MGB Project car done: idle variable

[joepf1]

To all:

This is my second post this am. Just finished a project car and want to discuss issue #2;

My car is a 1978 MGB Zenith carb. I've spent a lot of time with this carburetor and actually have respect for its design. It has a water choke which is have re-calibrated necessitated because of its age, I presume. The carb is reconditioned. The issue I'm having is related to the idle. I have the carb set fairly lean but not so lean as to affect the performance of the car. I've played with different idle setting primarily using Twist's instructional. In its current setting the car starts great! I have the high idle set to 1800 but it doesn't get there until it's somewhat warm, but not hot. As the car reached operating temp, when I blip the throttle, it goes to about 900 rpms. I think the idle is a little rough but its acceptable. When I run kt for a time, I notice that the idle can go up to as much as 2000 and sometimes go to 500 or 600 rpms. I can't seem to get it stable. All the while the engine winds smoothly and powerful, for its size. I think the issue is related to the heat mass/choke assembly. I've checked the water level. It has a new water pump and therm. Installed is a new electronic distributor. The valves are adjusted and I've replaced most of the emission tubing. I have restored the car to its original specs and equipment to get the it "smogged" to California standards. It passed with flying colors, no small feat.

Does anyone have an suggestions as how to alleviate this variable idle issue?

Thank you, Joe Ferrari

 

[DrEntropy]

Start with the throttle return springs. Another possibility is the throttle shaft itself. If it has any play in its bore, you may be looking at re-bushing it.

 

[joepf1]

This is a Zenith Carb. The return springs on the throttle shaft are intact. This carb does not have the springs that were hooked to the air intake. The bushings seem tight. When is push on the shaft while running, no change.