idle mixture screws question .....and update on new motor

Ok guys I just finished putting a new 460/C6 combo in my 71 F100 short bed. Kinda mild 400hp / 500 ft/lbs of torque. The C6 is a broader performance wide ratio. I've got 3.0 gears in the 9" for now.

I finally got it started and broken in. I kept it around 2200 rpm for 20 minutes. After that, I tried to let it idle down so i could put the transmission through the gears (its new too). It wouldn't idle so I had to hold the gas so it would stay at 1000 rpms. The trans went in all gears, so I then shut it down. No leaks, good oil pressure, and didn't over heat so I called it a good break in. When I got out of the truck, there was so much gas in the air, my eyes started watering. I'm thinking an overly rich idle circuit is why it wouldn't idle and why I kept getting back fires through the carb.

Now, what is a good setting for the idle mixture screws? Holley 3310 750VS. I know you screw them in to make it leaner and I know how to adjust it w/ a vacuum gage. I just need to know where to start. Also, if you tell me 1.5 turns out from seated, then I need to ask a second question. What is a turn? Is it 180 or 360 degrees? I think a turn is 360 degrees so maybe I have the screws way too far out.

thanks
Austin

Austin....the watering eyes and backfire you are experiencing may be a lean condition. First of all verify that you do not have any vacuum leaks. Check all hoses and connections ensuring that they are tight. Make sure you do not have a leak at the carburetor base gasket. Start the mixture screws at 1 1/2 turns out from gently seated. One turn equals a full 360 degrees.
Also ensure that your timing is correct and set with the distributor vacuum advance unplugged at the vacuum canister and the hose plugged. You'll have to get the idle down to around 650 or so to properly set the mixture screws. Connect a vacuum gauge to a vacuum source (not ported vacuum). Now slowly back out the screws a 1/4 turn at a time watching to see if the vacuum increases. Move back and forth from the screw on the drivers side to that passenger side and vise versa to obtain the highest vacuum reading. You didn't mention any camshaft specs etc so it is hard to determine the correct idle speed/vacuum readings that you should be shooting for. Set the ignition timing at 10 degrees and go from there.
Good luck.

I just helped my freinds dad brak in a Z28 (origional 68 too!) 302 and wer could not figure out why it sould not idle or made our eyes water. Turned out to be huge vacuum leaks because my reiends dad did not plug the vacuum ports on the carburetor. If you have no vacuum leaks do what rcodeenewf says and you should have it all tuned up.

holley tech line will tell you that the idle screws will not react over 900 rpm.
[addsig]

Thanks guys. I'm going to try it out and see what happens. Now, I'm leaning more towards vacume leak. Since it was a new motor, I wanted to use a known good carb. I have a parts 74 lincoln w/ a 460 that runs like a top. I took the carb off of it and tried in on the new motor in the truck and all I got was backfires out of the carb. Never could get it to run. I thought it strange that the carb ran so well in the lincoln, but not at all on my new build. So I got out a holley 750VS that I had rebuilt about a year ago. It had never been used and was sitting in a bag sealed. I put in on and still got some back fire, but got it to run, but not idle.

I will check all the vacume ports again and see if I can find a leak.

HI
Yes sounds like a vacuum leak It not nessaryly at the vacuum ports. check the intake and the carb base. use carb cleanner and spray around the intake where it bolts to the head and carb base and all vacuum lines if the rpm pick up then theres your vacuum leak..
Tim
[addsig]

From my experience, backfire through the carb is usually a lean condition. I've never figured out WHY that is, but that's the way it always words. Either that or your plug wires are on in the wrong order.
[addsig]

HI
I kind of re-read thourgh your post's and I'd check timming, fuel mixture, as well as the vacuum leak.. a 180 deg is only a 1/2 a circle.
Timming at about 2,000 rpm would be around 25 to 30 deg's. When you get it to idle timming should be 6-8-or 10 deg. Also make sure your timming is right, pull the dizzy cap and see which way the rotor turns.
Tim

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ALWAYS THE RED NECK WAY
61 Merc. Comet, 200 3spd.
1964 International 4door, short box W/466


<font size=-1>[ This Message was edited by: pedal2themetal45 on 2/1/06 12:18am ]</font>

Hey pedal,

Yeah, the timing is dead on. When I was asking about turns, I meant the idle screws. I adjusted the timing both ways and it didn't help any. Initially when I couldn't get it to start, I checked the timing multiple times. I even pulled the valve cover to make sure I was on the compression stroke. I set initial to 10 BTDC. Yes, I did make sure the balance matched #1 piston as far as TDC on balance and TDC on piston.

If I had a leak at the intake manifold base, wouldn't I get an oil leak?

Is there a torque spec on nuts that hold the carb down?

thanks
Austin

A turn is 360 dergree. The torque spec on the carb nuts is snug but not real tight. Too much and you'll warp or crack the baseplate. You can have a leak under the intake and not be pulling that much oil. For a 3310, start with the mix screws out 1 1/2 turns from lightly seated, the primary blades set to expose about .030 (small paper clip wire) of the transfer slot and the secondaries just below the rear transfer slot. You verified the timing, verify the firing order too.
[addsig]