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MG MGB Technical - dizzy setting

I'm a relative newbie to this MG world. I own a 1975 B & am trouble setting the advance curve.
I have got the white mark painted, have set the plugs, have set the dizzy & now want to finalise the ignition.

Here's my question.
When I turn the dizzy clockwise the white indicator moves ant-clock, which I have learned is correct.
The problem is, that I need to turn it quite a lot clockwise to achieve a nice steady rev (1500rpm). The best setting, appears to be approx 15-18 ATDC.
Why can I not be like everyone else & set it to BTDC?
Could the carb have anything to do with it.

Aubrey
Carl W French

aubrey schneider aschneider@shaw.ca

Talk about a newbie. I have no idea how Carls' address came up. Sorry Carl & all others
Carl W French

Don't adjust the timing for best running or idle.
Make sure the timing advances with rpm increase (centrifugal) with vacuum advance removed and total advance is more with it attached.
Set the timing to spec (ususally at specific rpm with vac. adv. removed) get rpm right by adjusting idle on carb(s).

After timing, then adjust carb for idle spec and mixture.



Mike Phillips

Just re-read your post, are you sure you're on the #1 wire? Its closest to the front of car.

Mike
Mike Phillips

Thanks for your response Mike.
Yes I am on #1 plug. I will try what you suggest, although I have done that before. Maybe I need to adjust the carb lean/rich setting as well. I just found it strange, that when I run at about 15 deg retarded, the car performs so well in all categories.
Thanks again

Aubrey
Carl W French

You might want to check that your damper pulley is still OK - take out No 1 plug and use a bit of wire to feel when the piston is at its highest - then check that the mark on the pulley is on or close to the TDC mark .

It is not unknown for the outer part of the pulley to move round on the rubber.
Chris Betson

Thanks Chris,
I'll check that as well.

I have to ask the question again however.

Why does the motor run so sweetly at 15 deg ATDC & so poorly at 15 BTDC? This of course assuming that TDC is correct.

Thanks again
Aubrey
A.R Schneider

Aubrey. It should not. The spark plug fires before the piston reachs top dead center so that the flame can travel across the top of the piston and creat the violent burning/explosion as the piston is on its downward movement, providing the downwards thrust to translate, via the crankshaft, into rotational movement. The time it takes for the mixture to ignite is fixed. It does not change with engine speed. However, the piston's speed changes with the engine rpms and the mixture has to be ignited earlier in the cycle to have its maximum effect at the same point in the downwards movement of the piston as the engine speed increases. This is why we have an "ignition advance curve" which increases the advance as the engine rpms rise.

Problems associated with retarded ignition timing (such as 15 deg ATDC) would include loss of power and overheating. The latter because the exhaust valve is opening as the mixture is still burning and the burning mixture is forced out of the pistons through the exhaust valves and exhaust ports on the head. With an actual 15 deg ATDC ignition timing, I would expect a top speed of less than 35 mph and a cylinder head temp reading of max. (This is from actual experience when a mechanic set the engine on my Bronco in a similar way to get it through emissions testing.)

So, if you are actually showing a 15 deg ATDC ignition timing and you car will actually run, there is some form of problem associated with the timing. As Chris notes, the delamination of the crankshaft pulley/harmonic balancer would be the first thing to check. Les
Les Bengtson

I've heard of the timing chain slipping. To be certain, you can remove the #1 plug and turn the engine by hand with a finger in the #1 hole. You can tell roughly if the cyllinder is topped out when the mark is in place.
Dan

I have heard of an odd situation where an engine can be assembyled/rotated 180 degrees out of the normal and still run, just not very well. How to check for this I am not sure.
RLF Fairey

Firing AFTER TDC? Naw!

Take a look at the two valves on #1, when they're both closed, and piston is at TDC, mark the crankshft pulley.

Or, Is it possible that the timing marks on the timing cover have been bent out of shape? so what you see is not really what it is?
glg gimbut

glg. When the two valves on number one cylinder are closed, it is on the compression stroke. That does not, however, indicate the piston is at TDC, nor is it accurate enough for timing purposes. The archives contain a number of ways to determine whether the timing marks are accurate and to determine what is actual TDC if the timing marks are inaccurate. Les
Les Bengtson

Thanks to all who responded. It seems as if my fingers are going to be telling.

