running rich

[waterhouses]

Been out in the car today and noticed a stronger tan normal petrol smell accompanied by lumpy(ish) tickover and higher than usual fuel consumption. What would cause the car to run rich and what may requires adjustment or checking?

Thanks

Richard

[stuart]

Been out in the car today and noticed a stronger tan normal petrol smell accompanied by lumpy(ish) tickover and higher than usual fuel consumption. What would cause the car to run rich and what may requires adjustment or checking?

Thanks

Richard

 

Assuming its a PI car then check to make sure the choke lever on the side of the metering unit is returning properly.

Stuart

[Risso]

I had a similar problem with my PI TR6, high fuel consumption, discharge from the exhaust black soot, poor running ie miss firing. It turned out to be a split diaphgram in the metering unit. Replaced metering unit problem solved.

 

Risso

[peejay4A]

Also, check for air leaks on the metering unit to inlet manifold vacuum tube. Or indeed any air leak on the induction side which could reduce vacuum.

[bcthom]

Having had a split diaphragm in the metering unit resulting in black plugs I have thought long and hard about the possible results of air leaks in the manifold system.

I am willing to be corrected, but my view is that a small air leak into the manifold is of little consequence. It will lower the manifold vacuum as many people have noticed, and this will cause the metering unit to inject more fuel into the engine, but there is also more air going into the engine as a result of the leak, and the most noticable result will be a faster tickover and possibly uneven running if the leak is closely associated with one cylinder only, ie inlet manifold leak. If the leak is, say, in the brake servo then there would be no uneven running since all cylinders will receive the same increase in both air and fuel.

The only situation in which a leak will cause a large change in fuel to air ratio is if the metering unit diaphragm is split, in which case variety of odd effects can occur. In my case the diaphragm was split along the crease where it folds as it works up and down, and this restricted the leak under some circumstances and not others. Hence sometimes all seemed well, but lift off with the right foot and down again or get stuck in slow traffic and the leak opens up and there is no power at all, and soon one or more plugs stop sparking and it is a case of ET phone home.

 

I would be interested to hear other comments on this

 

Incidentally, the diaphragm can be replaced fairly easily without altering the settings of the metering unit.

 

Bryan