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MG Car Repair/Burnt out ignition warning light on 67 MGB


QUESTION: I was checking the point gap and timing (still positive ground, no mods) because my car was not running well.  When I restarted the car I noticed that the ignition warning light was dimly lit at idling RPM, OK.  But when I started to rev up the engine, the light became brighter.  At about 3000 RPM the light went out.  The bulb is burned out, checked it with an ohm meter.  I have a Bosch voltage regulator, at least 20 years old.  Could this be the problem? Can you make any commments as to the value of electronic point replacements (Petronix, i.e.)?

ANSWER: Hi Larry.  
Before you do anything else, try replacing the bulb.  They do eventually blow with old age.  A dim light getting bright and then going out is a classic description of a bulb blowing.  If the replacement bulb fails, get back to me.

As regards electronic points replacement, both my MGs have Pertronix conversions, so Yes, I do recommend them, so long as your distributor is still in good condition.  If the vacuum unit has failed or there is wear in the advance mechanism, then a Pertronix unit will not solve these faults.  If you do buy one, please remember to buy the positive earth version for your MGB.  

[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: I put a response on your site, but received no answer, so here it is again. I forgot to say that I put on the head lights after the bulb blew.  They didn't get bright when engine was revved, so the charging circuit is not working.  Later I shorted out the dynamo leads and measured the voltage, approx. 12 volts at 1500 rpm. I also took the cover off the regulator (it's a Lucas), but points look OK.  Any ideas?

Hi Larry.  Sorry I did not pick up your follow-up question.  I was not notified by the system and I can find no trace of it.

Have you checked the ignition warning bulb?  If the bulb has blown, then the charging system will no longer operate properly.  The solution to your problem could be no more than a blown bulb.  

You did the right thing connecting the two dynamo terminals together then running the engine, but keep the speed down.  The 12 volts at 1,500 rpm is about right, possibly a bit low.  Try raising the engine speed to about 2,000 rpm.  You should get a higher voltage.  Do not allow the voltage to exceed 14 volts when you do this test.  If you cannot get more than 13 volts, then the dynamo needs an overhaul.

Your MGB should have a Lucas RB340.  This type of voltage regulator rarely goes wrong.  It is permanently connected to the battery, so do not clean the points without first disconnecting it from the battery.  Just pull the two (brown wire) connectors off the B terminals.

The RB340 has 3 sets of points.  They are:

Left - Voltage regulator.

Middle - Current regulator.

Right - Cut out.

When cleaning the points, DO NOT use emery cloth, it is much too harsh.  The cut-out should only be polished with fine glass paper.  The other two can be polished with 800 grade wet-and-dry paper or a fine carborundum stone.  Finish by cleaning them with alcohol.  

If you need to adjust the points, get back to me for further advice.

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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