No spark at plugs

Hi,

I have an old pre-war 6cyl MG . It uses an old non-original Lucas distributor (probably from a Healey 3000.

The problem I have is this :

No spark at the plugs! I have current right through to the distributor end of the HT lead to the coil, where there is a very healthy spark. After that there is no life.

I have renewed the rotor arm, the distributor cap ,all the leads and plug the caps. I have fitted a new coil, new points and condenser. I have tried breakerless ignition but still cannot get a spark at the plugs.

The fault occurred suddenly --it just seemed as though the ignition had been switched off. There was no banging or spluttering or misfire. The engine just stopped. Any Ideas please, I can't think of anything else to check. Regards

Reply to
mike green
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Hi.

Thinking back (logically!)

You've got a spark at the coil output/dizzy input lead.

You've not got a spark at the dizzy output.

This says to me that your dizzy has somehow become *very* "out". If you've got a spark, but there's no output, then the rotor arm isn't anywhere near the posts in the cap when the spark arrives.

The spark is generated IIRC, when the points open.

You'll probably find that when you look closely at the dizzy with the cap off, the position of the rotor arm when the points are just open, is completely wrong.

Why it happened : The dizzy could be breaking up inside I suppose, but IME, they're normally fairly simple. Check where the bottom plate connects to the input shaft.

What to do : Take it off, and check for slack between the plate and input shaft. If there is none, then it's probably OK. Is it loose? Fiat engines drive the dizzy from a gear from the camshaft. There's a nut/plate over the shaft, used to lock it down once the timing's right. If this moved, you could get the spark seriously out of step with the valves/rest of the cycle.

OK, carrying on from that. Inspiration time : I don't know how your distributor is timed, but make sure that is OK. If it's driven from the camshaft, if your timing chain has skipped quite a few teeth, this would explain it.

HTH

Pete.

Reply to
Pete Smith

Sounds like a knackered rotor arm tracking down to the shaft.

try this: invert the cap and place the rotor arm inside, preferably on the end of a piece of dry dowel, line the rotor up with a plug connector and flick the points, you will probably be able to see where the spark goes, if you can't then it is probably tracking across the cap somewhere, if it looks perfect then it may be that the arm has failed and when it is on its shaft it is tracking down to the shaft, I have seen this a lot on montego. another possibility is that under extra load the coil is breaking down internally

mrcheerful

Reply to
mrcheerful

There should be a spring loaded carbon brush sticking out of the inside of the middle of the dizzy cap and making contact with the middle of the rotor arm. The HT current is led into the rotor arm from here and the rotation of the arm feeds the current into each plug lead from inside the cap. The brush may have worn out or broken. If it's not there try replacing it with a bit of twisted tin foil from a choc bar (tin foil, not the modern silver wrapping that's only plastic really.). This has got to be the problem.

Rob Graham

Rob Graham

Reply to
Rob graham

Set the engine on the timing mark for No1 cylinder, or when the points just open. Does the rotor arm line up with the contact on the cap for No1 plug lead?

Is the rotor arm locating peg ok?

Seems to me the only fault could be is that the rotor is between 'contacts' at firing time.

Reply to
Dave Plowman (News)

Are you sure the new rotor and cap are right? Check both against the old ones, particularly for height/depth, and make sure the centre contact is in place in the cap.

Reply to
SimonJ

I'd suggest a better way to check the rotor arm for tracking, is to hold it in position inside the cap by stuffing a little finger up the hole for the shaft, instead of using dry towel, or other form of insulator. If you get a belt when you flick the points, the rotor arm is tracking. If you don't, it's fairly safe to say the rotor arm is not the problem.

Could be, but doubtful, as the OP has fitted a new coil. IME if the coil is is delivering a spark to the coil king lead, it's usually possible to hear or see where it's being lost, between it and the plugs, if the rotor arm is held inside the distributer whilst flicking the points. It has to go somewhere, and you can usually hear a click, as it shorts to earth somewhere. Locate the click either visually or aurally, and you've found the problem. Mike.

Reply to
Mike G

There is either a break or short in the electrical connections in or to the distributor. Check continuity in the LT and HT circuits with an ohmeter. Make sure the points have a plastic washer under the moving terminal so they aren't shorting out to the baseplate.

Is the points cam actually turning when the crank does?

-- Dave Baker - Puma Race Engines

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Reply to
Dave Baker

The message from "Mike G" contains these words:

Do it at night and you can often seen the blue glow of the sparks escaping.

