9 replies to this topic

[huw_jenks]

1990 MG Metro, totally standard (HIF44, electronic dizzy).

I want to do a bit of tuning on my MG Metro and I want to know whether I really need to get it rolling road-ed. Last time I took my car to get set up on a RR it cost a bomb, and although it was set up very well, I don't really have that much money to spend at the moment.

I have a spare HIF44 which I've gas flowed and a spare metro inlet manifold which has had the same treatment (as recommended in The Yellow Bible...). I've just bought on Ebay a massive K&N with a stub stack included too (£4.99 - Bargain!). I'm not bothered about the extra noise of a pancake filter either, there's a bit more soundproofing on a Metro!

So having typed all the relevant info into WinSU it has told me that the needle I will be needing is B?L. So what I want to know is, is it going to be safe just relying on the software, buying the right needle and using my ColourTune, or should I be looking at a rolling road setup?

I do realise I will need to pay to register software, unless someone with the full version would like to work it out for me!

[998dave]

winsu is designed to get a starting point, it can't hope to accurately predict the exact condition of your engine or vehicle.
You'll still want to go to a Rolling Road, but with a closer start, and by ensuring your car is suitably prepared, you should save a big chunk of cash once there!

Dave

[huw_jenks]

Thanks Dave, any other opinions?

[Guess-Works.com]

It's probably a BDL needle which is a pretty standard choice for an HIF44 on a mildly tuned engine ( basically MG metro spec ), actually I think a BDL is the stock needle for the MG Metro.

* Just checked... it is the stock needle for a leaded MG Metro.

Edited by Guess-Works.com, 04 February 2011 - 06:54 PM.

[huw_jenks]

It's probably a BDL needle which is a pretty standard choice for an HIF44 on a mildly tuned engine ( basically MG metro spec ), actually I think a BDL is the stock needle for the MG Metro.

* Just checked... it is the stock needle for a leaded MG Metro.

Thanks Guessworks, I've just checked and I've got a BDL needle so that's a good starting point at least. But I've been thinking, would it be okay in that case to stick with the standard needle and richen the mixture screw? My logic is thus.

The engine is sucking in a mixture of air/fuel at a ratio set by the mixture screw. Swapping the paper air filter and air box for a K&N will make it easier for air to be drawn into the engine, meaning less fuel will be sucked in and the mixture will be lean, so the mixture screw needs to be richened to compensate for this.

If the ratio is set correctly at idle then it should be the same all the way up the profile of the needle. The amount I need to adjust the mixture at idle should be proportional to what is needed at full throttle, as the only change has been to increase air flow at all revs.

Is this correct, or am I just making stuff to suit my own needs?

[Guess-Works.com]

It's not far off....

but, it's advisable, when ever changing an item in the induction or ignition system, then it's a wise move to get it tuned... but just changing the air filter will not make a whole lot of difference.

[Ethel]

A carb senses air speed, but better flow will increase the air density leaning out the mixture. The catch will be a better filter makes more difference as the amount of air going through it increases - it will lean out more under load than near idling speed. Even so I don't expect the effect would be that much to make a tweak of the mixture screw inadequate.

http://www.mintylamb.co.uk/suneedle/ is also handy for looking at carb needles.

[bmcecosse]

The beauty of the SU carb is that it automatically increases fuel flow to match air speed! The BDL is the starting point - the mix screw only affects idle mixture and the colour tune is also only for idle. You don't need an expensive RR session - just run the car hard after optimising idle mixture and timing - then switch off and whip the plugs out and inspect the colour. If white = too weak, black = too rich and light beige is just right. IF they come up weak - you can simply file a flat on the middle and upper section of the needle to increase the fuel flow. Or buy a richer needle of course!

[Ethel]

The mixture screw lowers the jet, so it does alter the mixture for any degree of piston lift (airspeed). It does depend on the needle profile so isn't linear though.

[huw_jenks]

The beauty of the SU carb is that it automatically increases fuel flow to match air speed! The BDL is the starting point - the mix screw only affects idle mixture and the colour tune is also only for idle. You don't need an expensive RR session - just run the car hard after optimising idle mixture and timing - then switch off and whip the plugs out and inspect the colour. If white = too weak, black = too rich and light beige is just right. IF they come up weak - you can simply file a flat on the middle and upper section of the needle to increase the fuel flow. Or buy a richer needle of course!

Not quite correct, filing the needle near the middle and upper section, wont richen it up in the correct area. This is usually done to richen up the bottom end/mid range.

Depends which way up you're holding the needle right?

Anyway thanks for the replies, it seems that I should be going for a RR setup if I want the job done exactly. However, running the standard needle and richening the mixture willl be unlikely to cause any damage, I will just have the knowledge that the car isn't running qute at it's optimum. I think I'll have a look on WinSU and see what needles I can find that are slightly richer top end than the BDL.