You are here:

MG Car Repair/MGB runs rough


QUESTION: hi Barrie
After installing the new cylinder head, and setting all valves at .015 I started the car, and found it idles ok, but stumbles and runs rough with gas. It also backfires out the rear carb intake when giving it more gas. My carb settings were perfect before so I do not suspect them. I have electronic ignition.
I checked the firing order and the vacuum port which comes off the carb.  Is backfiring an ignition related item? Could it be valve timing?

ANSWER: Hi Paul.  There are several reasons why an engine may backfire.  

Here are a few to check:  

1) Lack of oil in the carburettor dashpots

2) Ignition timing.

3) Air leak in inlet or exhaust system.

4) Two of the HT wires to the sparking plugs have been swapped by mistake.

5) The two LT wires to the coil have been swapped by mistake.

Let me know what you find

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

QUESTION: Barrie - you were right on with swapped plug wires! 2 and 4. Runs smooth and project cylinder head a big success. One issue for your comment: When I was torqueing down the head to about 40 lbs., the #10 stud snapped off at the block! Needless to say I was shocked as this was a new stud. I managed to back out the broken bit from the block, and replace stud with the original. I have torqued head to 45 lbs. Engine was run hot and cooled down.

Q: should I loosen all the bolts and start over or can I simply tighten up to 55 lbs?

thx P

Hi Paul
Glad it was so simple in the end.

I am a bit worried that a stud snapped at 40 ft lbs.
My book says to tighten to between 40 and 50 ft lbs, so 55 would be too high.

Having heat cycled the head, you can now re-torque all the nuts.
Do not loosen them first.
Starting with the middle two and working outward in a circle,
re-tighten them all to 40, then to 45 and stop there.

I recommend a pattern like this:

7  3  1  4  8

10 6  2  5  9

MG Car Repair

All Answers

Answers by Expert:

Ask Experts


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).

©2017 All rights reserved.