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MG MGB Technical - Eratic idle after cruise

1979 mgb roadster, stock engine, stock ZS carb (rebuilt 6 mo ago); choke rebuilt per John Twist procedure (with excellent results, by the way);just flew through smog test with flying colours. New cat converter and exhaust system, rebuilt vapor canisters, all new vacuum lines throughout engine bay, no vacuum leaks detected at carb to manifold joints, or at manifold to head joints. Air pump running, anti run-on valve functions perfectly, vacuum advance connected via solenoid as required by smog laws, functions fine. New coil, wires, cap, rotor; electronic ignition fitted. Carb damper topped up with 20w50 oil (same weight as engine is running at present).

After cruising along 45-55mph for any duration, when vehicle returns to idle, the engine runs very "lumpy" and eratic - a blip of the throttle seems to smooth things, but if left alone, will return to this eratic state again. Seems to pull just fine going up through the gears, and cruises along without a miss.

Any bright sparks ?? With all of the above work done, I am starting to run out of ideas !!

(And please, no "isn't "Eratic" the brother of Eric?" cracks !! I know how strong the temptation must be...:)

Cheers !

Chris
Chris

Chris,

How does it idle, *before* the drive?

I play with SUs but just a guess -- a dirty jet/needle on the idle side. Perhaps a worn O ring, diaphram. Something small you'd only notice now, at idle, that'll go real wrong later.


I'll look forward to more smarter people's input -- so I canalso learn.


glg
glg gimbut

The gulp valve may be leaking, or the restrictor on
the inlet side of the valve may be leaking. This
will excessively weaken the mixture by injecting
air into the inlet manifold on overrun, i.e. right
after a high speed run.

The EGR valve is also a suspect.

I think the gulp valve is the strongest suspect,
but as glg suggests, the problem could also be
dirt on the needle or main jet.

I like to take off the cover on the Z-S carb
along with the air piston/diaphragm, and
spray carb cleaner directly into the
jet to blast things out. This seems to help
with minor uneven running sometimes.
Ronald

This thread was discussed on 28/03/2003

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