3 replies to this topic

[gdcarpenter]

1990 Japanese Spec Mini Mayfair 1000 that received a John Cooper Garages Anniversary Model Conversion when new, some 54,000 miles ago. Low compression engine producing 150 psi across the board with 'Cooper' Head, twin SU2's, LCB, and stock cam.

After installing AFR gauge, and setting idle around 13.5, in town cruising and hard acceleration all well and fine. Highway cruising, at 70 MPH and 42,50 RPM with 3.44:1 final drive, was around 17 and sometimes leaner. Enriched idle to about 11.5, and though AFR gradually gets leaner as speed increases from 50-70 MPH, it pretty well stays below 17, but I would feel more comfortable with it a little less lean.

As a guideline I found the following in my research regarding needle profile stations:

"The first two dimensions (1 and 2) govern the idling mixture. The next five dimensions: 3 to 7 govern the pick up in fourth gear, from 30 to 70 kph (approx 20 to 40mph). A cruising speed of 60kph (35 mph) will lie somewhere around the fourth dimension, a cruising speed of 80kph (50mph) will occur around the sixth dimension. The dimensions from 8 to 13 affect top end rev range of the engine."

and

"Stations 1-4 cover idle to cruise at 4000rpm.
Stations 6-8 cover the point where you are cruising and you squeeze the throttle.
Station 12 what
Stations 1-5 are the cruise spots and when you floor it from cruise stations 6-7 are the first ones to come in"

I know none of this is conclusive or etched in stone and will vary but it seems to be a general consensus/ballpark out there from what I have found.

Given the above I think I may have found a possibly more appropriate needle than my current AAP to richen highway cruising AFR, the ADT.

There is one diameter only comparison table and a couple comparison charts with graphs, from 'Minty' and 'Haystack'.

The two needles are virtually identical top and bottom and the ADT only has slightly different diameters stations 6-11.

Then I tried to find some way of quantifying how much any specific needle diameter reduction increases fuel flow. In another forum I found a link to yet another British Mini Website offering a free spreadsheet download. Unfortunately I don't know if things have changed or my lack of tech skills prevented me from downloading it.

I did get some examples, and in the first forum someone posted some of his results. What I had never seen before that made brilliant sense to me was that, when comparing two needles, at each station the chart gave the annular area. That annular area is measured in square one thousands of an inch and is the 'ring' shaped area between the needle and the jet that the fuel must pass though

I used the concept to manually compare my current AAP needle to an ADT needle.

To start with a sample calculation.

If you have a 0.0900 jet and were to install a needle with a station 1 diameter of 0.0880" instead of one with a diameter of 0.0890" you are only decreasing the diameter 0.001", or about 10%. When you calculate the two different annular areas the smaller jet creates twice as much (100% more) area than the larger jet, and twice the amount (100% more) fuel will flow through it!

Yep, we know that would be a bad idea without the chart, but what happens lower down on the needle.

Doing the math comparing my current AAP to an ADT at station 8, the needle diameter again decreases by 0.001" as in the above example. But when you do the math this only increases the annular area by about 6.5%, only increasing fuel flow by 6.5%. As I understand it a 6.5% increase in fuel flow would knock down the AFR about one point richer, just what I am looking for.

The $27,000.00 dollar question is, would stations 6-11, especially 8, coincide with steady state cruising around 70 MPH!

So finally a question or two if I may.

Firstly, thoughts on replacing the AAP with an ADT.

Secondly, should I replace AAP with the ADT first, so if it's closer but still not right on, it should tell me which way i could profile the old AAP needles, or should I try profiling the APP needles first? At least now I have a baseline of sorts AFR wise as to where I am off and would recognize any improvements and what direction further improvements should take.

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Edited by gdcarpenter, 25 May 2020 - 01:26 PM.

[Spider]

A cruise AFR of 17:1 is about as lean as I like to got, but as long as I can get 12.5 - 13:1 on what from around 1200 to Red Line, I'm happy. I feel this is the part that needs to be sorted out and as long as the bridge in Carb hasn't been modified, the Cruise will usually sort itself. The charts with the needle stations 'suggesting' certain parts of the needles along their length I'd found to be total fiction. On what going from Idle to Full Noise, if the Carb is set up right and appropriately sized for the engine, it will use all stations as the engine 'air' damage increases throughout it's rev range. It's not proportional to revs but is proportional to Air Flow, by Area of opening under the Carb Dashpot Piston..

I'd suggest trying the two needles and see which your happier with or not and continuing the search.

 

Personally, I don't like custom profiling needles as a replacement is not ready to hand or easily replicated. This is important with twins and also with Biased Needles as these do wear and require routine replacement.

[Turbo Phil]

If you’ve got a wideband fitted build yourself a piston lift indicator. This takes all the guesswork out of it and with some time/practice makes profiling needles a relatively simple task.

Phil.

[gdcarpenter]

Thanks, better to be Lucky than Good!, Figured why spend $70 or so for two new needles that most likely wouldn't be right anyway, and there's nothing I can find needle wise even remotely close other than the ADT. Bit the bullet and dove into profiling.

First round started around station 5.5 and took it easy. Once I drove and saw I was slowly gaining on a richer mixture had at it. Wound up taking it back another half station and wound up removing about 0.003" at station 5 in the end. I could have gotten away with less but that would have left my idle too rich, so I wound up taking off more so I could raise the jets. It still tends to progressively get a tad leaner as I go from 60 to 70 MPH, but still below 16.5 at the worst. Being able to raise idle from 11.5 to 12.5 made the engine happier and I had to back the idle screw off a bit, quieted the rich 'burble' in the exhaust. Turned out station 1 has already worn 0.003" so I'll have to live with 12.5 at idle.

Thanks again to all.