AMGBA website home page or AMGBA Photo Gallery or AMGBA Club Blog
Subscribe in a reader

Join or renew today and receive a free t-shirt or tech CD, see details in the join the club section at!

Author Topic: Loss of Power  (Read 310 times)

0 Members and 1 Guest are viewing this topic.


  • An AMGBA Club Member
  • ***
  • Posts: 30
Loss of Power
« on: February 14, 2021, 12:53:51 PM »
I have a 1974 MGB that has a loss of power at about 2500 rpm.   The car starts fine and runs fine at low rpm but as soon as I get it up to 2500 rpm it chokes out.  If I take my foot just slightly off the gas it is OK.  But if I try to go faster it does it again.  I had this problem last year already so this year I decided to remove the carbs and clean them and set them back up. Also lapped the valves and adjusted. None of which fixed the problem.

I was wondering if it could be the fuel pump or coil. Please let me know what you think.

Heinz Kettler


  • AMGBA Club Tech Staff
  • ****
  • Posts: 121
    • University Motors
Re: Loss of Power
« Reply #1 on: February 14, 2021, 01:08:36 PM »
here are many problems you could be having, but from here, Id put my money on a faulty fuel supply.  Remove the fuel feed to the carb, put it into a can or bottle, turn on the key, and expect to pump ONE pint per minute.  If the line just dribbles, then there is a problem with the fuel filter, fuel pump, hoses, or the tank.

If the fuel supply is just fine, then I would look at the point gap next.  Ensure the points are opening 0.015.

After that, back to the carbs as the floats in the HIFs can be installed left to right and that results in low fuel in the carb float bowls.

Hope some of this helps!
John Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids MI 49506-2410
Cell  616 307 6737
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


If you have found my technical advice helpful, please consider making a contribution to help defray my online expenses.  Go to my website and click the PayPal button:


  • chfwrench
  • AMGBA Club Tech Staff
  • ****
  • Posts: 290
  • Membership Number (if known): 91-10014
  • MG information: '73 red B roadster
Re: Loss of Power
« Reply #2 on: March 18, 2021, 08:06:26 AM »
Hi Heinz,

I'll add another to John's comments:  Check the vacuum and mechanical timing advance systems of the distributor.  Both the older 25D4 and later 45D4 distributors can have the vacuum advance pots fail and that is usually in conjunction with (or because of) binding in the mechanical system inside the distributor.  Either of which could be in your B. 

The classic signs of the advance system failure is the engine stumbling, sputtering and bogging under hard acceleration, just as you describe.  In the 25D4 unit, the typical fail is that the actuator arm connecting the moving plate in the distributor to the vacuum pot disconnects from the pot, so leaves the distributor working on mechanical advance only.  That only performs better at low speeds and slow acceleration.

In both distributor instances, if you open the cap and can move the plate easily by hand, chances are the vacuum advance unit is faulty.  If you can't, it should still warrant taking it further and hooking a vacuum source (brake bleeder or Mighty-Vac unit) to the vacuum line of the pot and see it moves then. 

If it still doesn't move, maybe dismantling the distributor to see if it is the pot and/or if the plate is seized.  The plate being seized can happen from rust or even from using the wrong length screws in securing the points or condenser.  A long screw can jam the plate to the distributor and prevent it from moving freely.

I hope that helps you.  Good luck.

Art Isaacs
AMGBA Tech Staff


AMGBA website home page or AMGBA Photo Gallery or AMGBA Club Blog