Reply to
Guy King

In message , mike green writes

Thanks everyone for the benefit of your experience. I will try some of your suggestions during next week and let you know how things go. Or don't:-) Regards

Reply to
mike green

Sounds very strange if you have juice on the king lead then you should have juice down the spark plug leads unless it is conducted away -duff rotor or arcing down cap body- since both are new its not so likely! however if this is only when you turn the engine over and it cuts when the car runs suspect the earth braid to the distributor base plate, the lucas dizzys are buggers for it is a short braid with woven insulator and they are poor conductors at the best of times beware of replacing it with thick wire as that interferes with the advance curve Derek

Reply to
Derek

Off the top of my head, I'd suspect the crankshaft sensor. Had similar problems on my old '88 2 litre Carlton, and after having replaced literally everything from all plugs and leads to king lead to coil, to dizzy cap and rotor arm (put the rotor arm the wrong bloody way around as well and somehow ended up melting a nearby bit of plastic - I put it down to the rotor arm slightly knocking something out of balance - that bit of plastic cost me another £20 or so from Vauxhall and involved phoning around about 3 or 4 chains of dealerships to find one vaguely nearby), including replacing the coil again, "just in case" this brand new one was a duff one, and in the end I took out the crank sensor to find lots of carbon deposits on it - gave it a good clean with some sort of abrasive paper and the thing fired up straight away.

Peter

Reply to
AstraVanMan

Did many pre war cars use a crankshaft sensor?

mrcheerful

Reply to
mrcheerful

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "mrcheerful ." saying something like:

The driver's finger?

Reply to
Grimly Curmudgeon

In message , Grimly Curmudgeon writes

I' m rather pleased that this one doesn't if they are £100+ per throw.

As a matter of interest I was quoted £140 for a new dizzy cap by one guy. I then found ,and had to order from another supplier who priced their s at £50 or £60 when including delivery. The day of delivery I found the same cap new for £7 pounds in Pratical Motorist.

Reply to
mike green

In news: snipped-for-privacy@4ax.com, Grimly Curmudgeon decided to enlighten our sheltered souls with a rant as follows

David, you disappoint me greatly.

As Chairman and Chief Butty Maker of the Spagthorpe Enthusiasts Association, you should know perfectly well that the crankshaft sensor on the early Spagthorpe Obelisk Mk12 was the first in series production on any British / Malaysian manufacture. The design of which was unusual, even for a Spagthorpe. If I'm not mistaken, and I'm pretty sure you'll correct any detail lapses I may be presently suffering from, the design centred around a certain tribe of Indonesian Throwing Dwarf being paid 2/6 per week, by Spagthorpe as part of the servicing costs, to live in the engine compartment of the Obelisk and count the revolutions per minute of the 14, horizontally opposed, piston engine and to stoke the coals for the cigar heated footwells.

Reply to
Pete M

Heh. I forgot to say "obviously this is going to have a completely different ignition system, so my suggestion will probably be bugger-all good."

:-)

Peter

Reply to
AstraVanMan

Are you sure that it's identical to the old one which you know worked at some time? The contacts relative to the locating points? And that it's the same height so the centre brush is making contact with the rotor arm?

Reply to
Dave Plowman (News)

In message , "Dave Plowman (News)" writes

Yes , I am sure that I'm comparing like with like as far as design and dimensions go. The question is :- Was the distributor ever the most suitable for this car. I suspect it is at best,-- the best available compromise and has performed very well . In the past the car with this dizzy and appropriately numbered cap has raced at Silverstone and elsewhere .It held and quite likely still holds it's class record for Wiscombe Hill climb. A new original design distributor would , even if I could find one, cost the earth --the car is 71 yrs old .

After trying to start with my new cap etc. unit , I can see that the carbon insert in the dizzy cap does make good contact with the brass of the rotor arm ,both components have visual proof of contact but there is no sign of burning at either the six dizzy contact points or at the tip of the rotor arm.

I can see no obvious fault with the cap/arm combination that was in use at the time of the breakdown and I would have thought it was perfectly OK .For some reason it performs the same as the new one. That is, the current is not getting from the brass rotor arm strip to the plugs using either new components or old.

I don't have time to take a good look at the problem for a few days but I am inclined to follow those including you Dave, who suggest that for some reason the rotor has become out of sync with the distributor cap/rotor contact points . It is the suddenness of this happening which worries me ----something happened fast and just cut the engine. I can't see any obvious problem nor hear any damage to the distributor drive or timing gear. At this stage though, all other suggestions are valid and valuable.

I guess I need to look deeper for damage and then attend to the timing properly. Thanks for your interest , I have got lots of good advice from this NG.

Reply to
mike green

But even if something had slipped in the timing, you'd still get a spark if the distributor is turning. And assuming a normal type of Lucas distributor, the rotor arm is fixed to the cam that operates the points, so if the correct one and not damaged must line up with the cap correctly?

You've definitely replaced the king lead too? Many just do the plug leads. If this was faulty, it's conceivable you'd get a spark from that but not at a plug due to the extra resistance.

You can do a basic check on plug leads with a DVM. Suppression types have a resistance of about 5000 ohms per foot of length.

Reply to
Dave Plowman (News)

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