Misfiring at Tickover

[ashleywakeling]

Hi there, just fitted a new unleaded head and for a while it stopped the problems of my car misfiring on cylinder 3 (second back from the battery), as it had always done this. i was really happy. However, its started to do it again, as soon as a little bit of power is put through the car the misfire goes, runs lovely and smooth, but at lights etc its VERY irritating and just sounds like its struggling. If you turn the tickover up a little its fine, but it seems to be too fast for a tickover. If you unplug HT lead when its running it barely makes any difference so i know its got some form or problem there... The exhaust is very pattery but very little smoke (sometimes a little white but thats just condensation.) Done 200 miles since the head rebuild so very confused as to why this problem has come back. Once shes running she has no problems, will happily sit at 60 and more if pushed. i suspect plugs or HT leads but they dont seem to make any difference when changed, i dont wanna fiddle with the mixture really becuase in personal experience it just creates more problems. Thanks Ashley

[chickenjohn]

Have you done a compression check since fitting the head?? Possible the head may need re-torqueing.

Of course other causes could be ignition problems such as bad spark plugs/HT leads, even a worn distributor cap or worn rotor arm- could give mis-firing as you say at idle that goes away at speed.

Fraid you may have to "fiddle with the mixture" if ignition is fine and the car has a new head as either a very rich or very weak mixture can cause mis-firing (or "hunting") at idle.

[ashleywakeling]

We've retorqued the head down to 46psi all round and its holding fine its had a new distributor cap and rotor arm so i reckon theyre fine new spark plug as well, so i reckon ill change the HT lead, if that doesnt solve it, its gonna be mixture isnt it :/ hate fiddling with it, do you have to turn the engine off to change it and then turn it back on every time youve moved it one face of the bolt? thanks for your prompt reply

[Dean]

hate fiddling with it, do you have to turn the engine off to change it and then turn it back on every time youve moved it one face of the bolt?

Simple answer.... no.

Once engine is warm, I speed the engine up slightly sing the screw on the carb, turn the nut underneath one face, rev the engine once using the throttle cable on the carb, then walk to the exhaust and listen to see if the phut brrrrrr phut brrrrrrr phut phut brrrrrr has become a constant brrrrrrrrrrrrrrrrrr. If it hasn't.. repeat, but don't go too far and lose yor current setting incase it isn't that.

Make sure you eliminate everything electrical first, including making sure all HT leads are properly connected. Mine did this and it was a dodgy spark plug.

[chickenjohn]

We've retorqued the head down to 46psi all round and its holding fine its had a new distributor cap and rotor arm so i reckon theyre fine new spark plug as well, so i reckon ill change the HT lead, if that doesnt solve it, its gonna be mixture isnt it :/ hate fiddling with it, do you have to turn the engine off to change it and then turn it back on every time youve moved it one face of the bolt? thanks for your prompt reply

good idea! Change the ignition leads if the car misfires and they are old.

As for the mixture adjusting nut, run the engine till warm, no need to switch the engine off, in fact adjustments need to be made with the engine running. Then gently lift the carb piston lifting pin a tiny amount (to just lift the piston), if the revs drop away, richen the mixture (screw the nut down- clock wise from above) , if the revs raise a lot, weaken the mixture, screw the adjusting nut in (anti clockwise from above) when the revs just raise VERY slightly when you lift the pin then the mixture is correct.

please let us know how you get on,

cheers

John

[bmcecosse]

Have you reset the valve gaps - 12 thou inlets and 15 thou exhausts. Compression check will be a good idea to find out what's going on. Mixture etc doesn't just affect one cylinder. If you swop plugs around - is the one from #3 cylinder any different colour to the others? And after swopping - is the idle any better, even for a few minutes? Or just exactly the same ?

[mike.perry]

Don't bet on a new dist. cap and rotor arm being OK, I have come across both new ones being faulty, in fact I would rather use an old rotor arm, there is not much to go wrong with them except faulty construction. I fitted a new cap on a friend's car and it wobbled with the spring clips fitted and rocked around as the rotor arm hit the contacts. To fit the coil HT lead the carbon brush and spring had to be removed to get to the clamping screw