I assume I have the original Zeniths since I am only the second owner and have all the repair receipts from the first owner. I have two different carbs, the one closest to the firewall has an idle mixture bypass screw and idle mixture screw while the other only has a bypass screw. Did they actually put one carb with and without a mixture bypass in the same car and for what reason?
Another Zenith Carb question
- Thread starter bavbob
- Start date
sounds odd
can you post some pics ?
can you post some pics ?
Doubtful, either a mistake from the assembly parts bin or a replacement carb at some point. What year is your car? Maybe a transitional thing for emissions. Engine would not benefit from having one and one since 3 cylinders are fed from each.
I have attached pics. There are different cold start valves (thermo valves) on each as well which would confirm two different types on my 1972 Bavaria. The car runs well, idle sometimes rough when cold but I have it now at about 750 when warm and it is fine. First and fourth images are from the carb closest to the firewall.
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My '74 (4310008), before I converted to FI, had two zeniths each with air adjustment and fuel adjustment screws, and the thermo valves. Sounds like one of your carbs was replaced with an earlier year's model...aren't POs fun?
The question then is to swap out or not. The enemy of good is better. I have 3 other carbs that I can reconstruct to achieve at least one newer one. How is the idle mixture controlled in the carb with no idle mixture screw, manipulation of the idle bypass screw?
Here's some info on the 2 carbs, I suppose if you turn the bypass screw all the way in you effectively turn it into a non-emmisions carb like the other. The bypass screw allows the emissions to meet the standards of the day. The mixture screw on the emmissins carb is the smaller of the 2 screws so both can still be adjusted there:
INAT 32/40 and 35/40 two-stage carburettors with mixture recirculating system
Apart from the mixture recirculating system and a few other minor changes, this carburettor is identical to the INAT without mixture recirculation. Apart from the additional mixture recirculating system, the carburettor still retains its conventional idling system (Fig. 1).
Fuel for mixture recirculation is taken from the float chamber, metered through a fuel jet and supplied to a bore into which the air correction pipe projects from the top. The pipe opens into the mixing chanter. In this bore the fuel and air are blended to form an emulsion. A jet governs the emulsion flow rate. The emulsion passes into a passage leading down and is there mixed with fresh air drawn in via the mixing chamber. The recirculating mixture regulating screw has a taper which alters the cross-section of the passage without affecting the basic idle setting (the airflow through the throttle butterflies, preset on the carburettor test bench). (Fig. 2.)
The fuel-air ratio remains largely constant at both small or large flow volumes, and complies with the exhaust emission regulations.
Adjustment of engine idling speed and carburettor synchronization while remaining within the exhaust emission limits for each carburettor (with exhaust probes 13 0 020 in both exhaust manifolds) must only be undertaken at the mixture recirculation regulating screw (for engine speed) and the mixture regulating screw (for exhaust emissions). Use the air cleaner simulator 13 0 000.
Warning:
The settings of the throttle stop screws must not be altered, or else the throttle butterfly gap and thus the airflow preset on the carburettor flow test bench for the basic idle speed will be lost.
The carburettor manufacturer supplies plastic caps to protect the throttle butterfly stop screws.
INAT 32/40 and 35/40 two-stage carburettors with mixture recirculating system
Apart from the mixture recirculating system and a few other minor changes, this carburettor is identical to the INAT without mixture recirculation. Apart from the additional mixture recirculating system, the carburettor still retains its conventional idling system (Fig. 1).
Fuel for mixture recirculation is taken from the float chamber, metered through a fuel jet and supplied to a bore into which the air correction pipe projects from the top. The pipe opens into the mixing chanter. In this bore the fuel and air are blended to form an emulsion. A jet governs the emulsion flow rate. The emulsion passes into a passage leading down and is there mixed with fresh air drawn in via the mixing chamber. The recirculating mixture regulating screw has a taper which alters the cross-section of the passage without affecting the basic idle setting (the airflow through the throttle butterflies, preset on the carburettor test bench). (Fig. 2.)
The fuel-air ratio remains largely constant at both small or large flow volumes, and complies with the exhaust emission regulations.
Adjustment of engine idling speed and carburettor synchronization while remaining within the exhaust emission limits for each carburettor (with exhaust probes 13 0 020 in both exhaust manifolds) must only be undertaken at the mixture recirculation regulating screw (for engine speed) and the mixture regulating screw (for exhaust emissions). Use the air cleaner simulator 13 0 000.
Warning:
The settings of the throttle stop screws must not be altered, or else the throttle butterfly gap and thus the airflow preset on the carburettor flow test bench for the basic idle speed will be lost.
The carburettor manufacturer supplies plastic caps to protect the throttle butterfly stop screws.
Attachments
I think I am getting this now, thanks for the info. I have a plan and hopefully that plan will not lead to another post!