Morris Minor Owners Club

Recently came owner of two door 69 moggy passed mot no bother but was idling very fast and the mixture was rich, i purchased a gunson colortune and proceeded to weaken mixture to i nice blue was observed, and reduced revs untill the red light on dash came on then increased slightly to went of. Now the problem i have now is it stalls when the vechical is slowed to a stop at lights it will stall, turning the slow idle up a bit might sort this, but i noticed after the event that when i was adjusting the mixture nut at bottom below spring, the locking jet nut above spring was turning in sink with mix nut (ie when i turn the mix nut down the jet locking nut came down by the same amount). So my question is 1 should i have made sure the jet locking nut was locked tight to base of carb before turning mix nut 2 is it possible i need jet centering now. Thanking You Scott

Check if the carb piston rises OK and falls with an audible clunk.
If it doesn't you'll need to centre the jet.

Yes - you do need o tighten the locking nut - and then check if the piston rises and falls nicely. The colourtune is useless - just use the lift pin on the carb - that's what it's there for! If you have a dynamo - the red light should be flickering at idle......

As above - colourtune is useless as the burning is only measured at idle. Err on the side of rich and increase the tickover (but not ridiculously fast). Check spark plug colour and should be a nice grey or brown colour at the tip with some soot round the threads.

Minors aside, every B.L. transverse car I have ever had has been prone to cutting out when rounding sharp corners in neutral, or even changing down. Methinks it due to swirling in the float bowl and mechanical fuel pump, but cure eludes me.

The 'cure' is the HIF type SU carb ! Horizontal Integral Float - specifically designed to eliminate surge effects on roundabouts/stopping quickly etc. It works.

Thanks for that - I'll keep my eyes peeled at the autojumbles.

I have took of cone air filter to view piston and when lifted to top and released it falls and clonks to bottom. Am i right in thinking that i should screw the nut closesed to the carb bottom up tight make sure piston is still fallen smoothly with clonk. stick a spanner on it so it dosnt move and with an other spanner unscrew the nut under the spring so both nuts are not turning in Unison. The car never used to stall it just wanted to go every where in a hurry i know thats hard to believe for a 1098 and u could smell the excess fule sometimes.

Starting point for mixture nut, is it screw it up tight slacken 12 flats.

Colour tune should i ignore this and and just use lifting pin

The jet should move up and down when you adjust the mixture nut, that is how it works. If it stalls at tickover then either or both the tickover is too slow or the mixture is too weak. All Minor engines are slightly different

As you suggest - tighten the nut and ensure the piston continues to drop cleanly. Then indeed - hold it as you wind the mixture nut up/down to get best /smoothest idle. And check with the lift pin.

Thanks every one for there input. I think i got away with not disturbing the jet with having the top nut slack due to the piston being in the down position cos it was idling . I will have a go a setting it with piston lift tomorrow. Is 12 flats down from top position correct for a starting point for mixing nut on a su h2 carb seeing im not going to use colortune this time, pity thought it was to easy turn nut untill gos blue lol

12 flats means nothing, it's just a guess! Adjust it to get smoothest idle -at full working temperature of course.

If you haven't checked them for a while make sure the tappets, plugs and points are all adjusted properly too.

Got a good tip online this morning bout how to check if the jet is centered correctly. 1choke in listen to the piston drop and clunk 2 choke out listen to piston drop and clunk 3 any difference in clunk jet not centered correctly. So i did as it said and with choke in it was a clink rather than a clonk so followed centering instructions both sound same now started car screwed mix nut down making sure top nut secure, untill obvious to rich adjusted the slow idle screw and proceded to turn mixing nut up bit by bit and adjusted the slow idle when required, and lifted piston on every turn untill revs increased slightly and settled to what it was, then adjusted slow idle untill red light for dynamo flickered every now and then, car sounds lovly now like a kitten purring and not an angry lion, so decided to have a look at the colortune to see what that shows, and the result is, any one want to buy a Gunson Colortune only used twice

Excellent! Well done. Now after a good hard run - switch off and pull a plug out to check the colour. Should be light beige. If white then too weak, and if brown too rich.

Hello BMCE,

"12 flats means nothing, it's just a guess!" I have to, once more, disagree, all else being correct there should be little or no more adjustment required. That's how some carburettor manufacturers can supply carburettors with sealed idle adjustments as
it is set to a physical datum.
Colourtune is a lot more useful than you make out and it is not just for the idle either; particularly with a multiple carburettor set up.

Alec

Oh dear Alec- here we go again! How can 12 flats be 'right' - there are many different needles and many different engine specs - not to mention float level variations - and wear/tear in the jet orifice! It IS just a guess - possibly a starting point, but the idea of counting flats is ludicrous anyway. Even on multi carbs (well, two -for me) the idle is easily set by lifting the pistons. The colourthing must NOT be used at anything above idle - and in any case, just revving up an engine with no load is no way to judge running mixture. Ideally a 12 volt powered gas probe in the exhaust used while running the engine under load (up a hill etc) - or for most of us - spark plug colour checks, are the best guide to mixture strength. Or an expensive rolling road check of course - but beware - there are many cowboys out there with rollers keen to trouser the money for little benefit.

Hello BMCE,

simply because the carburettor is a precision device, there are many different needles but if you look at the dimensions chart the idle dimension is virtually identical whatever the needle. It is a very good starting point and even with minor wear in the jet will be virtually correct assuming all else is. That is where people go wrong, by not ensuring the carburettor is correctly assembled and adjusted.
Of course you can rev an engine with Colourtune plugs in, where did that idea come from?
The best way of road tuning is to fit a oxygen sensor in the exhaust and monitor the readings, that is the most cost effective way, but relatively expensive.
You are happy with your ways as I am, but there is not one correct way of doing things to the exclusion of others.

Alec

The colourthing really doesn't have the strength to be revved....take care -it may explode on you! And yes - I know virtually all needles are the same at idle - but to blindly assume that 12 flats will be correct is not good practice. I can accept it as a starting point....but 12 flats suggests some precision, whereas ' a couple of turns down ' gives more of an idea that it's just an approximation.....

Hello BMCE,

"but to blindly assume that 12 flats will be correct"

I did not say that, you say that "12 flats means nothing, it's just a guess", which is simply not true, it's a very good starting point and if much deviation is required from that then there is something else amiss.

Alec

I accept that Alec -just that sometimes '12 flats' is quoted as THE setting - and close the bonnet..