Intermittent Spark

[ADO16]

Hello all,

I'm not sure if this is the right venue to ask this question, but here goes. I'm working on a 1098 transverse engine car (wife's MG 1100) and found that after she drove it she complained that it seemed to be running rough at idle and felt like it had no power, so I checked it out and found it only running on 3 cylinders. I checked spark on each cylinder and found that no. 1 had the miss, so I checked the amount of spark there and found it not sparking at idle but only at higher revs. Does this mean that the ignition wire lead is bad or could it be a cap or something else? It's been awhile since I've played troubleshooter (can't remember).
Thanks,
Steve

[PSL184]

Steve, I'm not saying that there maybe other faults here with the parts mentioned above but the last time I had a misfire at idle and it running OK other times was down to the ceramic nose of the spark plug being broken - took me a while to find that one !!

[ADO16]

Steve, I'm not saying that there maybe other faults here with the parts mentioned above but the last time I had a misfire at idle and it running OK other times was down to the ceramic nose of the spark plug being broken - took me a while to find that one !!

Thanks for that....this one is a bit of a head scratcher, I've tried a different set of plugs, cap and wires but it runs better at higher rpms. It's very possible that the aforementioned items were bad because they were used ones. I should start fresh with new components. How I found out that it was no.1 is by isolating each plug buy pulling it off and then reconnecting it to find the bad one, but what is puzzling me is that the spark is dead at the plug connector when touching it until I rev it and the spark comes back strong. Don't know!

Steve

[PSL184]

Is it the same cylinder that fails with each try of different componants?

[bmcecosse]

I suggest you get a compression check done - and report back. Also inspect and adjust the valve gaps!
This failure to spark until high revs - is this with a plug connected to the HT lead - and lying on top of the cylinder head where you can see it ?

[mike.perry]

It could be a compression problem, head gasket or valve, hopefully not a broken ring. If the compression is low on one cylinder it will misfire at low revs and pick up at higher revs or at least not be so noticeable. If it was an electrical misfire it would be more noticeable at higher revs because the engine is trying to work against a compressed cylinder which is not firing and is effectively working against the other cylinders.
Well that's my theory.
To eliminate the electrical side swap the plugs over and if the same cylinder misfires, swap the leads.
It could also be a cracked distributor cap which can have some strange effects

[ADO16]

Thanks, I've had the same thoughts about this problem. Give me a few days to sort out and I will report back.

Thanks all...

[ADO16]

I suggest you get a compression check done - and report back. Also inspect and adjust the valve gaps!
This failure to spark until high revs - is this with a plug connected to the HT lead - and lying on top of the cylinder head where you can see it ?

Yes it is. As the engine is running with a plug in the no.1 hole and the plug lead with another plug lying on the head I'm getting the no spark situation until I rev the engine and see spark. It's just on that no.1 lead.

[bmcecosse]

Well - it really HAS to be something to do with the lead or the cap or the plug! i know you say you have changed them - but it MUST be something to do with these parts. Swap them with #2 lead !!

[ADO16]

Fun stuff....
compression:

#1 190 #2 170 #3 180 #4 190

I will get a new set of points, cap, plugs, rotor and wires in the next couple days to start fresh (no pun intended).

Thanks....I will report later!
p.s. feel free to comment while I ponder it.

[mike.perry]

I would rule out points and rotor arm as they would affect all cylinders not just one. A faulty plug or lead can be tested by swapping them around. A faulty dist. cap should be swapped to test. Careful examination may reveal damage, also inspect the brass terminals and the carbon plunger inside.
Run the engine in the dark to check for any random sparks or leads shorting out
Change one component at a time then test.

[bmcecosse]

Compressions are pretty good! Obviously #2 is not quite so good - but still acceptable.

[katy]

Have you checked point gap?