The marking is accurate. That was verified by an authorised (?) MG dealer.

Lots to try.
Will keep you all posted.

Again thank you very much for all your help.

Aubrey
A.R Schneider

Mr Bengtson,

When the intake and exhaust vales are close AND the piston is at TDC -- two seperate elements --- check the crank pulley as to where it lines up with the timing marks on the timing cover.

As a static test -- which you may know mans the engine is not operating -- it was suggested ONLY as a means to determine if the timing MARKS on the cover were not akimbo.

In my 35 some years of tinkering with the internal combustion engine, I can not recall any engine firing at 15* ATDC.

It was not a suggestion on how to adjust OR set the timing.

glg
glg gimbut

glg. I am somewhat reminded of the story told about the general who came up to Napoleon and wanted to be raised to the rank of field marshal. The general pointed out that he had been in three major battles and, thus, deserved the promotion. Napoleon pointed to a nearby mule and said, "That mule has been in seven major battles. Should I promote him over you?"

The key question here is the "top dead center" point of the piston travel. This point is reached twice in the four stroke engine's cycle. At the top of the compression/firing stroke and at the top of the exhaust/intake stroke. Thus, the position of the valves have nothing to do with it except for the determination of ignition timing, which takes place at the end of the compression and the beginning of the firing stroke. Since the question here deals with the accuracy of the harmonic balance markings in relation to the timing markings on the timing chain cover, what part of the four stroke process, as indicated by valve position, the engine is in does not matter. The only thing that matters is finding the actual TDC to compare the groove in the harmonic balancer with the pointer on the timing chain cover. Position of the valves is, unless you are attempting to indicate an out of time cam, irrelevant to the discussion.

If, in your "35 some years of tinkering with the internal combustion engine" you cannot recall any firing at 15 deg ATDC, it is only because you have not tried. You can, if you wish, align the harmonic balancer/crankshaft pulley zero mark with the TDC on the timing chain cover, then make a second mark at 15 deg BTDC on the harmonic balancer. Then, hook up your timing light, start your engine, and adjust the second mark to TDC. You may have to have your assistant give the engine some additional throttle opening, but the car will run. I suspect that the reason you have never seen this is the fact that, normally, no one would ever choose such a timing setting.

Now, let us discuss another matter, one on which we may agree. How did the original poster determine that his timing was 15 deg ATDC? There are no timing marks after top dead center, only top dead center and before top dead center. Thus, unless one has made a second timing mark on the hamonic balancer, there is no way to determine, exactly, 15 deg ATDC. How did the original poster determine that the timing was 15 deg ATDC? Even an adjustable timing light will not read ATDC, only actual timing or BTDC. This is worth considering. Does he not, truly, understand the orientation of the timing marks, or, is he the "trickster" who shows up here periodically?

I closing, let me thank you for spelling the last name correctly. Most do not and, over the years, I have developed a great affection for those who take the time to do so. But, I am "Les", not Mr. Bengtson. The honorifics are Captain, Professor and the Jarl. Most of which are only used to impress the masses. As my Father used to say, "They are good for sh*ts and grins", but not to be taken seriously. Les
Les Bengtson

Les & others,

I was horrified to see myself mentioned as a trickster.

Being a genuine OF shocks me that anyone would try & play tricks on such a subject.

To repeat. I am a newbie. I guesstimated the amount of ATDC as being about the same distance from TDC as the BTDC.

I am however genuinely pleased that so many people are trying to help me make sense of this.

Now. The piston on #1 pushes air out the plug hole at the right time. I didn't use a coat hanger as suggested as I was apprehensive. I don't want to do any damage to the crown of the piston. I used my finger ( the thin one)

Secondly, I have jacked the car up really high, so I can see all the markings. The white paint (which was verified by professional MG blokes) is the only place that was marked & has not changed at all.

To repeat my seemingly silly statement.

TDC is right on. When I turn the distrib anti-clockwise the white line moves clockwise & the car runs poorly. It misses a beat often & has no power at 15deg BTDC (third notch). When I turn the distrib clockwise the white marker moves anti-clockwise & runs beautifully.Starts right away,lots of power, no missing AND no run on.

There is obviously something that I am not doing right, so I think I will simply take it down to Octagon Motors here in Vancouver BC & let them solve it.
This is all too peculiar.

