Vintage Cars: MGB Ignition, combustion chambers, lean mixture

Question
QUESTION: Jo,

After I switch off the igniton, the motor "runs-on" jerkily with "pinking".

I have tried upgrading to 94 octane but this still happens.

Any suggestions?

ANSWER: Sorry, I was away a few days.

I'm afraid you likely need a head-cleaning job, but try a few other things first.

1) Cooling. It's possible your head is not getting full water flow and getting too hot. Make sure your water pump is really pumping, and of course that the radiator is topped up.

2) Mixture. Lean mixture can lead to this. If the spark plugs are  pale and flaky, the mixture is too weak. Do a carb tune up.

2-1/2 (It can't be anything to do with timing: the ignition is off!)

3) Spark plugs. Crud on plugs can sometimes be the cause of ignition. If cruddy, scrub with wire brush, re-gap; or replace with new. Make sure you get the right plug type so they don't run too hot (check the manual, not the plugs that are in it).

Otherwise, I'm afraid the head should come off and scrape the combustion chambers clean, and the carbon ring at the top of the cylinders above the rings. But before you do that, do a compression check on all cylinders, just in case you need to combine the de-carbonizing with a valve job while you're at it. (Although obviously the dieseling won't be coming from a low-compression cylinder)

You don't tell me the year. A head job on an early B, pre-emissions stuff, will be pretty easy. The later they go, the more complicated.

Jo

---------- FOLLOW-UP ----------

QUESTION: Jo,  Thank you very much for your comprehensive advice.  Do you think any of the carb or cylinder cleaning additives may do the trick to get rid of carbon build-up?  The car is a 1968 MGB so there is no emission stuff.

Tony

Answer
I'm afraid I've never used them. There's certainly no harm in trying one or two, and at fairly high doses. Then take it out for what I call an Italian tuneup, preferably a long hill, well warmed up, at least 3000 rpm and foot on the floor for as long as possible! Then repeat. If clouds of brown stuff come out the tailpipe, be optimistic. Jo