djessence

First off let me say I am completely disappointed in my ability to use the search function and actually find the information I am looking for. I didn't used to have such trouble! Part of this is why my thread title is so explicit.

I am looking for information on how to go about tuning the car/carburetor using a wideband AFR gauge. We have recently done some modifications to the carb (I don't want this to be a mod thread) and car and cant really find any tuning philosophy. Can someone point me in the right direction or towards some publications/manuals or threads? Unfortunately Sterling's website is gone. I did find many threads that dealt with tuning to feel (pull, smoothness, drivability etc) and many saying get an AFR but not many stating how to get the correct data.

The questions and information I have so far:

1. The target AFR is 13.4 (ish)?When performing the tuning, is one doing a long full throttle pull? Several short and different sized pulls?

I understand that the actual "tuning" aspect involves potentially changing mixture, hets, air bleeds but what I am fairly uncertain of is actually obtaining the data I need to make the right decisions on what to change.

Hopefully this makes sense and someone can point me to the right documentation.

Not that it should matter but the car is:
Aggressive street port, racing beat exhaust/dual muffler setup on an endurance racer FB.
Modified Nikki.

Thanks!

Jeff20B

iTrader: (13)

You left out the most important part as far as I am concerned. Where you say modified Nikki, what are you talking about? How modified? Stock venturis? What jets and air bleeds does it currently have installed? This is relevant info and should be posted in this thread.

As for your questions about target AFR, I think only wankel=awesome can help with that. He has more NA tuning experience than I do. But I'll take a crack at it.

Idle - give it what it wants. If it idles best at 12.2, there you go.

part throttle cruise - 13 seems to work.

full throttle - 11 or maybe 12

That is all I can help you with. I'm coming from a turbo tuning perspective. I need to keep my engine alive in boost so I've been told to aim for 11.something at WOT. If I see 12, I back off. If I see 10, I know it's safe but I'm not making as much power as I could at 11.

Was that helpful? Probably not. Good luck!

t_g_farrell

iTrader: (1)

So in general rotaries don't do well at ideal mixtures, they need to be a bit richer. So 13 and
above is too lean, this is my understanding. I've also seen that somewhere around 12 is usually
ideal for power, as Jeff mentions, for boosted applications. But for boost you add in a bit more
fuel to make sure detonation doesn't occur. For a NA, non boosted application it will not hurt
the engine but of course you want to avoid it for performance reasons.

I haven't gotten an AFR gauge yet but this subject interest me all the same.

j9fd3s

iTrader: (3)

the AFR you end up with depends on a few things, like the port, engine health, ignition system, etc etc, so we're generalizing a little. with a carb, the AFR gauge actually complicates things, as it allows for "accuracy" to 0.01 but generally an engine isn't that picky, anyways.

idle, set it where it idles best, without an air pump, this will be anywhere from 11.5-13.5. an air pump, going to port air, like stock will read 2-3 points leaner. so its 12.5-16.5.

part throttle cruise: you want to be as lean as you can be without misfires here, an EFI car will be 14.7:1, generally a carbed car will be a little richer, if its richer than about 13.5 you are going to be too rich and mileage will go down the tubes.

WOT: start rich, and lean it out until its as fast as it is going to get and then go a little richer. this will probably happen around 12.5-12.8 on the street. the way it works is that the power to AFR chart is a bell curve, and so there is an AFR where it makes peak power and then +/- maybe 1.0 power isn't going to be much different, but going further richer or leaner power will take a big nose dive. so what you want is to find the optimum, and then be on the rich side of that.

picture is a power vs mixture, and its sideways from how we would expect to see it, but we want to be on the rich side of max power. basically we want a little margin for if the engine is really hot, and goes lean for whatever reasons, detonation isn't a worry, but it is possible, so better safe than sorry.

DreamInRotary

iTrader: (49)

That is an awesome pictorial explanation - thanks for sharing that!

j9fd3s

iTrader: (3)

its from Mazda, we've had it for a while, but its sideways! so i didn't figure out what it was until a couple weeks ago

Jeff20B

iTrader: (13)

j9fd3s keeps harping on how once you find the ideal tune, to then go a little richer for safety. I need to do that. I've just found the ideal tune but I haven't gone a little richer for safety yet.

I think my secondaries are good where they are, but the primaries could go a little richer. They're fine at the current ambient temps outside but when the weather cools down it will need more fuel as air density (oxygen content) and power goes up.

To put it another way, the current 120 jets are perfect for right now on a warm carb at 13.something at part throttle cruise, but the carb runs worse when it's cold making it less fun to daily drive.

It would be better to go to the next size up which is 124. However its warm part throttle cruise goes richer at 11 or 12.something instead of the 13.something it's at now with the 120s. But it does run better over a broader range of conditions.

