Idle mixture screws have no affect on synchometer calibration

R

RandyMettler

Senior Member
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
 
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Re: Idle mixture screws have no affect on unisink test

do the throttle shafts wiggle around? usually they are worn out, leaking air, and need to be rebushed to control the idle air
 
Re: Idle mixture screws have no affect on unisink test

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.
 
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Re: Idle mixture screws have no affect on unisink test

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.
 
Re: Idle mixture screws have no affect on unisink test

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
 
Re: Idle mixture screws have no affect on unisink test

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?
 
Re: Idle mixture screws have no affect on unisink test

If the idle screws do nothing, you have a air/vacuum leak somewhere.
 
Re: Idle mixture screws have no affect on unisink test

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
 
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Re: Idle mixture screws have no affect on unisink test

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.


.
 
Re: Idle mixture screws have no affect on unisink test

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.
 
Re: Idle mixture screws have no affect on unisink test

You'll get it figured out Randy. Look forward to your updates.
 
Re: Idle mixture screws have no affect on unisink test

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.
 
Re: Idle mixture screws have no affect on unisink test

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.
 
Re: Idle mixture screws have no affect on unisink test

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.
 
Re: Idle mixture screws have no affect on unisink test

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.
 
Re: Idle mixture screws have no affect on unisink test

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
 
Re: Idle mixture screws have no affect on unisink test

The unisync is not a vacuum gauge its a flowmeter. You say it is "*******" 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?
 
Re: Idle mixture screws have no affect on unisink test

"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
 
Re: Idle mixture screws have no affect on unisink test

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.
 
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