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MG MGB Technical - Just spent an hour at the roadside...

Running smoothly at 80 when BANG from the exhaust, tacho drops and engine is dead. Coasted to a halt on the side of the A40. Between trucks, I checked for spark/fuel (seemed OK) and changed rotor arm and the car ran badly, with no power. I then changed the plugs, coil, dizzy cap, and condensor to no effect, but on putting the dizzy back in messed about with the timing. By advancing it considerably (I had marked it in 2 places before I took it out) the running came good and I got enough power back to get home.

I've just checked with my timing light and I had to advance it another 10 from my "get home" position. The ignition had retarded by about 20 degrees in the blink of an eye.

What's happening?!
Steve Postins

Very unlikely for something to break inside the distributor and cause sudden retardation, as when spinning everything is trying to advance it. Did it just jump out of the clamp? Changing the points and/or their dwell/gap will change the timing. If the tach dropped to zero and the ignition warning light was off before the engine had stopped spinning then that is definitely a break in the ignition LT circuit or something stopping the points opening and closing.
Paul Hunt

Good job you had all those parts in the boot!
Dave
David Steverson

Just for info - I have come across several sets of points that are badly made - the plastic part is too tight on the pivot and they stick open under certain conditions - very frustrating on a cold, wet windy night.......
Chris at Octarine Services

I'm a little calmer now! Nothing like that to start your day, but I made my meeting and it did make me feel better about lugging those used parts & toolkit around for all these years! I was determined not to go home on the back of a truck!

I should have said it's a GT '74 with 45D4 dizzy and points.

Here's what I know: I don't remember seeing the ignition light come on until the engine stopped spinning as the tach hit its stop. The dwell showed 50* even when it wouldn't run (I had my dwell meter too) so I think that rules out the points? The dizzy seemed tight in its clamp but it's the only explanation that seems to fit so I have to question that. Come to think, I can tell from the scratches I made across it and the clamp that was it was fully home, and the clamp nut was tight to release. More than that, I can see the dizzy is physically turned to the right (advanced) from it's usual position as the vacuum can is in the way of my spanner. Now I'm stumped. Could a loose timing chain cause it? Probably not. I'll have a look inside the dizzy tonight and see if there is something obviously kaput internally.

Thanks for the help.

Steve

Steve Postins

Is it possible that the timing chain skipped a tooth? I replaced my well worn chain and found that the timing had been retarded by about 15 degrees after the new chain was installed.
Bill Young

I would think it would have be be one stretched timing chain and the tensioner would have to be shot for it to jump a gear. How about the Distributer gear. I know my TD has a pin that attaches the distributer gear on the shaft. Could this be loose or perhaps broken?
Bruce Cunha

I am wondering about the timing chain. I'm trying to get my head around what would happen to the car's running, apart from the ign side of things.

Update

I've just had a look inside the dizzy. As I took off the top plate heard a ping and found the (slightly bent) secondary spring rattling about in the bottom. Can't quite see how it came off. However, as Paul said, I also can't see how this would cause retardation? I know from previous work that the secondary spring was played no part in the advance curve I'm using (it was never under tension even at full adavance) so I guess I'll have to put it back without the secondary and will see where the dizzy sits.
Steve Postins

Have a look at the base of the distributor casting, where the clamp goes around it. I had one where the clamp had been so tight that the cast metal had broken. You could never stop that from turning by itself. Just a thought.

Mike
Mike Howlett

I have known a Triumph 1147 (herald) have a timing chain jump a tooth , ran like a pig of course
S Best

Perhaps this shows even more that I'm a home amature and not a pro but if my engine made a loud bang, first thing I'd do is a compression test. Did you change the points? Your post doesn't mention. Dwell of 50 is ok but my cheap dwell always reads wrong. Feeler gauges on the bench work best.

I too am impressed with your onboard stash!
Do you keep hydualic parts too? Clutch slave and MC's seals are a must.
Paul Hanley

I'll look at adding them!

I did a quick visual check on the points gap when I changed the condensor and they were OK so I didn't move them, and the dwell was set at 50* a few months ago and hadn't moved. I'll certainly do a compression check in the morning.

