Diagnosing Varying Idle Speed Problems

from the Spitfires List

Bob Bollinger asked:

I have a stock 79 Spitfire (ZS carb with water auto-choke). When I start it up it idles about 2200 - 2500 rpm then will drop down to about 900 rpm which I believe is just what it's supposed to do. After driving the car for awhile (10 or 15 minutes) the idle goes back up to about 2500 rpm. I've tried to rev the engine really quick and occasionally that seems to get the idle to drop down, but most of the time it doesn't help. Any suggestions? Could something just be sticking? I have a carb rebuild kit that I was going to use in hopes it would help.

William Davies (from across the big pond) responded:

I've no experience of this particular carb installation (we had twin SU on all Spits in the UK), but the 2 main causes of varying idle speeds in my experience have been:

1 - Sticking throttle cable

2 - Air leak on the induction side, ie., around the carb mounting or the manifold to head joint. I guess air being drawn through a worn throttle shaft could give a similar symptom. DPO's have a habit of not tightening the two almost inaccessible manifold nuts between the Intake and Exhaust manifolds on the 1500.

Victor Michael added:

The process of adjusting the automatic choke is quite simple. I'm not sure I can explain it as well as the Haynes manual, but I'll try.

- remove the entire automatic choke assy from the carburator
2 coolant hoses
3 screws
- remove the center bolt from heat sensing chamber (couldn't figure out why)
- remove the heat sensing chamber and heat shield (black plastic piece)
3 screws
- inside the main choke linkage housing, inspect the cam and 2 pistons for breakage or gunk I also cleaned everything and put some grease in there
- with a new gasket, mount the main choke linkage back onto the carb
- set the gap between the cam and pin to .500mm by adjusting the fine idle adjuster and/or fast idle screw
- make sure the pin is not holding the cam at this point
- place the heat shield onto the main choke housing, positioning it carefully - making sure the tab protruding from the main choke inserts into the square notch on the bimetallic spring. Test by turning the heat shield slightly to tighten the spring, it should recover or you should feel some force moving it back to position
- replace the heat sending chamber onto the heat shield, leaving the 3 screws slack
- align the notch on top of the heat shield with the notch on the main choke assy
- tighten the center bolt
- replace the coolant hoses

You can now start the engine and adjust the fast idle by turning the heat sending chamber and heat shield. When you're satisfied, tighten the 3 screws on the sensing chamber/heat shield. Check out the Haynes manual on this, it's more concise.


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