Intermittent misfire - new ideas?

[abri]

I'm hoping someone can think of something I haven't thought of that I should check.

The car is a 1310 Cooper S with engine rebuilt about 2000 miles ago. Rebuild included new valves, guides, crack test of the 163 head.

Symptoms are:
Intermittent misfire at cruising speed with the engine up to temp. Cruising at ~ 4000rpm it would start misfiring, I'd switch off immediately and coast to a stop, switch on and it's gone. The last few times I've kept the engine going and the misfire continues as long as you keep the rpm up. If it comes down to 2000ish or below it becomes less and then disappears.

Things I've done/checked:
1) Fuel filters (several)
2) Swapped fuel pumps
3) Checked float bowl contents next to the road - normal
4) Fuel tank breathers - clear
5) Three different sets of plugs, different temp ratings
6) Three different condensors
7) Three different sets of points
8) Three coils (2 Lucas & 1 brand new Bosch)
9) Two distributors (complete swap, both S, timed the same)
10) Two solenoids
11) Two sets of plug leads (copper and carbon)
12) Two dizzy caps
13) Three rotors
14) Hot compression test, identical across all cylinders and within spec
15) With and without brake booster connected
16) With and without PCV valve connected
17) Air fuel ratio tuned with a Lambda sensor
18) Earth straps cleaned up
19) Coil wires on the correct way around (positive earth car)

Like I said, unfortunately it's intermittent, so I haven't been able to open the bonnet and look and listen with it misfiring.
It pulls well and there's no other unusual sounds.

I'm starting to think a valve getting stuck in a guide, but don't know if that will tend to come and go like this.

Any thoughts/advice would be appreciated. I'm out of ideas

Thanks

[Polarsilver]

Crap E Petrol ?

[abri]

Crap E Petrol ?

95 Octane Unleaded and the same octane Lead Replacement. None of my other cars have an issue with the petrol.

[imack]

I too was thinking sticking valve/s.
Possible lean mixture causing exhaust valves to over heat and become tight in guides.
Has the car been set up on rolling road?

[Herbert]

Check compression. Change plugs and leads. Make sure no loose wires in dizzy. Change coil. Have a huge glass of red wine and be safe! R

[Herbert]

Just read your messages. I suggest you go indoors and get drunk.

[abri]

Just read your messages. I suggest you go indoors and get drunk.

Excellent advice!

[abri]

I too was thinking sticking valve/s.
Possible lean mixture causing exhaust valves to over heat and become tight in guides.
Has the car been set up on rolling road?

That's the only thing I can think of. I've tuned the AFR on the road with a lambda sensor, so I know it's not running lean. It was a little lean before but I adjusted it.

Hasn't been on a rolling road.

[Polarsilver]

If your near Southam SMMC ..Colin will sort it out on their Rollers.

[abri]

If your near Southam SMMC ..Colin will sort it out on their Rollers.

Thanks. I’m in Cape Town, South Africa

[iain1967s]

Ignition switch contact bounce?

[Catmint]

Have you got a tacho fitted ? if you have try disconnecting it - they can cause problems at certain rev ranges if faulty

[GraemeC]

Fuel starvation - vacuum forming in tank or pick up pipe picking up crap and blocking.

[jerry]

I had intermitant misfire. It was NGK sparkplugs. Changed to champion and problem cured. Tried a whole box of NGK sparkplugs, and they all misfired. Will now never buy NGK plugs again.

Jerry