I have a 360 with 10.9 Scr, aluminum heads. It is a zero deck with the pistons .005 max out of the hole.
Years ago,I ran this with a 270/280/110 Hughes cam. Lift with 1.6 arms was .538/.549listed. I run the AirGap and TTIs( with H-pipe at the time). I also run an A833 with a same starter gear as you.
No MSD, but similar timing; I ran 14*
It also was my DD,: but in winter I stuffed a 318 in it, sometimes with a 904. So we're practically twins. Well except you are Duster,I am Barracuda.
>I am positive your tune is off, or at least way different from mine.
The primary clue is the sluggish mixture screws, with correct T-port sync.
I believe your running float level is too high, making pull-over too easy. The back-up to this is the secondary cracking. If your timing really is in the 16/18 zone, that little cam should not require much if any cracking. Mine didn't. Mine also was fine with a 700 idle.
Your idlespeeds is too low. You're trying for the sound aren't you.
Here is a test; does it pull 1st gear at idle by itself-no slipping the clutch, on flat level hard ground, Down to 600/550 rpm, before it starts getting jumpy. If it does that, ignore the AFR.In fact ignore the steady state AFRs below about 1200 entirely.
>This cam has an approximate ICA of 60*. That means the piston is about 1/3 of the way up the compression stroke before that intake finally closes. And that means at slow engine speeds, it pushes some of it back up into the intake. The slower the speed, the more of it gets pushed back up into the intake. You can see this on the vacuum gauge. By about 2200rpm the vacuum has peaked. It may be 20 to 22 inches. But slow it down and see what happens. Sure enough every 150 rpm sees a drop of about 1 inch. That is the ICA in action.
So now this A/F charge has backed up into the intake, and is just hanging around. The intake valve in the next cylinder in the firing order opens up, and puts a tug on the carb and pulls some A/F in and that mixes with what's already in there, and down the hatch it goes. But it's now fat. And your AFR sees that. So you go lean it out. But wait that ICA strikes again. The next cylinder, now fat, pumps some of it's charge back into the intake, and the next cylinder gets a double fattened up charge.And on and on it goes. And you keep leaning it out according to what you see on the gauge, and eventually you get it working pretty good. At idle speed.
But your engine is not a one rpm engine.
Here comes my point. I find very little solace in the AFRs below the vacuum peak.
Every changing rpm with the manifold full of reversion, requires an AFR correction. IMO forget about nailing a number, Instead let the engine tell you what it wants.
The very FIRST thing you have to do is get those mixture screws working.
I assume that carb has NO idle fuel going into the secondaries right? Go look for the discharge ports. Cuz what I'm about to spout depends on NO SECONDARY IDLE DISCHARGE.
So here is where we
start
With a working PCV, and with the idle timing fixed at 16*, and a decent timing curve in the dizzy (about .7 to .8 degree per 100rpm),and a Vcan coming in early with at least 14 degrees in it, put a vacuum gauge on it to read manifold vacuum,and make sure that TDC mark is correctly indexed on your balancer.
>Start up the engine and while she is warming up,put a timing lite on her.Stop the engine. Close up the secondaries tight. Remove the PV and put a PV plug in there. Crank in the curb-idle screw 4 full turns. Set the mixture screws to 1.5 turns out. Start her up again. If the idle speed is too slow, crank in some more speed. Keep track of the number of turns so you can go back to your T-port sync.
>OK;Rev it up until the vacuum peaks. This should be about 2000 rpm maybe 2200. This is the very first rpm that the engine has stopped pumping charge back into the intake. Set the throttle onto the fast idle cam, whatever cam will get you 2000rpm. If you can't get close to 2000 rpm, stick a wedge between the curb-idle screw and its stop. Check the timing. The total from all sources should be 16+
8+
14 in the can = 38. It doesn't have to be exactly 38.But I estimated
8*in the centrifugal, cuz that is what mine liked. And I estimated a minimum of 14* in the can.So to my way of thinking 38* should be the minimum.
Disconnect the Vcan To see what's in it. If you have this amount we are good to go.
>Now go mess with the mixture screws. Screw them all the way in, then back them out 1/2 turn at a time until you get the highest rpm. If they don't seem to do anything check your AFRs, I bet she's fat. Too fat on the LOW-SPEED circuit. Prove it by sticking your thumbs on the boosters; there should be no change in rpm. Here is the proof; grab the choke blade and slowly begin to close it.If the rpm rises she is lean. If the rpm falls, she is fat. Return the engine to idle. Let it stabilize. Shut it off
What to do?
Well if it's lean, the first thing you have to do is prove the engine is not getting air somewhere it's not supposed to. But with the secondary cracking, this is hardly likely.
So I'm gonna assume it was fat. That is a good thing to know.
OK go find something to pinch your fuel line closed. Set the mixture screws to about 1.5 turns out. Start the engine up, set the speed to
about 1200 using the fast idle cam.Write down the rpm number, and install the clamp. Then wait. And wait. And wait. One of two things is gonna happen; A) the rpm is gonna begin to fall almost immediately, or B) its gonna rise for a while as the fuel level falls, then it is gonna hang there, then it is gonna fall to a stall as the bowl runs dry.
