12g940 head

[bmcecosse]

The 15 thou is fine - leave it! If the needle is not fitted with the shoulder completely flush with the base of the piston - it could be impossible to set the idle weak enough.

[horrace]

yep i got it now as it wowuldnt "seal" against the jet ill take the dashpot off and the piston out annd have a look, change the jet as well though?.

[bmcecosse]

I wouldn't......

[horrace]

What needle do you have? I Have an AAu needle. As BMC (Merry Christmas from a loyal reader!) says, check that the jet mounting collar is flush with the bottom of the piston when you tighten the clamping screw. (not done this yet) When you take the damper pot off to check that, see how the jet position looks, it should be a little under-flush with the surrounding brass sleeve. the jet looks to be several mm below the surround brass sleeve. You may just have been screwing the wrong way! Screw inwards/ clockwise to richen, and the reverse to weaken.if you screw in as much as i have the jet is closest to the needle so must be leaner yes? Or maybe your timing could be too retarded?

i have the needle, spring and needle fixing collar here, it seems bit odd the way it is set up as the collar has a shoulder which makes the needle sit to one side so will wear one side of the needle or jet? is that how it goes back together? i assume with the collar flush with the damper?

[bmcecosse]

Yes - the idea is that it is spring loaded to one side rather than worrying about 'centering' it perfectly as with the earlier type. And yes - it can wear the jet slightly after much use.

[horrace]

ok so thats about right then what do you think about my other findings? red writing in the quote...

[bmcecosse]

I use AAA needle - but I do see AAU suggested on the Mini forums - it will be fine for now. The other highlights seem about right - just put it together and run it!

[horrace]

the jet seemed a little bit low against the collar, so i investigated..... the wire link arm had straightened and was snagging at its attachment to the jet so i bent this which shortened it and allowed it to move more freely. the mating surface where the jet assembly touches the bottom of the carb had a layer of crud on it which i cleaned off. both of these things didnt allow the jet to seat high enough thus making it run rich!

when all back together i started the pick up and it wouldnt even run under throttle as it was now running much leaner, i enriched the mixture by a few complete turns and bingo away it went! i did the old lift the piston technique to tweeak it and im going to take it for a spin tomorrow see if we have got the mixture anywhere near right

[bmcecosse]

Well done finding the problems ! I admire your detective work!!

[horrace]

well its so far so good.... i havnt quite got the mix right yet, still running a bit lean, every time i go out for a drive i check the plugs on return and adjust a couple of turns on the carb, nearly there. There is a definite improvement in performance i especially notice above 60, it still pulls well at 60 which it didnt before, (remember i have a 3.7 diff). and when i get it running perfect on the plugs i would think it will be even better!
should i adjust the timing at all now or leave as is?
when i put a new unleaded head on the engine when i first bought it i was told to change the timing by a degree or 2 which i did, now ive changed the head again i wonder whether it should tweak it another degree or 2? or what should it read on the timing gun?

[Alec]

Hello Horace,

yes you need to retime it, it shouldn't need much to get it to optimum. Changing mixture means a slight change is needed to the timing, so get that right first. Advance it until it is pinking then retard so it just in not, which should be the best setting.

Alec

[bmcecosse]

Yes - as Alec describes - just 'not quite' pinking is the way to go. Don't worry about timing gun - sell it on ebay!

[horrace]

last time i adjusted the timing my chum did it with his gun and i wasnt looking.... so is it a case of loosening the clamp on the distributor and twisting clockwise or anti clockwise? i guess check the points first.... and pinking is a sound like what?!!

[Alec]

Hello Horrace,

it's a light metallic rattle from the engine, getting the engine to pull hard at slow speed in high gear is the best way of getting it to 'pink'.

Alec

[bmcecosse]

Yes - twist it very slightly clockwise to advance. But check the little vernier adjustment on the dizzy (knurled knob) - maybe there is some adjustment there you can use first. The timing gun is useless - because you would be timing it to...what ?? Pinking is indeed a light rattle - like a few small nuts shaken up in a tin can!

[horrace]

ok thanks ill try that at the weekend so long as the plugs look ok...........