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MG TD TF 1500 - 3rd cylinder spark plug makes problem

The engine of my TD suddenly ran bad. I have now determined that probably the ignition of the 3rd cylinder makes the problems.

I have checked the following:

Points set to 0.3mm - because of the earlier design of the distributor camshaft with 0.25 ... 0.3mm.

All ignition leads are measured separately - all have 0 Ohm.

All ignition leads including plug measured - all have 8 kOhm.

All contacts of the distribution cap are measured - all have 0 Ohm.

All the contacts of the distribution cap, including the ignition leads and the plug measured - all have as expected 8 kOhm.

Ignition lines and plug of 3rd cylinder had been exchanged against other ignition line with plug - the error remains on 3rd cylinder.

New spark plugs measured - all have 0 Ohm.

New spark plugs used - points and distributor fingers are relatively new (less than 10,000km).

After start (jumps immediately) the engine runs out of round and it bangs from the exhaust.

Pulley mark flashed with strobe - mark stays quiet (10 ° on the pulley) and increases when turning up.

Each line tested with the strobe - all flash evenly; except line 3 which flashes only sporadically.

Line 3 disconneted from the spark plug and connected to an external spark plug - no dramatic change in idle. If the external spark plug has no connection to ground, the stroboscope flashes. However, if the spark plug is grounded, the spark is visible but the stroboscope does not flash.

What could be the cause of a deviation from cylinder 3 when everything is the same from each contact of the distribution cap?
W_Mueller

IF the fault stayed on 3 with swapping plug and wire, the cap has an internal short. It may ohm fine, but the high voltage of the spark may short.
George Butz

Pull the valve cover. check if the exhaust valve spring is broken and stuck just open.
Or do a compression test.
M Grogan

I agree with MGrogan. Pull the valve cover and check the valves are opening. I have the same issue with a broken rocker arm on #2.
Rick

If the plug leads at long enough you could turn the distributor through 90degs so the contacts in the cap are firing different cylinders. If number 3 is then OK and one of the others isn't the problem is with the cap or lead.

Have you tested the compression in number 3. Simply putting your finger over the plug hole and cranking the engine would give you a good indication.

Banging in the exhaust suggests unburnt fuel is getting into the manifold and would indicate an ignition problem.

When you sort the problem out let us know how you sorted it.

Good luck

Jan T
J Targosz

@M Grogan & Rick

I've done a compression test. All cylinders have nearly the same pressure with 9 bar. Is it possible to see a good compression with a broken rocker arm? I'll open the cover and have a look.

On the other hand the strange behaviour of the strobe is seen whith the external plug.

@ George

that was my thought too. But the material looks good and there is so much plastic there and the next ground is far away. However it is possible and worth to test it, because it became more faulty during testing.


Just an other thought. If the shaft of the distributor is worn (I can feel it a little bit) - is it possible that the points do not open correctly only at position 3?

W_Mueller

Sounds like the spark is tracking inside the cap to me
Have a good look in there for cracks or pencil lines from the no.3 post
willy
William Revit

Because I can continue working not before tuesday or wednesday I wanted to purchase a new cap. Bad news here in Germany. There is a problem with the stuff on market depending on the middle contact (coal and spring seating) and good sellers don't bring them out until the problem is resolved.

Someone in D who can help with a good part (NOS)?

@ Jan T

good idea - but fixing the distributor of my TD is a little bit tricky. I'll look for a cap substitute. It is not expensive.


W_Mueller

Get one from the Distributor Doctor in England.
Dave H
Dave Hill

If you can see where it's tracking in the cap
For an emergency repair you can sandpaper it till there is no sign of the marks and paint it with fingernail polish
willy
William Revit

Thank you William

I'll try to find an evidence of a wrong way for the high voltage.
W_Mueller

Hi

A look under the valve cover shows no surprise.

I'd purchase a new cap, finger and new points and after mounting the situation became a little bit better. The old cap shows nothing. I've tested with 1000V but I guess that was not enough. It seems to be an early Lucas original cap.

Is it true that the original cap has the leads at the side and not on top? Since now I never saw a picture with such a distributor cap.


Once again I'd opened the carb and adjusted the needle. Seems it run better. Yesterday it was too late for a drive.
W_Mueller

The plug wires for an original cap come off the top, not the side. Many other cars used the side-wire cap which will fit, but is not correct.

Tom Lange
MGT Repair
t lange

I have examined the old distributor cap with a magnifying glass. In the region of the contact for the 3rd cylinder, there is a crack outside which can hardly be detected viewing without a magnifier.

I'm sad because it is an original LUCAS part. One part more consisting modern plastique.

W_Mueller

From the Frame Up has original caps, as does Kip Motor company. George
George Butz

Thank you George - I can't see it at FTFU but Kip Motors is new for me and I guess it will be a hit.

W_Mueller

Winfried,
You can always contact Martin Jay-the "distributor doctor" in the UK.
http://www.distributordoctor.com/distributors.html

Regards
Declan
Declan Burns

Thank you Declan

at the moment I'm a little bit helpless. The new cap solves only a part of my problem. The engine runs rough (rear carb is too rich with jets on top end) and the overhauled gear box makes noises while changing into 3rd gear and the remote lever rattles more than expected.





W_Mueller

Item #EL115, pg 13 in catalog
George Butz

Thank you George - now I've found it too.
W_Mueller

Winfried,
The problems are non related.
Obviously the first question-
Who overhauled the gearbox and what was done?
Is the clutch adjusted properly?
Does it make noise shifting up and down into third?


There are several causes when the remote rattles.
It could be the bushing.
It could also be the plate buffeting under the spring.
It could also be the plunger.
It could be a combination of the above.
I found replacing the paper gasket with a thin rubber gasket also helps reduce rattle.
All are relatively easy fixes.

Regards
Declan


Declan Burns

Thank you Declan for your advice via telephone.

Here the answers fo all:

Who overhauled the gearbox and what was done?

A specialised gear and overdrive workshop with a very good reputation for old English cars. New are lay shaft, all bearings including many needles, rod for 1&2, a speedi sleeve for 1rst motion shaft and some little parts.

Is the clutch adjusted properly?

If I press down the clutch there is much amount until the gear makes any audible or mobil reaction. But I'll follow the hint of Declan an will go with free play less than 19mm.

Does it make noise shifting up and down into third?

Yes - not allways. It is independ from pressing clutch twice or blipping between.


I've forgotten to ask Declan (or forgotten if I'd asked ;-)):

Does a high idle affect this item too?
W_Mueller

This thread was discussed between 14/09/2017 and 27/09/2017

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