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Mikuni Tuning - The Wideband May Fool You.

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I've been doing some tuning of my Mikuni 44 set up as of late with the assistance of a wide band o2 sensor, and I have learned a few things that may be helpful to some others.  Generally, I recommend determining these in the order listed:

 

Pilot Screws:

One bit from Honsowetz in the How to Modify Your Nissan Datsun OHC Engine which is a very specific instruction: run your pilot screws 1.5 turns out.  I have mine set at 1.5 dead on, and the engine likes the 57.5 pilots at that setting.  AFRs are where they should be for idle and low speed operation.  The engine runs well in both of these conditions.

 

Mains (Fuel Jets) and Air Correction (Air Jets):

This bit came from TonyD... and that is stay out of the pump nozzles when trying to determine what the engine wants for main and air jets.  Accelerator pedal movement may engage the pump circuit and if it does, this will throw fuel into the mix from that circuit.  This will "mask" what is happening with the main circuit and you won't know what is what.  According to the Mikuni manual, accel pumps are in play from 0 to 30% of throttle.  So use inputs like 50% or 100% and ignore where the 0-30% is most likely occurring.  Since the pump circuit has a known volume... and the duration of the fuel "injection" from the nozzle is specific to each nozzle, you could time the length of the injection duration if you are so inclined.  Then you could ignore that time duration (from throttle open) in your Wideband AFR (Air Fuel Ratio) plots.

 

Pump Nozzles:

This one came from my engine builder.  Know that sometimes wideband o2 sensors/loggers may show you rich readings when actually the engine is lean.  This can occur when there is any engine misfire condition. The rich reading is resulting from a chain event of the flame going out too early, and the subsequent passing of remaining unburnt mixture through to the sensor.  When you open the throttle quickly, if the engine is sluggish, but doesn't misfire, and your wideband shows rich, you are rich.  However, if the engine misfires, and your wideband shows rich, you could be lean.   

 

Regarding this last one, I think that is what I am currently experiencing.  I will be testing that theory out soon.  

As I have changed the pump nozzles from 50 to 45 to 40, I am fairly certain that I have been experiencing a more prevalent misfire when applying large throttle inputs (I haven't been doing much full throttle yet and there has been a fair amount of time involved from when I swapped nozzles one to the next to the last).  I was going smaller and smaller because of the rich readings I was getting on the wideband when "throwing large throttle inputs" at the engine.  Last tuning session, I was getting a large amount of misfire for the first couple of seconds of full throttle... even when in first gear.  I am going to switch back to the 50's next and see if that situation improves.

Edited by inline6

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Whenever you have a misfire the air/fuel ratios go rich on all measuring devices (Lambda sensor, AFM sniffer, etc.) The fuel in the misfiring cylinder is not burned. You have to cure any misfire before tuning via any air/fuel mixture measuring tool.

 

One tip, if you're data logging you can look at AF just before the misfire occurs to help diagnose the problem.

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Great info, thanks for taking the time to share it.  

I seemed to be always be running rich when on the throttle hard with my Mikunis last autumn, and misfire might explain some of it.  I'd go from mid 13's straight down to mid 10's.  Running the jetting I am, it's a curious thing - 57.5 pilots, 150 fuels, 200 airs, 40 pump nozzles, and 34mm venturis.  

I think I am going to do some plug chops when I get the car out this spring, and it might help give me some answers.  The wideband seems like a great tool, but on it's own it can indeed be misleading.  

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As another thought, I recall reading something about measuring the temperature of each header pipe with an infrared temperature gun.  Basically, measure each header pipe, and when things are ideal, they should all be very close to the same, but if one is lean or rich, it will show a difference.  Anyone have any experience with this?  

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Within 25F ideally.

You can buy engine Analyzers from Aircraft Spruce that operate on 13.8v that have six EGT's, TIT, TOT and adjustable differential alarms. As JeffP said "that would be a bitchen addition in the center console!"

