Carburetor settings?

[Nessumsar]

I need desperately to tune my engine. Idle speed stutters, and runs way too fat at WOT. Now, I don't have an O2 sensor; so I understand that I will only be able to get close to an optimal tune, but I need some of your guys/gals experienced input.



My engine is in my sig., and would like your input on how to set me carb so that my car will run well. Right now I have a stock 4150 750cfm Holley carb, and my timing is 18*int. and 36*total at 3000rpm. While sitting at a stop, you can smell fuel, and if you sit at a light long enough, it can burn your eyes (my fiancee hates this!). And at WOT my friends can't stand to be behind me. And when I punch it, from a roll or stop, it sputters and then goes.

So, what peices do you recommend I put in my carb. Right now I've only thought about jet sizes; a size down on primary (77?) and two on secondaries(78?)?

[Nessumsar]

bump

[Shawn MacAnanny]

WAY too rich primaries. My engine is about 500-550hp and i am running 72 primaries, 76 secondaries, a 6.5pv and i did have to install a 50cc acclerater pump because of my off idle stumble with the victor jr. I also have the long yellow spring installed for my secondaries. I am running a dual wideband setup so i have mine dialed in just about perfect now. What is your idle vacuum in gear? what power valve are you running?

I would try my exact setup and it would get your darn close. I would run the purple spring instead though if yours is vacuum secondary. Whatever your idle vacuum is in gear (drive holding down brake) i think you drop it like 2 points or something (i.e. 8hg, run a 6-6.5pv). For a manual trans whatever your idle is in nuetral you just cut it in half.

Right now with my setup i am running 14.6 at idle, 13.8 at cruise, and 13.2@WOT.

If you do get a wideband, get this one. It's pretty amazing.

http://www.rpmoutlet.com/mt.htm

[CarlC]

Is that a double pumper? If so, then I agree to the above as a starting point.

Carburetor tuning is very dependant on the ignition working properly, a clean well-functioning carb, and a good clean fuel supply.

If the above is OK, it's time to adjust the mixture screws, located on the front of the carb baseplate. Easily turn each screw in until they seat, then back out 2-1/4 turns. Start the car. With the car fully warmed up turn each screw SLOWLY to the right (lean) until you notice the engine just start to stumble. Back it out a 1/2 turn. Repeat on the other side. Repeat the proceedure several times. You may find that you do not need to back out (richen) 1/2 turn, but only a 1/4-turn. You will need to play with it a bit. Usually for a well balanced package (engine size, cam, carb) about 1-1/2 turns out is about right.

Main and secondary jetting do not effect idle mixtures.

You may want to check this out for some additional help.

http://www.bob2000.com/carb.htm

[6'9"Witha69]

THe mixture screws, accelerator pump, cam and lash all have an effect on idle and off idle characteristics. You can also enlarge the metering circuit if need be, but play with the others like mentioned above first.

Even without a wideband O2 setup, you can use the plugs as a gauge to how rich/lean you are running.

[Nessumsar]

I will try to get a jet kit and some springs on Monday, and hopefully have the time to screw around with it.

Does my timing seem to be alright, I've read elsewhere that my timing curve is a little slow.

[CarlC]

Timing is basically fine.

Do you have the advance connected to manifold or ported vacuum?

[Nessumsar]

I have no vacuum advance, I know I should have it on manifold, but have not hooked it up.

[Shawn MacAnanny]

To set idle mixture get a vacuum gauge and turn each screw in or out equally until you get max vacuum. This is the BEST way to do it. I had set mine this way before i got my wideband, hooked my wideband up and it was idling at 14.8.

[yellow heap]

My 383 is close to yours but a bit more cam
It likes 16 in 34 total at 3000,so I think your is fine.
The vac advance will help you run a leaner idle.
A good tach and vacuum gauge are great to set idle.I keep the rpm constant(say 750 or so) and as I'm moving the idle metering screws I keep the speed the same with the throttle screw while looking for the highest vac reading-at that point I turn each idle mixture screw 1/16-1/8 turn rich to aid drivability.
You will have to be carefull on the vacuum advance with your comp and iron heads,Iwould guess no more than 10-12 dedrees is all your motor would take
for a total of 46-48.
Nice lookin nova by the way.

[Nessumsar]

Thank you guys for all the input!

So far I have down size 72 primaries, 76 secondaries, hook up manifold vacuum, bust out a vacuum gauge, and maybe an accelerator pump.

I do have a question about my tach(s)? When I first got the car running, I had a cheap-o Sun Pro II mini tach hooked up. I have since then installed an Auto Meter 5" cobalt. I have noticed that the Auto Meter reads ~20% higher than the Sun Pro. I have my tach's signal wire hooked directly to the tach output on my 6AL box, is it sending the right signal to tach? I have also noticed during "gnarly on-ramp burnouts" my tach jump up to 7k rpms, I have a 6k chip in my 6AL. So if the tach is reading correctly, can the rev limiter chip be bad; or is my tach reading incorrectly?

I will try and get some up-to-date shots of the car Yellow, she looks a lot better now!

[Shawn MacAnanny]

Sounds like the tach is just off. I wouldnt think it would be but it must be. I would also replace your plugs, or atleast clean them once you fix your jets lol. Try a purple spring as well.

[6'9"Witha69]

Make sure the tach is in 8 cyl mode. It should ship in 8 cyl mode from AM but I have seen them in 6 before. Your pickup point is correct.

[Nessumsar]

There is not a setting on this tach as far as I am aware. I will take a double check then.

[6'9"Witha69]

It's on the back, usually under a black cap.