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MG Midget and Sprite Technical - Timing Questions

Please excuse a couple of basic idiot type questions but I'd rather seek some knowledgeable advice before I proceed and potentially muck things up!

Nearing the end of my re-build, I set about sorting out the ignition timing and this has thrown up some anomalies.

The picture shows my distributor leads as they have always been, doing their job perfectly; the zip ties indicate the cylinder number. I assembled the engine by the book (Haynes mainly) and installed the distributor drive as directed, with the off-centre drive slot at the bottom, ending up on the roughly 2 o'clock/ 8 o'clock diagonal. This placed the rotor arm in the 2 o'clock position which, in my case, aligns with the number 4 lead.

I'm sure I assembled everything correctly, so surely the leads should be as per my numbering on the cap. I can only think that the PO fitted the distributor drive 180 degrees out.

Am I right in assuming that just switching the leads to match my numbering will sort this out?



A bit more technical and pushing my boundaries: As advised by a local engine builder, I replaced the cam gears with a vernier set-up. In the search for better acceleration, I also set it with 4 degrees of advance. I assume that, as a starting point, I will need to set the ignition to the normal static setting (7 degrees BTDC for my distributor) and not, as Haynes describes, according to the rocker action on number 4 cylinder. I'll do a dynamic timing check with the light once the engine is running and warmed up.

Thanks in advance, Colin



C Mee

Sounds like you have the drive in the right way and on mine the rotor arm pointing about 1-2pm is No1 , like you said perhaps the drive was originally 180deg out .
Q2 beyond my knowledge!
Mike Fairclough

Depending on which version of Haynes you have, it could be wrong. In the past, I’ve fitted the dissy drive 180 degrees out by following Haynes.

The procedure I now use is to set the engine to TDC with no. 1 cylinder on the firing stroke (both valves closed), orientate the dissy so that the rotor is pointing to where I want no.1 lead, then insert the dissy drive to match.
Dave O'Neill 2

It does sound like the drive was originalĺy 180° out. I can't comment on the 2nd question.
Bill Bretherton

Colin

I follow David's procedure.As a possible explanation I think in the dim mists of my memory the 948 and possibly the 1098 were timed 180 degrees from the 1275. Hence the discrepancy with Haynes.
Bob Beaumont

Colin,
have you changed the dissy cap and if so where did you buy it from?

On the cap, depending how you look at it, 1 & 3 HT leads should be at one level and 2 & 4 on the other.

Your cap markings are not as convention.

If 180 degrees out firing for Ht lead/cylinder one would be at about 7 o'clock instead of about 1 o'clock.







Nigel Atkins

Colin,

With that type of cap hopefully you use some sort of sealer around the leads as in my experience without sealer they're very prone to damp and dirt causing tracking and misfires. Often very visible when viewed in the dark.
David Billington

It may be 'convention' but it doesn't matter what level the specific leads are at, so long as they go through the correct sequence 1342, on clockwise rotation.

Colin, are you sure you had #1 piston on the compression stroke when setting static timing?
GuyW

Colin. It is possible for both the distributor drive (the part inserted into the engine) and the distributor internals to be assembled 180 degrees out of phase. It is also possible, since the engine rotates twice for every rotation of the distributor, to try to time the engine at the end of the exhaust stroke rather than the end of the compression stroke.

My process begins by removing the rocker arm cover, then rotating the engine by hand to watch the intake valve open and begin to close. The intake valve closes just after the beginning of the compression stroke, so keep rotating the engine over, by hand, until the timing marks are aligned with your static timing requirement. Then, install the distributor and see where the rotor is pointing. It should point to about the 2 O'clock position and this is where the number one lead from the distributor cap should be. Now that you have verified that the proper (number one) lead is at the two O'clock position, you install the number three lead to the left (anti-clockwise/counter-clockwise) terminal, the number 4 lead to the terminal at the eight O'clock position, and the number two lead to the remaining position. Then, after timing the distributor, you are ready to start the engine.

As to timing, the traditional British method is static timing and, back in the old days (before almost everyone could afford a strobe light) timing was performed in this manner by everyone including the majority of professional mechanics. Today, with strobe lights being so common, it is just as easy to recruit an assistant, attach the strobe light to the distributor, have the assistant crank over the engine, and rotate the distributor as the engine is turning over to the old static timing mark and watch the engine start more easily and with less fuss.

