Mixture adjustment

[KyleMorrisEvans92]

Hi,

My Morris misfires when under load travelling up hills, I have adjusted the mixture and there has been some improvement and it hardly does it now. Do you think this is related to mixture ? It does still do it occasionally when travelling up hill, does it need further adjustment?

Thanks

Kyle

[beero]

If it only does it uphill then you could be looking at fuel starvation. Is it okay for the first few yards of the hill?
Possibly filter blocked, poor fuel pump performance, or rubber fuel hose collapsed internally so restricting flow.

[KyleMorrisEvans92]

Yes, it is fine in first and second, when in third it misses and only occasionally does it on the flats. Could it be timing related?

[bmcecosse]

It could be almost anything! Tell us the plug colour - and is the vacuum advance working correctly? Are the valve gaps correct - and the points gap? How old are the plugs ? Has the ignition timing been optimised - not just set to some random figure from a 40 year old Manual?

[KyleMorrisEvans92]

The plugs are a light brown colour and the timing was set by an engineer at a Classic specialist. The plugs are about two moths old. The Vacuum advance is fine. I altered the mixture today and it seems to be running better, but with the odd misfire

[bmcecosse]

Was the timing set by road testing? Make and (heat) number of plugs ? Valve gaps? Ideally - a compression check will tell us the general engine condition. But as others have mentioned - it could be a fuel flow problem. Pump a pint of petrol into a container and tell us how long that takes.

[KyleMorrisEvans92]

After adjusting and strengthening the mixture, it seems to have done the trick, for now at least. It seems to have a bit more power when pulling off etc...

Thanks folks

I also cleaned the filter in the pump, as there were a few deposits in there.

[bmcecosse]

I think the filter may be the answer....mixture adjustments do little for the running mixture - most effect is at idle. Take it for a good thrash - switch off and coast to a halt - and pull out a plug to see what colour you have then. You should also advance the timing till you hear a little pinking - and then take it back very slightly so it's 'not quite' pinking. It's the best you can do without adjusting the advance curve internally.

[Alec]

Hello BMCE,

"mixture adjustments do little for the running mixture - most effect is at idle."

True for virtually every carburettor except for S.U. and Stromberg where it affects the mixture over the whole range.

Alec

[bmcecosse]

Only by a TINY amount - which is why I said 'little'. If you really want to change the running mixture - you select a different needle !!!

[Alec]

Hello Bmce,

no, that's not correct, altering the mixture nut changes the mixture over the full length of the needle. Needle changes are only required if the mixture is not consistent over the full range.

Alec

[bmcecosse]

The change with the 'nut' is tiny - assuming just a few flats of turning of course ...... there are hundreds of different needle profiles with relatively HUGE differences in the taper profile -some at specific parts of the air flow range - some throughout the whole range above idle. And in 'other' carbs too - the idle mixture adjustment DOES affect the running mixture - because the idle jets stream fuel all the time the engine is drawing air... However - as in the SU - the idle adjustments are relatively unimportant to the running mixture.

[Alec]

Hello BMCE,

the amount of the adjustment is irrelevant, the fact is it does affect the mixture over the full range. Yes there are many different needles to suit the many different engines with different air flow characteristics.

Alec

[bmcecosse]

Yes -I said right bat the start that it makes a slight change - but it's not significant - unless you turn many flats - and then the idle mix will be well gone. It's principally to set the idle mix. If the running mix is not correct then a different profile needle is called for.

[Alec]

Hello BMCE,

the mixture adjustment is just that, the mixture adjustment, saying it is mainly the idle mixture is simply misleading. It is adjusted at idle because it is convenient but it is not the idle mixture adjustment in isolation.

Alec

[bmcecosse]

Alec - you are obviously not going to be convinced of this -but honestly - the only SIGNIFICANT change is at idle - where the tiny amount of fuel going through the annular gap is much changed by a flat or two of turn on the nut. But when the piston rises during running the annular gap is MUCH larger as the taper needle rises out of the fixed jet - and so the tiny change made by a few flats adjustment is of course there - but is really not significant. Virtually all needles are the same profile at (or just above) idle - but then the profile changes as the air flow increases and the needle rises out of the jet. So - the adjustment is used to give a nice smooth idle and/or low readings to scrape through the silly MOT emissions test where necessary - but the running mixture if not correct will need a needle change. Even BMC always quoted a weak and rich needle along with the standard 'best guess' needle that was fitted on the production line.

[Alec]

Hello BMCE,

I understand your point but bear in mind that the adjustment affects the running mixture progressively,, stages 3 to 5 (Normal running area) will be much greater than at stage 12 to 13. Those early stages only have a 2 thou difference so gap change is only a thou per step.
The reason I posted was so that those inexperienced in S.U.s don't just look on the adjustment as an idle adjustment but that it is the mixture adjustment. You did say it makes a tiny amount of difference higher up but I wanted to make the point that it can make a bigger difference than you suggest?

Alec

[bmcecosse]

I don't believe it is a big adjustment at all when running. Hence the need for Rich and Weak needles (and many others of course).. And so - I want to make it clear that adjusting the nut really doesn't make any appreciable difference to the overall running mixture - just the idle - which should be set as smoothly as possible - testing with the handy 'lift pin' .