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MG MGB Technical - Distrubutor alignment TDC

I am changing out my 25D for another and prior to removing I have read many postings.My Dilemna,
Currently with timing marks aligned to TDC the rotor points to #4 plug wire, 10 oclock
The engine runs fine save for the fact that I cannot get an idle below 1500 and when I retard to get good idle I have no power. Also intermittently the car dies at idle no matter what I set.There is considerable slop in the Dist mechanism.
Everything I have read says that the rotor should point to #1 at TDC,hence when I install the new dist I am wondering if I should simply align as is,and is it possible for timing marks be out of sync. The engine has been rebuilt prior to me owning 10yrs ago, and I have driven it seasonally every year.
any thoughts
Greg
g e granstrom

Hi Greg,

The dizzy is fitted correctly or the car would not run. What you need to do is to turn the crank another 360 deg and you will find that now the rotor points to #1.
TDC is true for both #1 and #4 at the same time, they move up and down in sync, however, the difference is when the spark is fired, when #1 is on compression #4 is on exhaust.

Herb
Herb Adler

Hello Herb,
Thank you for the response! Once again I tried to make things more difficult than they are!
Cheers
Greg
g e granstrom

You don't adjust the timing to control the idle, you adjust the idle screws. If you find they are backed right off but the engine is still running (it should have stalled) then something else is holding the butterflies open, like sticking throttle cable, insufficient slack in the throttle cable, misaligned interconnecting shafts such that one butterfly closed is holding the other open, choke fast idle screws holding the butterflies open, butterflies not seating properly in the carb throat, or throttle shaft and bushings so worn that mixture can get past the fully closed butterflies.

Slop in the distributor, if you mean the rotationally in the rotor, means the springs are stretched or the short spring has become detached. If you mean sideways in the shaft then the bushings are badly worn.

Dying at could be any number of things. If it doesn't happen when driving then it's quite possibly a sticky throttle cable (causing a high idle) suddenly releasing.
Paul Hunt 2010

Hello Paul,
Thank you for your input.I have pulled the Dist today and on investigation the weights were not moving freely and not going to either stops.The vacumn advance moving plate was also worn and had very little travel. As to slop the shaft had play up and down as well as side to side.

Needless to say I have swapped the Dist for a new one, I just finished firing the car up and will fine tune tommorrow.

As to idle thanks for your advice. I have backed off the adjusting screws right off, the cable has plenty of slack,choke screws backed right off, pistons move freely. Your suggestion re throttle shafts is probably my next move.However

Apparently the Dist was due for a rebuild at any rate and my efforts have not been in vane.

Again thank you for your help
Sincerely,
Greg
g e granstrom

Hello again Paul,
Just to reaffirm your suspicions.I had thought I had tested every joint for vacumn leak, however today I sprayed carb cleaner between the block and the intake manifold and low and behold,leak. The manifold bolts behind the heat shield were loose to the fingers. I am amazed it ran at all. But then it is a MG.Thks
Sincerely
Greg
g e granstrom

OK, you found a vacuum leak. But I would still reiterate that the idle screws should be able to 'compensate' for that. If the engine was still running *at all* after backing off both idle screws then one or more of the other problems still exists.
Paul Hunt 2010

This thread was discussed between 07/06/2010 and 14/06/2010

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