How much does 1 jet size affect A/F?

-
67cuda360

67cuda360

Well-Known Member
Joined
Nov 11, 2009
Messages
462
Reaction score
23
Location
St.John's NL, Canada
I'm fine tuning my Edelbrock AVS 650 Thunder series carb and I'm showing 10.5 A/F at WOT on my wideband and I was wondering how much a 1 step leaner jet for the secondaries would affect the A/F. I read somewhere that 1 jet size is 0.5 A/F point but I'm not sure if that is correct for this carb.
 
Boy, hasn't technology changed. Great tuning tool for both power and efficiency. Subscribed! tmm
 
well... there are only about 100 factors that would change that "number". that said if your at 10.5 i would pull three and do another run
 
Get the tuning manual for the carb from Edelbrock. The main jets and metering rods work in concert, so it isn't as simple as just changing the main jet. The manual will tell you what jet and metering rod to change.
 
Yes I have the tuning manual but when I switch out the jet I want to make sure I put in the best jet choice otherwise I'll have to take it apart again lol.

Here's how my carb is currently set-up:
Primaries:
- Purple step-up springs (7"):
- 2 stages lean on power section of the metering rod
- stock setting on cruise section of metering rod
I think the above metering rod is 68/52 with the stock jet .095)

Secondaries:
1 step lean (jet .095)

I'm reading around 13 A/F at cruise, 13.5 A/F at part Throttle and 10.5 A/F at WOT
so I'm thinking of going one step leaner on cruise and maybe 2 steps leaner on the secondaries.. or will 2 steps (.089 jet) be too lean for the secondaries?
 
67cuda360 said:
I'm fine tuning my Edelbrock AVS 650 Thunder series carb and I'm showing 10.5 A/F at WOT on my wideband and I was wondering how much a 1 step leaner jet for the secondaries would affect the A/F. I read somewhere that 1 jet size is 0.5 A/F point but I'm not sure if that is correct for this carb.
Click to expand...

I have that same carb sitting in a box and I'm planning on installing it soon. What A/F meter are you using? Also, what are the specs on your engine? I'm subscribing as well because I'd like to see where you end up with as far as jets and metering rods go.

I don't know if you bought the tuning kit, but it does seem like a trial and error thing. I don't have an A/F meter yet, but I plan on getting one soon once I get some other work done on my engine.
 
Yes I'd like to end up at around 14-14.5 at cruise, 13.5 at part throttle and 12.5 at WOT, or as close as I can get LOL. So to go from my current 10.5 to a 12 or 12.5 I could at least go an extra 2 steps (8%) leaner correct?

My current jet and rod combo is listed above. (post #5)

engine specs:
360 LA engine, not sure what cam (told it was RV cam) with solid lifters, RPM airgap intake, stock manifolds with 2.25 inch dual exhaust with H pipe. Cleaned up J heads (slight port and polish), flat top pistons and a 650 AVS thunderseries carb backed by a 727 transmission with stage 2 transgo shift kit and 8.25 rear with 3.23 suregrip.
 
Just don't let the A/F ratio meter do all your thinking for you. Once you get it good at this time of year, wait 'till that first cool, crisp dry fall day and get the engine warmed up and do a WOT throttlle run or 2 and listen for pinging. Dry, cool air acts like a loss of about 3 to 4 octane points vs warm, humid summer air (more oxygen content, and less water to moderate the combustion temps), and the internal air flow and mixing can change a bit too with air temps.
 
Sorry, but you are not likely to get it right the first time. I have a Holley and have had several Carter/Eddy carbs and there is no one formula for jetting. Take a big swipe at jetting and then come back. If you only do it 2 times you are really lucky!!!
 
You'll never get it right because weather and temps have a lot more effect on max. power.
You need EFI for this.

Besides, the time it took you to write your posts in this topic could have been spend on actually working on the carb and get first hand experience.
 
well...10% of 10 is about 1.0.
I use a 650 Thunder on my warm 340 and WOT is 12.5 right on.You are way too rich.
..there is something else going on here.You may need a hotter plug,more timing and a sharper tune.
 
thanks for all the feedback, I have already tackled the timing, it's dead on and double checked everything including making sure the TDC mark was correct on the dampner, weather wise it is currently a good time to do it. Early spring here (still feels more like winter) and the temp is still below the 10 degrees Celsius, so If I set it up correctly now, it will only get richer the warmer it gets. I'll go with what I suggested doing in the previous post, and I'll post how it goes :)

en voor m'n nederlandse broeder, dank je voor je suggestie altijd hulpzaam.
(the above was for bigblockmopar, a fellow dutch member)
 
nm9stheham said:
Just don't let the A/F ratio meter do all your thinking for you. Once you get it good at this time of year, wait 'till that first cool, crisp dry fall day and get the engine warmed up and do a WOT throttlle run or 2 and listen for pinging. Dry, cool air acts like a loss of about 3 to 4 octane points vs warm, humid summer air (more oxygen content, and less water to moderate the combustion temps), and the internal air flow and mixing can change a bit too with air temps.
Click to expand...

UHH WHAT? winter air depending on the location can be dry, and if the temp isn't to low its good dense air. In the summer the air is thin, less dense, with humidity making it worse due to water content.
 
rklein383 said:
Get the tuning manual for the carb from Edelbrock. The main jets and metering rods work in concert, so it isn't as simple as just changing the main jet. The manual will tell you what jet and metering rod to change.
Click to expand...

This is great advice.

Having to pull that much jet out of an ede carb is not a normal situation. One thing that I like to do, especially on street driven cars, is disconnect the secondaries and tune only the primaries. Once the primary side is right, reattach secondaries and tune that side.