Thank you one & all for your input. I will post the results from Octagon as soon as I have them.

Thank you all so very much!

Aubrey
Carl W French

Aubrey. Sorry if I suggested that you may be the "trickster" who has plagued these boards for several years now. We have, in the past, had some people, or, perhaps, one person, who has seemed to take great delight in posing some seemingly impossible problems. This individual/these individuals, have posted on the B, the midget and the MGA boards, often with a seemingly possible problem. Once, it was the "last MGB built", in Leona, from scrap parts, for which the poster claimed a Heritage certificate! Thus, those of us who have been posting here a while tend to be somewhat wary. The fact that your heading is "Carl French" and the name is "Aubrey" may be a problem with the system, which you have already noted.

As to your particular problem, (to get it into the archives, if nothing else). The earlier MGs had a one piece crankshaft pulley. Unless it broke, it could not change its relationships. When MG went to the harmonic balancer, the three piece unit having an inner rim, a rubber portion, and the outer rim, things changed. The bonding of the rubber to the steel could become corrupted and the relationships change. I have such a pulley in my collection which had the outer segment sitting 1/8" back from the inner segment. Totally useless for any form of precision tuning.

There is only one, true, way to determine whether the harmonic balancer/crankshaft pulley is in proper relationship to the piston movement. That is to pull the cylinder head and put a dial indicator on the piston. Note when the piston is .001" below the top dead center and .001" after the top dead center. The point half way between the two readings is the theoretical top dead center used for timing purposes. In actuallity, the crankshaft will move quite a number of degrees while the top of the piston is stationary. However, this can only be determined by the use of a degree wheel on the crankshaft while using the dial indicator.

The concept of an "authorized MG dealer" may be different in Canada than the US. When MG went out of production, in 1980, the dealerships lost their authorization. The last time I saw an "authorized MG dealership", in the US, was in 1983. It was in bankruptsy and had been taken in by a Nissan dealership. It soon went out of business and I had to have my MGB serviced by a "British Car Specialist" in Tucson. Perhaps it was different in Canada. However, if it were my car, I would pick up a dial indicator and magnetic base (Rutland Tools has them available), pull the cylinder head (FelPro gasket sets have the English Payen head gaskets) and determine, for myself, exactly what is happening when. As I said before (posted before?) the timing of after top dead center, results in a, quite massive, injection of heat into the cooling system and a major loss of power to the engine. The problem you describe cannot be due to miss-matched parts (such as an early crankshaft pulley and a later model timing chain cover). It can only be attributed to a delaminated harmonic balancer or to a basic engine assembly problem. The only solution is to tear into the engine and accertain exactly where the problem lies. Les
Les Bengtson

Aren't the degree pointers on the top of the timing chain cover on the 75? I'm not sure why the car had to be jacked up.

Mike
Mike Phillips

I think Mike is on to something. On the 75 and later,
as he points out, the timing marks can be easily
seen from the top.

Aubrey states earlier

"The problem is, that I need to turn it quite a lot clockwise to achieve a nice steady rev (1500rpm)."

This corresponds to advancing the timing. So
Aubrey is definitely running BTDC even though
the white mark does not line up properly with the
timing marks.

He also states

"When I turn the distrib clockwise the white marker moves anti-clockwise & runs beautifully"

This again confirms he is advancing the timing since
the white marker is moving anti-clockwise
when viewed from the front of the engine.

I think these are the most likely possibilities.

1. The white mark is in the wrong position on the
balancer. Look for a notch on the balancer and
see if it lines up with the white mark.

2. The harmonic balancer has delaminated as per
Chris and Les.

3. Again, the 75 engine should have timing marks
visible from above. Thus maybe the engine is
from an earlier car, or the wrong timing cover was
installed on the engine?

Ronald

Hello all,

You are right that the mark is seen from the top. I jacked the car up, 'just in case' I had missed something.

To clear things in my mind a trifle.

Ronald states I am doing the right thing by moving the dizzy clockwise, why does the white marker go beyond TDC?
Les you have been very patient & extremely helpful but your latest suggestion anout taking off the head etc, is just a wee bit advanced gor me at this stage.

I, however am going to absorb this information (albeit slowly OF remember) & will give it a go.
Also, the authorised dealer part was a slip of the fingers.
You are right no more 'authorised' but Octagon (NFI) have been in biz for close to 35 years & are THE place to have problems solved.

