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MG MGB Technical - Exhaust note on 79 MGB LE

The exhaust note on our 79 MGB LE has always been a smooth and regular beat. I notice now at idle the exhaust note seems like it is spitting. Carb is original Zenith Stromberg and have never touched it as plugs have been toasty brown. Now they are a greyish brown. Car still drives well, but am I right in assuming that maybe I now have a lean mixture. What would cause it to change? Diaphram in carb looks ok. Just like some comments before I start playing with it.
Kim R.
K Rutherford

You certainly can get a lean mixture from no or low oil in the dashpot. Try adding some oil and then rechecking the level after driving to see if you need to replace the "O" ring and star washer due to a leak here... a common problem with the ZS.
Frank Mooring

Have replaced the o ring, and keep an eye on the level.

Kim R.
K Rutherford

Is the centrifigal advance and retard still free to turn? They can sieze on the shaft.
Stan Best

Kim,
Have you checked the valve rocker clearances lately? Sometimes a case of valve seat recession can reduce the clearance on a few valves, giving a slightly choppy exhaust note, more noticeable at idle.

Ralph
Ralph

Dashpot oil level will only affect mixture during acceleration, not at a steady idle or driving.

Make sure the timing is correct, also points dwell/gap if you have points. Remove the vacuum advance pipe from the manifold, plug the manifold port, and suck hard on the pipe. You shouldn't be able to pull any air through, if you can the diaphrgam is punctured which will give a weak mixture. Some vacuum advance plates were pinned in position if that was the only way the car could get through emissions. If it isn't pinned then by sucking on the pipe with the distributor cap off you should see the points (or whatever) plate twist clockwise and spring back when you release the vacuum. Also check the range of centrifugal advance, although wrong timing is less likely to affect plug colour than mixture. Only when ignition is right do you start checking carb mixture.
Paul Hunt

Thank's to all, checked timing, dash pot,dwell,vacuum advance was not working as the line from the manifold was damaged and was not immediately visible, until closer inspection. I replaced all the rubber lines when we got the car as it was 1 owner and all was original. However these things do happen, replacement rubber was probabley off spec and does not seem to be high quality. Exhaust note has lost that splashy sound. As the car has A/C it needs all the power it can get. However cruising along in O/D at 75 mph it meets all our needs for long trips.

Kim R.
K Rutherford

Sounds like a punctured diaphram to me!
allan

Car is running fine now , replacement line from manifold to vacuum advance did the trick as I mentioned.

Kim R.
K Rutherford

This thread was discussed between 26/07/2008 and 30/07/2008

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