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MG Midget and Sprite Technical - ignition timing: big detonation in the exhaust

Hello,
I do not succed in starting the midget, cause of ignition timing.

So what I have done:

Cyl 1&4 at TDC
valves 1&2 have got play at the rockers
valves 7 just before opening
the rotorarm (metal part) is in front of slot of the N°1 cylinder
distributor turned on the left, and with a 12V lamp, I turned right (clockwise) to have the lamp lights on.

When I try to start the car, I have got a big detonation in the exaust silencer.

What is the trouble?

laurent
ld derancourt

Are you getting a spark at the plugs?
Brad (Sprite IV 1380)

how to test?
ld derancourt

Put no.1 HT lead onto a spark plug which is resting against the block, crank the engine, you should see & hear the spark.
Brad (Sprite IV 1380)


As I didn't suuceed in with this DM2 distributor. He worked well, cause it came from my previous engine, I have swapped with a 25D, a coil and HT leads I had on stock.
I did the same process and with this one, it works fine.
I think.

I now have to understand if it s the coil or the distributor wich was faulty.

many thanks Brad.
ld derancourt

Laurent. There is a tech article on checking out the ignition system, and another on timing the engine, on the MG section of my website, www.custompistols.com/. Between the two, you should be able to find out where the problem is if it is ignition related.

Les
Les Bengtson

thanks,
I have now to do my best to understand every details of the articles.
laurent
ld derancourt

Laurent. But, you can contact me through the website and, hopefully, I can answer any questions you might have.

Les
Les Bengtson

I think I was mistaken with the advance.
currently, the engine does not go more than 3500 rpm.
I have used the TDC mark on the pulley to check the distributor position with the lamp.
If I have well understood, I have to put the pulley on the 5° advance and then chek the right position for the distributor.
Is it true?
ld derancourt

Laurent, sounds like you have timed it for TDC rather than 5BTDC.
Brad (Sprite IV 1380)

If it is detnating in the exhaust, the timing is a lot more than 25 deg out. Most common cause of this is plug leads in the wrong order or wrong position (Don't ask me how I know!).
Not the sort of thing you want to admit to, but even the most experienced of us can get the odd bit of brain fade. With that symptom I'd definitely check that out.
Paul Walbran

Laurent, do you have a vacuum advance on your ignition? This should be located at approx. 11 o'clock. If not, the timing might be wrong (no problem, run like that for a couple of years, as long as you now)
Alex G Matla

I have remove the distributor, cause the vacuum advance was at nearly 9 o'clock, and touched one backplate screw.
The driveshaft was turn left at nearly 90° compared to the normal position.
I have cured this.
I have now retimed the distributor with the lamp and the puley just before the first small mark (4-5° advance?)near the big one (TDC).
It is a 3 marks front cover.
The distributor is now at nearly 11 o'clock.

But the HT leads are now wrong:
old 1 is now 4
old 3 is now 2
old 4 is now 1
old 2 is now 3

So I have to do no mistake when I am going to plug the sparks.
Don't laught, I can still mistake at that stage.

As we say in France, I have 2 left hands!!!!!!
ld derancourt

I know this thread is kind of finished, but today I set out to do some timing on Little Blue. Last time I put the engine back in (not long ago) I timed it by swinging the dizz till she ran best. Since when she's been making a hell of a lot of exhaust noise, and vibrating quite a lot.

I remember recently somebody saying on here (not to me), "if you leave it like that, you'll have far too much advance". So that's been niggling at me, and today I set about it properly.

I checked my up-top timing marks against the old (almost invisible) set-up down below, then fired up the strobe. And by the time I got the marks opposite each other, I had dialled in 35 degrees of advance! This was at 1000 revs.

Swinging the distributor back seemed to make it run terribly, but when I got to about 7-8 degrees, and adjusted the throttle, it ran fine. Noise gone, vibration gone (well, nearly), pinking gone. And great on the road.

So, what I want to know is - how? why?
Nick

Nick

Advancing the ignition at idle will almost always cause the idle speed to advance. This is because there is very little air/fuel getting into the cylinder, so effective CR is very low. The vac advance unit does not fully compensate for this, because:

- the vacuum unit cannot be made rev senstive
- so it is limited to around 10-20 degrees total max advance
- otherwise, if it could, it might give 30, 40 or even 50 degrees advance with max vacuum, and this might at high rpm add onto 32 degrees mech advance, which would give ignition before compression had started / cam still open.

For reference, my Emerald gives max advance with zero throttle and max revs of 50 degrees; this represents max mechancial (incl static offset of course) say 32deg and max vac advance say 20 degrees.

But at idle, it gives around 10 degrees advance; representing static (7) plus little mech advance (3) and little vac advance (0?) (even tho vac is high). The vac component of advance is able to be rev senstive.

So on conventional systems, vac adv is limited to an overall low max, since it can't be made rev sensitive.

So it mean timiing at idle is not optimum for efficiency; which means you can make the engine more efficient for idle if you give the engine mroe static advance - but then the engine is far to advanced for the rest of its rev range and so pinks and detonates (= vibration).

Hope that's explained reasnably well.. meal is calling.

A
Anthony Cutler

Thanks very much. Yes, I did understand that (surprisingly).

I hated that 'all mouth and no trousers' noise from the exhaust. Now I have a nice strong steady pull up to 5500 revs, and no pinking.

Was my turn to cook tonight, hence the belated thanks.
Nick

Well I have certainly told that to many people on these boards although most ignore it to there peril. However I have never explained the reason for it happening as well as Anthony has done. May I use that explanation in future threads Anthony? perhaps explained in that manner may get through to the doubters.

I have noticed that at work if I or anyone else tries to explain difficult theories then most times they are ignored but when someone comes along with a computer andd explains the same theory it is grasped by all present. I put that down to looking proffessional, what a strange and difficult world we live in.
Robert (Bob) Midget Turbo

Hi Bob

By all means; please correct the spellers incl the 'to advanced' (should be too).

... was under pressure to present myself in the dining room!

A
Anthony Cutler

This thread was discussed between 11/06/2009 and 23/06/2009

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