Jetting my carb??


Active Member
Apr 20, 2004
Salt Lake City, Utah
Hey guys I will be installing my exhaust soon.(Check my Sig) I know I'm gonna have to change the jetting on my carb for the increased airflow. What size jets should I run? I am in Utah at 4500 ft elevation. I don't know the current jetting on the carb. Its a 600 cfm Holley 4010 on a 351 Windsor Mach 1. What other things will have to be tuned for the car after I install the exhaust. Im new to this so bare with me. Thanks in advance.
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I can't give you the exact jet size for your Holley carb since I only ever tuned the Carter/Eldelbrock type carbs, but I don't think that you are going to have to make that large a change in jet size if any to see a gain. Get a strip kit with a good assortment of jets and try the next size see what that does and then try something else.

Basically I always tune my engine by making a change and then timing the engine's acceleration. I compare the times for each set of primary and secondary jets and then install the ones with the lowest times. Um the drawback to that is I was always tuning for max power so I was running the engine on the fat/rich side, and although I had my stang running great it is really time consuming and maybe a little inaccurate.

I've got an air fuel meter now that I'm going to install and try tuning my carb with. I should allow me tune my mustang in a lot less time.

Good Luck!
Well Ill do that but I still need to know what jetting to be around. There are supposed to be sites that show the jets to be used, but I can't find any. Plus nothing beats the opinions of fellow stangers. Keep in mind this is with an exhaust system.
I am running same dr. gas, 3" all the way, 347, victor jr. with victor jr. heads and a 4150 815 CFM carb at 3000 elevation and get best dyno power with 88 rear jets and 80 fronts. If i were to guess I would try mid 70s jets to start with your set up.
Okay that sounds cool. Killer setup btw. What kind of #'s you getting(off topic, but interesting)?? Ill try out the mid 70's. Im very new to tuning carbs myself and to tell you the truth they intimidate me. Last time I tried tuning the float level I cracked an o-ring so when i started gas shot out. Been scared eversince. So whatever tips you guys can give me please give. And if anyone has other suggestions and closer estimations feel free to chime in. I have a holley book and know how my carb works basically and I know where everything is, but Im no expert like u guys. So anyways chime on in guys.
First off, jetting does NOT increase airflow. Also, the higher the elevation, the smaller jet you need. Since the air is less dense with oxygen at elevation, less fuel is needed. Until you get your AFRs on there, I think mid-70s jets will be way too rich. Most Holleys in the 600cfm range usually have 66-68 jets from the factory, which is often a little rich for a stock motor.

Since you are not changing heads yet, I would guess that your stock jets may be just fine. You could always call the Holley tech line for advice.

Once you change heads and cam, you will be changing jets again anyway.

You may want to consider a 750 once your motor is done.
Leave the jetting where it is and drive the car. A header change should not affect jetting. Increased airlow into the motor will. As an example I recently installed an carb tray to increase flow into my set up. The AFR changed from 12.8/13.0 to 13.7/13.9 to 1. That required an increase in jet size. The headers will NOT get rid of the "bog" when you go WOT. That is most likely caused by a lack of fuel. It could be caused by a number of things.
Your bog is probably an accelerator pump problem, but could be many things. Throw a kit in your carb, and check the jet size when you are in there. Then run it for a while and read the plugs.
I would take a vacuum reading as the next step, and make sure that you have the right power valve, based on the vacuum. There may be a delay in opening the valve causing a "bog", or it could be dumping too much fuel.
I looked at what BG ships their 650 CFM with as far as jetting, and they suggest 71 & 79. So I might start with a 68 & 74. Always try to keep a six size spread between the P and S. Run the car and read a couple of plugs.

I just started having major problems with my 650V Speed Demon tonight, I was barely able to get the car home. About 4 miles from my house, I had to stop for a light to turn left, well the car started running really rough and almost stalled. I quickly put it in neutral and put my foot on the gas and I was able to keep it running.

At part throttle (or idle) it was running REALLY rough, like it was missing a cylinder or something so I thought to myself "Oh great, I have an ignition problem". At every stop light I had to either put it in neutral and stand on the gas, or two foot it to keep it running. A block away from my house it finally died, and would not start again so I pushed it home.

When I got home I checked the ignition, and it was getting spark. When I tried to start the motor again, I would get nothing unless I gave it full throttle, after which it would pop a couple of times and then die. I looked at the carb and noticed fuel dripping out near the throttle shaft. This would lead me to believe the floats somehow are too high, but I have my float levels set perfectly so I don't know what's going on :(

I'm gonna let it sit overnight, and let any pooled gas in the motor evaporate and try tomorrow.
""Dang I don't have the tools to do the vacuum readings and I don't have any extra jets. I am gonna screw this up. Any other suggestions.""

Yes, buy the tools or bring it to someone who has them.