As it is, I'm becoming more inclined to suspect the timing chain or tensioner. Maybe the latter as it's a double row set. I remember one tooth equalling 18* so the 20* shift would make sense. I ran it very gingerly having rest the time so I don't know if it's pulling properly.

I'm just trying to think of an easy way of checking without opening up the cover, removing rad etc.
Steve Postins

Really not a difficult task. (famous last words!) Removing the radiator with shroud attached and the oil cooler together make it a 15 minute job. Just place lots of blankets on the car and let the entire assemply rest back on them.

Also, the timing chain cover is the one spot on my engine rebuild that got hylomar as a gasket dressing/sealant. Just in case I had to open it in the future.

Paul Hanley

If the tensioner came apart, I could see how the timing chain could easily jump a topoth. As for the Mechanical advance spring, without it you would have significantly less advance, which could cause an exhaust backfire. But all in all, that doesn't explain the change in distributor position. Looks like a timing chain is in order!
Jeff Schlemmer

No need to take the cover off - you can check the valve timing if you can get a timing wheel attached to the crank pulley using Blu Tack or similar - just line the zero up with the timing mark and check that full lift on number 2 valve occurs around 107 degrees ATDC
Chris at Octarine Services

An hour at the roadside is better than a week in the shop. Be glad it wasn't a modern car. ;)
Steve Simmons

I think if you follow a suggestion Chris made a few weeks ago you should be able to tell if the timing chain has jumped a link.

Turn the engine to place number 4 cylinder at TDC, compression stroke. Loosen valves 7 and 8 to approximately .125", making sure they are both at the same clearance. Turn the engine to one full turn, number 1 cylinder at TDC compression stroke. Measure the valve clearance of number 7 and 8 valves, they should be very close to equal if your timing is correct. Move the engine so that both valves have equal clearance and estimate how far the timing mark is from TDC. If it is around 20 degrees you have a timing problem.
Clifton Gordon

I can't believe that the timing chain has jumped a tooth but stranger things have happened. Have you looked at the condition of the distributor drive gear ?? Easiest way to check valve timing is to put No1 on TDC at which point cyl No4 valves should be "rocking" This is obvoiusly approx only but does eliminate the need for any measuring equipment or setting up and is good enough to confirm that the valve timing is approximately correct. On the assumption that it is correct and bearing in mind what you have already done then the drive gear/camshaft pinion could be the problem.
Iain MacKintosh

Well, I've reinstalled the dizzy without the rogue detached spring and it's back at its normal position and the car runs fine. I can't understand how the spring could have caused the timing to retard like that, even if it was jammed in the innards, but there you go. As Steve says, if it were a modern car I wouldn't understand either but it would still be broken! For good measure, and becuase I've finished about 4 hours ahead of time, I'm going to check the cam timing now.

Thanks everyone for the help and suggestions.
Steve Postins

Damm Steve,

With all those parts on board, don't ya just wish it had been one!! Glad you're fixed and it didn't cost anything!

Cheers

Paul Hanley

I've always known that the part you have a spare for is never the one that goes wrong, but that means the more spares I carry the fewer problems I'm can have!
The cam timing was spot on but I've marked full lift on the crank pully to allow easy checking in future.
Steve Postins

A note for any future occurence. If you used a digital dwell meter they have an internal battery and so may work whether or not there is 12v connected through the coil, so use a voltmeter as well. This will give a reasonable rule of thumb indication of dwell - and certainly if the points are opening and closing - as well as voltage. If the tach didn't drop to zero or the ignition warning light come on until the engione stopped spinning then the ignition LT was OK, making the problem HT (inclucing timing) or fuel. If, when you eventually got the car going again, the fuel pump chattered like mad when you switched on then you know it was a fuel supply problem.

I agree that a dwell indication of 50 degrees indicates the points are opening and closing as they should, as did the tach, and have the correct gap. Maybe the timing had been out for a long time? And there was another problem that you inadvertantly cleared while moving the distributor? If so expect it to happen again!
Paul Hunt

I have a feeling that's prophetic!

I'm going to ramble now in the hope of a "Eureka!" moment.

