Air/fuel gage numbers

[Just Joel]

Last night, I finished installing my new innovate air/fuel gage and I need help interpreting the data that it gives me. I understand the basics of AFR values for a gasoline engine but how rich is to rich and how lean is to lean? At idle I'm hovering around 10.5 cruising I'm about 15.5 slight load its up around 16.5 (and never above 18) WOT it drops to 12.5 how am I doing? By the way this is an l20b with 38mm su's with m66 needles...

 

Thanks

Joel

[datsundash]

Seems a little lean cruising and alot rich at idle. Im assuming you have the innovate mtx-L? Did you properly calibrate the sensor? Wot throttle is normal 12.5 is good but maybe a tad rich for a non turbocharged engine. Sounds like you have some small tuning to do. Can you run one size smaller main fuel jets and about 2 sizes smaller idle jets or possibly adjust idle mix screws. Youc an use the logging feature on a laptop if you get a usb to serial port adapter.

[Sealik]

Assuming you have the SUs tuned....you can raise the seat to lean the idle out a bit....but that just changes another stations.

My numbers are about the same....LZ23 with 46s.

Rich at idle....WOT is good.....getting a little lean at slight load/cruising.

I believe up to 17 is okay

I try and stay away from the low and high numbers.... :rolleyes:

 

No idling or cruising...just WOT... :w00t:

[Sealik]

Seems a little lean cruising and alot rich at idle. Im assuming you have the innovate mtx-L? Did you properly calibrate the sensor? Wot throttle is normal 12.5 is good but maybe a tad rich for a non turbocharged engine. Sounds like you have some small tuning to do. Can you run one size smaller main fuel jets and about 2 sizes smaller idle jets or possibly adjust idle mix screws. Youc an use the logging feature on a laptop if you get a usb to serial port adapter.

 

38mm su's

[ggzilla]

Great! I've always wanted to get one of those.

 

10.5 is WAAY too rich. Adjust the idle mixture to 14.

 

12 is max power rich, OK for full throttle

14.7 is least emissions and will still run excellent. Acceleration should be somewhat richer

15.5 cruising is lean, bad for emissions but should give good fuel economy

16.5 is too lean for part throttle acceleration. Slight risk of burning a piston.

 

I rode in a street-legal supercharged race Datsun on Monday. It was running 10-11 during most all conditions.

[Just Joel]

Gotcha, the only internal adjustments for su's are the needles and the m66's are the sss l16 and the largest needle released for the 38mm (or I'm told) I've heard that su's tend to run rich at idle but as you pointed out it does seem a little rich and I have no idea how to fix the cruising mix as there are no provisions for doing so other than oil weight and re shaping the needles. Am I in danger of hurting the motor with these numbers?

 

Thanks

Joel

[Sealik]

http://autospeed.com/cms/title_Tuning-AirFuel-Ratios/A_1595/article.html

[ggzilla]

You can adjust the idle mixture with the external screws. But first make sure the float levels are correct (use the clear tube method).

 

M66 is for a bone stock L16T engine, so perhaps a different needle is called for. The oil thickness affects transitions, but won't change cruising (steady state) operation. You can do tricks like raising/lowering the needle, but it is not the best way.

 

Gasoline engines will feel OK with a wide range of tuning (12 to 15), but that doesn't mean its tuned good. For example I see a lot of cars trailing black smoke yet the owners are proud of their mods because it makes good power. Others that smell really like gasoline at idle. You can have power and fuel economy both with careful tuning. It's not easy, although not too difficult, but it takes a lot of time.

[Just Joel]

but won't adjusting the idle mixture leaner make my cruise mixture worse (leaner) and I know all about the float level thing, went through that early in turning and they are now perfect and holding (confirmed by the clear tube trick) And according to z therapy, the m66 needles are the largest (smallest diameter needle by taper) available for the 38mm's

 

If I'm reading the posted article right, it appears 17's for light load driving is ok?

 

Thanks

Joel

[ggzilla]

won't adjusting the idle mixture leaner make my cruise mixture worse (leaner)

It shouldn't. I mostly familiar with the Hitachi flat-tops. They have improved idle circuit. Maybe for round-tops it might be as you say.

 

There are dozens of needles available, as you can use SU needles in the Datsun Hitachi sidedrafts.

[Sealik]

17 is getting up there....not really conducive for sustained RPM.

Until I find different needles......Idling for me at 10.8 is insignificant. I'm not sitting in traffic or at stoplights for more than a couple of seconds.

On the highway I have had readings of up to 17....but...then I'll shift and or change throttle postion/RPM...thus dropping the A/F readings.

 

Mark/take note of the position of the jet adjusting nuts....then turn them both 1/6 of a turn (one flat) CW and note the A/F readings during a run.

Try it again going CCW ...check the numbers.

Who knows....You might be able to richen the light load numbers up a bit

[Just Joel]

Yeah, so I adjusted the idle mix to about 12 and she idles MUCH smoother and cruise and slight acceleration are still between 15.5 and 17 she drives nice and smooth... No lurching our hesitation but I'm still a little concerned with how lean she gets. So, 17 is to lean huh? I need to figure out how to fix that (eventually when I have more time I will consider messing with the needles but for now I just need to get to canby this weekend without melting a piston) I have read you can mess with float levels to help lean cruise issues but I don't see how that could work, you just end up raising the nozzle after to compensate for the extra fuel. Thoughts? Can she drive to canby (about for hours and a mountain pass) like this our do I need top fix it meow?

