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MG MGA - Rotor arm shape

This is a request for some technical advice regarding the ignition system. In fact it relates to a 1959 Jaguar 2.4, but would be equally relevant to the MGA.

I have been trying to set the timing as the engine was not running smoothly and when checked dynamically was about 20-30 deg advanced at tickover. The distributer had been fitted twisted too far anticlockwise so that it would foul the block if I turned it enough to retard. So I remounted in the correct position and refitted the leads to the correct alignment (about 45 degrees clockwise). I was then able to set the timing statically. However when I started the car the timing was retarded by over 20 degrees. This time the distributer could not be rotated clockwise as it would foul again.

After much head scratching I have come to the conclusion that it is the rotor arm that is incorrect and am about to order a replacement thinking that should solve the problem. Am I on the right track?

Just to be certain I have confirmed that the timing marks coincide with TDC (easy to do on this engine as the spark plug hole is directly over the piston) and the camshafts are correctly aligned with the crank (again easy to do with the supplied tool).

Neil
Neil McGurk

The distributor drive gear is installed wrong. See here:
http://mgaguru.com/mgtech/ignition/ig103.htm

Of course your Jag will be somewhat different, but the idea is similar.
Barney Gaylord

Thanks Barney, that was my first thought too, I checked it and it was OK.

BUT, I believe I have found the problem - it's not the rotor arm, it's the distributor cap!

It seems there may be two types mounted 90 degrees differently, which, with the six leads will give a difference of 30 degrees. That should account for the difference between the static and dynamic and is exactly what I am after to be able to set the timing well within the limit of movement of the distributor.
Neil McGurk

Neil

I used to get some strange readings from my dynamic timing light (the type with the impulse detector that fits round the No.1 plug lead) when used with the old type wire cored plug leads. It works fine with modern silicon leads.

I think the rotor arm is a red herring.

Steve
Steve Gyles

The cap being 30deg off will not change the timing as seen by a timing light. It may cause poor or no running. Timing is simply when the points open. Rotating the disty relative to the engine and the drive gear are the adjustments for timing(with a bit of point gap thrown in). Whether you get good spark to the plugs is a different matter. It is necessary for the rotor to align with the proper plug lead when the points open to get the spark to the right plug.
Chuck Schaefer

I am now convinced the rotor arm was not the culprit.

I have been looking at distributor caps and there are two types differing by 30 degrees in the way they fit. Unfortunately I have not been able to find out if that is only due to distributor type or whether I can get an alternative type cap that fits the existing distributor. I suspect an earlier type cap may have been fitted on a later distributor as the engine was swapped for a later unit. I will probably modify the cap I have so that I can rotate it 90 degrees.
Neil McGurk

Chuck,
If the cap is rotated 30 degrees relative to the distributor that will effectively delay or advance the spark to the plug by the same amount. Will it not?
Neil McGurk

I am beginning to see where you are coming from Neil. If you are stuck with the one cap then the length of the rotor 'L' arm could compensate for the 30 degrees. i.e. A long arm could measure the 30 degrees radially, whereas an arm with no 'L' would be the other extreme.

Steve
Steve Gyles

Neil -
Changing the cap also has no effect on timing, exactly as Chuck described relative to the rotor. If the cap/rotor combo is far enough off, the spark may have trouble deciding which plug to go to, but the spark will still happen when the points open.
I think your problem is that the mechanical advance was stuck advanced, and no you have disturbed things it is either stuck retarded or moved after you set timing. Take the dizzy out and be certain the advance mechanism is free. Then be sure you are turning the dizzy
the right way when you set static.
In the static timed position, the cap terminal should be aligned with the rotor contact segment, and when advanced it should still be.

FRM
Fletcher R Millmore

FRM

I disagree with you. IF the distributor is installed and bolted tight (no rotation of the distributor body and rotor possible) and you put on a different cap which has the plug contacts 30 degrees different, then the timing will be 30 degrees different. The only way you can compensate for this is to slacken the distributor and rotate the body those 30 degrees or fit a different rotor arm which has an 'L' arm 30 degrees different in length.

