Help! Jet 850 down vs jet 670 up vs buy 750

[1bad340]

Hello gang,
"sputtering issue". Cruising down the road it just stumbles. Alot of the time it does it if I ease of the throttle. whether I'm going 30 or 80 mph.
I have tried several things. See below....

Holley 870 vac,: I jetted down to 78 primary/82 secondary from 82/82. Sputtering was better, but still did some.
Can I jet down further? How do I know if it's too lean?

Holley 670 vac: I jetted up to 70/71 from 63/64. Ran great!! No sputtering. Engine ran a little warmer. I don't think outside temp was much warmer. I'm worried it was running lean. Again, how do I know if it's lean?
I'm not experienced enough to tell if it lean. The plugs were spotless. I'm paranoid about running it too lean and cooking my new motor. Can I jet it further?

Edel 750 factory jets. It ran good. It stumbled just little at start, but fine after that. I can give more gas and delay the clutch to make smoother starts, but no fond of that. Performance seemed less than both the Holleys. The plugs were spotless and does not appear to be rich.

Buy a new Holley 750? Or Edel?
I feel the Edel is more of a "driver" carb. But I don't want to have to tinker too much with the Holley.

I'm friggin tired and in the dog house buying carbs. I am ready to buy one more and sell the others if need be.
Help!!!

The motor is kick butt!!! I love it.
Specs:
Vacuum at idle is 10 to 13. Not sure what it is at higher rpms.
Intake is Edlebrock performer dual plane (spreadbore) has 1" adapter plate to square bore.
Mallory electonic distributor.
360 block with 318 heads. computer specs with this build shows 450 peak HP (5000 RPM), 540 peak torgue(4000).
Lunadi hyd cam 30405 , Duration (.50) 224/234 , lift .318/.338
Bored .30 KB 107 pistons. High polished cylinders.
New Carter mechanical fuel pump 120 gph. 3/8" to carb. 6 psi.
5/16th line from tank to pump. 3/8 from pump to carb.
160 degree t-stat.
Headers
3" stainless magnaflow.
4 speed / 3.23 SG.

Hopefully this gives enough info.
thanks!

[Oldschool]

If it stumbles off idle and when throttle is applied with all the carbs, it might be in the ignition/timing.
If you are certain those are correct, you need to make sure you have no vacuum leaks. Once you are sure you have it sealed up, then you need to adjust your accelerator pump shot....

Oh yea, I'm thinking the 850 is a little much for your set up...   

There ya go....  clear as mud....

[jimynick]

Cheaper than buying another carb, would be buying a wide band O2 sensor and welding in a bung into the header. THEN, you'll know exactly where you stand mixture-wise. The old rule of thumb runs something like 14.2 : 1 on cruise and 12.5-7 : 1 on power. Stops guesswork and you don't have to be Dick Landy (RIP) to read plugs. 

[72 cuda]

I had the same problem once.
It was the vac-um advance.
In order to check to see if that it (the vac-um advance) is the problem.
You should suck on the vac-um line going to the distributor with the motor at idle.
It should rev up approximately 1,000 rpm give or take.
If it does rev up ,the problem is probably the primary accelerator pump in the carb.
Mark

[bandt]

850 is to much carb. 750 will be ok.

 Did you buy that motor assembled? Never heard of that combination before "318 heads". Do you mean 308 heads? Those seem like big numbers for a 360 no matter what head.

Brian

[PlumCraZRT]

850 is to much carb. 750 will be ok.

 Did you buy that motor assembled? Never heard of that combination before "318 heads". Do you mean 308 heads? Those seem like big numbers for a 360 no matter what head.

Brian

I agree... 450 hp (not to mention 500+ ft-lbs) from a 360 with stock heads?  Not with a tiny cam like that either.  Those heads flow 170 at .600 lift (at the valve) which gives you no more than 400 hp... actually closer to 320-330hp.