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MG MGB Technical - eratic idle ??

Some advice, please. I am at the end of my rope...
1979 MGB, US CAL spec (all the emissions goodies, including air pump(functioning and connected), vapor canister, gulp valve, etc.) Vehicle has 118,000 miles indicated. Has been going through steady process of renovation this past year, including replacement of all rubber on car, such as vacuum lines, including brake booster. Also new (closed) oil filler cap.
Vehicle has rough idle, engine rocks violently from side to side at 800 to 1200rpm. Above 1500rpm, seems to smooth out. Replaced all engine mounts, thinking they had perished along with other rubber parts on car. Rocking is so violent, cat converter smacks steering column, even with spacers on drivers side engine mount. Carb has been cleaned; new glass fuel filter, with fuel visible. New fuel pump. Car starts fine, no hot starting problems. Crane Electronic ignition fitted by previous owner. New HT leads, cap, rotor fitted. Starting to run out of ideas !! Car has original Zenith Stromberg carb fitted. EGR valve functioning correctly. Car recently flew through state-required smog testing, very low emissions reported, so assume emissions control systems are all working properly. What the devil could be causing such a violent thrashing about at idle ? I am all ears.

Cheers
Chris
C Craven

Eratic Idle, wasn't he in Monty Python?
philip may

I've had lots of problems with air leaks on my 79. You might
first check with a vacuum gauge -- below 17" at idle would indicate
an air leak, which if bad enough could cause a really rough
idle. At higher speeds, the air leak makes little difference in
mixture hence the engine smooths out.

Check EGR valve, gulp valve, and anti run-on valve, as
well as distributor vacuum capsule for air leaks. I've had all these
items fail on my B. You can use
a mityvac pump to check all devices except the anti run-on
valve which won't hold much vacuum. Blow gently
through the manifold connector on the run-on valve, if you
can feel air escaping through the bottom its leaking.

Also check for intake manifold gasket leaks by spraying with
carb cleaner at idle, if smooths out or change in vacuum that's
the problem.

I'm assuming ignition is fine and no bad valves since you
passed emissions. I doubt that a missfire severe enough to
cause your symptoms, or a bad exhaust valve would allow
you to pass the test.
Ronald

Chris. I agree with Ronald on most of what he says. I had a similar rough idle and it was tracked to an intake manifold leak. In this case, where one of the vacuum lines attached. This caused a visible miss in the engine, but not such as you describe. The engine was not thrashing about sufficiently that the cat was hitting the side of the car, nor was the car capable of passing emissions testing at that point. I agree that a vacuum check is in order. I would also recommend a compression check and a timing check. If there is a good mechanic in your area, I would put the car on an engine analyzer to see what it shows. There is a significant problems somewhere if your engine is shifting sufficiently for the cat to strike the steering column. I measure about 1/2" clearance between the two on my car. This is a very significant amount of free movement and the cause needs to be investigated and the problem cured or engine damage could result. The cost of paying a professional to diagnose and fix the problem is far less expensive than correcting any engine damage that might result while you are trying to figure out what the problem is. Les
Les Bengtson

I agree with Les that this is a good time t o take the car to
a mechanic.

Also check out www.mossmotors.com. Select tech tips -- MGB --
engine -- rough idle (you have to log in but registration is free.)

Describes vacuum leaks that are bad enough to cause
shaking and destruction of motor mounts.
Ronald

Thank you, Ron, I had forgotten about that tech article at Moss. I could use a refresher on this problem too.
Bob Muenchausen

Apologies if I insult you but are you absolutely sure that the firing order is correct (1342 *anti*-clockwise) and all cylinders are firing?
Paul Hunt

With respect, if the firing order is wrong then at least two cylinders won't be firing so the engine is unlikely become any smoother as the revs rise.

One cylinder not firing is probably not enough to make the engine shake so violently unless the mounts are already shot.

With all of the emissions gizmos on the US cars, then a vacuum leak has got to be a high probability. I had an inlet manifold leak on my UK car; the engine ran fine at speed but ran very rough at idle. Depending on where it is, an air leak could make the engine run weak and hence very low emissions?

Neil

Neil

As the revs rise it still missfires as much but the engine cannot shake as much - inertia, and all that.
Paul Hunt

Submitted reply, but may have messed it up.
Just in case - your suggestions paid off. Using the old carb cleaner ploy, I located a major leak at intake manifold to head joint near cylinders 2 and 3. Did an exhaust system replacement earlier; must have not torqued those nuts tight enough. Anyhow, thanks for the advice and support. This is by far the best MG BBS out there. Cheers, guys.
CRC
C Craven

This thread was discussed between 07/12/2001 and 10/12/2001

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