Misfiring - spark problem

Why do you think it is ignition? my first thought was an airleak.

mrcheerful

fuel mixture is rich car is ok-> mixture not rich enough -> air leak like our collegue has already pointed out.

Yes, an HT electrical fault would be more obvious at tickover than higher revs so I would say an air leak is most probable, amazingly common on Volvo V70's and the like because of the size of the plastic intake system.

Spray WD40 from intake manifold up and test for a change in revs.

Andy

Is it a carburettor setup? Sounds like a weak mixture at low throttle, but not tickover. Carbs tend to give a weak mixture at small throttle settings once the butterfly wears. Or you could have a split rubber vacuum pipe going to the inlet manifold which would do similar things.

Thanks all,

I've had it checked for an air leak and nothing was found - but I don't regard this as conclusive, I'll be looking into it again.

I still suspect sparks. After the last service I found the mechanic had bundled together the HT leads and secured them with tie-wraps. Misfiring was bad. I snipped off the ties and separated the leads - result: misfiring reduced but not eliminated.

JP

It is possible you have a hairline fracture in the dizzy cap, these are not always visible, even to the trained eye.

I had this exact same problem on a vaux cavalier, got so fed up with it I took it to a tuning garage who changed the dizzy cap and all was fine, turned out the Halfrauds one that I bought to replace the broken one I had was also damaged.

Get another set of leads and try it, if it makes no difference take them back to halfrauds.

Andy