3.8 Ignition timing- position of rotor arm?

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PPV
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3.8 Ignition timing- position of rotor arm?

Postby PPV » Fri Jun 07, 2019 4:09 pm

Hi all, I have had a problem with my car for years and it is annoying. I have commented on other threads as you may have seen about the fact that my car starts and idles and runs well but when accelerating quite hard it coughs and spits back through the carbs. I have electronic ignition and my latest attempt at trying to correct the problem has been to rebuild the carbs.
No difference has been made although my diaphragms were a little worse for wear! I did not change the needles though.
Going through everything again by setting my car at no6 cylinder at TDC and the timing marking on the front crankshaft pulley at TDC mark (there is only one mark on my pulley so I assume it is TDC) I thentook off my distributor cap and noticed that the rotor arm points towards the block. The manual says this should be pointing at no 6 HT lead. I checked and in fact it is pointing at no 1 (i.e the cylinder at the back of the engine). I have checked the timing using a strobe connected to no6 cylinder and the timing mark shows about 8 degrees BTDC when idling and it seems to advance ok when the revs are increased.
Can anyone tell me where on their distributor either where no6 or no1 cylinder HT lead is on their distributor cap.
In fact, I am getting so confused that I cannot even think what differnce it would make as long as the firing order is correct.
If my distributor and timing are ok then my last port of call is to change the needles to the new ones provided in the rebuild kit.
Many thanks.
Paul V
Mk2 3.8
Mercedes A45 AMG
Mercedes AMG GLC43

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piman
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Re: 3.8 Ignition timing- position of rotor arm?

Postby piman » Sat Jun 08, 2019 6:11 am

Hello Paul,

not only does the piston have to be near TDC but it must also be on the firing stroke, if not you will be 180 degrees out on the distributor.
It sounds to me that you are not on the firing stroke?

Alec
Mk 2 3.8 (long term restoration), MK1 Triumph 2.5 P.I.,2.5 X Type Estate, 564 Hymer Motorhome

PPV
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Re: 3.8 Ignition timing- position of rotor arm?

Postby PPV » Sat Jun 08, 2019 10:11 am

Hi Alec, I thought that but the only way I could tell was to put my finger over the plug hole and feel the compression rise so that is why I asked which plug lead on the distributor face the block. Having said that surely it would not run if 180 degrees out?
Thanks for reply.
Paul V
Mk2 3.8
Mercedes A45 AMG
Mercedes AMG GLC43

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piman
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Re: 3.8 Ignition timing- position of rotor arm?

Postby piman » Sun Jun 09, 2019 10:49 am

Hello Paul,

yes, correct it will not run. Although, in practice all manufactureers align the distributor in one way for that model, it really doesn't matter. You can set the distributor wherever it fits and replace the leads to order starting with number one when the engine is set to number one firing. In other words you can have a different visual layout on the same engine with no problems. This is not possible on all engines, physical constraints, usually the advance part fouls some other component but the principle is true.

Alec
Mk 2 3.8 (long term restoration), MK1 Triumph 2.5 P.I.,2.5 X Type Estate, 564 Hymer Motorhome

PPV
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Re: 3.8 Ignition timing- position of rotor arm?

Postby PPV » Sun Jun 09, 2019 11:45 am

Thanks Alec, that has got it straight in my head now! I shall try a slightly different advance and then if no joy check my needles. The new ones provided just say '100' on them but I can see nothing on the old ones but I have not removed them, just lifted the dashpot off and then the damper etc.
Any idea where they should be marked? I have had a look at various websites but no indication which my needles correspond to. My carb says AUC 7081W but cannot see this on Southern Carbs website and looked at this book (http://mk1-performance-conversions.co.u ... t_1965.pdf) but no joy.
Regards,
Paul V
Mk2 3.8
Mercedes A45 AMG
Mercedes AMG GLC43

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piman
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Re: 3.8 Ignition timing- position of rotor arm?

Postby piman » Sun Jun 09, 2019 5:25 pm

Hello Paul,

the needles are stamped on the shank, so need removing from the pistoon to identify them.

