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MG MGB Technical - '77 B- Backfire and then runs rough

I recently purchased a 1977 MGB. It ran fine for awhile and then suddenly started running rough at idle, and then not running at all. I rebuilt the carburetor, replaced the plugs and air filter, and things were going great again...until last night. The car drove great, I turned it off and it dieseled slightly and then made an unusual-sounding backfire. When I started it again, it was missing, running rough, and had a faint metallic-sounding grinding noise (similar to worn brakes).

It was idled up a little high, and needed to be adjusted. My guess is that it's either the timing belt or I threw a rod? Any suggestions on where to start?
C Black

CBlack;
The MGB motors use a timing chain settup, so suspecting belt is out. If it broke, it would not start at all, and very possibly cause a few valves to be bent .I think I would remove the valve rocker cover and check to see if anything is amiss, broken valve spring, stuck valve, broken pushrod.
However first pull the spark plugs out and bring each piston to top, and take a careful look at each cylinder thru the plug hole with a flashlite. You'll be able to see part of the piston top.You may be able to see if you have a cracked or holed piston. Or, there may a broken piston ring which has galled the cylinder bore(which you cannot see). Turn the engine to lower the piston and again look thru the plug hole to see what you can of the cylinder wall.Inspect for galling.This is not conclusive, as you have a limited view. A thrown rod most likely would cause a definite knock rather than a grinding noise.You will know this if a piston does not come up to tdc in one of the cylinders. If nothing is visible by now,take a compression check. If cylinders are developing equal compression, valves timed ok, I would drop the oil pan first to look for trouble. If nothing found there, then remove the head.
Please let us know what you find.

Pete H
Pete Haburt

CBlack;
I have not followed my own rule of looking for simple and obvious first.
After writing the dissertation of major takeapart above, I remembered my experience with the air pump on my 76B several years ago. On the passenger side of the engine, above the alternator is a smog air pump. Remove the belt and start the car, if grinding noise stops, that is where that is coming from.It is possible that dirt or carbon has been blown loose in the air pump from the surge of the backfire, causing the noise. Or one of the phenolic vanes within has broken. You can run without the belt connected. If you have to have smog checks, you'll have to repair or replace before the next inspection.
Next, check all the vacuum hoses coming off the intake manifold and carb. They become brittle with age, and the backfire may have blown one off, or cracked one, creating a leak which would cause it to idle rough, and possibly stall. There is a larger hose coming off an elbow on top the intake manifold,forward from center of the intake manifold, that goes to a gulp valve. Check that elbow, it is a press fit into the manifold, and has been know to come loose, again creating a sizeable air leak into the manifold. If it is loose, smear some Permatex or RTV on it and push it back in snugly.
These are much easier to check, and are more likely to be the culprit. After I posted the previous text, I found it hard to believe a backfire could cause that much damage.I apologize if it caused unnecessary anxiety.

Pete H
Pete Haburt

This thread was discussed between 19/04/2002 and 20/04/2002

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