I am 100% the problem was the timing retarding immediately and massively, I just can't see how. The engine is pretty sensitive to timing changes so I can tell if it's more than a couple of degrees off and I'm used to making tiny adjustments of the dizzy to bring it back. In this case the crank pulley marker was almost out of sight of the timing light.

I think I underestimated the shift at 20*. Once I'd reset it to 10* BTDC, the dizzy body was about a centimetre turned clockwise from it's usual position, measured at the clamp. I've just done some rough working and that makes about 20* of retardation at the dizzy, so 40* at the crank! So my best guess is that it was running at 30*ATDC. That would make a bang!

I can't explain it. Fishing that spring out of the workings seems to have put things back to normal but maybe it's an illusion. The spring is bent and battered, but I can only see that any interference it caused would give more advance. Unless it broke some drive gear teeth? Twisted something?

Sorry, no Eurekas but I will certainly let you know if there are any further developments.
Steve Postins

The drive to the distributor is a helical cut drive arrangement to turn the corner. I have seen a dizzy lose it's vertical restraint (on a TD ) and it was impossible to time as varying drag and centrifigal force caused the base plate to move up and down by 1/2 " , and as the gears ran round each other to advance and retard by about 20 degrees , at the dizzy. Perhaps your jammed sping forced the base plate up , amd so altered the timing ? Another hard to diagnose problem in this area was where the slot is machined in the drive spindle , one half had cracked and was not registering the distitributor drive, although the other half was still turning it . All became clear when in desperation the distributor was removed .
S Best

Yes, this can happen but usually in the situation is an offset tongue and either the tongue can shear or the lower half of the tongue drive shears because it is a small quadrant. I can't see the baseplate lifting but just because Steve's had this off and put it back on again, but by any chance has the roll pin which pins the drive collar to the shaft sheared and altered the alignment between the drive dog and the distributor shaft. That's a possibility and it would still be tight on the shaft and perfectly capable of driving. It would also account for the bang which I'm not sure anything else does. If its not that I would check the orientation of the oil pump drive as I still have a hunch that it may have slipped but that would have caused damage to the drive and maybe also the camshaft.
That loose spring has been the source of the problem.
Iain MacKintosh

Interesting...I had the dizzy off again this morning and checked it over thoroughly but spotted nothing except a gouge in the bottom of one of the centrifugal weights. I had a quick look at the roll pin but maybe should have looked harder.

I'm wondering if the sping caught under the weight, forcing the advance plate upwards to a point where it could twist independently of the drive shaft. I could replicate this on an old dizzy, and get a shift of about the right amount before it locked against one of the weights. Thought I'd cracked it but then couldn't get the actual dizzy to do the same (different advance plate). Still, I'm pretty sure any damage was to the dizzy as having replayed events in my mind, I reckon the first bang was a backfire, as there was more backfiring as I tryed to retart the car on the clutch as I coasted.

Once I'd stopped, the car would not catch at all from the starter, until I pulled the rotor arm off and pressed the new one on. After this it ran, if horribly. I'm wondering if by pushing the rotor arm back on I may have forced the advance plate to a better position, and if the ping I heard as I stripped the dizzy wasn't the secondary spring coming off, but the primary pulling things back into place.

All guesswork, but I didn't have to use a big hammer on anything to get it back to normal (the dizzy now sits back at its original pre-problem timing position) so I'm assuming (Ass-U-Me) that it wasn't heavy hammer damage I am looking for. All the same, I think I'll keep that old dizzy into the toolbag for a while.
Steve Postins

Steve, Maybe you should put a camshaft in the bag as well just in case you have to change it at the side of the road !!
Iain MacKintosh

I've got a spare block somewhere... might fit the passenger footwell...
Steve Postins

Or you could tow a spare MGB behind using one of those special frames used by some motor caravanners! :o)

Laurie Webb
L Webb

Possibly the secondary spring jammed the centrifugal advance mechanism. Doesn't it provide about 20° advance? At 80 mph, a sudden 20° retard could certainly cause a backfire. -G.
Glenn G

But that wouldn't have caused the ignition to be retarded at lower revs as well, which is what Steve found.
Paul Hunt

This thread was discussed between 16/11/2004 and 20/11/2004

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