Zentih Carb Adjusting screw ID help.

[BKC]

I have an 81 SC that has been fitted with what I believe are Zenith TIN 40 carbs. I'm going to "monkey" with their tuning, if for anything else to get my feet wet with adjusting them.

I have Wayne's book in which he describes adjusting Webers. On the outboard side of each carb are 3 screws with springs wrapped around them. Can I assume that these are the "mixture adjustment" screws? Also there is a smaller, set screw looking screw very close to and slightly above the "mixture adjustment" screw. I'm assuming these are the "air correction" screws?

My local auto parts store has a nice Unisyn that I can borrow while I'm making adjustments.

My goal is to make the car run leaner. The "mixture adjustment" screws are turned out approximately 3.125 turns. I'm hoping that by turning them out more it will lean out the mixture and minimize the smoking that occurs under heavy throttle. A tad better MPG would be nice too.

[neilca]

Brian,

Those screws are for the amount of mixture introduced into the venturi. To change the mixture you need to change the idle jets. these jets and screws only effect the car at idle.

The smoking on acceleration is more your air correctors and main jets. It sounds like you car needs some air. The standard Zenith runs a 27mm choke, this is way too small for a 3.0. I am running 32mm chokes on a 2.0 and thinking of increasing those to 36mm for the race track. The chokes are found just under the venturis and above the throttle plates. If you look down the throat you will see the diameter is restricted, this is the choke, not to be confused with a choke for starting th engine. These carbs don't have that kind of choke.

Your car probably has a lot more horsepower if it could only breath. I think I would run 34 mm or 36 mm chokes on a stock 3.0. Now the bad news they do not sell them. You have to make them. I think there is a guy on this board that was selling them. Try doing a search on Zenith.

Hope that helps,
neilca

[BKC]

Looks like my ignorance is showing pretty badly!

I'm not sure if there's a difference between venturis and chokes but I'm sure I'm running 34 mm venturis per the car's PO. Guess I'll have to school myself on carburetor parts. It's been tough trying to find pictures of these carbs with there parts called out!

[1QuickS]

The 34mm "chokes" are the main venturis just as the little tubes (auxiliary venturis) that are concentric with the 34mm main venturis are venturis as well. The mixture screws do have the springs wrapped around them and the little screws to the 2 o'clock position are the idle air adjusting screws.

Mixture for idling operation is provided by fuel passing through the main jet to the idle jet where it is mixed (emulsified) with atmospheric air drawn through a pressed-in idle air jet located on top of the main throttle body. The emulsified fuel is drawn into the throttle bore by the low pressure created by intake vacuum against the closed butterfly. Once the butterfly is opened additional fuel is provided in incremental fashion from the progression holse (hidden by the big slotted brass screw/plug directly above the mixture screw) plus fuel from the idle mixture screw.

As the throttle plate is opened the vacuum decreases while air flow increases thusly activating suction through the auxiliary venturi. Fuel from the main jet is drawn into the aux venturi after passing through the emulsion tube and emulsifying with atmospheric air drawn through the air correction jet which sits on top of the emulsion tube. The outlet for the aux venturi is at the waist level of the main venturi which is the point of maximum vacuum due to venturi action.

As air flow increases fuel flow increases but so does the air that emulsifies the gas which tends to lean out the fuel mixture ar higher RPMs.

If you have read Wayne's book I suspect that tuning the carbs is a sequence of checks including verification of fuel pressure, float level heights, idle air correction, idle mixture adjustment, side-to-side air balance at both idle and at 3000 RPM. Also, valve settings, ignition wire condition, spark plug type and condition and ignition advance are all part of the carb tuning equation.

To jump into carb adjustments is admirable and a worthy effort, there are few carb tuners available so many owners opt to perform the task themselves.

Diagnosis of tuning issues follows a careful system tuning to assure you don't correct something that is mechanically OK but where tuning errors provide a false diagnosis.

Black smoke from acceleration may be the result of several items but it isn't a result of idle mixture jetting or settings.

You might want another source of tuning and/or troubleshooting info so I recommend visiting:

Weber Carbs Technical

Best of luck on your new path.

[bfunke]

Hope this helps. Agree 27mm venturis on Zeniths are too small for any performance application.

[BKC]

Thanks for the great replies, guys!

Perhaps while I try to figure out how to lean this thing out, someone could recommend a spark plug that would withstand the excessive fuel flow. Car is running MSD ignition with recurved distributor. When JW checked it out he mentioned a plug that may help but I can't remember what it was. Iridium maybe?