changing the points : MGB & GT Forum : MG Experience Forums : The MG Experience

Re: changing the points #21
kimmie Kimberly Rae Briar Hill, Victoria, Australia 1970 MG MGB MkIII "'Blue'" 1998 Mercedes-Benz C-Class "The Merc"	Posted: July 11, 2012 12:23AM Registered: 2 years ago Posts: 511

If the rubbing block is always in contact with the cam, then the moving point would always move the same distance, and the gap would have no adjustment. At the lowest point of the cam the rubbing block is not in contact.

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Re: changing the points #22
the omega man phil wilkins staffordshire, United Kingdom 1973 MG MGB GT	Posted: July 11, 2012 12:28AM Registered: 2 years ago Posts: 1,738

Isn't that why you have to adjust the points gap,because the rubbing block wears away over time,and the points will then not open as much as they should.
Didn't realise that the timing on a B was so sensitive tho.I must admit.

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Re: changing the points #23
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: July 11, 2012 12:34AM Registered: 4 years ago Posts: 5,284

In reply to # 2116907 by kimmie If the rubbing block is always in contact with the cam, then the moving point would always move the same distance,That is correct. and the gap would have no adjustment. That is incorrect. The adjustment of the gap is done by moving the other contact. At the lowest point of the cam the rubbing block is not in contact. It should be. It's held in contact by the spring. If it isn't something is wrong.

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Re: changing the points #24
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: July 11, 2012 12:39AM Registered: 4 years ago Posts: 5,284

In reply to # 2116908 by the omega man Isn't that why you have to adjust the points gap,because the rubbing block wears away over time,and the points will then not open as much as they should.
Didn't realise that the timing on a B was so sensitive tho.I must admit.

You're right on why you need to adjust the points occasionally, and you're right that the timing on a B, or any other car, isn't that sensitive.

Member of The International Society of Luddites (Unrepentant Chapter).

Take the time to understand what a part does and how it does it, then you'll have a better understanding of how to fix it when it goes wrong. Beats the scattergun approach every time.

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Re: changing the points #25
kimmie Kimberly Rae Briar Hill, Victoria, Australia 1970 MG MGB MkIII "'Blue'" 1998 Mercedes-Benz C-Class "The Merc"	Posted: July 11, 2012 12:55AM Registered: 2 years ago Posts: 511

The moving point travels from the other point to its highest, a distance of about 15 thou, if adjusted correctly. However they may not be adjusted correctly, and the travel maybe other than 15 thou. The reason this adjustment is possible is because the inner point stops the travel, and holds the block off the cam.
All this is beside the point. I know the Kettering system is not that sensitive, the car will run reasonably with the timing 5 or even 10^o off, but if you want it right you need to set it.

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Re: changing the points #26
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: July 11, 2012 01:12AM Registered: 4 years ago Posts: 5,284

Phil, are you up for an experiment?

Seeing as you're going to change your points how about

Put the timing light on it and note where it is.

Change the points and check the timing again.

Go for a drive.

If the timing is different after changing the points, set the timing correctly.

Go for another drive over the same route as before.

Does the butt dyno notice any difference?

Member of The International Society of Luddites (Unrepentant Chapter).

Take the time to understand what a part does and how it does it, then you'll have a better understanding of how to fix it when it goes wrong. Beats the scattergun approach every time.

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Re: changing the points #27
RAY 67 TOURER Ray Marloff Fort Bragg, CA, USA 1967 MG MGB "My Girl"	Posted: July 11, 2012 01:23AM Registered: 3 years ago Posts: 6,032

Technically, both camps are correct, but the difference in the overall change in timing is minute. RAY

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Re: changing the points #28
JohnTF John F Northeastern Ohio, USA 1970 MG MGB 1970 MG MGB	Posted: July 11, 2012 03:19AM Registered: 1 year ago Posts: 843

Did a lot of these on my pre MG cars, have electronic ignition now.

Often when the cam follower wears, the point gap closes down, dwell/timing changes. In a perfect world, when you clean the cam with alcohol, give a little lubriplate on the cam and put in new points carefully gapped , there should be little timing change from the original setting.

It is, however, sop to check the timing after changing points.

Static timing can be done by setting No. 1 on TDC after the compression stroke and adjusting the distributor so that the points are just opening.

However, if you have a timing light and can find the timing marks, -- you will at least know where it is set.

