contact breaker gap setting

[scotthesniper]

hi guys i have a morris minor 1000 and was adjusting the timing and the points for some unknown reason have closed i have looked in the book and its saying between 36mm - 40mm im used to setting the old stuff at 25mm, so im just asking this to make sure of the setting of these points and if it matters it has a black distributor cap on it

scot

[simmitc]

Your typing is very difficult to read, but I think that what you're asking is simply what gap to set for the points. I don't know from where you are getting the wild metric figures that you quote, but the points gap should be 15 thousandths of an inch. To set the timing correctly, you must first set the gap for the points.

[bmcecosse]

As above - the correct gap is 12 to 15 thou (0.012/0.015) of an INCH!! The gaps close up because new points these days are poorly made - and the plastic heel that runs on the 4 point cam wears away very quickly.. You can 'help' it by apply a tiny spot of grease to the cam - and by setting the gap on the 'wide' side of correct......

[horologiumwatches]

I set my gap correctly first, then check the dwell. I then tweak the gap until the dwell is sitting smack through 60.

[Bit of Both]

Forgive my ignornce, but how do you measure the dwell?

[bmcecosse]

It's a waste of time - don't bother...... The wear on the dizzy shaft will vary it (and the gap) more than it's ever worth worrying about. The gap is not that important - as long as there IS a gap!

[horologiumwatches]

I beg to differ. You will notice a difference with different dwell angles. Yes the dwell will vary according to wear on the shaft, but you can get an idea of the ball park that it's in. You measure the dwell using a dwell meter, clip it on the battery and on the low tension lead, and it will give you a read out on the gauge in degrees.

[bmcecosse]

Hahahaha = definitely 'old skool' thinking there! The gap is the thing - set for 15 thou and all will be well - for a while, till the 'heel' wears down.

[bmcecosse]

So - when you tediously set your '60 degree' dwell - what gap does that give? I'm also interested to know just how the 'dwell' is measured - how does it know the rotational angle of the dizzy cam - or the speed of rotation?? I suspect it's charging up a capacitor - something like that??

[IslipMinor]

The correct dwell angle or % is the actual requirement to give the coil its full 'charge' - setting the points gap is merely a simple way of achieving it, but it gives the same value for all engine speeds, which is not necessarily optimal. I.e. more charge time at low engine speeds and less time at higher engine speeds.

We are going to fit an new Accuspark 45D distributor to my son's MGB this weekend, complete with their programmable 'black box unit'. One of the programmable parameters is the dwell % over the complete speed range - their recommendation is less dwell at lower speeds (20%) and increased dwell in the higher speed range (75%). The A-Series has a set 60° dwell angle, which equates to 67%, so using Accuspark's recommendations, it has 'too much' dwell at lower speeds and 'too little' at higher speeds.

Once this has been fitted we will be doing a full rolling road session with Peter Burgess to set it all up (it is a newly built very well modified engine), and it will be very interesting to see the effect of different dwell settings on torque and power - I have no idea what to expect, so we will see??

[smithskids]

Beat that

[stag36587]

Indeed and a great explanation

[horologiumwatches]

So when my dad taught me to set the gap using a 60 degree dwell he wasn't wasting his time then? Are we suggesting that other people also have views that may be of worth before bmcecosse steam rollers them?

[bmcecosse]

I bet the famous '60 degree' equates to 15 thou.........still no-one has mentioned how it is measured, and to what accuracy...I'm waiting!! It's the amount the condenser gets charged, dependent on how long the points are closed - and yes at low speeds there will be plenty of time - although I doubt it's possible to 'overcharge' the condenser... When the points snap open the condenser then discharges back through the coil causing the HT to appear at the dizzy centre tap - and hopefully be led from there to a spark plug by the rotor arm..... I wouldn't steam ROLLER anything - although I could steam RAIl it!

[MarkyB]

'60 degree' equates to 15 thou

It probably does, but seeing where a needle comes on a gauge is a lot less subjective than what a sliding fit of a feeler gauge between points amounts too.
As has already been said the gap is a compromise, ignition mapping is the way forwards.

[bmcecosse]

The Mini lads who fit Megaspark claim good improvement in torque even with wide timing cams fitted.

[MarkyB]

Engine design has come a long way since the A series, unfortunately cars have got bigger and more full of gadget too in the same period too.

[katy]

Roy,
The dwell meter that I have has 2 selector switches, 1 for 4, 6 or 8 cylinder engines, the other for RPM or dwell. It measures the dwell by how long the points are closed, in doing so it also takes into consideration the speed between the pulses so that you get the same reading at 500 RPM as at 2000 RPM.

[bmcecosse]

And so - when you set the 'dwell' to 60 degrees - what points gap does it make ?? This sounds as pointless as a Colo(u)r Tune..........or a Vacuum Gauge !!

[MarkyB]

It makes the points gap right, probably 15 thou but why bother with 2 methods?