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MG MGB Technical - Losing points gap

Hi all and thanks for having this site here.

I acquired my 1973 mgb roadster last November. It is in basically good shape with the body having been restored with a down to the metal restoration about 10 years ago. It has since been kept in a climate controlled garage so is has remained in pretty good condition.

Although the body was well cared for, things are a bit rough under the hood. The compression in all cylinders is reading 105 and I think that would be 160 in a new or rebuilt engine. Also, one mechanic thinks he can hear an air leak between cylinders 3 and 4 when doing a compression test with all plugs removed. He is recommending the head be pulled to see what is going on. I'll do an air pressure leakage test on all cylinders before I get into that though.

Before I do that I would first like to get it running relatively well and use it for a while. The problem is that I lose the timing within a quarter mile of driving after having set it correctly. Vacuum advance appears to be working correctly as when I rev the engine I see about 15 to 20 degrees of advance with the timing light. I'm setting the timing to 10 to 11 degrees before TDC.

When I first got the car it was running VERY poorly. We found the points were almost fully closed so adjusted them and the car ran pretty well for a while......but the choke needed to be out about an inch or it would sputter and have no power.

Since then (and the car was parked for the winter so I'm just now beginning to use it a bit) I found that no matter how tight I turned the locking screw on those points the gap would close up when driving..... especially under high (say 3500 to 4000) rpm.

So, I replaced the points and thought they were holding. As such I finally adjusted the timing and expected that to end my troubles. As I said up front though, the timing isn't holding and I'm now suspecting the points remain the culprit.

If I'm correct and even the new points are closing down under use does anyone have any ideas on what is happening? Again, the points locking screw is dead nuts tight.

Thanks,
BH Davis
BH Davis

Hi, BH - the most obvious would be a very rough distributor cam that is wearing down the rubbing block on the points as the car runs.

If the points are, indeed, moving with the locking screw fully tightened, you can verify that with a scribed mark around the outside of the points onto the base plate. Never heard of that, though I'm too old to believe that nothing is impossible on our lovely little cars. Maybe another washer under the head of the screw?

If it's the block wearing away, the only solution I know of is to polish the cam surface with succeedingly finer grits of emery paper, finishing up with 1500 or 2000 grit for an absolutely smooth surface. Pay particular attention to the outer 20% of each lobe, and don't polish one more than the others, to keep dwell consistent from cylinder to cylinder. Then, a quick smear of grease on the cam, and you're good to go.

Best of luck - - Alec
Alec Darnall

I would also check for worn distributor bushings
as it can give errant points gap settings.

Grasp the distributor cam and wriggle it. There should be very little sideways slop.

Are you using a high voltage coil? Some coils
require a ballast resistor - without it, the rubbing
block on the points can siften and melt.
Daniel Wong

Here's another possibility. Your dwell will be all over the board of the center pivot on the points plate is worn. If you can remove the points plate and squeeze the two halves near the condenser until they touch, then the center pivot is worn to a point where you will never be able to keep a consistent dwell adjustment. When its worn that far, you will also have an issue of the condenser screw rubbing on the lower plate, slowing the rate of advance. For $75 plus shipping you can convert to a pertronix system and never have to worry about that again!

Another points question for you - do you have the lockwasher and flat washer on the points screw? many times one or the other will get lost or discarded. Also, are the weights free and well lubricated? Binding/sticking advance weights will also make the car run poorly.

Also check the dashpots on your carbs if you run SUs. Make sure they move freely and the oil is topped up!
Jeff

Jeff Schlemmer

Most likely the bushings are worn
Sandy
Sandy Sanders

Wow.....all this response in just a couple of hours! You guys are great.

It's late here and been a long day. I'll read all these again in the AM and give it some more thought.

Quickly though, I thought the cam felt smooth (so shouldn't be wearing the points rub knob that fast) and I greased it as well. I too thought it might be wearing down.

Also, I didn't think the cam was loose, but will look much more closely at that. I have a friend coming over tomorrow who is much more of a mechanic then I so that should be a help.

Thanks again for your time and help!

BH
BH Davis

According to Haynes manual and I believe the official factory manual, a spring and flat washer should go under the screw holding the points down on the distributor. Are they really needed to keep the points in place? Could you substitute a lock washer for the spring and flat washer?
jerry19

A moving gap will alter timing due to the physical design of cam and points, which is why timing must always be checked/adjusted after dwell/gap. Poor quality points can have the heel wear very rapidly. Is the screw is still tight when the gap has closed up? Can you lever the points open/closed using the cut-out in the fixed part and a screwdriver with the screw tight? This is the cut-out to be used to make fine adjustments to the gap with a partially tightened screw. If so maybe it is the wrong screw and isn't screwing far enough in to fully tighten them. I always use dwell for checking points adjustment, avoids transferring oil from a feeler gauge to the points which can cause a misfire, and only takes a moment. I find my points need no adjustment between replacement intervals, and currently both cars have done over 10k miles on their present sets.
Paul Hunt 2

This thread was discussed between 06/05/2006 and 08/05/2006

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