Idling too rich...

[BigBird]

Our '77 MGB, with a single Zenith, is running too rich at cold idle. Once the car warms up to normal operating temperatures, it is fine. I have attempted to adjust the course and fine idle screws, but they seem to have little effect at correcting the problem. Any suggestions would be greatly appreciated. Thanks.

 

[Nunyas]

Adjusting the choke richness is a task that requires more than just adjusting the idle speeds. I've not attempted to adjust the choke richness on my ZS equipped car, but if I'm not mistaken, both the Haynes MGB manual and the Haynes Weber Carburetor (includes SU and ZS carburetors) manual describe how to adjust the choke richness.

Of course, there may yet be someone else on these boards that's done it before and may be able to help you out directly.

 

[DrEntropy]

Aren't there "index marks" on the bi-metal spring housing/carb body? I can't recall the exact process, but I'd be first making certain all the bits were working and in good nick by disassembling the ZS and going thru it carefully/completely. They're fine when they're right but can be a bugger if some internal bit goes on th' blink (read: spring, diaphragm, by-pass valve seat, etc.). Running too "fat" (over-rich)when cold does sound like a choke malfunction, but I'm surprised it clears up when the engine is up to normal temp.

 

[AlanT]

I see most of the forum is staying away from this topic and for good reason. Here is what I would do if this car was brought to me with the symptom described.

First remove the three #2 posidrive screws from the top of the 'automatic fuel enrichment unit' . Two screws will be easy and one will be half covered by the original air filter housing. With the engine cool , see where the machine-fit piston is in the bore. It should be close to the top. Make sure it will easily move up and down in the housing. Next step is to start the car and see if said piston pulls down into the bore. If it does not pull down all the way then either the mounting gasket is bad or the vacuum orifice is clogged. If it does pull down then you will need to follow the adjusting procedure in the Factory or Bentley manual . If you moved the 'idle' screw with the locknut then you must go through said procedure anyway as this screw should never be moved except when performing a full overhaul. Try this and write back. There is plenty more to cover if this does not help.

Alan T

 

[DrEntropy]

Alan: This is why I "hinted" at an R&R for careful/complete rebuild. Better to "know" everything is clean and clear, all settings and adjustments in accord with spec's. It actually takes less time for removal and rebuild than it would to try and troubleshoot it bits at a time, doesn't it? I'm not trying to kill a fly with a shotgun here, just figger it'd be less agro in the long run. Bentley's step-by-each descriptions are great and the secondary effect is (for all practical purposes) a brand-new carb.

 

[AlanT]

Right Dr. E , It is very easy to remove the carb. But checking the choke pull-off should be done while on the car as it is difficult to simulate a running engine on the work table unless you have a flow bench handy. The shop-vac works well , though.

Alan T