Technical TDC cylinder 1

[Bubins]

Hello,

I know setting points has been covered a lot in this forum but I can't find anything on my exact question (perhaps it's too simple for anyone else to have needed to ask).

I've read the gap should be 0.5mm when at TDC for cylinder 1. People have said the rotor arm should be pointing at cylinder 1 when the timing mark is level with the arrow on the casing. Can anyone confirm if my picture is showing this having been done correctly?

Thanks.

 

[fiat500]

Hello,

I know setting points has been covered a lot in this forum but I can't find anything on my exact question (perhaps it's too simple for anyone else to have needed to ask).

I've read the gap should be 0.5mm when at TDC for cylinder 1. People have said the rotor arm should be pointing at cylinder 1 when the timing mark is level with the arrow on the casing. Can anyone confirm if my picture is showing this having been done correctly?

Thanks.

View attachment 437811

That looks about right; but equally, it could be pointing at cylinder 2. It depends on which way the distributor has been put in.
The gap isn't set at TDC, but at the point when one of the lobes on the distributor shaft pushes the gap to its widest opening.

 

[Bubins]

That looks about right; but equally, it could be pointing at cylinder 2. It depends on which way the distributor has been put in.
The gap isn't set at TDC, but at the point when one of the lobes on the distributor shaft pushes the gap to its widest opening.

Ah ok, everything I've read so far seemed to say measure the gap at tdc for cylinder 1.

Do you mean I should turn the pulley till the points seem to be at their widest and then measure the gap?

Thanks for your reply.

 

[fiat500]

Ah ok, everything I've read so far seemed to say measure the gap at tdc for cylinder 1.

Do you mean I should turn the pulley till the points seem to be at their widest and then measure the gap?

Thanks for your reply.

Exactly.....and then adjust them if necessary. The optimum would be 0.5mm, but it is specified within a range that allows the gap to be fractionally wider or narrower...but we're talking a few hundredths of a millimetre leeway.
But then things still need to be done. Have you read and understood the next procedure of manually rotating the engine clockwise until it reaches the point where it is ten degrees before TDC?

 

[Bubins]

Exactly.....and then adjust them if necessary. The optimum would be 0.5mm, but it is specified within a range that allows the gap to be fractionally wider or narrower...but we're talking a few hundredths of a millimetre leeway.
But then things still need to be done. Have you read and understood the next procedure of manually rotating the engine clockwise until it reaches the point where it is ten degrees before TDC?

I've only read that the mark should be 10 degrees before TDC and then 18 degrees at 3000rpm. But from what you've said the engine must be off, so I'm not sure what you're suggesting should be done when I manually turn engine to 10 degrees before TDC?

 

[fiat500]

I've only read that the mark should be 10 degrees before TDC and then 18 degrees at 3000rpm. But from what you've said the engine must be off, so I'm not sure what you're suggesting should be done when I manually turn engine to 10 degrees before TDC?

Once you have gapped the points as above, and turned the engine on the pulley clockwise so that you are sure that it is ten degrees before TDC, you then need to loosen the distributor so that it can be rotated, but still has a bit of friction to hold it in place. You turn the distibutor anti-clockwise, without rotating the engine, until the points are at the point of just opening. That can be verified by the use of a test-lamp or continuity-tester. You could also do this by inserting fine tissue-paper or cigarette-paper into the opened points and then rotating the distributor until the points just grip the paper...or simply do it by eye. The final decision should be made when rotating the distributor anti-clockise. You should not need to turn it very far. Once set, you obviously then secure the clamping nut.

At this point the engine is in a condition where we say that it has been statically-timed. The rotor-arm will point slightly to the left of the position where you had it previously. The engine should start very well. If it doesn't, then it's worth swapping the positions of the two HT leads.

 

[Bubins]

Once you have gapped the points as above, and turned the engine on the pulley clockwise so that you are sure that it is ten degrees before TDC, you then need to loosen the distributor so that it can be rotated, but still has a bit of friction to hold it in place. You turn the distibutor anti-clockwise, without rotating the engine, until the points are at the point of just opening. That can be verified by the use of a test-lamp or continuity-tester. You could also do this by inserting fine tissue-paper or cigarette-paper into the opened points and then rotating the distributor until the points just grip the paper...or simply do it by eye. The final decision should be made when rotating the distributor anti-clockise. You should not need to turn it very far. Once set, you obviously then secure the clamping nut.

