Well...it's technically a Summit 600cfm carb, but a Holley design. Anyway...

Tuning with AFR meter. Stock jetting P67/S73. Re-jetted to P67/S67. Idle is at 12.8. Off idle when I begin to accelerate slowly, it hits a lean 14.9 and higher, and then back down to around 12.7 - 13. Does that mean I need to go up or down with the accelerator pump size? It currently has a 65. Or is it something else? Jets seem good size.

Second question - dropped 6 jet sizes on the secondaries and it still runs the same rich mixture at around 11.2 to 11.5 - I can't get it to go up! What am I doing wrong? Please help.

 

You have it backwards. Below about 15:1 is rich. Above is lean,

 

Sorry...its been a long day. Edited.

 

Everything is fat. Try 72 primary, 76 secondary to start. Don't mess with pump nozzles till you get the mixture correct.

Read this, or get an updated copy:

http://www.nastyz28.com/~ericf/tech/htune.pdf

 

What AFR Meter and how is it connected ?

vacuum secondaries or mechanical ?

You need to get a base line for 4 different driving conditions.

#1 Slow acceleration from ~ 1500 RPM to ~ 1000 RPM below redline
#2 WOT from 1500 - 1000 RPM below redline
#3 steady state ~ 2500 RPM light engine load ( simulate highway driving )
#4 idle RPM

if test #1 is in the range ~ 12.5 - 13.5 AFR leave the primary jets as is
if test #2 is in the range ~ 12.5 - 13.5 AFR leave the secondary jets as is


if the range steady state is lower (rich) for #1 decrease the primary jets by 2 sizes.
if the range steady state is high ( lean ) for #1 increase the primary jets by 2 sizes.

dont increase or decrease from stock by more than 6 sizes.

once #1 & #2 are in range you will find #3 test should be in the range ~ 13 - 14.5

to adjust for test #4 adjust the idle mixture screws unit ~ 13.5 - 14.5 provided the engine can idle smoothly.
I usually adjust idle mixture to get MAX vacuum and just record what AFR that is ~

What you will notice if you graph the AFR on a carbed engine the AFR will first go lean (high AFR number ) then it will go rich ( low AFR number)
then will start a steady state through the RPM range. Dont worry about the initial changes. Look for the steady state averages.


Once you get these numbers then you can start testing throttle response.

Now using a AFR on a carb is just reading a average of the cylinders so it is better to error on the rich side vs lean.
A lean cylinder can lead to higher temps and burn pistons and valves.

Once this is done you should clean your plugs and take a short test drive using a varity of driving methods. Then check your plugs
they should be a nice tan color. anything all black would indicate overly rich still.


For throttle response you will need to adjust accerator pump linkage, discharge nozzels, and accerator pump cams.

This for the most part is trial and error.

If from a 1500RPM to instant WOT produces a bog / hesitation / back fire you usually will need to increase the discharge nozzle.

 

I just got that same summit carb! I'll be doing more to dial it in this weekend, I'm in the process of switching intake manifolds.

What AFR meter are you using?

 

in order to see what the primary jet is doing you will need to cruise at a steady 2600 rpm or in this range, anything below this you will still be on the idle circuit. You need to be above around 2500 rpm to get a good idea of what the primary jet is doing.

Don't adjust your secondaries until you get the primaries figured out or you will have to start over with the secondaries. At WOT all circuits are feeding fuel so work from the primaries to the secondaries.

Neal

 

The A/F gauge is helpful BUT if it drives correctly , no hesitation , idles no stall and cruses with no misfires then what are you correcting ?

So what driving issue are you having ?

 

I currently have 67/67 - wouldn't 72/76 make it even more richer? I've read that tuning paper, but it has some conflicting info regarding PV's and determining best size.

Wow - lot of info! Thanks. AFR meter is AEM Uego Wideband. Vacuum secondaries. I pretty much followed all those to get where I am at. Also using vacuum gauge for tuning. I have tuned idle for best vacuum which is around 17"/AFR 12.5 in park, 11" in gear after the car is warmed up. Problem is when I drive the car and then come back to the garage, the idle mixture seems to lean out and hits 13.5 or higher. If I shut the car off and then restart it, it runs even leaner at 14 or 15 and wants to die in gear unless I feather the throttle.

Right on - it seems like a great carb so far. Meter is AEM Uego Wideband.


Looks like I am gonna have to get a copilot and do some more runs today. I will report back these numbers.

Just trying to tune for the middle with fuel economy and power. Also trying to tune out a lean flat spot when accelerating and rich WOT.

 

Your correct. I will bow out now.

 

What about going from idle to part throttle and then reading goes from 12.8 to 14.9 and higher and then back down to 12.7 all within seconds. If I am cruising and lightly accelerate, the reading goes high again and then back down to 12.7 - is that an accelerator pump issue?

 

No this is normal operation using a carb. It goes lean due to throttle plate opening before the gas reaches the cylinders, then you get a flood of gas from the discharge nozzel quickly giving you a rich condition, then you get your steady state based on the jet sizes.

Remember this is a mechanical device delivering fuel and not a injector for each cylinder.

If you could log your readings it may look similar to this.




A good way to see if your sizing is correct place your car in 4th gear running at 1500 RPM, then go to WOT and watch your readings. Forget about what happens in the first second or so.

Do the same WOT starting in 3rd and 2nd in the same RPM Range.

The only time you worry about the first second is if you experience bog hesitation or a back fire.

Because you cannot log the readings the gauge is usually only useful to see steady state readings.

 

Thanks cagotzmann. Is that logging software you are using? I tried hooking up my meter to my laptop and using the logging software from AEM, but couldn't get it to read the meter. Mixture is good right after the initial lean out, so looks like the primaries are right on. I guess the secondaries will need to be dropped a little more and we will see what I get. I might change out my PV to 5.5 to match my vacuum signal better as well.

 

My Software doesn't connect to a meter but a controller. You connect the controller to a PC or a meter.
My unit also has other inputs so I can read more sensors as well.

http://www.zeitronix.com/

Using a PV 5.5 your vacuum at idle is only 11 ?

My ZZ383 is 17-18 at idle. I am using a 8.5 PV

Changing items like the discharge nozzels, vacuum secondary spring , accelerator pump cams are a trial and error to get best throttle response only.

The order I used was vacuum spring , discharge nozzel , accelerator pump cams.

 

Thanks for sharing that link!

Yes, the 5.5 would be at about 11" which is what my vacuum is in gear.
I will follow that order and see how it goes. Thanks again.