Not firing on all cylinders

[antijam]

Took the '71 to a local car show today. It's actually The local Austin 7 Club annual bash but they invite a few more modern 'interesting' cars to join in. It's held at an NT house and grounds only a couple of miles from home so I set off this morning in anticipation of an enjoyable and interesting day. Immediately it was obvious that at least one pot had decided not to join in the fun but since it wasn't far I pressed on and arrived without real problem.



On the way home it was clear that at least one other pot had joined the strike. Made it home sounding embarrassingly like a clapped out tractor and quickly checked all the header pipes with an IR thermometer. Nos. 5&7 showed no signs of life but the rest seemed to at least be firing. There was no obvious electrical problem and since cylinders 5 and 7 are adjacent, a head gasket failure is an obvious suspect. The was no sign of overheating or coolant loss so it may be just a break between the two cylinders. Looks like a compression test is the order of the day tomorrow! (.....and I'm probably unlikely to be making the Nationals - ho hum )

 

[Mad4slalom]

Took the '71 to a local car show today. It's actually The local Austin 7 Club annual bash but they invite a few more modern 'interesting' cars to join in. It's held at an NT house and grounds only a couple of miles from home so I set off this morning in anticipation of an enjoyable and interesting day. Immediately it was obvious that at least one pot had decided not to join in the fun but since it wasn't far I pressed on and arrived without real problem.

View attachment 23287

On the way home it was clear that at least one other pot had joined the strike. Made it home sounding embarrassingly like a clapped out tractor and quickly checked all the header pipes with an IR thermometer. Nos. 5&7 showed no signs of life but the rest seemed to at least be firing. There was no obvious electrical problem and since cylinders 5 and 7 are adjacent, a head gasket failure is an obvious suspect. The was no sign of overheating or coolant loss so it may be just a break between the two cylinders. Looks like a compression test is the order of the day tomorrow! (.....and I'm probably unlikely to be making the Nationals - ho hum )

Ooh! Makes my noisy clutch pedal seem trivial ! Hope you get it sorted soon

 

[Roscobbc]

Took the '71 to a local car show today. It's actually The local Austin 7 Club annual bash but they invite a few more modern 'interesting' cars to join in. It's held at an NT house and grounds only a couple of miles from home so I set off this morning in anticipation of an enjoyable and interesting day. Immediately it was obvious that at least one pot had decided not to join in the fun but since it wasn't far I pressed on and arrived without real problem.

View attachment 23287

On the way home it was clear that at least one other pot had joined the strike. Made it home sounding embarrassingly like a clapped out tractor and quickly checked all the header pipes with an IR thermometer. Nos. 5&7 showed no signs of life but the rest seemed to at least be firing. There was no obvious electrical problem and since cylinders 5 and 7 are adjacent, a head gasket failure is an obvious suspect. The was no sign of overheating or coolant loss so it may be just a break between the two cylinders. Looks like a compression test is the order of the day tomorrow! (.....and I'm probably unlikely to be making the Nationals - ho hum )

As they are adjacent is there any chance of a couple of plug leads arcing to each other or earth?

 

[teamzr1]

Pull the sparkplugs and see what they're color is
Check to make sure the 2 plug wires arc if put the sparkplug end of wires to ground when cranking
Check the underside of distributor cap and if contact points of 5 & 7 are good

Or were you a bad boy causing a valve to get tweaked ?

 

[antijam]

Well, I seem to have resolved the problem, though I'm not quite sure how.
I pulled all the plugs and checked their resistance . I've got Accel shorty plugs fitted to improve clearance with the headers and reading the american forums they don't get a very good press. Resistance of all was fine though. I know the car is running rich so most of the plugs were pretty sooty except for Nos 6 and 8 which were a nice tan colour - odd? Checked the HT lead resistance, giving them a good flex to ensure no broken cores - all OK. Did a cold compression test and no compression loss in either 5 or 7
1.....145 psi
2.....155 psi
3.....160 psi
4.....150 psi
5.....165 psi
6.....155 psi
7.....165 psi
8.....165 psi
Compression is pretty consistent. The low value of No. 1 might have been due to a tired battery - I was checking them in numerically reversed order.
Carefully checked the distributor cap for signs of tracking between 5 and 7- nothing detectable.
The obvious remaining possibility was cross firing between leads 5 and 7 as Rosco suggested. The two leads do cross over very close to one another and I've fitted a bit of insulating rubber tube over each lead to maintain good separation.
Whatever the reason, so far so good - keeping my fingers crossed that it stays that way!

 

[Roscobbc]

If you have any doubts about ignition lead 'tracking' (remembering that electricity always takes the path of least resistance) - and this is a real faff to do.........remove the hood - take a drive out on unlit roads - compare normal cruise speeds (for any tracking) with engine under load (so gas pedal to the floor) when engine receives a full blast of enriched gas plus squirter load plug will struggle to fire the enriched mixture.......creating a high resistance and may possibly highlight an issue......sometimes even sharing a 'spark' with a specific plug AND a tracked earth.