Morris Minor Owners Club

Just wanting advice on carb needles for a SU HIF38 carbie with water heated manifold and electronic ignition. Ive got an AAU needle fitted at the moment but i think this maybe a bit weak as the plugs are whitish grey, but more whitish tho and the mixture has been set using gunsons colour tune so is fairly correct at idle. Any suggestions

Is this colouration from just normal pottering around - or is from a 'power run' where the engine is worked flat out uphill - and then the plugs inspected ?

What exhaust manifold/system are you using?

Is the rest of the engine standard 1098?

The engine has a Howley waterheated inlet manihold, but the rest of the engine is standard other than what described. The whitish plugs is are seen at all engine uses, but are more obviously so after long runs which i assume is normal anyway.

Right, well. Are you sure that the breather pipe on the side of the carb is blocked up? (or connected to an engine breather).

If you set the mixture using a colourtune then did you set it to a blue flame or just into the yellow?

I would try richening the mixture a bit first then checking the plugs again.


Does engine 'hold-back' at any particular rev range? or does it have smooth power all the way?

If you need a richer needle for your set-up then the AAM is a good one to try after your AAU as it is richer mid to top end.

The engine seems to be quite smooth up to about half throttle but sounds like it leans out if i put my foot right to the floor at accelerating away. When i set the mixture i first got a yellow flame and then weakened the mixture until the yellow just goes and the blue is present. The engine breather is fine and all the connections are airtight. Initial acceleration is good and pulls away well but going into third and forth the engine seems to hesitate slightly.

Right, well I set the mixture until you JUST get the yellow/blue mix but I usually move to yellow moreso than blue.

That sounds like it could be dizzy related. if you move into higher gears (lower revs) and it struggles, unless you have too thick oil (or non!) in the dashpot.

What's the POWER like when accelerating in low gears through the rev range?

If the power drops off when at high revs and at wide-open-throttle, if all connections are tight and everthing is OK then perhaps a richer mid to top end needle is required. The AAM should suit your needs.

If the power doesn't drop off then I would say the needle is OK and the fault lies elsewhere.

Set up the ignition timing so it 'just' pinks on hard acceleration in top gear - then wind it back very slightly. How does the carb react when you lift the piston with the jiggle pin ? This is the best way to set the idle mix on an SU - it should not die , or rev up when you lift the piston about 1/4". Richer needle will mean higher petrol consumption.