Intermittent mis-fire

[Jaywhy]

If the exhaust connection against the exhaust manifold is slightly blowing will this cause an intermittent misfire when you try to accelerate? My reason for asking is that my 1098 mis-fired and stopped on the M1 and then instantly re-started. It intermittently misfired all the way home. I changed the points and it lasted a short while and started mis-firing again. I re-checked the points and they seemed fine.

I re-sealed the joint on the manifold and it seems to have cured the problem. If this can cause this to happen please can you explain

[bmcecosse]

It can upset the overall running - but I would have thought unlikely to stop the engine !

[philstill]

Do you get this intermittent mis - fire when the engine is cold, or is it only after you've driven a few miles? If the engine mis fires then cuts out after a few miles, then you let the engine cool down, try it again and it's ok again for a while, it's usually a faulty coil.
Phil

[bmcecosse]

It's NEVER the coil.........

[philstill]

Why?

[bmcecosse]

Because they are deadly reliable...

[Jaywhy]

BMC - The mis-fire causes the car to stop start very quickly, effectively jumping. Can you give me any other suggestions? I have changed points, checked plug gaps lifted bonnet in dark with engine running to see if there is anything arcing to earth but nothing. Plugs and points done one at a time to try and eliminate the problem. Tick over smooth, but seems to do it when hot.

[bmcecosse]

It's always difficult with these mysterious misfires! And this is where a complete 'known good' dizzy is handy - drop it in and see if the problem is solved. It 'could' be the little 'low volt wire' inside the dizzy - it's flexible to allow the baseplate to move - and it doesn't last for ever. And it CAN be bad/loose connections at the coil.

[Jaywhy]

Thanks BMC, will check these pointers you raise tomorrow and report back

[Jaywhy]

I said I would report back on my mysterious mis-fire. Well I took BMC advice and checked the small low volt wire in the distributor -ok checked all connections on coil -ok. Went out in it this afternoon ran fine for about 20 miles then started to jerk and mis-fire. When this occurs the engine seems to cutout out and instantly re-start. ( giving the jerk action) Can anyone give me anymore pointers? As said before it only does it when hot

[Jaywhy]

Also when I got home took coil off and check the coil using multimeter - read 3.5 ohms positive to negative also checked centre connection ( coil lead output) to negative read 8000 ohms does this seem ok?

[bmcecosse]

Spot on for that coil.... Jerk like that is a bad / loose connection somewhere.... unless you think it could be a sticking valve...is there any suggestion of a backfire through the carb ?

[Jaywhy]

No backfire thro carb, can the LT wire in distributor be changed?

[LouiseM]

Yes, easy to change and worth trying.

[kennatt]

Whilst I accept that its Never or very rarely the coil,that only applies to old original coils,some of the later ones are known to be poor quality,it could be heating up and failing,It has happened to me once on a mini and others on here had had the same. not many but a few. It would be worth getting an old known good coil and trying that before looking further.Whereabouts are you,I have a good spare if you are in the north east you can borrow it.There's bound to be someone near to you with the same spare.

[Jaywhy]

While I know there has been a lot said on this site about ignition coils. In order to help me eliminate the problem I have changed one thing at a time to get to the bottom of the issue. Having changed most things like points, condenser rotor arm cap check plug gap and tightness of connections I decided to change the coil. Our old mutual friend BMC some months ago persuaded me to buy an old lucas coil off ebay. What good advice. So now was the time to test it. Fitted it and bingo the fault (touch wood) has gone. Done several miles and all seems ok. The coil I have taken off has only been fitted two years and pricing this particular one up it was not cheap.

So working on the basis of it's never the coil I would agree that if it is of the older type then this is probably right, since the one I have now fitted is still going strong and it is a very old coil, however if the coil is a much newer type in my case two years old, then there is always the possibility it is the coil !

[bmcecosse]

Can you put a meter on the 'failed' coil and report the readings?? And - sure it's not bad connections at the old coil? Bosch coils are usually reliable !

[kennatt]

Exactly as said.

[Jaywhy]

Sorry for the late reply to your question BMC. As said in earlier mail the coil removed is 3.5 ohms positive to negative and 8000k when measured across coil centre to negative. I am currently running on an old Lucas LA 12 coil which seems to be working fine. Without doubt when I was running on the faulty coil I would go about twenty miles and the missing would start. The coil was also extremely hot.

[Jaywhy]

Should read centre ( coil lead input) to negative terminal 8000 ohms