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Varying A/f Ratio - Needle?


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#1 gdcarpenter

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Posted 19 May 2020 - 05:50 PM

1990 Jap Spec Mini Mayfair 1000 with original dealer installed John Cooper Garages Conversion.
Final drive 3.44:1 and testing done in town up to 40 MPH or so, pretty sure stock cam.

Did not want to drill hole in the dash I built so found suction cup mount gauge pod.

After some tweaking overall happy, save the rapidly and constantly changing readout.

Averaging 13-13.75 idling smoothly at 800 RPM, will tick lower/richer ratio the longer the idling time.
Averaging 14.5-15.5 steady state throttle cruising.

All good so far, but first run, using manifold vacuum to distributor advance, hitting 17 and higher on hard acceleration. Swapped to ported vacuum and down to about 15 on hard acceleration. My conclusion is the extra spark advance from ported over manifold vacuum giving increased advance under heavy load allows more complete combustion before exhaust opens. Changing from ATF to engine oil in dash pots helped reduce hard acceleration ratio slightly.

Carbs Pistons have about 7 second drop test with AAP spring loaded needles and 'Red' spring. Wonder if switching to 'Blue' spring would enrich the A/F ratio on hard acceleration? Jets seem fine elsewhere.

Also my max advance was limited to about 20 degrees with the 65D I had, the replacement I installed is very similar. Anyone know of Mini distributor curving facilities stateside? I would like to have my old 65D recurved, any suggestions on an appropriate curve? Understand I should be shooting for 30-34 max.

Apologize if I can't figure how to get my darn IPad to post photo right side up!

Thanks🙃

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Edited by gdcarpenter, 19 May 2020 - 06:03 PM.


#2 Steve220

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Posted 19 May 2020 - 06:01 PM

Hmm. The idle and cruise seem ok. what should be about 13 on a NASP engine.


Edited by Steve220, 20 May 2020 - 08:41 AM.


#3 DamoMini

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Posted 19 May 2020 - 06:42 PM

Love this topic!  I am hoping to fit the same AFR unit to mine soon.

 

On the advance item, I don't believe you can get "more" vac advance from ported over manifold, it just comes in at different times.

 

On manifold, at idle or throttle closed, you will be at high vacuum so max vac advance.  As you open the throttle the vac will drop.

On ported, at idle or throttle closed, you will be a no vacuum, so no advance.  As you open the throttle the vac advance increases.

 

At partial to full throttle the ported and manifold vacuum will get closer to each other and drop out as the carb opens up.

 

Keep the data coming!



#4 gdcarpenter

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Posted 19 May 2020 - 07:24 PM

I drove my Mini with a vacuum gauge teed into the manifold vacuum and then with the ported vacuum teed in. The biggest difference, outside of idle, is at what. When you put the petal to the metal the manifold vacuum drops off precipitously, almost to zero, whereas the ported vacuum 'comes on' under that same circumstance with the increased air flow thought the carbs.. While the ported vacuum isn't all that strong, my vacuum advance unit kicks in at about 3" hg. and reaches it's max advance, 8 degrees distributor advance (16 degrees C/S advance), at about 8" hg., so it doesn't need a strong vacuum source.

I cannot think of any other reason why my AF ratio at heavy acceleration was significantly lees running ported versus manifold vacuum. The extra advance would allow more fuel to be burned off before the exhaust valve opens, 'leaning' out the A/F ratio. That's why I feel I need more than the 20 or so degrees max centrifugal advance I am currently getting. Also wonder if going form a Red to a Blue spring would slow the rise of the dashpot and richen the hard acceleration A/F ratio.

Really wanted an analog A/F ratio gauge but couldn't find one with decent reviews. The never ending rapid changing of the display is almost dizzying, and you just have to 'interpret' what the overall/average reading is. Other than that the install went well & fired right up.
Getting a 12 degree off horizontal install of the O2 sensor in the Mini was tight, like everything else about the Mini!