Something is causing the rich condition, high fuel pressure or something. Is there any fuel dripping into the carb bores?
 
^ To add to this further here's a copy of what I posted on a similar thread awhile back:

You also might want to try this trick out of the q-jet tuning manual By Lars Grimsrud:

When set up with the correct metering rod for cruise & idle, will produce a slight hesitation upon acceleration if the
accelerator pump is disconnected. I do this with the engine running so I don’t have any trouble starting the engine without the
accelerator pump. With the pump disconnected and with the engine running in neutral, “flick” the throttle just a little. If the engine
actually feels more responsive with the pump disconnected, your cruise metering area is too rich, and you need to install a fatter set of
rods. If you get a severe stumble, or if the engine dies, you’re on the lean side and need smaller rods. When the rods are correct for
the jets in use, you will get a slight hesitation when the pump is disconnected.


After your done verifying the jets and rods are good, watch this:

[ame="https://www.youtube.com/watch?v=Kw-8lv4QIfQ"]Edelbrock Carburetors - Troubleshooting - Hesitation and Stumbling - YouTube[/ame]


Once you have set the rod size up like this, using the disconnected accelerator pump, a road test is in order. If the car is a little “flat” on light acceleration, or if it has a slight “surge” at steady cruise, you need to richen up the metering area slightly. If it is smooth and responsive on light acceleration, and feels smooth at cruise, you have the rod size nailed down.

Again, use the charts to keep all changes limited to 10% at a time. This will prevent you from “over-shooting.” Remember, with the
main jet size determined, your rod sizing is affecting idle, off-idle, light acceleration, and cruise. In most cases, when there are
problems with stumbles, poor idle, idle speed that starts out high and then degrades, and surging at cruise, the rods are too big and are
causing a lean condition. On the other hand, if the rods are too small, causing a rich condition, the throttle will feel “lazy” or “slow”
when you rev the engine, and you may get a puff of black smoke with a hesitation when you “flick” the throttle. Correct rods will
produce crisp, clean and instant throttle response.
 
The manufacturing tolerances from one jet size to another can vary enough to make enough of a difference between any two given adjacent jet sizes to negate any real effect. In other words, if you tried a dozen of the same sized jets an measured each ones effect there would be a significant range of A/F ratios for the same sized jets.

Some companies offer blueprinted jets that have a lot closer tolerances, but I believe the are for the Holley type carbs.
 
805moparkid said:
UHH WHAT? winter air depending on the location can be dry, and if the temp isn't to low its good dense air. In the summer the air is thin, less dense, with humidity making it worse due to water content.
Click to expand...
That is what I am saying, maybe just not saying it in the right way. We are talking about the equivalence of octane, or maybe I should say, the octane that is needed. Humidity displaces the the other air molecules and thus lowers the oxygen content, and heat makes it less dense. The two factors allow you to get by with lower octane in warm humid weather, and get away with it; with less oxygen, the mixture is 'less lean', hence lower octane is needed, and the water moderates the combustion temps, which again lowers the octane needed. Go to cool and dry, and the octane needs to go up; research in the 80's found this change to be up to 3-4 points (it's engine dependent too). I know of one turbo tuner who didn't leave a margin in for changes like this; it ran great in summer and then kablooie in the fall. I don't know of any EFI system that has a humidity sensor.....???

So if you tune A/F to the high edge in summer, you may not have any knock margin for dry, cool conditions. That is why (IMO) production cars tend to run rich for power....it takes care of all the variables inside and outside of the engine.
 
Just as a side note, I'm at sea level here and the edelbrock carb in stock condition are always on the rich side so I'm not surprised if I'll have to tune it down a bit. Also my engine runs a pretty strong vacuum, 20-21" of vacuum at idle and around 19" at cruise. I really appreciate all the feedback, I'll keep you guys posted :)

Oh and no, no fuel dripping in the bores. A/F ration at idle is around 13.5 and fuel pressure is around 4.5 - 5 psi.
 
And keep in mind that the o2 sensor in the exhaust or in each exhaust is reading the overall composite of all the cyclinders fed to that exhaust. If an individual cylinder is a bit leaner or richer, you won't see it from a common o2 sensor. So, that is aonther thing to keep in mind when tuning with an o2 sensor; you can think you are doing great but in reality, 1 or 2 cylinders are getting a bit too lean, with no margin. (And I bet SOMEONE has put o2 senors in each exhaust tube!)

As said, don't let the AF meter do all your thinking......
 
nm9stheham said:
And keep in mind that the o2 sensor in the exhaust or in each exhaust is reading the overall composite of all the cyclinders fed to that exhaust. If an individual cylinder is a bit leaner or richer, you won't see it from a common o2 sensor. So, that is aonther thing to keep in mind when tuning with an o2 sensor; you can think you are doing great but in reality, 1 or 2 cylinders are getting a bit too lean, with no margin. (And I bet SOMEONE has put o2 senors in each exhaust tube!)

As said, don't let the AF meter do all your thinking......
Click to expand...

Good point. I want to stay on the rich side but not as rich as I'm currently running lol.
 
I'm a little late, but hopefully this will help someone too.

Let's say a jet increment is, for example, 0.0005".

If you have a jet of 100 and it has, again for example, a hole diameter of 0.0500", then the 101 is 0.0505", a 1% increase.

If you have a jet of 200 and a hole diameter of 0.1000", then the 201 hole diameter is 0.1005", a 0.5% increase.
So the larger the jet, the less an effect the size increase will have. (FWIW, the 100-101 area change is 2% change in area, the 200-201 is 1% change in area)

It also depends on who makes the jet and how they're rated (flow vs. hole size) and if they've been reamed or not, but that's another discussion.
 
You must log in or register to reply here.
-

Back
Top