As a side note, your installation of the distributor drive (assuming you are measuring on the end of the compression stroke) seems to be perfect and indicates that both your installation and the build of your distributor is perfectly correct. The hook up of your distributor cap, it would seem, was incorrect originally (due to someone else's mistake) and has caused you to doubt the excellent quality of your own work. Ensure you are on the compression stroke by watching the intake valve open and close while bringing the pointer up to 7-10 deg BTDC position. If you still see the rotor pointing to the two O'clock position, you did your work correctly and now need to take care of someone's former mistake.

Les
Les Bengtson

Guy. We were posting at the same time. Am I mistaken and the A series engine has a distributor that rotates the opposite direction than the B and C series engines which rotates counter-clockwise? I think you meant "anti-clockwise" rather than "clockwise) for installing the plug leads.

Les
Les Bengtson

Les,
yes he did, the image I used is from a B I think.

Guy,
true it doesn't matter as long as the sequence of the layout of the posts terminals inside matches firing order. I was trying to get the cap to match Colin's photo of his leads to make it easier for him to see, neither photo sits well with me.
Nigel Atkins

I don't really help myself, do I!

This time with the leads numbered correctly - or as I think they ought to be but aren't. Sorry for the confusion created.

Thanks for all your comments, especially for your detailed responses Dave and Les. As it sits at the moment, No 1 is at TDC, No 4 valves are rocking and the rotor arm is at 2 o'clock.

My intention now is to shuffle the leads around in the cap as per the corrected numbering and to set the timing at 7 degrees BTDC.

Dave, thanks for the suggestion about sealing. The original set of leads gave quite a good light show when I ran the engine in the garage with the lights off! That's the reason for the zip tie on the coil connection, a prevention rather than a cure. This set has been in place for a few years without giving any trouble. Incidentally, I just dug out the original set and the old and new arrangements match, so I definitely lay the responsibility on the PO, who also used mis-matched distributor mounting bolts and chewed the points adjusting screw (replacements on my next shopping list Nigel!)






C Mee

Colin,
that looks a lot better for the cap, the photo and numbering to the cap I put up didn't sit well with me.

Is the rotor arm a bit chewed up or just how it appears (I don't see a DD on it).

I'd also change the HT leads if they're a good few years old and fit the 8mm ones which will make a sealed (tight) fit to the cap. You also want the boots and their terminals to be a good tight fit in our land of slight misty now and again. Times I've had or seen leads that look alright and "work so don't need changing!" for them to cause problems. Tired ignition parts can detract from reliable instant starting and running taking away the crispness, cheap parts for motorsport use.

Yes best to replace that screw as you may well need to readjust the points after a few miles.

Your AH Spridget seems a bit like the AH Spridget I had, a couple of things were backwards, I used to say it went better in reverse.

I hope you're not also going to throw in four odd spark plugs you've found in your toolbox or shed drawers because they look alright and you can't remember them being used much - we want you at the top of the hill well ahead of your previous times. 😁
Nigel Atkins

Indeed Les, my brain thought anti-clockwise, but fingers forgot the anti bit!

To check it's on the firing stroke I usually just remove plug #1 and put my thumb over the plughole to check it has compression as the timing marks are coming round to TDC. I then tune a portable radio off channel and with the ignition on, turn the dizzy until I hear the "click" of the spark interference on the radio. Its more accurate than trying to see the points contacts down in the gloom, it saves having to take the dizzy cap off and anyway I don't have the required resources to use the traditional cigarette papers these days.
GuyW

It was annoying me too much, I had to change it.



Nigel Atkins

Or if you have a meter put the leads from dist connector to earth, on low (continuity bleep test) resistance range, and gradually turn dist clockwise (from a deliberately more anticlockwise position) until a change from zero (bleep on low ohms range) to infinity ohms (bleep stops) indicate points have opened and stop.
Bill Bretherton

Hi Colin, what did you advance the cam from and what to in total degrees?
Peter Burgess Tuning

Hi Peter.

