misfire

[ROBBIE]

that's what I've got, intermittent misfire that started on the day of my MOT I've changed the plugs, leads, dizzy cap and rotor arm,coil was done last year so was dizzy.
everything seems fine except when under load you can hear a slight change of note very occasionally what im wondering is could the fact that i have a cherrybomb backbox, been on for about 2 years and now have a cherrbomb up front (just done for mot) be causing a problem?
engines aren't my department so now eagerly awaiting some help
cheers robbie

[Jordi]

Any possibility that the distributor has been disturbed and disrupted timing?
Head gasket? Try a compression tester to make sure each cylinder is reaching the standards.
Valves gone?

Could be a number of things. Try and narrow down which cylinder isnt firing and work it from there.

Jordi

[ROBBIE]

cheers jordi ive put a timing light on it and theres no probs there, all yhe plugs have a good spark ive taken them out and their sparking fine even under load. the head gasget seems fine theres no water in the oil or vise versa. i havent tried a compresion check cos i havent got a gauge but ill try and get 1 today
robbie

[ScottieJ]

Also check the inlet manifold gasket for air leaks by spraying wd40 around it, you will notice a change in the engine running when you spray it on the leaking bit if there is one, also check the small vacuum pipes on the inlet manifold are sealing properly (if they have them on the 413??)

[ROBBIE]

hi i now have a constant misfire on no2 cylinder today i done a compression test and all 4 cylinders are the same. i sprayed wd40 all round the manifold and there was no change? also all the vac pipes are OK. i know there is a spark at the plug i checked by tacking the plug out grounding on the engine and turning the engine over spark was good
so now I'm totally confused and i need of help.......
thanks in anticipation

Robbie

also been told it could be fuel,carb but wouldn't that affect all cylinders?

[muddy kev]

the plug may have a good spark when removed but doesn't mean its good when under compression. have u tried swapping the plug to another cylinder and also the ht lead to see if the misfire moves to that cylinder?
what colour was the spark? should be blue.
also try getting an old plug and open the gap right up to see if keeps a nice colour/strength when jumping a larger gap.

cheers kev.