 

Thanks

Joel

[Just Joel]

The only time I hit 17 (and its momentary) its during light rpm change... The majority of cruising is 16-16.5

 

Thanks

Joel

[ggzilla]

See how it idles better at the a normal gasoline A/F ratio? 12:1 is the richest, any more than that and power does not increase but fuel usage does. For idle you can go down to about 14 and still have it run just as smooth as at 12. 14 is still lean. 14.7 is the nuetral point (neither rich nor lean).

 

It might be OK at 16.5. But even a stocker doesn't run that lean. But probably a lot of carb swaps are way off, as very few check their AF ratios or even read their spark plugs. If it doesn't ping and the engine stays cool ...

 

Raising the float level will richen it across the board. You don't have to raise or lower the needles after changing the float level. If you do it'll probably negate the float-level richening. It's not the corect way to tune it, but should work in a pinch.

[Just Joel]

Crap, double post...

[Just Joel]

I'm only a turn and a half out right now (according to the fsm for su's I should be 2.5 turns out for my elevation)... I think the nozzle adjuster will be tight before I get to 14 at idle... On the plus side, if I get in a pinch where I'm running to lean while cruising over the hill this weekend I can always set the choke to compensate :P

 

Thanks

Joel

[Kisor]

16-17 is like taking a blowtorch to your pistons and exh valves. Cruise you want no more than 15.5, WOT on a mild NA motor should be 13.5'ish. I do a lot of fuel injection tuning and carbs are much easier by far... lol

 

I'm not familiar with SU's and what all they have as far as adjustments but keep working at it. You want idle to be as close to 14.7 as possible. Tip in should see a drop to 12's then level back to 14's at part throttle cruise, WOT should be again in the mid 13's depending on compression and cam size/ignition timing.

 

That lean on cruise will most likely hurt something. Ignition timing also can affect AFR's. Before changing a bunch of stuff check timing and maybe advance by 5* and see how it drops. Late timing can cause a "lean" reading on a wideband. It's not recieving to little fuel, it's the fact that retarded timing causes an incomplete burn and leaves a larger amount of oxygen in the exhaust. That's what pushes the reading high on the gauge.

 

The sensor only tells you how much oxygen is in the exhaust, it doesn't know what happened in the cylinder to cause it.

 

Too little fuel and all of it gets used and that leaves more O2. = 15-18's

Too much fuel and more O2 gets burned. = 10-13's

 

Hope that confuses you more... Lol

[Just Joel]

Timing is dead on spec 12* btdc. Last night I attempted to take her for a drive with my new leaner idle set of about 12.5 and right of the bat, light acceleration went through the roof (18!) So I shut her down for more work today... The only thing I can think out doing (aside modding needles) is to go thicker on the dampening oil and see what that does for me. Any other ideas?

 

Thanks

Joel

[bonvo]

richen them things bro im not sure if you have noticed but i run pig rich at idle but under driving conditions its almost perfect the point is that you want that a/f ratio as close to 14.7:1 as you can get it while you have your foot to the floor hauling ass. until you can play with needles and such its better to be rich at idle then lean under load

[Just Joel]

That's the plan for the weekend if I can't figure it our but even rich as shit at idle I'm still seeing 16's while cruising...

 

Later

Joel

[ggzilla]

The oil won't richen cruising. It only changes transitions, temporary changes like accelerating.

 

As a short-term hack, you can raise or lower the needle with the set screw. The choke lever does this too (raises and lowers the needle).

[ggzilla]

you want that a/f ratio as close to 14.7:1 as you can get it while you have your foot to the floor hauling

Actually emission controls want it close to 14.7:1. You won't make maximum power at 14.7:1. Not even maximum fuel economy.

 

For max power you want to 12-13. Even an emission-controlled car will do this because of the power valve. Foot to the floor bypasses emission controls even on a a totally stock Datsun!

 

This is all typical carburetor tuning. Even a Weber needs this kind of tuning.

[Kisor]

I'll probably get reamed by everyone but seriously, bump the timing up. Spec is what the factory had to compromise at to reduce emissions. Try 15* and see if it pulls the AFR's down some.

 

It's not going to be a fix all but I bet it helps, plus probably make noticeably more power. I run 18* initial on my A14 and its base is 10*. I have absolutely no starting issues and only slight ping on a 95* day under moderate load.

[ggzilla]

18 initial on A14 and centrifugal advance of 20 to 28 = how much total?

That should be marginally OK if you have one of the 20-degree A14 distributors. But 27 or 28 degree advance is more common with A14.

 

So it depends on which distributor you have.

[Just Joel]

Alright, changed the oil to 20 weight and like said, it doesn't significantly help the high end light load but I did see the numbers drop a little. Then I advanced the timing to fifteen and that seemed to help a little as well. Now when cruising, I see mostly fifteens and occasionally sixteen but at this point I'm happy with those numbers enough to take the trip to canby this weekend. Acceleration is perfect and I don't have any serging issues while cruising. That and with the 12.5 idle, the car is much smoother and overall happier.

 

Thanks

Joel