Thanks all.

Aubrey
A.R Schneider

Les,

Perhaps, what we have here is failure to communicate. The root cause, if in fact true, is far beyond my ability to categorize in the "fun department".

Your knowlege is respected. The firing 15*ATDC was fun to contemplate. Perhaps I suffer from AADD, but I'm moving on to other "problems".

Always a pleasure to discuss the car we love.


glg
glg gimbut

Aubrey. You might research the archives under Ignition Timing or Alternative Ignition Timing. There has been some extensive discussion of this in the past, including how to use a vacuum gauge to set the timing. Since it seems clear that your car is actually showing about 15 deg ATDC it is a probable indication of the harmonic balancer delaminating. If so, the marks could move again and will probably do so before you need to set the timing again. Hence, one of the alternative methods of timing would probably be your best bet. You might also want to take a little white paint, such as model paint, and paint a straight line from the center of the pulley to the edge. If the marks on the center section and outer section of the pulley move out of alignment, you have confirmed the problem is delamination. Les
Les Bengtson

"TDC is right on. When I turn the distrib anti-clockwise the white line moves clockwise & the car runs poorly. It misses a beat often & has no power at 15deg BTDC (third notch). When I turn the distrib clockwise the white marker moves anti-clockwise & runs beautifully.Starts right away,lots of power, no missing AND no run on."

Aubrey, You say no power at 15deg, third pointer. Then you go on to say, "when the distributor is moved clockwise the timing mark moves anti-clockwise and the engine runs great. When the timing mark moves anti-clockwise, to the left as observed from the front of the engine, the timing is advanced. So based on your quote it appears your engine runs best with about 30deg advance not retard as you said. Did you unplug the vacuum hose from the inlet manifold before checking the timing? You did not say but I assume you still have the OE manifold and carb.

You can verify the accuracy of the timing mark with a simple piston stop I will describe. Look at my Yahoo photos at; http://photos.yahoo.com/flash1929
Click on the " Positive" album and you will see a tool I made from an old spark plug yrears ago for finding TDC. I removed the guts of the plug and tapped the hole for a bolt, 3/8" NC tap. I screwed a bolt into the electrode end, sawed the bolt off about one inch from the plug thread and rounded the end so it will not damage the piston. To use the tool, disconnect the battery ground cable, you want to be sure the starter cannot turn the engine with the tool inserted. Remove all spark plugs. Insert the tool in no. 1 cylinder. Make a pointer from a coat hanger or other stiff wire and fasten it with a timing cover bolt so the pointer is over the balancer pulley. Turn the engine over clockwise slowly until the piston bumps the stop. Make a thin mark under the pointer on the pulley. Turn the engine slowly anti-clockwise until the piston bumps the stop. Make another thin mark. You now have two marks, they will be close together. Accuracy is very important for the next step. Measure the distance between the two marks and make another mark exactly half way between them. Remove the stop tool and turn the engine until the pointer is under the center mark you just made. If the balancer mark is correct it will be lined up with the timing mark on the timing cover. If it isn't make a new timing mark opposite the TDC on the timing cover. If done carefully you should not have more than a 1 or 2 degree error, close enough for checking timing. Good luck, Clifton
Clifton Gordon

Clifton,
Thank you so much for your input.

The location of TDC, the notches & the results have to be refined, so I can understand why this is happening.

The teeth are 'sposed to indicate TDC & the deg of advance. Why then, does the car run so well when the distrib is moved clockwise & the white mark doesn't move towards the notches but away from them, where there are no notches.

I have had the car for 4 years & it has run beautifully after my engine re-build 2 years ago. Nothing else has changed. Here is another thought. When I adjusted the tappets, maybe I made an error. That would certainly cause the missing, bad starting & run on, right?

Everyone is right however as far as 15-18 deg advance is concerned. Originally it was just that & no prob's.

Someone had suggested the coat hanger/spark plug mod & I did feel the pressure when #1 piston came up. I have not fashioned the spark plug. At first glance it appears to be way more than my current knowledge. Some of the advice I have been given, simply flys over my head.

Will keep all posted.

Thanks again
Aubrey
A.R Schneider

Clifton,

I think you may have solved one part of my dilemma.

