Moggie Running Rough...

[bloodyank]

Help!!
So, car starts fine, idles good, but in stop and go traffic, it'll sputter and almost stall. Seems like low revs are what makes it run rough, as once I get it on the highway it runs well.

It's a 1275 with I think an HIF 44 SU carb. I have never really adjusted the carb at all, and I'm at 7,000 feet above sea level, if that helps.
Thanks in advance--
Dave

[bmcecosse]

Well - you really DO need to set it all up. Was it ok before and this trouble has developed - or have you just started using the car and it's like this from the start? First move - lift the idle speed! Then when warm - adjust the mixture for best idle - all the time adjusting the idle as necessary. The engine needs to be right up to working temperature before trying this.. And of course - ignition timing may well need fine tuning as you do this.

[IslipMinor]

The standard needle will be way too rich at that altitude. We were at the top of the Stelvio Pass (9,000ft) a couple of years ago in the Minor and had exactly the same symptoms. Pulling at low revs was hopeless, hill starts were a nightmare, but once going ran fine.

What needle is in the carburettor? Do you have a picture of the carburettor so that we can identify it?

[bmcecosse]

But Richard - virtually ALL needles are the same profile in the early stages - it just needs the idle setting adjusted. The SU works on air mass flow - it doesn't know what altitude it's at - the amount of air going in determines how far the piston lifts and so the amount of fuel delivered. It's not affected by altitude - EXCEPT when idling when it works just like a 'traditional' carb . And so - the idle setting will need some attention. Of course - overall engine power IS affected by altitude - since there just is not enough air to make the power !

[IslipMinor]

virtually ALL needles are the same profile in the early stages

Roy,

Looking at the SU needle chart for the complete range of 0.100" HIF 6/44 needles, there are 16 diameter stations down each needle, and for the first station (the largest diameter) there are 12 different diameters, 25 for station 2, 46 for station 3, 57 for station 4 etc. With 268 needles in the range, the commonality rapidly runs out after the first station, and by station 4 of the 16 there are odd groups of commonality, but that is all.

As you say, the principle of the SU is that if the profile is right, then all that is required is to set the idle mixture correctly, and the rest will follow suit. I would very much doubt that the standard sea-level profile will still be correct at 7,000ft, even if the idle mixture is adjusted to compensate?

Burlen suggested going to this website:

http://www.mintylamb.co.uk/suneedle/

You can enter different needle codes and see the profiles compared graphically - it does not suggest alternatives for 'weak' or 'rich', but at least it is slightly more friendly than trying to do it with the SU needle table!

[bloodyank]

Thanks, guys- I'll get a pic posted for identification help.

[bmcecosse]

Yes of course -I use minty regularly. But mass air flow is mass air flow - sure with wide open throttle the engine won't be able to 'gasp' in as much air - so then the piston won't lift and the petrol flow will be right for the mass flow. .

[MarkyB]

This makes interesting reading on this subject;
http://www.redrivertriumphclub.org/altitude.htm

[bloodyank]

Here's a couple pics for identification....what do I have here?
Thanks![frame][/frame][frame][/frame]

[bmcecosse]

It is an HIF carb - could be 38 or 44 - only a ruler will tell - or take the needle out. If it is Axx = 38 , and if Bxx = 44.

[IslipMinor]

Isn't an HIF 6/44 is held on with 4 studs and an HIF 4/38 with 2 studs?

[Matt]

I know one of the Imperial hifs is only held on with 2 bolts... I think its the 6.

The HIF44/38, which are much more common are both held with 4 bolts.

And thats a 1275 A+ from a later Marina or Ital with a Metro alloy inlet manifold

[MarkyB]

Should there be a Bakelite type spacer between the carb and manifold maybe?

[bmcecosse]

Yes - and there is !

[bloodyank]

Idle speed seems fine.. high in fact. I'd be scared to undertake changing the needle I would think...
Can someone walk me through adjustment, though? A diagram, maybe, to show which screws to adjust....

So you think the engine is out of a later Marina? What makes the engine A+?

Could my problem be excess heat? The carb has what looks like a heat sheild on the bottom of it, but that manifold looks awfully close.

Thanks as always!

[bmcecosse]

The Mini engine operates with the exhaust and carb in close proximity - it's not heat......

[IslipMinor]

[frame][/frame]
In the picture above:

1. Idle speed adjustment
2. Fast idle adjustment
3. Fast idle setting - when the line is opposite the centre of the fast idle adjustment screw, adjust speed to 1,200rpm
4. Distributor vacuum take off

Now it gets interesting!
5. Should connect to the distributor vacuum unit, but appears to loop round and connect directly to the inlet manifold - WHY?
6. Inlet manifold vacuum connection - block this off?

Unless there is a special reason (?altitude) there is no purpose in linking the distributor vacuum connection to the inlet manifold. It is likely to cause very odd behaviour on part throttle??

As a first change I would disconnect this tube and block the inlet manifold end (6) and connect 5 to the distributor vacuum unit and see how it runs. At altitude an engine generally needs more advance than at lower levels, so getting some part throttle vacuum advance should help matters.
[frame][/frame]
The diagram above shows the opposite side of the carburettor to your picture, with the connections for fuel, crankcase ventilation, float chamber ventilation and mixture adjustment.

11. Should be connected to the timing cover oil separator.
8. Vent to atmosphere via a small diameter tube
2. Mixture adjustment - screw clockwise (in) to richen and anti-clockwise (out) to weaken. Adjust with engine at normal working temperature and adjust to get the smoothest idle. Try screwing out until the engine slows, that is the indication that it is too weak. If screwing out just makes it worse, then screw in first a couple of turns and screw out until it slows. Once you find the 'weak' point screw in about 1/2 turn or until the idle is the smoothest.

[bloodyank]

Wow, Thanks--
Will try to mess with it today after the big parade in town. We're celebrating our separation from a certain tyrannical monarchy many years ago. I, for one, however, am happy we are friendly allies in this day and age. I'll post up a pic if I can.