#5 sonscar

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Posted 19 May 2020 - 08:50 PM

Roughly speaking with sweeping generalisations.AFR of 17 at what is engine damage territory aim for 12.5 to 13.1.timing and AFR are linked lean mixtures need more advance rich mixtures less so.Total advance about30 to 34 degrees.Heavy accn may need less advance.It is relatively easy to damage the motor guessing.I have spent countless hours tuning and experimenting with my Mgb1800 and Mgb3500 and these are only my observations,your mileage may vary.Experiment carefully,Steve..

Edited by sonscar, 19 May 2020 - 09:06 PM.


#6 timmy850

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Posted 19 May 2020 - 09:46 PM

I’ve recently put twin HS2’s in my car and going through the same process. I’m still learning and happy to be corrected!

Some of it works the complete opposite to what you’d think. For example the spring keeps the piston down and the lower the piston is, the smaller the opening and the faster the air is moving. The faster moving air through the carb will suck through more fuel than slower moving air (despite the needle being in a richer portion)

The same theory goes for the damper oil (which only slows the piston on the way up). If they open up quickly they’ll raise the piston up quicker and be leaner due to the reduced air speed

Do you have any PCV or emissions/breather hoses on the intake manifold? Mine had a problem with lean running at cruise, this turned out to be partly due to air leaks sending too much air through the PCV.

I assume you’ve got a 998 with twin HS2’s? Where do you have the O2 sensor and is there any possible leaks between the head and sensor?

Edited by timmy850, 19 May 2020 - 10:08 PM.


#7 DamoMini

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Posted 19 May 2020 - 10:09 PM

I drove my Mini with a vacuum gauge teed into the manifold vacuum and then with the ported vacuum teed in. The biggest difference, outside of idle, is at what. When you put the petal to the metal the manifold vacuum drops off precipitously, almost to zero, whereas the ported vacuum 'comes on' under that same circumstance with the increased air flow thought the carbs.. While the ported vacuum isn't all that strong, my vacuum advance unit kicks in at about 3" hg. and reaches it's max advance, 8 degrees distributor advance (16 degrees C/S advance), at about 8" hg., so it doesn't need a strong vacuum source.

I cannot think of any other reason why my AF ratio at heavy acceleration was significantly lees running ported versus manifold vacuum. The extra advance would allow more fuel to be burned off before the exhaust valve opens, 'leaning' out the A/F ratio. That's why I feel I need more than the 20 or so degrees max centrifugal advance I am currently getting. Also wonder if going form a Red to a Blue spring would slow the rise of the dashpot and richen the hard acceleration A/F ratio.

Really wanted an analog A/F ratio gauge but couldn't find one with decent reviews. The never ending rapid changing of the display is almost dizzying, and you just have to 'interpret' what the overall/average reading is. Other than that the install went well & fired right up.
Getting a 12 degree off horizontal install of the O2 sensor in the Mini was tight, like everything else about the Mini!


Thanks for the vac data. I have been running a vac Guage on my ported just to see what it does. Gonna switch it to manifold, again just out of interest.

When (if) I chuck an AFR in mine I plan to use the data out to collect and log the data using an Arduino so I don’t have to worry about looking at the guage constantly. I currently have it recording my GPS speed onto an SD card and I’m working on RPM. In its current state it can also calculate BHP.

#8 gdcarpenter

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Posted 19 May 2020 - 10:27 PM

Thanks to all for feedback.

My Mini has twin SU's, 1 1/4". The O2 sensor is in the intermediate pipe with a flex joint between the end of the LCB and the start of the CAT that has been gutted by a previous owner. Currently running into existing single box RC40 at th back while I Wait for delivery of a Maniflow CAT BACK twin box system.

I 'leak tested' all around the upper end of the engine with Propane gas and found no indications of unwanted air entry, plus all othenrA/F ratios seem to be spot on except for heavy acceleration/load, and I can idle her down to 800 RPM smoothly.

