'68 Dart with a stock '76 360 out of a motorhome, edelbrock performer rpm intake, and unknown brand headers, a Holley 750 4bbl vacuum secondary and a 1/4" phenolic spacer.
it idles in park at about 950-1000 rpm and about 800 rpm in gear, and the engine has seemed its happiest at about 14-15* initial timing. The carb has been jetted down 3 levels from its original 76 jets to 73.
Why, why, and why?
Why does the stock engine idle so high.
Why does it need 14/15 * idle-timing. and
Why did you need to downjet?
But first;
>> Ok so your problem could be ; mechanical, or it could be ignition, or it could be fueling, or it could simply be the tune.
I always look at the
mechanical first; Next up is
ignition. Then
fueling. And finally the
tune.
Ok I see a lotta this has already been covered so back to the whys;
------------------
Last first: as for the jets;
73s are still too big on the primary side, but the secondary side should have a plate equivalent to about six/eight sizes bigger than whatever you run on the front. I would diagnose the Primary MJs by removing the PV and installing a plug, installing 68s, and then roadtesting in second gear, with a vacuum gauge, by rolling into the throttle at various rpms from 1800 to 2800, to various vacuum settings; to see at what manifold vacuum the primaries run out. You will feel it as the engine lays down.
You want this to be at about 10 inches. Change the MJs until you get it.
Now install a NEW PV, either a 12.5 or a 10.5 and roadtest for a smooth transition from 12 to 8 inches.
As for the high idle-rpm;
Make sure your secondaries are fully closed but not sticking.
Make sure that you have NO vacuum leaks, including into the Crankcase.
Make sure the PCV is plumbed to the FRONT of the carb.
Make sure the Brake Booster, if you have one, is plumbed to the back of the carb or to the Plenum, but NOT to just to one intake runner.
Make sure the Vacuum-advance can is plumbed to the spark-port.
Make sure ALL other vacuum ports are plugged.
Make sure the fuel-level is correct and stable. BTW, your carb does not care about the fuel pressure; it only cares about a stable design-correct wet fuel level. Pressure is what the system needs to get the fuel from the back and to maintain the wet fuel level.
For now; if you have an EGR, defeat it, so it doesn't confuse the tune.
Now back up the timing to 8 to 10 degrees, and reset the idle-speed to 650 in Neutral.
Now before you do anything else, check your PowerTiming. Unplug the Vcan, and just rev it up until it stops advancing. Could be 3000, or 3500, or even 4000. Wherever it stops, it should be less than 36*. Record the number. Rev it up and down gently a few times while watching the timing marks, to prove it is advancing smoothly and not sticking.
Next, with a Holley-type carb, set the mixture screws to between 1/2 and 3/4 turn.
Here's the thing;
The slow speed system of this carb consists of the Transfer Slots and the Idle enrichment screws. These two are synchronized to eachother by the Idle-Timing.
If the Idle-Timing is set too high, then the idle speed will be too high. If you then slow the engine down by using the speed screw, this automatically reduces the Transfer Slot fuel. Which makes the engine run rough; so you open up the mixture screws. And finally it idles. But, as soon as the engine comes OFF Idle, the AFR goes rich, because the mixture screws are out too far. Another thing that happens, in the above condition, is most times the engine will have a Tip-in; stumble, hesitation or sag. This is because the transfers are lean, they take a few milliseconds to come back up to speed. The cure is simple; just open the throttle some more, and reset the mixture screws. If the speed goes outta sight, you just retard the timing.
If the IdleTiming is not enough, your Idle speed will be too slow. So you will use the speed-screw to speed it up. This will open the transfer slots dumping in more fuel. So then you will shut off some of the fuel coming from the mixture screws. Now it idles. But as soon as the engine comes Off Idle, the AFR goes lean because of the dried up mixture screws.
You can play around with this for hours and hours searching for the perfect setting.
Or you can try the settings I gave you above.
Or you can just get it right the first time with
this method;
Remove the carb and drain it. Then with the throttles on the curb idle screw, keep it there and flip the carb upside down. Now go find the transfer slots, on the engine side of the blades and adjust them with the speed screw, to be square, to a little taller than wide but Not shorter than wide......... BOTH the same.
After that, make sure the secondaries are closed up tight but not sticking.... BOTH of them. Then reset the mixture screws to 1/2 turn from lightly seated.
These settings will get you daymn close.
NOW, after this, do NOT touch the speed screw. If you have to adjust the rpm, do it with timing advance in the range of 8 to 14 degrees, to get about 650 in gear or whatever does not BANG when shifting from Neutral, and is half-azzed smooth.
Now remember; the idle fuel is synchronized between the transfers and the mixture screws.
So you can use the mixture screws as diagnostic tools. If the engine runs better with a lil more mixture screw opening, then you can adjust the transfers richer and put the mixture screws back to .5 to .75 turn. But if the engine wants less fuel, then tweak the speed screw the other way, and reset the screws again. Do not tweak the speed screw more than about 1/2 turn slower, nor more than 1 turn faster.
Now you are really close.
Reset the timing to recover your desired in-gear idle-speed.
Finally, road test it for a tip-in hesitation.
see note-1
If you consistently get one, reset you accelerator pump per the factory instructions.
If you still get one, check/reset your float level.
If you still get one, add 2 degrees Idle-Timing but do not reset the speed screw setting.
If you still consistently get the stumble; ADD ONE FULL TURN to the speed screw fueling, reset the mixtures to .5 turn, take the two degrees timing back out, and try it again.
Note-1
I am talking about a Tip-in hesitation.
So Tip-in is what it implies. You get into the car, you put it into gear and let it idle, like you were sitting at a light behind another car. The light goes green, the guy in front of you starts moving, then it's your turn. So then you very gently tip in the throttle, and drive away.
In the next year, you will be doing this thousands of times, so if it stumbles, it will make your life miserable. So, IMO, you must not have a tip-in stumble. Take whatever time you need to get this set up right.
Ok so, that's my story and I'm sticking to it......