I have set some by ear as well, and the TD manual said someone should be riding on the hood while you go up hill -- advancing until it knocks and backing off.

Ya pays your money and takes ya choice. I personally prefer the guy riding while going up hill. ;-)

+1 on the magnet, somewhere I have a tool with a magnet with a spring loaded slot screw driver on the other end to hold that little screw that seems to be drawn to falling down inside of distributors.

Never timed my old Chevy, the timing marks were on the back of the engine and I could not see the marks.

Regards, John

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Re: changing the points #29
dickmoritz Dick Moritz Philly 'burbs, PA, USA	Posted: July 11, 2012 10:38AM Registered: 3 years ago Posts: 7,412

Kimberly is exactly right. If you adjust the points so that the gap is very small and the dwell likewise very small, the rubbing block does not stay in constant contact with the cam, and when the cam finally does move the points open, it will be much later, retarding the ignition timing. The key is that the rubbing block does not stay in constant contact with the cam surface, and so if the point gap is set too small, it will take less "cam lift" to open the points, meaning that it won't be until a higher point on the cam separates the points, triggering the spark at a later time and retarding the ignition timing.

Similarly, if the point gap is set too wide, the cam will contact the rubbing block sooner to provide more "lift" to the rubbing block and movable contact, and when the cam opens the points sooner so it can open them farther, the ignition timing will be advanced.

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: changing the points #30
the omega man phil wilkins staffordshire, United Kingdom 1973 MG MGB GT	Posted: July 11, 2012 12:12PM Registered: 2 years ago Posts: 1,738

So Dick,you mention setting the gap too small or too large,but what if I was to set the gap spot on to what it should be?

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Re: changing the points #31
dickmoritz Dick Moritz Philly 'burbs, PA, USA	Posted: July 11, 2012 12:56PM Registered: 3 years ago Posts: 7,412

Well certainly that's what I would recommend. I was just trying to illustrate how point gap/dwell can affect ignition timing. IMHO it would be difficult to precisely, within a degree or two of dwell, to exactly replicate point gap/dwell before and after points service or replacement, especially with used points likely no longer at the precise specification, and that's why I recommend checking the ignition timing after points service and adjusting as necessary.

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: changing the points #32
pinkyponk Adrian Page Nova Scotia Canada, Canada 1971 MG MGB GT 1973 MG MGB MkIII	Posted: July 11, 2012 01:14PM Registered: 2 years ago Posts: 2,896

I think the best setting is by driving the car under load and adjusting it until it just stops pinking. A timing light is useful for setting "static timing" though. winking smiley

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Re: changing the points #33
JohnTF John F Northeastern Ohio, USA 1970 MG MGB 1970 MG MGB	Posted: July 11, 2012 01:32PM Registered: 1 year ago Posts: 843

Especially if you have a guy to ride on the front adjusting it while you drive uphill as in the TD manual.

;-)

John

In reply to # 2117291 by pinkyponk I think the best setting is by driving the car under load and adjusting it until it just stops pinking. A timing light is useful for setting "static timing" though. winking smiley

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Re: changing the points #34
mac townsend Fairfield, CA, USA	Posted: July 11, 2012 04:08PM Registered: 7 years ago Posts: 18,411

a change in dwell will change the timing. One degree change in dwell gives a degree change in timing.

As you say, dwell is how long the points are closed. If this is changed the points will remain closed for longer or shorter duration. that means they will be open for shorter or longer duration, respectively. and that in turn means the point at which they open must change. The total of degrees open plus degrees closed is a contsant value for a given distributor.

In reply to # 2116872 by oily-hands There seems to be some confusion between timing and dwell, or points settings.

The spark occurs when the points open. This is controlled by the cam on the shaft, the design of which is set by the manufacturer and is not adjustable without changng the cam. This is timing.

The dwell is the length of time the points are closed to allow the charge to build up in the coil before being discharged when the points open again. A wider gap means a reduced dwell and and less time for the charge to build up and vice versa. This is controlled partly by the profile of the cam, which isn't adjustable and the points gap, which is.

Changing the dwell will have no effect on when the points open.

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Re: changing the points #35
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: July 11, 2012 04:30PM Registered: 4 years ago Posts: 5,284

In reply to # 2117470 by mac townsend a change in dwell will change the timing. One degree change in dwell gives a degree change in timing.

As you say, dwell is how long the points are closed. If this is changed the points will remain closed for longer or shorter duration. that means they will be open for shorter or longer duration, respectively. and that in turn means the point at which they open must change. The total of degrees open plus degrees closed is a contsant value for a given distributor.

