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MG Midget and Sprite Technical - Ignition timing

I'm still trying to get optimal timing on my 998 engine. The distributor is a 59D4 from a 998 Metro, with Powerspark kit. The Lucas book says the maximum advance on the Metro should be 5 at 1200 rpm, 10 at 2000, then rising steadily until topping out at 20 at 4800. I have used the time-honoured method of advancing until the engine pinks when well warmed up, and backing off until it doesn't. The timing light then tells me that the advance is 15 at 1200 rpm, 15 at 1500, and 20 at 2000. Vaccuum disconnected of course. It then stays at 20 - I haven't run it above 3300 on these tests to spare the neighbours the racket.

The setting I have gives the best throttle response and torque. If I retard it the engine goes soft. The downside is that when starting from cold the engine usually fires instantly but doesn't catch - it kicks back judging by the whirring from the starter motor. This is just as it would do if over-advanced, but as I say backing off hurts the performance.

Is the advance hitting the limit too early? The curve I am getting seems odd, as it just advances once to 20 without a steady progression from the secondary spring. The engine build followed Vizard's recipe as far as possible, and he recommends more top end advance. I have bought a few different springs and will play about with those, but would appreciate advice from those in the know before I do that.

I use regular unleaded fuel. I have serviced the distributor and the whole mechanism is in good nick and well lubricated. Look at my profile for the engine spec if you are interested.

L B Rose

Setting the advance curve can be a challenge without a rolling road. but I think you have may have too much advance too early. If you look at the Aldon Yellow distributor advance curve its 9 degrees at 2000 rpm, 14 degrees at 3000 rpm and 24 degrees at 5000 rpm. Aldon recently rebuilt my distributor which was an old Cooper S and the primary spring is clearly much stronger than the original.
Bob Beaumont

I did put it on the rolling road at Brian Slark's when I first built the engine. I was expecting to get the advance curve sorted out for the 100 but they just said it's advancing OK. That was years ago though.
L B Rose

Calling Peter....

Bob was ahead of me. Sounds like a weak spring to me.

Or worn spring. The secondary spring could be taking up too late due to wear. That would allow it ticadvsnce further on the weaker primary spring and in some distributors be at nearly full advance at idle. MGB is bad for this. Doesn't take much for this to happen, only a few thou.
Address by closing the hook of the worn spring a little. Need to make the hook shorter, not narrower.
Simple to do, just remove the baseplate to access the springs.
From the info you have given, the 2nd spring should take up after half or a bit less of the way before the cam meets the stop.
Paul Walbran

Took the words out of my mouth Paul
I had a B where the springs in the dizzy had rusted and the smaller of the two had fallen apart leaving just that last bit of advance from the fat spring
Exact same results as Les
Maybe take the cap off and give the rotor a twist and see if it springs back -- if the springs are an issue it will just be foppy and twist around and not return
William Revit

Thanks guys, I will do the rotor twist test and report back.

L B Rose

The distributor is on the bench now. The thick spring is very loose and is still slightly loose when the cam is on the stop. The next job is to replace it, but the ends are very tight round the posts and I can't get them out of the grooves. How do you get these things off? Will I have to get the shaft out of the body?

The stamp on the cam has been very poorly done and I can't read the maximum advance figure.

L B Rose

The thicker spring usually has some travel in it
What condition was the little spring in

Just roughly to put you on track
The light spring should be strong enough to just hold the advance mechanism back against the stop
The other spring usually has an elongated eye on one end of it and is not under tension at rest

How it works is, as the revs increase from idle the weights start to swing outwards which advances the top shaft and stretches the light spring in the process which controls the advance rate
As it gets more advance(more revs) the peg reaches the end of the slot in the eye of the heavier spring(until now loose) and then the weights have to stretch both springs which slows the rate of advance from then on till the top shaft hits the advance stop

I was expecting an issue with the little spring, either rusted out or rusted enough to have lost it's tension, Was the top shaft returning back ok when you twisted the rotor and let it go

The springs are as you say a bit of a tight fit on the pegs, but usually you can twist them up on one side and roll them off
William Revit

Both springs were in good condition. The small spring holds the cam back OK, and looks like new. Yes the top shaft returns OK with the new spring. With the old one there was slight backlash. I have a range of springs to try.