Steve
Steve Gyles

Fletcher is correct.
Timing is set when the points open. A wrong cap only makes the spark try to jump further.
Repeat The spark happens WHEN the point open. The spark goes WHERE the rotor points. ALL that will happen if the cap is rotated, either physically or with a wrong cap, is the spark goes further or not at all. ANY other statement comes from an incomplete understanding of the system.
This is not intended as a "flame" but incorrect info should be countered for the benefit of all.
Respectfully
Randy
R J Brown

FRM and Chuck, I think I am beginning to understand! You are absolutely right about the spark to the plug only occurring at the point when when the points open.

I think that the basic problem is that with the current distributor I cannot position it in such a way that I can adjust in both directions. Neither this problem nor the timing can be caused or solved by cap or arm. The distributor has to be aligned so that the points are opening on one of the lobes when the engine is at TDC. There are two positions 60 degrees apart (six cylinders) where this occurs and I cannot fit the distributor body in either as it fouls on the block or inlet manifold. I need another position between the two.

Looks like I may have the incorrect type distributor for this engine. I will compare with my Mk2 Jag tomorrow.

I have had the distributor off and the advance mechanism seems to work fine, no sticking. I will have to look further to see why I appeared to have such a big difference between static and dynamic timing.




Neil McGurk

I was never contending when the spark occurs, I don't have an issue with that. It is where the spark goes. In the scenario we are talking about with Neil it could well go to the wrong plug lead. With 6 cylinders, his terminal posts in the distributor cap are 60 degrees apart. Assuming he has the engine timed correctly and he then changes the cap that is 30 degrees rotated, the spark will occur mid way between 2 contacts. To which terminal post will the spark jump - let's toss a coin.

Neil's problem is a rotational obstruction of the distributor to put the correct terminal post back in contact. One likely option developed by the manufacturer was to use a rotor arm that was set at a different angle or had an extra 30 degrees length on its 'L'

Steve
Steve Gyles

Neil -
Sort of embarrassed by this, as a former resident Jag expert, and it's among my favorite engines, but I can't recall how the distributor drive is retained. But then I never had need to reset it other than when rebuilding an engine, and that was 30+ years ago.
If the engine has ever been apart it is possible that the drive is not lined up right.
General idea is that the drive is off a skew gear keyed on the crank. The take-off shaft has a mating drive gear. If this is installed a tooth or two out, you can have this problem. The distributor drive is also the oil pump drive, so if you need to move it, you must be absolutely certain you get the oil pump part properly engaged.
The OP drive is a tanged affair. When you pull the drive gear, it will turn and so will the OP tang. If you rotate the gear a tooth or two, you need to rotate the OP tang a similar amount, so that when the gear is engaged and lowered into position, it picks up the OP tang. This is absolutely critical, since you can lose the OP drive, usually on a cold start but possibly anytime. Had a customer who started her near new 4.2 in zero F weather; smart girl was watching the OP gauge and saw it come up and drop off. I discovered that the factory had not aligned the drive correctly, and it had pressed the OP shaft through the OP rotor It had enough engagement to drive the pump for about 8000 miles, but the cold start sheared the end of the tang off. Also had an identical failure on a 504 Peugeot, which has a similar arrangement, in that case the gear is attached to the distributor, so if you take it out you must line up the pump tang - factory mistake leading to a wiped engine at around 40,000.

FRM
Fletcher R Millmore

Thanks Steve, but maybe I didn't explain well enough, because I wasn't thinking clearly enough about the problem. The cap is not actually the problem, that is just the indicator as to which distributor is used.

In theory you are right, but in practice I don't think that you could find the parts.

I have been back to examine the evidence! The distributor has been fitted in the wrong position and was indeed about 25 degrees advanced, with no room for correct adjustment. It is an early distributor on a later engine. This was done some six years ago and the car has been running badly (albeit only about 400 miles) ever since.

When I refitted in the "correct" position I had set the timing correctly, but forgot to check the setting on my timing gun! Hence the appearance of being 20 degrees retarded! The engine was still hesitant to accelerate, but I have found other problems with the ignition system that may account for that (intermittent shorting of contacts to baseplate, poor HT connections and rapidly burned points indicating condenser failure). The reason that the distributor runs out of adjustment space is that the intake manifold was changed on later models. Therefore the later distributor was rotated 30 degrees.

Thank you Randy, Fletcher and Chuck for straightening me out! I knew I could rely on this forum!

Neil

Neil McGurk

This thread was discussed between 06/03/2010 and 07/03/2010

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