I used the two large ported vacuum connections on the twin carbs and routed the one tapped cover breather through an air tight Oil Catch Can into the carb 'CCV' ports.

As to lean mixture burning slower and needing less advance and rich mixtures burning faster and needing less advance I totally agree. The caveat is that that does not factor in RPM. The higher the RPM, the faster the piston sped, the earlier (BTDC) you need the spark to reach peak pressure at the appropriate time. This is why I feel the extra advance from using ported vacuum, as opposed to manifold vacuum, reduced my A/F ratio by more completely burning the mixture and,'dumping less unburnt fuel down the exhaust pipe.

I understand 30-34 max total timing is the most common agreement. Unfortunately I am not getting that and need to investigate options to improve that aspect.

#9 timmy850

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Posted 19 May 2020 - 10:47 PM

The AAP needle is very similar to the GY which was factory fitted to the 998 Cooper, although they used a blue spring. You’ve checked for air leaks around the carbs and intake manifold?

What’s stopping you from increasing the total timing?

#10 gdcarpenter

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Posted 20 May 2020 - 01:15 AM

Yep , checked for air leaks using a MAP/Propane torch, and one would figure idle would be irratic, fluctuating, or difficult to adjust with air leaks, and I have none of those problems.

As for total timing I had a separate thread on that. Lucas 65D with weight stamped '16' and reluctor arm moves through 16 degrees with distributor out and shaft rotated manually, My best guess is primary spring weak so advance is perhaps already half in by idle speed. Tried a new Lucas knock off described as identical to the one I had, and actually got it from your side of the pond. Still only getting around 20 degrees max advance. Now looking to find a stateside company that might be able to recurve my old genuine Lucas.

Will dio my first real road trip this coming weekend, 400 mile round trip, and report back. She's in amazing condition and cannot fault her overall performance, just want he to BA all she can be. My last Mini was in Bermuda over 30 years ago, good to have ne again.

#11 gdcarpenter

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Posted 20 May 2020 - 08:45 PM

Recently installed A/F ratio gauge and just did first highway run. To get reasonable steady cruising ratio (15-16) (60 MPH @3,800 RPM)I need to set idle ratio around 10! If idle is set to 13.5 of then cruising is pushing 17 or more at times.

In another way of explaining it, when idle ratio set at 10 if I rev up while parked @ 4,000-4,500 ratio is right

1990 Mini Mayfair 1000 with original John Cooper Garages Conversion, twin SU2's, Cooper head, LCB.
Current needles are AAP's. Perhaps they needle selection was overly optimistic? Perhaps AAA's?

#12 cal844

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Posted 20 May 2020 - 09:59 PM

So you need richer at the idle area but no change at cruise?

#13 gdcarpenter

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Posted 20 May 2020 - 10:54 PM

If I set idle at about 13.5 then at highway speed cruising I am dancing around 17 and sometimes over that, too lean!
If I set idle about 10 then at highway speed cruising I am around 15 +/- 1/2 or so.

#14 Moke Spider

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Posted 20 May 2020 - 10:56 PM

I've merged this with your previous topic as they are on the same subject



#15 Moke Spider

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Posted 20 May 2020 - 11:02 PM

Generally speaking changing needles is what's done to adjust the mixture and tune it to get it right across the board of load and revs.

 

The first thing that's generally done is to sort the spring in the dashpot. These are selected such that at peak air flow, the Dashpot Piston(s) will be fully raised. Then run through the process of sort needle profiles. Finding a car with similar Torque Curve to what you expect yours to have is a good starting point. Then go back and check the Dashpot Springs and back and forth a couple of times between these, though, usually, the springs can be picked first go.

 

There's a lot of factors here that can give the unexpected results you are seeing, not the least of which being an instrumentation error.

 

I'd suggest saving yourself from going around the bend and putting it on a rolling road and having it sorted. At the same time, check your gauge against theirs.






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