I had to knock out the roll pin from the key and push the shaft up to get at the springs. I have replaced the big spring and now just have to remember which way round the key goes!
L B Rose

It may be the small spring is still too weak allowing to much advance. The problem is that it could mean quite a bit of experimentation with different springs to get the optimal performance. I had this problem which is why I sent the distributor away to be overhauled as Aldon were matching the curve to the CR, Camshaft profile and capacity. It was certainly much better when it came back.
Bob Beaumont

Yes Bob I remember Vizard writing that an engine with this spec had to be kept from detonating at low to mid revs, and then brought up to much more advance at the top end. I hope to get the higher speed curve right before fiddling with the lower speeds.
L B Rose

If the shaft is returning with your new little spring, where it had backlash with the old one, you are well on the way to making it better, -The old spring must have lost it's tension
The shaft ;has; to return back on it's own whatever the setup is otherwise you will have unstable / varying idle making it hard to tune
William Revit

William, it was the big spring I changed first. This morning the engine started instantly, but the ignition was more retarded than before. On a test drive it was less responsive. I changed the small spring and frankly I'm hard pressed to detect much difference. I have the advance at 12 at 1200 rpm and the throttle response is OK but not as good as it is at 15. But that gives me detonation at 2000-3000 rpm. I suppose I need a stiffer primary spring and maybe weaker secondary.

I find I can't get the springs off the posts without damaging them!

L B Rose

Ok Les
If the timing was more retarded at startup with the dist. in the same position as before then that prooves that you previously had a week primary spring problem andthat the dist had been retarded to compensate for which in turn had reduced the amount of max advance
12deg at 1200 should be about right, but if it feels flat at that and better at 15 and you want to run it at 15 then yes you will have to go slightly stronger with the primary spring
I'd only change one spring at a time though so you know what the result is

Really if the engine is basically std 12deg should be fine though--and if it doesn't ping at that 2000-3000rpm at that it can't be all that far out-Is the vac advance part of the dist. ok and hooked up and working properly--?
William Revit

Yes, as per Willy's post you can't be that far out at 2-3000 if 3 deg more advance makes it ping.
It could be that it is teaching Max advance too soon or advancing at the wrong rate. It would be worth measuring the advance every 1000 rpm until it stops advancing.
If we know that, the compression ratio and camshaft type then a quite accurate assessment of what you need to do can be made.
Paul Walbran

If you are not far from Salisbury area you could contact mini Guru A.C.Dodd via Fb and ask him to set up your advance curve to suit your engine.
Alan Anstead

The dizzy Aldon rebuilt for me was a Cooper S type without a vacuum advance. The original S springs were a light primary and stronger secondary. It now has a much stronger primary spring than before. Aldon explained the burn rate of modern fuel is very different these days and more gradual advance is needed. The engine certainly picks up and runs much better. I am sure they could help on spring rates as well.
Bob Beaumont

Thanks again guys. One major improvement is that the engine starts instantly from cold - no coughing or kicking back. That's a relief. I have to rely on the mechanical tach though, and I know that isn't very precise. I'll try to record the advance curve I'm getting and let you know.

I'm not on Facebook. I live in Salisbury, but there are no contact details for AC Dodd on his FB page.

L B Rose

Went for a run yesterday. I found that if I open the throttle at 2500 rpm there is very slight pinking, but only when very hot. Worse when going uphill of course. No pinking above 2500 at any throttle opening. The engine is running hot, but I can't be sure if that's because of the ignition timing or that the pinking is because of running hot, ie the other way round. I just checked the gauge (hot water and cooking thermometer) and it seems pretty much spot on. The gauge shows 190 of so around town but about 200 on open road. If the temp rises only a few degrees the pinking starts, and when it cools it goes away. I have scrupulously cleaned the cooling system as discussed in another thread, and fitted a new thermostat. A plug test tells me the timing is OK, but to be honest I really need to switch off the engine at full throttle under load for that to be a good guide.

If this gets too boring just tell me to stop!

L B Rose

Sounds a bit like your thermostat is the wrong (too hot) one
William Revit

Yes its running a bit too hot. I would aim for around 165/170 when on the open road.
Bob Beaumont

It's, is your compression ratio standard, or is it up a bit?
The temperature sensitivity like that can indicate a spark plug temperature issue. Generally I have found that for cr around 9.5 plugs need to be BP7ES, and BP8ES for 10 or higher.

The risk with cooler running plugs is of course that they might foul, but for an engine with rings etc in good order I have not found it to be an issue.
Paul Walbran

CR is 10.5. Plugs are BP6ES. I will try BP8ES. Thermostat is 75C.

L B Rose

Mine is 10.5 and the BP6ES are fine. I don't have a thermostat though just the blanking plate. It runs about 165 when hot and will creep up to 190/200 in heavy traffic. Standard vertical flow rad with 6 bladed fan
Bob Beaumont

For the price of them I'd be trying a new thermostat, yours could be playing up
75 should be ok but a 72 or 70 might be worth a try
I think the original spec was 71/160f
William Revit

M*ss offer a choice of three 'stats, depending on climate

Thermostat, wax type, 165F, 74C, hot climate

Thermostat, wax type, 180F, 82C, standard

Thermostat, wax type, 192F, 88C, cold climate
Dave O'Neill 2

Have you checked your valve clearances? They may be a bit tight, especially if you have a performance cam fitted.

