 Oakland Roadster show. Won the Pasadena Reliability Run. Its in a bunch of early hot rod magazines. It belonged to the guy that invested the glass pack muffler.

[aussie merc]

ok having read this it sounds like you have a vacum leak from another source IE wiper motor hose or even a bad intake gasket that is leaking from within the cam valley so now to track it down 1 remove and plug all vacume taps ie wiper motor etc if no change attach vacume gauge to manifold and check eng vacume take notice if steady or pulseing refer to charts for vacume gauge[ from net] them if unable to locate problem go back to basics and start again there are several good posts on setting up and tuning multi carbs on this site

[Tinker]

Looks like you found your air/vacuum leak. nice work, way to stick to it.

[cas3]

bob c, that aerotype carb was on my ww2 white halftracks too. i think the flathead caddys too