I advanced the cam from 110 degrees to 106 in line with my interpretation of what it says in Vizard.

Sincerely hoping that's OK!

Cheers

Colin
C Mee

Brill, just checking you had gone the right way, not trying to catch you out. Will you fine tune on a rolling road?
Peter Burgess Tuning

Phew, you had me worried for a moment, Peter!

Am I on the right lines with the ignition timing if I set it with reference to actual TDC and ignore the cam advance?

I'm hoping to book a rolling road session with you when it's got 500 miles on it and we're allowed out for non-essential stuff - though, to my mind, a rolling road session with you is a definite Essential!


Thanks to everyone for the tips on ignition timing. I haven't tried Guy's radio method yet, but I did try a modified version of the finger over the plug hole test.

The Sprite's up on stands at the moment, and the hex nut on the PM competition half shafts was useful for turning the engine over. As I couldn't reach the plug hole to put my finger over it and turn the wheel at the same time, I came up with a simple compression indicator - it's the thumb of a vinyl glove if you're asking! It confirmed that Cylinder 1 is definitely on its compression stroke!








C Mee

Nice one Colin! 👍
Patent it!
GuyW

Yes to ignoring the cam advance, just use TDC as reference point.
I like the TDC balloon as well.
Peter Burgess Tuning

Thanks Peter, that's what I was hoping you'd say!

As for the TDC indicator, Heath Robinson is myinspiration! :)

Colin


C Mee

Colin,

Would it be possible to obtain distribution rights for North America for your invention? ;-P
Martin

By all means Martin.

What are you offering by way of royalties? :D
C Mee

As far as royalties goes, my people will get in touch with your people to iron out the details. I never deal with numbers.
Martin

My version of the high-tech blue vinyl glove pressure tester in use checking on crankcase pressure.

Photo 1 at 3000rpm, photo 2 at tickover.





GuyW

I guess I'd better get started on designing the left-hand drive version for you! :)
C Mee

Guy,

Reminded me of the Otto the Autopilot in the film Airplane !
richard b

I thought I still had a video of it which was rather more amusing as the crankcase pressure starts from negative and gradually rises with the glove inflating, rising up and starting to wave at any passsing spectators!
GuyW

Colin,

We could sell both since there are some right hand drive cars around here although you don't see them too often. Mostly at British car shows.

Guy,

Is yours available for marketing also?
Martin

Ha ha ha--reminds me of a practical joke we played on a work mate once .--She rode a tiny little scooter thing to work with about a 1/2 inch exhaust pipe---Plan was to stretch the end of a balloon over the pipe and when she started it ,it would blow up and 'pop'---
Didn't work, the thing must suck about a tenth of a cc at idle, she started it , off the prop and away and got about 50 metres away before the balloon blew off and did a wizzy flight behind her, much to the amusement of us onlookers but unknown to her
Good fun though all the same-
willy
William Revit

We have lift off! - Well, Start-up anyway!

Firing up my Sprite's engine after its rebuild.

Not quite the first turn of the key, there were a few coughs and splutters first, but it didn't take long:

http://youtu.be/lqruDt6ddgo

The smoke had me worried for a moment but it was just a bit of oil on the exhaust manifold burning off.

Now to get the carburettors tuned and timing checked and 101 other little jobs, not least of which will be bleeding the clutch, and we're on the road again!

OK, it's only a BMC A series, but it's my A Series and it's the first engine I've rebuilt in almost 35 years, and only the third rebuild ever.

The proof will, of course, be in the pudding when I get it back on the road, which will be as soon as possible on a day when it's not raining. If it hadn't been for several unwanted interventions by 'My Friend' I would have been here a couple of months ago! :)

The main reason for posting today is to say a heartfelt Thank You to everyone here who as very patiently answered my sometimes very basic questions and helped and encouraged me along the way.
C Mee

The engine sounds good. Great job. Ready just in time for the weekend ;-D
Martin

Well done Colin.

Sounds and looks lovely.

Lovely to hear all those birds too. Looks like your heater blower has been to the vet's, I hope its collar can come off soon.

