Annoying intermittent misfire

[moggiesgalore]

Bought a cheap tatty '68 traveller last week to add to my fleet. Bravely drove it from Oxford to Devon, car did really well though the brakes are terrible. Find it hard to believe they were free when it was mot'd last December (no handbrake at all!).

When buying it the guy mentioned it had developed a misfire on initial startup. This clears within 10-20 seconds and the car didn't miss a beat all the way back.

On stopping for fuel, toilet etc the misfire would briefly return then clear.

Over the last week I've been attempting the easy task (or so I thought) of getting rid of it.

It sounded like a bad plug, so I replaced those. No luck. Then did the HT leads. No luck. Since then I have changed the points, condenser, cap, rotor arm, distributor (vac advance shot), coil. Have also checked earthing.

If anything its worse now. When dropping to idle it sounds like its running on three cylinders. Rev the engine and its like a switch has been flicked and it runs smoothly

I think the next checks will be tappets and a compression test. Am wondering if the head gasket might have failed, though find it strange that it isn't misfiring all the time.

Anyone else had similar?

Happy days!

[LouiseM]

I had a similar problem. Have you tried changing the low tension lead?

[Dean]

I had similar, although it was a miss most of the time. My issue was down to one plug not giving off a strong spark.

[beero]

Open the bonnet in a dark garage with the engine running and see if there are any stray sparks.
When it sounds like it is running on 3 pull the spark plug leads off one by one (with rubber gloves on) and see which cylinder is affected. The lead that make no difference is the suspect cylinder.

[bmcecosse]

Put the old coil back on - it's 'NEVER' the coil!
It sounds like a sticking valve to me... remove rocker cover - start it up and inspect all the valve movements...

[moggiesgalore]

Never say 'never' Roy! In twenty odd years I have had two coils fail, most recently on my 1275 Midget. Granted though, this is a very rare occurrance and these symptoms don't suggest that anyway. It's really been a case of changing the easy parts first (raided from a rather large collection of spares) before moving to the bigger potential problems.

As you suggest, I will remove the rocker cover and check tappets/valves etc, as well as doing a compression test. It does sound more tappety than usual. There is also a loud tapping/rattle from the water pump/alternator area that needs getting rid of. I have tried another sh alternator but it's no better so looks like the pump (fine with the belt removed). I'm avoiding spending money where possible at this stage!

Thanks for the suggestions so far.

[bmcecosse]

Did the coils REALLY fail - or was it just bad connections ??

[tomas.geoghegan]

VERY Rarely the coil Granted but quite a few people have said now that its was the coil!
Tomas

[bmcecosse]

And in 9 of 10 cases - I don't believe it! Show me the tests on the coil with a meter........then I will believe.

[moggiesgalore]

Well, hats off to you Roy your theory was spot on... Have just done a compression test which revealed nothing on cylinder number two. After whipping the rocker cover off the problem is revealed; exhaust valve stuck.

I guess the reason it has been missing on startup is because when switching off the engine there isn't enough inertia to close the valve. Then when restarting it is stuck open until the engine is revved enough to generate some force.

Nice one Roy!

[bmcecosse]

Thank goodness it wasn't the coil...........

[beero]

Well done BMC. That should put a stop to people pulling your leg about it never being the coil.

[bmcecosse]

I'm happy to have my leg pulled - if it brings home the message that it's 'never' the coil!

[tomas.geoghegan]

Well, hats off to you Roy your theory was spot on... Have just done a compression test which revealed nothing on cylinder number two. After whipping the rocker cover off the problem is revealed; exhaust valve stuck.

I guess the reason it has been missing on startup is because when switching off the engine there isn't enough inertia to close the valve. Then when restarting it is stuck open until the engine is revved enough to generate some force.

Nice one Roy!

Im Not saying that Roy is wrong on your car As Roy knows hes always given me Super advice.And im for ever Gratefull.
And my car is a MUCH Better Running car Because of it. Just had a coil go on me thats all.
Tomas

[moggiethouable]

Did the coils REALLY fail - or was it just bad connections ??

I have just for fun, googled "its never the coil" youll never guess what came up

[MarkyB]

Unplanned pregnancies?

[bmcecosse]

If you still have that 'failed' coil Tomas - let's see the test results on it with a meter.

[tomas.geoghegan]

I wish i had kept the coil.
Looks like we will never know??
Very Best Regards and Thanks for your patience . Your a Star.
Tomas