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Author Topic: 1972 MGB acceleration problem in 3rd gear  (Read 6081 times)

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1972 MGB acceleration problem in 3rd gear
« on: August 12, 2009, 08:11:53 PM »
My 1972 B has developed an acceleration problem in 3rd gear.  It has plenty of power in 1st and 2nd, but will seem to starve out in 3rd under heavy acceleration. If you let up off the gas pedal, the engine will "catch up", power will return and the car will begin to accelerate again.  Timing, plugs, wires, and distributor all are ok, as is fuel flow with new pump and clear hoses/lines.  Any thoughts of what's left to check/replace/rebuild??    MGB176


  • chfwrench
  • AMGBA Club Tech Staff
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  • Memberhsip Number (if known):: 91-10014
  • MG information: '73 red B roadster
Re: 1972 MGB acceleration problem in 3rd gear
« Reply #1 on: August 18, 2009, 01:33:23 PM »
I know I have been saying this often, but you need to check the advance systems in the distributor.

That the timing at idle maintains the mark only shows one phase of the distributors' operation.  Under acceleration, it is the dynamic systems that make it all perform well and the Lucas distributors are especially subject to wear and failure that cause issues like you ar experiencing.

Set-up like you are going to set the timing.  Make sure the idle is low and the vacuum line to the distributor is disconnected and plugged.  Looking at the timing marks, accelerate the engine.  Under light (slow) accereation, the mark should move (advance) smoothly and recover to the original spot when you let off the gas if the mechanical system is functioning.  If it does not move, the mechanical system is not functioning.  Accelerating hard ("blipping" the throttle), the engine might stumble and the mark not advance so quickly.  That is to be expected as the mechaincal system is not curved to work that way.  The vacuum system should be.

Now reconnect the vacuum line.  The idle speed should pick up and timing mark advance slightly.  Now compare the perfomance.  Slow acceleration should be similar to the line being removed, but blipping the throttle or jamming the pedal down should be smoother.  If there is no difference, the vacuum advance system is not functioning correctly.  Either the pot is leaking or the arm is possibly disconnected or siezed.

If you have a "Mighty Vac" or similar tool, you can test the pot by pulling vacuum and seeing if it maintains or recovers with the tool still connected.  If tit recovers fairly quickly, the diaphragm is probably leaking.

You should also check for wear in the shaft and weights and if the arm to the pot (under the sliding plate) is binding or disconnected from either the plate or the pot (not uncommon).  Depending on what you find, you can get replacement parts or a new distributor.

Wheter you go with a Lucas replacement (the 45D is still available new) or one of the aftermarket units, look at going to an electronic conversion as well.  They are definitely more reliable and easier to maintain.  They can save you  some money and grief over your ownership of the car.

I hope that helps you.  Let us know what you find and post any question that might arise as well.  Good luck.

Art Isaacs
AMGBA Tech Staff


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