I know it's passed but I can suggest to you from personal experience it's good practice not to leave items resting in the engine bay (rad cap, deep socket(?)) particularly with the engine running, although it's unlikely at the moment you'll close the bonnet onto a tool you've left. I usually end up knocking the items off and into difficult to get at recesses or the only fitting I have of that type wedged or lost until my wife locates it.

I've just seen the rotor you have is a DD, so does that mean the points and condenser/capacitor are also from the Doctor or are all three from Moss?

Sorry but another word of caution, I split quite a lot of 3-in-1 onto my exhaust manifolds without realising how much and when I lifted the bonnet and looked for only a second just to check it was that the fumes affected me for the rest of the day, only mildly but enough and very unexpected as I drove straight off with the roof down so got plenty of air.

Sorry, well done again.
Nigel Atkins

C Mee,

Good work, sounds great. Don't forget to remove the torch before putting the bonnet on or you be wondering where it went.
David Billington

Well done Colin! It's good isn't it, the first time it starts. The smoke is nothing, I had that. Did you reface the cam lobes and followers? If so you should keep revs above 2000/ 2500 rpm for a while to bed the cam in.
Bill Bretherton

Now David has pointed it out I can see it's a torch, and it moves about a bit, I was fixated on the rad cap and smoke, even watching repeats I'd wonder if the cap might fall or smoke get worse.

I did notice that Colin doesn't lack for rubber hose of at least two sizes - well stocked Colin.
Nigel Atkins

Good stuff Colin--Like Nigel ,I was fixated on the radiator cap, it's going in, it's going in, but it didn't---
Congrats on your result
willy
William Revit

Thanks for your comments. The one about leaving tools etc. around the engine bay is noted, though the torch was well contained within the bonnet hinge, and I have an electric fan otherwise I wouldn't have left the rad. cap where it could fall onto the fan. That torch has been really useful for peering into nooks and crannies throughout the rebuild.

Yes, Nigel, a DD rotor arm and capacitor which cured a cutting out when warm problem.

As for the smoke, one friend said he'd be more worried if there wasn't smoke when starting up an A series! He also told me to get rid of the quick release battery terminals - thinking about it.

I didn't touch the cam or followers and kept everything in a bit box/organiser so that all the valve gear when back exactly where it came from. I did, however, follow Peter Burgess's guidance on starting up and ran it at above 2000 rpm for 20 minutes. Delighted to find there were no leaks, not even any blowing from the exhaust joints.

The engine is definitely much quieter than before but there is one new noise I'm trying to track down - might be associated with the timing gear.
C Mee

Col.
Don't forget to retension your manifold studs after it's been nice and warm, they usually move quite a bit, and your timing chain noise, if it has a new chain with a joining link and the previous chain didn't the joining link might be touching the oil slinger---? that would be a little click click click type of noise

And go to bed it's half past one----------when you posted, and 3am for you now--2pm here
William Revit

Can I add my 2cents on the topic of loose items around a running engine?
One morning last summer I replaced a blown dip beam lamp in the dreaded Volvosaurus. Dead easy job - the whole headlight unit pulls out of the front of the car but to release it you have to pull out two flat metal retaining bars.
These I had simply set down on top of the airfilter.
Having replaced the lamp I started the engine to check the lights. (Being a Volvo it turns on the dip beams as soon as it starts which annoys me but hey-ho...)
One of the bars dropped down and hit the alternator belt and was instantly flung up and hit my sunglasses. The result was a wrecked (expensive) pair of Raybans and a cut on my nose but if I hadn't been wearing the sunnies I would almost certainly have lost an eye.
I'm much more careful now.

Brilliant news about your engine Colin. Well done indeed.
Greybeard

Thanks Greybeard, I'm really glad you suffered nothing worse. I guess even the best of us make the odd mistake.

I thought my engine bay in the video wasn't too bad, you should have seen it before I set the camera up!

Points and tappets checked today, all in order and still no leaks. :)

Attention turning to carburettors tomorrow.

The return to the road is delayed again. Brake shoes one side have got oil on them from an axle weep, probably down to having been on the stands for over a year. Fortunately I've got spare gakets and O rings but have had to order the shoes.
C Mee

This thread was discussed between 14/02/2021 and 20/02/2021

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.