In reply to # 2116872 by oily-hands There seems to be some confusion between timing and dwell, or points settings.

The spark occurs when the points open. This is controlled by the cam on the shaft, the design of which is set by the manufacturer and is not adjustable without changng the cam. This is timing.

The dwell is the length of time the points are closed to allow the charge to build up in the coil before being discharged when the points open again. A wider gap means a reduced dwell and and less time for the charge to build up and vice versa. This is controlled partly by the profile of the cam, which isn't adjustable and the points gap, which is.

Changing the dwell will have no effect on when the points open.

The spark occurs at the moment the points open. How long they remain open after that isn't relevant to that spark as it has been made, but is relavant to the time for the current to build for the next one. The point at which the spark occurs is when the heel of the rubbing block is on the highest point of the cam, assuming things are set correctly. It cannot be otherwise.

Member of The International Society of Luddites (Unrepentant Chapter).

Take the time to understand what a part does and how it does it, then you'll have a better understanding of how to fix it when it goes wrong. Beats the scattergun approach every time.

Ignition testing made easy.

Making your MGB handbrake work

My You Tube Channel

Life with an MG TA and an MGB GT in the UK.

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Re: changing the points #36
dickmoritz Dick Moritz Philly 'burbs, PA, USA	Posted: July 11, 2012 05:09PM Registered: 3 years ago Posts: 7,412

Owen,

The spark occurs at the moment the points open.
True.

How long they remain open after that isn't relevant to that spark as it has been made, but is relavant to the time for the current to build for the next one.
True.

The point at which the spark occurs is when the heel of the rubbing block is on the highest point of the cam
Not true.

You must be smokin' the wacky weed, bud. If the points opened only when the rubbing block was on the highest point of the cam, then it would also close at the same instant, resulting in, essentially, continuous dwell and virtually zero time open. Rather than going back and forth any more, suggest you grab a points dizzy, and stare at the points while rotating the shaft, and the cam along with it. You'll see that the points open on the ramp, stay open through the peak of the cam, and close on the down side of the ramp. Don't take my word for it; just grab a dizzy and see for yourself...

And, all comments offered in good cheer... smileys with beer

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: changing the points #37
JohnTF John F Northeastern Ohio, USA 1970 MG MGB 1970 MG MGB	Posted: July 11, 2012 08:13PM Registered: 1 year ago Posts: 843

You may want to rethink that last sentence, dwell and point gap are directly related, and changing either will certainly change when the points are open as the only way I know to change point gap and dwell is to move the cam follower position. When the cam follower wears, the points close and if they close enough, you will certainly notice the changes in timing, etc. that follow. On GM cars, they had a window in the side of the cap that allowed you to use an allen wrench to adjust the points while watching a dwell / tach or other instrument.

Bottom line, normally when your car is otherwise in good repair, you can often get away putting in new points with a correct adjustment, which will restore normal dwell and timing. I had a car that probably did not have a timing light on it for 30 years because the marks were hard to see. I had another that was not running right, and I timed it by ear and a wood pry bar to turn the distributor.

Certainly if a pro is tuning up a car, he should swap the points, condenser, plugs, adjust the points, check the timing, and also check and adjust the carbs.

Regards, John

In reply to # 2116872 by oily-hands There seems to be some confusion between timing and dwell, or points settings.

The spark occurs when the points open. This is controlled by the cam on the shaft, the design of which is set by the manufacturer and is not adjustable without changng the cam. This is timing.

The dwell is the length of time the points are closed to allow the charge to build up in the coil before being discharged when the points open again. A wider gap means a reduced dwell and and less time for the charge to build up and vice versa. This is controlled partly by the profile of the cam, which isn't adjustable and the points gap, which is.

Changing the dwell will have no effect on when the points open.

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Re: changing the points #38
Robert97526 Robert Clare Grants Pass, Oregon, USA 1974 MG MGB	Posted: July 12, 2012 01:01PM Registered: 1 year ago Posts: 109

.......never realized so much could be said about changing points and timing the engine. Sounds like lots of differing reasoning for arriving at the same place i.e. a well timed and tuned distributer/engine. Remember first learning about static timing of my first MGB too many years ago.....what a beautiful thing it was untill my know it all drag racing brother introduced me to the notion of the timing light and dwell angles.....it was never the same after that.

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