Larger jets or larger carb cfm?

[Lanman70]

Bought the 1970 Se 383 auto a month ago and have been trying to get it running better every day. Sometimes I think I've won and then it fails. Engine has unknown cam and has headers, stock ignition and carb (new plugs, wires, cap, rotor, coil, voltage reg, ballast resistor, carb rebuild,). Using vacuum gauge only way to get good vacuum is idle mixture screws out 5.5 turns and timing very advanced. Do recommend larger primary jets or larger cfm carb? Thank you
Previously posted that engine died when put in gear and got great help sorting that issue out.

 

[rklein71]

Jetting is the last thing to work out with a carb and has virtually no impact on idle quality as jets limit gas flow at wide open throttle.

Make absolutely sure you float levels are set correctly and you have no vacuum leaks.

Idle mixture screws turned fully in and when turned fully out should be able to kill the engine. If not, your idle circuit is not functioning correctly, and you will need to figure that out before doing anything else.

I would suggest doing a search on the internet on how carbs work. It helps to have a good understanding of the carb circuits before attempting to do any adjustments.

 

[pschlosser]

Using vacuum gauge only way to get good vacuum is idle mixture screws out 5.5 turns and timing very advanced. Do recommend larger primary jets or larger cfm carb?

Idle mixture does not really impact vacuum. If it idles steady (using the tach on a tune up meter) adjusting the screws for maximum idle speed is generally a good indicator of correct mixture.

 

[Deathproofcuda]

Could be helpful if you can provide some more details on how things are running. What exact carb are you running, what is your idle rpm, what are your vacuum readings, what are your timing settings? As others have stated, a bigger carb or bigger jets are not your issue at the moment.

 

[Xcudame]

Seriously it sounds like the carb could need a rebuild. Also, with an unknown camshaft, it's hard to know what "good" vacuum is. As Deathproofcuda mentioned, what carb is on it and what are your vacuum, rpm and timing readings?

 

[pschlosser]

The camshaft may be "unknown" but does it sound OEM factory or long duration? If its the latter, then yea, good vacuum at idle may not be easy. But if it sounds milder, like OEM, then you have potential for decent vacuum.

 

[Lanman70]

This is the battle, trying to get good vacuum. Currently running steady at 17 on gauge. Timing advanced to get to this number. Runs real good right now, so maybe this is where it's happy.

 

[Xcudame]

Heck, 17inHg is really good! My Challenger has been between 15 and 16inHg ever since I installed the big block high performance cam.

 

[pschlosser]

yea, 17-inch mercury is decent. Is it a long duration cam, or a mild one from the sound of the idle?

 

[Lanman70]

I am guessing when I say it has a mild cam, it ran so bad when I got it. Turns out it had 2 dead cylinders, one bad plug wire and one bad spark plug. Has a bad set of hammer tuned headers that I would like to get rid of in the future.

 

[Challenger RTA]

You say new parts.When? did it sit for awhile? Back to basics. Pull plugs check gap and inspect top and bottom. Is there one or more cracked? check wires did one or some get burnt or chewed?Check cap and rotor for cracks and corrosion. As mentioned vacuum. Carb gasket 17-inch mercury it should be ok check it. Check choke for proper operation. Check distributor vacuum advance working. Don't think a new part is good. Eliminate and move on.

 

[Challenger RTA]

Great mind must think alike. I had it typed earlier but didn't post it . I went to get an other cup of coffee and forgot. 20 min later. While in the meantime you already did it. Always think back " what the F**k did I do last" and think simple. Just my 2¢ and a another cup of Joe. Now I can understan why Chiefs aboard ship had a bent finger only a cup would fit in.

 

[Dodgeboy]

Just gonna say: What do you guys consider good vacuum if 17inhg isn't enough? I got 6inhg and yes it's a long duration cam, and I use my idle adjustment screws to set idle vacuum, and idle speed screw to set idle rpm.
But anyway, lots of good suggestions that I would tackle first, read up on carbs before you mess with yours, it's easy to screw it up and lean out your motor which you don't want to do

 

[ctaarman]

17" mercury says you have a mild cam. Hotter cam would never achieve that.

After cleaning up everything, The best way to tune and diagnose is with an Air/Fuel Mixture analyzer. Spark plug gap will only impact running if too small. 5 thousands over has no effect so don't under gap.

As others have commented, you need to characterize "runs like crap". The best simple old school way to see if your main jets are the correct size is to go out with clean plugs on a non busy highway, run your car evenly at cruising speed with no changes in throttle for 3-5 miles then immediately put it in neutral and kill the engine, and pull over to side of road in that order. You can't let the engine run while you decelerate.. Pull one or more plugs. If they are tan you are good. Black too rich. White, too lean.

But there are many more carb variables. There are decent videos to learn from online plus books on your carb type.