You pointed out (now I understand) that when I rotate the distrib clockwise, I am actually still advancing. I thought that once I had gone past TDC I was into retardation, wheras, I was actually putting on more advance.

Now, the other part. If I am running so far advanced, what dangers am I running into?

What a relief I understand. I did tell you all that I am an OF. Just takes longer to understand.

Thanks to all.

Aubrey
A.R Schneider

Aubrey, The main danger is if you have a spark knock/pinking it could cause some internal engine damage. If you don't hear any evidence of a spark knock on acelleration it should be ok. A spark knock is a rattle or clattering type noise. If your engine has all it's original equipment installed and you checked the timing without removing the distributor vacuum hose and plugging it the timing could very well be advanced beyond the timing marks and be within limits. I don't have the exact timing specifications for your 75 engine but I'm pretty sure timing should be checked with the distributor vacuum hose disconnected and plugged. One step at a time is how most of us learn and many of us are also a little slow.
Have fun, Clifton
Clifton Gordon

OK, this is the 'Real' Carl French. It has been interesting for me to fantasize owning another MGB here for a bit. Thank you Aubrey
Geez, Think of the things you could do with the access to another name ;->
Carl
Carl W French

You are aware that the engine turns clockwise as you're looking at it from the front.
william fox

The 75 MGB used the Lucas 45D4 distributor and the vacuum advance is listed as either "fitted, but not used" or "pinned" depending on the source. Thus, if the dizzy is original, the vacuum advance is not a player in the timing.

When standing on the right hand side of the engine and looking at the distibutor, the distributor is rotating in a counter clockwise manner. The crankshaft pulley, being 90 degrees offset, is rotating in neither a clockwise, nor counter clockwise manner when viewed from this setting. It is moving from the bottom of the engine then upwards. (When viewed from the front of the engine, when you are facing rearwards, it is moving in a clockwise direction.)There are five pointers on the timing chain cover. The upper most represents top dead center (TDC) and each lower pointer represents five degrees before top dead center (BTDC). Rotating the distributor, as the engine is running, in a counterclockwise manner retards the timing and causes the timing mark to move upwards. Moving the dizzy in a clockwise manner advances the timing and causes the timing mark to move downwards. Factory timing for the 75 MGB is 13 deg BTDC at 1500 rpm. Changing the timing will also affect the idle speed, which must be reset and the timing checked again. To do this properly, you need both a dynamic timing light and a dwell/tach. The tach on the car is not sufficiently accurate for such work. After the timing is set accurately, the idle speed is reduced to 800-900 rpm. Some go as low as 600 rpm.

Over advancing the timing at idle will throw off the factory designed centrifugal advance curve and will allow full advance to take place at a lower than specified rpm. This can cause both performance problems and engine damage. Detonation, commonly referred to as pinging/pinking is audible detonation and, when heard, means the engine is suffering. There is also sub-audible detonation which cannot be heard, but, over a long period of time, can result in engine damage.

Even though an engine may not ping/pink at idle, or when reved up while standing still, detonation normally is found when the engine is under load. That is when the engine rpms are low in relation to the throttle opening, such as when accelerating to pass or to go up a hill. This is why the old "run it up a hill and advance it until it pings, then back off three degrees" works as a general method.

As most know, and has been recently mentioned, cars having a functional vacuum advance are normally timed with the vacuum advance line disconnected and plugged. This is so the centrifugal advance system is set properly for performance on the road. We all know, at least most of us do, that, as soon as we attach the vacuum advance line to the vacuum advance can, the idle ignition timing is advanced by several degrees. This is easily tolerated because the engine is not under load at idle.

Thus, for most of us, the factory recommendations will work fine if we have a basically stock engine and original distributor in good condition. When changes are to be made and the factory settings deviated from, the use of a chassis dynomometer/rolling road is an excellent idea. Les
Les Bengtson

Les,

How precise!

As I said previously, I now understand, have set the advance at 20deg (she runs best), have set carb & she is rolling right along.

I think I have take a lot of time & space (bandwith?) & must way for someone else.

I do however want to thank you, one & all for this excellent & fascinating way you helped me walk my way through this unknown (to me at least) jungle.

You are truly a great bunch!

Aubrey
A.R Schneider

This thread was discussed between 20/03/2003 and 25/03/2003

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