Number 3 Misfire

[yatesfox]

Dear All,

I am looking for a bit of advice for a problem that I haven't been able to solve.

I have a standard '66 saloon which I only run on dry days and only did around 600 miles between MOTs last year. I keep it on the road over the winter and it was running fine until May time. It then developed a misfire and has been off the road ever since which is quite frustrating with the good weather we have been having.

I have done a compression test and all 4 cylinders are pulling around 150.

At idle, I have pulled off each lead in turn and it made no difference when I disconnected number 3. I tried swapping the leads which had no effect but narrowed it down to that cylinder.

I have checked and replaced points and plugs and with help from my father-in-law, we have stripped the head and checked the valves but all seated fine, holds petrol when tested so no problems there.

Thinking it has to be electrical fault now so going to change roter arm and distributor cap but not really sure why it should just be affecting number 3?

Any ideas? Really grateful for any help

[Alex'n'Ane]

Dizzy cap would be my guess if its electrical and only one of of the cylinders, as they have one contact point per cylinder. I presume the compression tester screwed into the head? In which case you can probably rule out damaged thread where the plug screws in.

[yatesfox]

Yes compression tester does screw into plug hole in head. For what it costs will change dis cap. Looks really good though and not that old - all contacts look clean and no hairline cracks I can see with naked eye in good light. Thanks

[MarkyB]

Try rerouteing the leads so they don't cross or touch anything metal.
Looking at it running in the dark can show up where sparks are going astray.

[mike.perry]

Do not change the cap and rotor arm at the same time. If you fix the problem you will not know which was at fault. Most likely the cap, is it tight on the distributor?

[chrisryder]

check the dizzy shaft for wear when you've got the cap off. if it slops from side to side a lot, it could mean that the rotor arm isn't quite close enough to the contact in the cap, or the points aren't opening enough at that point.

[bmcecosse]

It likely is the dizzy cap - but have you watched the valves moving when the engine is idling - valve on #3 not sticking open ??

[yatesfox]

Dear All - have now replaced distributor cap, rotor arm and leads with parts supplied by Bull Motif. Not made any difference. Have had rocker cover off and watched the tappets - can't see that number 3 valves moving any more slowly / sticking. Still spitting back through carb. Nice blue spark, starts first time and runs on the other 3. No idea what to do next Looks like being expensive garage bill

[bmcecosse]

What will the garage do ? At the moment there is no obvious fault for them to fix. Is this misfire just at idle - or when running ? Have you double checked the valve gaps - 12 thou inlets and 15 thou for exhausts. You 'could' also check the lift on each valve - I suppose it's just possible that a cam has worn away on the camshaft - or a cam follower has broken up - but i have only ever seen this on a hard driven Competition engine running stupidly strong valve springs! Try advancing the ignition timing till it 'pinks' on hard acceleration - then retard it very slightly . Also check if the vacuum advance is working on the dizzy.

[simmitc]

That's a subtle clue that you slipped in about spitting back through the carb. There must be a leak past an inlet valve. Double check the compression on all cylinders, but suspect number 3 inlet to be stuck or broken. Unusual (it's normally an exhaust valve) but not unheard of.

[bmcecosse]

Or a cross gasket leak - but the head has been off I gather........ I do suspect a sticking valve though.....