Damn, my last reply disappeared. Yes Guy I have checked the valve clearances to death. The cam is a Kent 276 and recommended gap is 0.40mm. Makes the engine clattery but I suppose that's a penalty for a performance cam.

I will try a new thermostat, and will check it in hot water first. How widely do these things open? I tested a few of them a while back and none opened more than about 3mm.

Should have mentioned, I have a thermostatic electric fan on the front of the rad and I wonder if it occludes too much. Might take it off and run without it.

L B Rose

lock out your advance

and use this to set your timing

The Stealth box is no longer available
Dominic Clancy

I fitted a new 72C thermostat, having tested it in hot water. The engine runs at exactly the same temperature, about 200F. Not right. But the cooling fan switch is 85F and almost never comes on. Confusing! Even though I have calibrated the gauge, either it is wrong or the switch is. The gauge sensor is in the head and the switch is above the thermostat. So if the stat is open at 72 the switch is sensing the actual water temperature.

On today's run the engine started instantly but it was back to that awful hesitancy when picking up from idle. Also it won't idle steadily but gradually slows until it stops, just as if it were running much too rich (which it isn't according to the plug colour). This is with the advance at 7 at 900 rpm. I tried giving it a few more degrees of advance and the throttle response was better, as was the idling (but still not perfect). However the pinking at 2500 rpm came back. After that point it goes like a train.

So I think I need more static advance but a shallower curve up to 3000 rpm. Hence a stronger primary spring. Does that make sense?

Thanks Dominic, the Black Box would be nice but it's a lot of money and would have to be shipped from the US.

L B Rose

74c (165F) water stat could start to open at 72c75.5c (162f168f) and should be fully open at 80.5c86.5c (177f188f) so within 85c (185f) thermoswitch and its tolerances probably.

To me the water stat, electric switch and gauge never seem to exactly agree with each other.
Nigel Atkins

Les, we recently had in a Sprite with worked 1275: 286, 45 DCOE, 10.8:1 CR.
Came to us running on badly and with a nasty gasp at low revs. Distributor and jetting had been set up on RR from 3000 upwards and was fine there, but clearly they hadn't considered anything belw that.
Once the timing curve was advanced to address the gasp, pinking set in between 1500 and 2000, but only after the car had been driven 400m or so. Stop, turn the engine off for 2 min and repeat the test: no pinking at all on full throttle until driving 400m. This was very consistent. To me that indicated plug heat range too hot, and that was already on BP8ES. Went a further step down (Bosch W2CC) and the problem went.

I realise that the Metro had 10.5 CR and ran BP6ES, and certainly some modified engines run OK on them at higher than std CRs. But quite a few don't, so plug type is worth following up.

The other thing we did was fit a 123 programmable. This allowed us to bring the idle advance right up and so reduce the throttle opening and therefore the amount of fuel feeding the run-on. At 1000 RPM idle, this did the trick and the run-on ceased. Until the owner increased the idle setting to get it to idle on its own immediately after start-up, which then brought it to 1300 hot and the run-on returned, though nowhere near as bad as before. He preferred to retain the idle on its own when cold (oh for a fast idle cam like the SU) so that forced us into an anti-run-on valve (which I usually regard as admission of defeat).
We then used the ignition advance to limit the idle speed as the engine warmed up, by reducing advance by 15 deg just off idle. That limited the hot idle to 1100 RPM which was a suitable compromise.

Having addressed all that we then set the timing at 2000 RPM on road test to minimise the gasp without introducing pinking.

The curve we ended up with was:
900 and below 25 deg
1200 10 deg (10)
2000 20 deg (15)
4000 21 deg (21.5)
5000 25 deg (25)
7000 30 deg (30)
The timing above 3000 is as set on the RR, no point in trying to reinvent the wheel. The figures in brackets (there for interest and comparison) are those for the Cooper S, 10:1 CR on standard BMC cam timing.

The end result was the engine will accept full throttle above approximately 1500 RPM pulling strongly without a gasp, and will pull away on light throttle from 1300 RPM even in 3rd and top gears. Below this speed, pinking will occur in 3rd and top in particular due to the advance ramping up under 1200 RPM to address idle conditions. But then, if all you want to do is lug away in top gear under 1300 RPM then you wouldn't run a 286 cam.
Paul Walbran

Thanks Paul, all very interesting. Maybe I'll try the plug change first.

L B Rose

This thread was discussed between 15/06/2019 and 19/07/2019

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