[Car] Misfire underload?

[mudcow007]

My dad has a W reg MX5, last week it developed a misfire. No worries i thought it will be a coil, lead etc...

So he bought a coilpack (without testing his old one) an some "go faster" red leads.

I fitted these on the weekend, initially the car sounded fine until he accelerated away/ put it under load - it started misfiring again.

I refitted the factory leads (with the new coils) an the car was working great again underload etc.

He came to use the car yesterday an its back to misfiring.

Ive got his old coils in work with me to test em today but i dont think its coils..

didnt change/ check plugs but they were supposedly changed last year, an the insulator does look shiny white so reckon they were changed.

any thoughts?

[Oldie]

You will have to check each plug for a proper spark. Even new plugs can be faulty. A weak spark will cause mis-firing.

[bugeye_bob]

as above plugs, age of plugs, ceramic intact and gap ok ?

[Rogerborg]

Probably plugs / spark, but I had a misfire under high throttle / low revs on my C3 due to a hooky electrical connection on an injector.
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Biking is 1/20th as dangerous as horse riding.
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[UncleFester]

As above, would also check plugs / gaps.

Equally ( assuming it has a diagnostic port) i'd do a scan and see if there is an o2 sensor fault or a blocked / dying cat.

A gentle tap from a rubber hammer will tell you if the Cat innards have become detached.
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2017 Z1000SX

[bugeye_bob]

a quick Google mentions the CAS is a popular problem.

[mudcow007]

Will whack some plugs in it tonight, so how that goes, might even give the MAF a spring clean too. It has a K&N fitted to it (has done for about 3 years)

cheers for the suggestions

[mudcow007]

sorted I think

Bunged some new plugs in, all looked ok except for No.1 which had loads of gung above the nut - the electrode looked a bit sooty...

tested the car, still had a misfire

So, replaced the lead to No.1 an the car now runs sweet as a nut - I will get some more new leads for him to replace the rest of them (they look original )

I reckon a mixture of new leads an plugs sorted it?

The gung looks like (an smells like) something has gotten onto the rocker cover an died an basically melted down onto the plug and onto the HT lead - the smell is/ was horrendous