Setting SU Mixture

[John P]

Hi,

I have an "Ulster" replica with a 1 1/4" SU Carb from a Morris Minor (I Think) with an AN needle which is fed from the scuttle tang by a 6 volt SU pimp.

I have set the timing to TDC fully retarded, it's a later DK4A distributor, but have always had problems with starting.   Mo matter what the weather it needs full choke to have a chance of starting but on the road seems OK.   However if you switch off re-starting is sometimes fine but more often problematic.

To try to resolve the starting issues I have dismantled and fully cleaned the carb, re-checked the timing, added fresh petrol, and made sure the float level is correct.   However, when I try to adjust the mixture at tick-over I can turn the jet up and down around two turns with no real effect on the running.

The plugs are normally black so it looks as though I need it very rich to get started but then it's too rich for normal running.

The engine was completely re-built by me a few years ago so I don't think there's any problems with compression etc and, as far as I can tell there's no leaks in the induction tract.

Any suggestions why I can't seem to adjust the mixture correctly?

John.

[Chris KC]

John, your set-up sounds very similar to mine - I'm also using 1.25" SU (AUD13) with an AN needle & an SU pump.

I'm not sure it's optimal but it certainly runs OK. Yes, always choke from cold (by which I mean cold engine, regardless of weather). It needs a bit of a warm up before you can let the choke out. However once warm I've never had a problem starting. In fact hot or cold one swing of the handle is all it needs. I seem to recall setting the mixture up by taking it for short drives and inspecting plug colour on return - that was years ago though when petrol was petrol.

If the plugs are normally black (and it's not oil) then you are running too rich. Make sure you've done a few miles though before checking or they'll still be black from running on choke. Should be a sandy colour.

If I have engine starting or running problems the ignition system is always my first port of call, and points are top of the list.

[Ian Williams]

If your carb has ever be used without the needle centred the jet will almost certainly be worn, if this is the case, as it probably is, you will never get the idle mixture correct. The problem with an SU is that the wrong needle can be made to work by adjusting the height of the jet so mixture is correct at cruising speed, however almost everywhere else will be a compromise. Most SU needles are identical at the first two measuring points, to set the carb up you should get the idle mixture adjusted first, if the mixture is incorrect at any other measuring point a different needle should be selected to correct this, NOT by raising or lowering the jet, a flat or two is probably acceptable but will still compromise idle. Where people fall down is recognising where in the rev range you are week or rich and which measuring post this equates to, however getting it exactly right pays benefits in overall performance, particularly noticeable with acceleration.

[Charles Levien]

Hi,

I have an "Ulster" replica with a 1 1/4" SU Carb from a Morris Minor (I Think) with an AN needle which is fed from the scuttle tang by a 6 volt SU pimp.

I have set the timing to TDC fully retarded, it's a later DK4A distributor, but have always had problems with starting.   Mo matter what the weather it needs full choke to have a chance of starting but on the road seems OK.   However if you switch off re-starting is sometimes fine but more often problematic.

To try to resolve the starting issues I have dismantled and fully cleaned the carb, re-checked the timing, added fresh petrol, and made sure the float level is correct.   However, when I try to adjust the mixture at tick-over I can turn the jet up and down around two turns with no real effect on the running.

The plugs are normally black so it looks as though I need it very rich to get started but then it's too rich for normal running.

The engine was completely re-built by me a few years ago so I don't think there's any problems with compression etc and, as far as I can tell there's no leaks in the induction tract.

Any suggestions why I can't seem to adjust the mixture correctly?

John.

Hello John,

Is your DK4A set up with electronic ignition?

It makes for easier starting and no more problems with stale pet4ol.

Charles