Petrol engines Diagnosing faults

General advice

Aidan: Just because parts are new don't assume they are good (lets face it nowadays what, if anything, is made better than it used to be). If anything is changed, especially if it's not because of a fault, keep the old to be able to revert if necessary. This applies particularly to engine ignition components e.g. HT leads, rotor arm, etc.

Misfiring/hesitation

Ignition causes

Aidan: Some may remember Project Caravelle had a problem once upon a time following fitting of new parts during service that he couldn't resolve and ended up throwing in the towel and selling the van at a big loss, then new owner replaced leads with a good old set and it was sorted for a tenner, just because new don't assume they are good, (lets face it nowadays what if anything is made better than it used to be) if anything is changed, especially if it's not because of a fault, keep the old to be able to revert if necessary.

Carburretion causes

Weak misfire/hesitation OR grossly overfuelling

Injection causes

Exhaust causes

Leaks and holes in the standard exhaust system can result in popping back when on the overrun, and slightly weak fuel/air mixtures that may cause hesitation on light throttles. Seriouly open exhausts can cause valve burning or overheating in the longer term, and can require mixture richening/re-jetting up, to get performamnce back and prevent damage. Of the three fuelling types, the injected MVs runnning closed loop (lambda connected) should stay in tune better.

Wear/Damage/Age causes

Burnt exhaust valves