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Not sure how to stay out of the accelerator pumps when accelerating unless you disconnect them. Having the carbs off and watching the nozzles you will see that ANY movement of the throttle causes squirting fuel from the nozzles.

Edited by madkaw

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On a Mikuni, pulling the spring, washer, and cotter key allows the Accel pump rod to just slide without moving the pump mechanism.

 

Really you get a long incline or straight level stretch of road and get the car into fourth or fifth at 1000rpms. Set your throttle position and leave it there, Even if the pump squirts some, that gas is out quickly and you are in the idle jet, sloooooowly accelerating through transitions.

 

With a GTECH-PASS and WBO2, you got VERY powerful tuning tools.

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Within 25F ideally.

You can buy engine Analyzers from Aircraft Spruce that operate on 13.8v that have six EGT's, TIT, TOT and adjustable differential alarms. As JeffP said "that would be a bitchen addition in the center console!"

Well, thanks to this post of yours, I just went and spent $225 on 6 probes, a 6-way selector, and a Westach EGT gauge.  :P

 

My wideband always left me scratching my head a little, since it told me the whole picture, but none of the individual parts.  This new toy will remove a lot of trial and error guesswork hopefully.  If one cylinder is running rich, I'll see a lower temperature on it, and if another is lean, a higher temperature.  Seems worth a go...  

 

Between playing with timing on the Electromotive HPV, changing jetting on the mikunis, and checking the wideband and EGT readings, it looks like I am going to be having a lot of fun this summer!  

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John,

Interesting article. Full disclosure, while I do have some direct experience and have observed lean readings with misfires, I'm mostly just repeating what I've read on the Internet. (Lean with a misfire seems to be the consensus on the megasquirt forums.) One thing to note is that the article is discussing narrow band sensors which reference the atmosphere. WB sensors reference a electronically controlled ion pump.

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I am working on correcting a periodic ignition miss on my L28ET with MS3.  It's a noise issue, and happens every 2 seconds or so, but every time it misfires my wideband goes full lean.  My understanding is the O2 sensor is directly responding to the oxygen in the exhaust, not the fuel.

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Well, maybe I'm not standing corrected.  This article from Motor helps explain what our LM1 says when we see a misfire.  Next time we have an engine on the stand I'll run a test.

 

http://www.motor.com/magazine/pdfs/102000_04.pdf

 

So, quoting from the article, this seems to be the reasoning for why during the intentional ignition misfire, the pre-cat O2 is showing rich spikes: 

"The O2 sensor is made up of two platinum thimbles (one exposed to the atmosphere, the other to the exhaust) separated by a solid zirconium dioxide electrolyte. The exhaust thimble, being made of platinum, is a catalyst much like a catalytic converter. If excessive CO or HC is present in the exhaust, it can be catalyzed on the thimble, using up the oxygen next to the thimble and causing a low-oxygen (rich) indication despite the fact that a high-oxygen content exists in the exhaust. This explains the mysterious rich O2 indication in my April 2000 column on using pre- and postcat O2 sensor readings to predict ignition vs. fuel sensor failures."

 

Interesting.  

 

This is followed a bit later by "In its normal environment, the O2 sensor is very accurate at measuring exhaust oxygen levels, thereby making an accurate assumption of fuel mixture. Outside of the normal range, however, the O2 sensor’s response can be somewhat unpredictable, rendering its assumption of oxygen content and fuel mixture unreliable."

 

I am thinking engine misfire from a lean mixture is an event similar enough to the above intentional misfire test... 

 

I am going to spend 2 hours on the chassis dyno this Saturday!  Should be quite informative.

Edited by inline6

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Hoping to do the same thing this summer.  Would be very interested in reading how it goes for you!

I'll be sure to post.  

 

By the way, my car is HLS30 - 16511.  Yours and mine were made pretty close together in the same factory about... 45 years ago.  

Edited by inline6

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