A) indicates that either the fuel level was just right, or at least it wasn't too high.
Fuel pull-over immediately became harder, so the engine slowed.
B) indicates that the fuel level was too high. Pull-over was too easy. As the fuel level decreased the rpm went up, as the AFR normalized. Then it peaked for a bit, and then pull-over became difficult so the engine began to slow as it leaned out too far. I'm guessing this is what yours is gonna do.
So stop the engine. Release the clamp. Start her up. Let the fuel level stabilize. Then back up to 1200 and pinch the fuel line. This time,as soon as the rpm has peaked,, tap the fast idle down,shut it off,and record the rpm.
Now, let's go see where the fuel level is. Wherever it is, that is what she wants, so see if you can give it to her. This may take several trys so don't get stressed. When you are finished it should be very near to the factory setting, perhaps a little below the sight-hole. If it gets to be too low, it may be trouble on the mains or at WOT. If what she wants requires a very low float level, then we will need to mess with the airbleeds. I have run my engine on a lower than stock fuel level for more than 15 years and over 100,000 miles, so A bit below seems ok.
There is a short cut for this.With the engine idling and a tach under the hood, crank the float level adjuster down 1/2 turn and wait a minute. If the idle speed came up, do it again. Repeat until the speed no longer rises, and back up 1/2turn. If the idle speed fell, back up the other way.Once that is done, repeat the test, to prove you got it. You are now looking for the previously recorded maximum rpm. Pinch the line and the rpm should begin to fall. Awritee tap her down shut her off and remove the clamp.
>Now back to the first test. Where we reved it up to 2000 and adjusted the mixture screws for maximum rpm. Try it again. The mixture screws should work now. But if they don't, stick your thumbs over the boosters; this should produce no change in rpm. But if it does, your boosters are already flowing,reduce the engine speed 200 rpm, and try again.
Surely the screws work at 1800. If not try 1700.
So once the screws are working, we are in business. Return the engine speedscrew to restore the T-port sync.
Now if you are a sharp guy, you have figured out that the last hour or two,was all about setting the NO-LOAD AFRs without a gauge, letting the engine tell us what it wants. It wants one AFR at 2000, and a different AFR at idle. And in between it might want a million other AFRs between those two.It ain't EFI so we can only do so much with that cam.
>OK now we have to make the engine happy at idle.
So #1, prove the T-port sync is still good. The Transfers have to be sq to a little taller than wide. The mixture screws should be about 1.5 turns out, or as previously established, at the hi-rpm test point. Now don't touch those after this.
Next we are gonna set the idle rpm with; timing and idle air bypass. My engine combo did not take kindly to receiving air from the secondaries DRY. No sir it didn't like that at all. So My target for this combo is 700/750rpm. My combo liked 16* same as yours. So if your idle timing is truly 16, and the rpm is not at least 700, then we have to give it air. But how much and where? Well grab your secondary shaft and tip it open a tad and see if the engine likes it. Mine did, but it then didn't like the primary tip-in. There is no fuel in that secondary air, or at least my carb didn't have any, and no matter what I did to the front, I just couldn't get rid of that tip-in sag.
>So here is what I did; I tee'd into the PCV line with another hose and started jamming Holley MJs into the hose until I found one that got my idle speed up. Then I went for a road test. Gone was the tip-in sag. I reasoned that since the PCV air enters the carb right near where the low-speed fuel comes in, that the fuel and air now has time to get all mixed together by the time it gets into the cylinders. I also reasoned that the dry-air in the secondaries, couldn't do that and perhaps the back two were running lean.
So now I calculated the area of that MJ, and divided that by two, calculated the size of 2 holes to equal that and drilled my throttle plates
If your engine requires more air than one MJ can supply, get creative. I have used golf tees and drilled those out. At this point the size of that hole is not critical, cuz we can fine tune it with the speedscrew; just don't make it too big!
(As a side note, I put this same 750DP on my next combo, and modified it to supply some fuel from the front idle circuit, into the rear holes, and no more sag!)
Awright so the point is; figure out how much air she wants, and give it to her. You can give it in the secondaries, if she accepts it there, or through the PCV system, or through the carb internally, which usually involves drilling the throttle blades. Just do whichever it takes; I have drilled a lot of plates. And I have filled a lot of plates. And I have redrilled those plugs smaller,lol. so easy does it.
But if you are real close to the target rpm, and since your transfer exposures are square, it is permissible to crank in the speed screw a bit. But be careful, this can cause a tip-in sag if the transfers slow down too much.So give it a half turn and check the tip-in. With a manual trans, a tip-in sag spells death. Remember to always be sure that with any speed screw change, you check the squirters for action.
So now we have optimized the float level, and the AFR at 2000 rpm. We have set the T-port sync, Set the idle speed, and maintained a nice tip-in. I think it's time for a roadtest. Try and keep up OK?....... Oh wait!
We have to reinstall the PV. If anything changes after the PV goes in, something is wrong with it. That is the reason I asked you to plug that circuit, to rule out PV issues. If you didn't plug it, and you made it this far, I guess it's working , so get moving already,
.