I have an (old) Burlen manual and the carburetor identification (Tags attached to one of each carburettor float screws) should be AUC 954.
Where is the number you refer to located? Funnily the same carburettor specification covers both 3.4 and 3.8 engines.Weak needle is an SJ, standard an SC and rich is a WO3. Incidentally the .100 is the diameter of the needle for 6 series carburettors.

What oil do you use, it should be SAE20 (I used to always use engine oil 20/50 grade)

There is no cross reference so I can't directly look up the number you quote.

Alec
Mk 2 3.8 (long term restoration), MK1 Triumph 2.5 P.I.,2.5 X Type Estate, 564 Hymer Motorhome

PPV
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Re: 3.8 Ignition timing- position of rotor arm?

Postby PPV » Sun Jun 09, 2019 5:56 pm

Hi Alec, the number is on the carb body. I have no tags or other numbers. When I do an internet search it says that my number is the one for a HD6 Jag carb (also fits other makes I understand). My car was worked on/rebuilt a bit by Robin Burden (think that was his name), Mk2 Jag Centre Wrenbury, Cheshire about 20years ago and seems pretty originally if a little aged. I understand he has moved on to Wales somewhere.
I do use 20/50 oil as reports say that is fine but perhaps I should try SAE20 and see what that does.
On the new needles there is no other marking on the shank. I shall have to go back to Burlen to see what they supply in the kit I bought, although I did specify the car and engine number etc.
Thanks for replying.
Paul V
Mk2 3.8
Mercedes A45 AMG
Mercedes AMG GLC43

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piman
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Re: 3.8 Ignition timing- position of rotor arm?

Postby piman » Mon Jun 10, 2019 6:59 am

Hello Paul,

that may just be the part number for teh carburettor body. AUC is a common prefix for all sorts of carburettor parts. The tag is a small aluminium tag. fixed usually to the float chamber and on some to teh dashpot screw.

Burlen should be able to clarify that for you?

Alec
Mk 2 3.8 (long term restoration), MK1 Triumph 2.5 P.I.,2.5 X Type Estate, 564 Hymer Motorhome

Martec
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Re: 3.8 Ignition timing- position of rotor arm?

Postby Martec » Tue Jun 11, 2019 11:36 am

May I just add to this post as I had a similar thing happen ten or so years ago to the point where I could only get to 45 mph on the motorway on the way to a guy with a rolling road to find the fault.

I had bought a more up to date Electronic Distributor set up specifically to suit my car (state of tune and axle ratio). It was found that the electronic pickup was incorrectly positioned within the distributor so that the rotor arm was not pointing directly at the appropriate plug lead, and as the speed increased the rotor arm moved plus the plate so that the rotor was pointing between two plug leads and fired which ever it suited.

The guy jury rigged it to work, but I went home and cut the centre out of the distributor cap (and bought another one) to adjust the pickup position better. I also found that not all rotor arms point in exactly the same direction.

The distributor is still in and works OK now that it is set up correctly.
Brian
MY2000 3ltr S type manual standard car with leather seats, cruise control and mistral blue metallic paint.

1961 3.8 MkII manual, Indigo blue, nolonger runs on LPG, everything else uprated

Tom1
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Re: 3.8 Ignition timing- position of rotor arm?

Postby Tom1 » Mon Aug 12, 2019 9:16 pm

Hi there, if you have electronic ignition make sure you have resistored spark plugs.
Try NGK BP5E S ( S is for suppressed) set gap to .025".
If you have 123 tune ignition I can give you advance and vacuum curves to try.
Check plugs first.
Tom.

PPV
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Re: 3.8 Ignition timing- position of rotor arm?

Postby PPV » Tue Aug 13, 2019 11:00 am

Thanks for responses. I do not have 123 Ignition, just a conversion inside the distributor.
I started again with the ignition timing, setting TDC etc and found that after restting everything the problem did not appear. I did also dismantle the distributor and clean all parts so perhaps something was not working right. However, I also noted that the position of my distributor was in a different place when the ignition was set to about 7 degrees BTDC.
Just shows that sometimes on an old girl a clean and tidy up does wonders.
Thanks again for the comments.
Paul V
Mk2 3.8
Mercedes A45 AMG
Mercedes AMG GLC43


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