I think I might go for it because it should also help to reduce the tiny secondary delay this carb has had ever since swapping to the 120s. And like I said the secondary jets seem ok when all four bbls open, produces 11.something in boost, but a similar setup on a different carb produced 10.something with only a different primary jetting (slightly richer) at 127. Both carb's secondaries were exactly the same.

Interesting how 7 numbers richer primaries but the exact same secondaries makes such a difference when the secondaries open.

j9fd3s

iTrader: (3)

for a turbo car, you want to go so rich it stops running, and then lean it out until it runs again, on an AFR gauge that should be about 11:1 give or take about half a point. for a street car.

Interesting how 7 numbers richer primaries but the exact same secondaries makes such a difference when the secondaries open.[/QUOTE]

for the most part a carb is simple, bigger hole in the fuel jet = more fuel = richer mixture. the only complex part is the emulsion tube, as it mixes some air in with the fuel, and when you mix a liquid with a gas, you get something called "two phase flow", which can flow like a liquid OR a gas, or some semi random combination. this can create some weird anomalies where jetting 7 numbers makes a bigger difference than you'd think, or mine where going from a 180 fuel to a 175 fuel has the AFR going from the 11's to the high 12's.

wankel=awesome

For the modded nikki, I like a 13:1 (ish) AFR for cruising NA.

Its a good mixture between power/economy and it helps the transition to the secondaries if they are mechanical.

You can squeak out serious MPG's by going lean, but its just not as smooth as the 13:1

For full power, I go rich until its not pulling any harder, then go back depending on the blackness of the sparkplugs and burnoff.

There shouldnt be excessive soot or wetness after an 80-100 MPH pull. (4th gear, also be careful.)

My engines porting really likes the SA intake manifold with a '74 carb. I ported my intake a bit too, but its not a huge deal I dont think.

My engine also smokes a bit on decel jetted this way, but it might be that the engine is finally tired after 10 long years of daily abuse.

Still runs great though, long live the 12A!

I post like this so it's easy to read. :P

Jeff20B

iTrader: (13)

It is easier to read. Especially since it looks like the forum cuts off the far right side of text on my browser now. I just noticed this earlier today. Weird.

I just completed some updates to a Nikki I prepped last year:

I solder filled a 40 air bleed for the air horn.

Three of four boosters were loose so I pulled all four to widen them at the bottom so they'll go in tighter and decided to clean up the airfoil mod a little while they were out.

It had my rare set of 80 primary air bleeds so I pulled those to swap in the more common and assumed to be better working and/or more appropriate 70 bleeds.

I left the current 118 drilled primary fuel jets alone. If they need to be bigger or smaller with the 70 air bleeds, I'll know right away.

The secondary air bleeds were updated to a set of 92 jets threaded in.

The secondary fuel jets were updated to a set of freshly drilled 137s.

The accel pump extension needed to be just a couple mm longer due to it running out of travel before the secondaries were fully open (must have been one of my earlier ones drilled 1 1/8" apart) so I filed out the upper round hole into an oval so I could move the contraption down 1/8" to get to the Sterling recommended 1 1/4" hole spacing. Tested good on the bench.

The OEM seats still had the original screens but they had fallen off and were bouncing around up in there so I had to pull the seats using the vice grip & hammer smack method. Works every time. Cleaned a little corrosion and swapped in new crush washers and reinstalled the OEM seats. If anyone reads this post, they will notice the bolded text first. Always reinstall OEM seats. NEVER install aftermarket seats. Throw them in the trash instead.

What else did I do?

The OMP nipples were loose fitting so I swapped in my last set. They went in with some difficulty so it seems this carb's OEM ones were machined a little small.

I think that's all the mods I did.

Jeff20B

iTrader: (13)

I tested the carb today and it was running at around 13.0 on the wideband at part throttle. wankel=awesome would say that is perfect but I wanted slightly richer for safer boosting so I swapped in a set of 120s. This brought it down to about 12.5.

It was the right thing to do because the carb has great transient response and tip-in. Its AFR in boost with foot planted to floor was 11.0 at about 75 degrees F. That's rich enough to cover when the sun goes down and temps cool as well. Even though it was a warm day, it still had enough to break them loose in 2nd gear up my test hill.

The carb didn't run any better with the 70 air bleeds so I figure on doing some more fine tuning once this carb goes on its intended engine.

Anything else about this carb?

Oh yeah, the OMP nipples worked perfectly! lol

Oh there is one more thing. The new needles in OEM seats still require babysitting to keep from flooding so I said screw it and swapped in a another air horn that had a known working old OEM needle and seat combo that allowed me to do some boost testing without fear of floodiness. It worked flawlessly. No more annoying floodiness to put up with!