At this point the engine is in a condition where we say that it has been statically-timed. The rotor-arm will point slightly to the left of the position where you had it previously. The engine should start very well. If it doesn't, then it's worth swapping the positions of the two HT leads.

Thank you. I'll give that a try!

 

[fiat500]

Thank you. I'll give that a try!

EDIT: The rotor arm won't move, but it will obviously be slightly changed in position relative to the distributor body.

 

[Bubins]

I forgot a measuring tool to measure 13 degrees before TDC so ended up just setting the points gap to 0.5mm once they got to the widest point. The car then started first time and I could actually drive it. Before, all I could get was slow, juddery acceleration before the engine would cut out. Hopefully, once I've measured 13 degrees before TDC it should be running even better.

So the points gap should never be wider than 0.5mm?

It seems strange that at the widest point the TDC markings were not lined up, but it worked.

Thanks for your help.

 

[fiat500]

I forgot a measuring tool to measure 13 degrees before TDC so ended up just setting the points gap to 0.5mm once they got to the widest point. The car then started first time and I could actually drive it. Before, all I could get was slow, juddery acceleration before the engine would cut out. Hopefully, once I've measured 13 degrees before TDC it should be running even better.

So the points gap should never be wider than 0.5mm?

It seems strange that at the widest point the TDC markings were not lined up, but it worked.

Thanks for your help.

As the points wear the gap will increase naturally, and that is why we adjust them every few thousand miles.
If the gap is too wide or too narrow the spark will occur a bit late or early, and this affects the efficiency and performance of the engine.
It is at the point when the cam just starts to change profile towards its high point that the spark occurs, and if all is set correctly, by the time the engine is at TDC it's likely that the points would be open or even fully open at the top of the cam profile.

Great news that the car started properly and you can move towards even greater precision.

 

[Goldnrust]

Glad to hear you're getting somewhere, always good when there's some light at the end of the tunnel!

It seems strange that at the widest point the TDC markings were not lined up, but it worked.

Not to be patronising, but I think you've ended up a bit mixed up about the factors relevant to the timing and the points gap The two are inter connected but they are also separate and perform separate functions.

To get a good clean burn, and maximise the power made by the engine, the spark need to be strong enough to ignite the fuel and air effectively, which is where the points gap come in, and at the right time, timing, to create maximum downward force on the piston.

The points gap should be set first, the points gap determines the 'dwell', the relationship between how long the coil charges and how long it has to discharge (spark) before it starts to charge again. Too small a gap and the coil will over charge, possibly damaging it, and the discharge time will be shorter leading to the spark jumping across the plug for a shorter time. Too big a gap and the coil will not charge for long enough leading to a weak spark. Setting the points gap to 0.5mm as its widest point, regardless of where this falls in the combustion cycle, should give the correct dwell. There are other ways to measure dwell with specialised machines, but for most home mechanics setting the points gap is all the accuracy we require.

Second we set the timing. The interlinking happens here because changing the points gap will make a difference to the timing. To adjust the timing, this is where the TDC mark becomes relevant as, in the case of the 500, we need to make sure the spark happens at the point that is 13mm before TDC on the pulley.

 

[Bubins]

Glad to hear you're getting somewhere, always good when there's some light at the end of the tunnel!



Not to be patronising, but I think you've ended up a bit mixed up about the factors relevant to the timing and the points gap The two are inter connected but they are also separate and perform separate functions.

To get a good clean burn, and maximise the power made by the engine, the spark need to be strong enough to ignite the fuel and air effectively, which is where the points gap come in, and at the right time, timing, to create maximum downward force on the piston.

The points gap should be set first, the points gap determines the 'dwell', the relationship between how long the coil charges and how long it has to discharge (spark) before it starts to charge again. Too small a gap and the coil will over charge, possibly damaging it, and the discharge time will be shorter leading to the spark jumping across the plug for a shorter time. Too big a gap and the coil will not charge for long enough leading to a weak spark. Setting the points gap to 0.5mm as its widest point, regardless of where this falls in the combustion cycle, should give the correct dwell. There are other ways to measure dwell with specialised machines, but for most home mechanics setting the points gap is all the accuracy we require.

Second we set the timing. The interlinking happens here because changing the points gap will make a difference to the timing. To adjust the timing, this is where the TDC mark becomes relevant as, in the case of the 500, we need to make sure the spark happens at the point that is 13mm before TDC on the pulley.

Not patronising at all! A great explanation. Very helpful, thanks.