So I'm updating my stance on needles and seats yet again. Leave your stock needles AND seats alone! If you must remove the seats to take out the strainer screens, reinstall the seats in their original positions with their original needles and do not adjust the floats. You'll save yourself a whole lot of headaches and get to start boosting far sooner than you thought you could.

lastphaseofthis

iTrader: (8)

man all these number seem like a hole point off from the efi setups, is this simply because of it being a carb? i mean for efi n/a i can idle at 14.2-14.5 good cruise is around 13.6, mean torque is around 13.2 no richer then 12.8 unless over 7k rpm.

t_g_farrell

iTrader: (1)

Yep carbs are not as exact as EFI. They don't atomize evenly, gas gets left on the walls or pools in
the intake. All this makes the numbers different for what works well.

wankel=awesome

Not even EFI runs at "stoich" for performance applications.

I have efi engines that dont even like to idle at 14.7:1 at idle happily.

The math will tell you that 14.7:1 is the "target" number, but that accounts nothing for variables in the actual fuel or timing.

Todays gas = trash. I dont have a 12A that runs nicely anywhere as lean as "stoich".

j9fd3s

iTrader: (3)

14.7 is only the target sometimes, and it is what the catalytic converter wants to see, so this is what most stock newer cars are targeted for.

if you tuned a car for economy, like the Civic VX, you'd be running much leaner, and if you tuned for power, you'd be running more richer

wankel=awesome

Even if you were tuning for economy with a carb'd 12a, you would have a crappy tune anywhere leaner than 14:1.

I tried it with various carbs, the engine surges and wont start cold, and runs too hot for the stock cooling system to handle without the fan being on 80% of the time.

With that, I got 28 mpg with a super lean tuned holley on the highway, but go so much more power and smoothness fattening it up. The mpg wasnt important to me after seeing how much nicer the engine ran.

Aaron Cake

iTrader: (1)

In EFI there is no "tuning a car for economy" and "tuning a car for performance". There is only "tuning a car".

Unlike those controlled fuel leaks into the manifold (carburetors ) an EFI system can select the AFR based on what it needs to be. Rich under boost, as lean as possible (accounting for cats, what the engine will tolerate, EGTs, etc.) under cruise, a good strong idle.

Carbs can only set a general fuel curve, with usually an idle and a secondary circuit to at least allow a somewhat selectable idle ratio and high load ratio.

LizardFC

iTrader: (1)

Quick note I'd like to add from my own experience with this:

If you live somewhere hot (like not Canada ), be careful tuning your cruise and idle AFR too lean. I noticed a significant difference in engine temps on the highway between low 14's and low 13's. It might be worth sacrificing some mpg for a cooler running engine.

Also note that when it gets cold again, your summer tune is going to be even leaner.

Aaron Cake

iTrader: (1)

It was 36C yesterday in my backyard according to the thermometer on the shop. I Canada we have to deal with a wide variety of temperature swings. Had a few -35C degree days last winter where I found out the 12V aux battery on my Insight was stone dead (Surprise! In a parking lot downtown Toronto at midnight after an auto show.).

Nice thing about those wacky EFI systems is that they can de-fuel or re-fuel the engine based on coolant temp. Or more effectively drop a bit of timing. You'd have to be quite lean to raise engine temps significantly though. At that point EGTs are through the roof.

j9fd3s

iTrader: (3)

i was taught by an old schooler to tune the carb on the coldest day of the year, and that way every other day its safer. now you may get the tune really close any other day you want, and we're in California and not Canada, so our low is only about 0c (although its foggy, turbo weather).

this was before widebands were really available too, so we were tuning for max power, and then a little richer. he had an Rx3 with a big street port 13B and a weber, with FC 4 piston brakes, and other stuff on it, in the early 90's.

djessence

I guess I will follow up on this thread. Unfortunately I didn't really learn how to properly "tune" a carberator as when we were doing the various pull conditions (WOT, cruise) in the various gears the AFR's were all bang on.

Car seemed to really perform for us: under WOT the AFR was right around 12.5 and on cruise we were right at 13.0-13.1

We also noticed there was basically no change in AFR's between 3000ft (where I live) and 300ft (where I was racing).

j9fd3s

iTrader: (3)

if you were on a dyno or something that could measure a small amount of HP, i would try a little leaner everywhere and see what it does. i wouldn't expect a lot, but if its a race car, its probably worth checking.

as far as the nut and bolty bit goes, with a carb you start at idle and work your way up, one circuit at a time. so play with the idle screw, and then the primary jets, and then the primary air bleeds, and then the secondary jets, and then the secondary air bleeds. you need to write it down and or keep a log, as mission #2 is to know what part of the carb does what to the AFR and when.

its a good idea to do things like disable the secondaries, and stuff, you need to know what they do