Initial timing
You gotta set your TransferSlot exposure underneath the primary throttle valves first. If this is not done first, then you will be chasing your tail all over the place. The starting point is from dead square to a lil taller than than wide, depending a lil on the engine combo. Then set the mixture screws into the middle of their adjustment range, which is about 3/4 turn on Holleys, and 2.5 turns on most others. Then close the secondaries up tight but not sticking.
This is your starting point. After this DO NOT adjust the idle Speed screw. After this, you will set the Idle-Speed with Idle Timing in the range of about 10 degrees to at most 18*, depending on the combo.
In order for this to work, the following must be correct; namrly
1) the float level is correct and remains stable at all times
2) The PCV is correctly installed and plumbed to dump air just below the primary throttle valves.
3) the brake booster does not leak
4) the engine does not have any vacuum leaks, including not into the valley.
5) the engine is in good mechanical shape
6) the installed centerline of the cam is within a few degrees of "straight up"
7) the pistons move up and down freely
8) the engine and it's oil, are up to operating temp of ~185 or more.
9) the convertor is properly slipping
10) some engines are gonna need some Idle-Bypass Air, to get the rpm up, and to get rid of the "burns your eyes exhaust stink..
I hooked Mighty Vac up to va, plates move at 9"hg and stops moving at 18".
if your Vcan has a hexagonal shape at the spigot, those are adjustable as to their cut-in/finished calibration.I like to set them to come in as early as possible, so long as it does not induce detonation.
Your engine/all engines, have a very small window of best timing. It is, generally, whatever it takes to get the peak cylinder pressure to occur at about 25 to 28 degrees AFTER TDC on the Power-Stroke. At this time, the crank is in the best position to receive the energy of the hot expanding gasses, and send it on down the line.
At full power WOT and beginning at around 3600rpm, this point is usually achieved with a Power-Timing of 34 +/- 2 degrees.
At Idle it usually occurs with MUCH more Initial/base timing. Usually in the mid to high 30s , but as Bewy discovered in post #7, can be even higher.
All your factory timing mechanisms are designed to hit that optimum crank position, as often and for as long, as possible.
You can easily find this position at idle, just by advancing the crap out of it while re-adjusting the speed screw to keep the idle speed down. However, by adjusting the speedscrew, you shut off the Transfer Slots, and this is bad news. Yes, you can readjust the mixture screws to compensate, and it will idle; but this introduces other problems like; the dreaded tip-in sag, or a hesitation, or an outright bog, and an impossible to cure low speed rich condition. You will end up spending ridiculous amounts of time with the accelerator pump and float levels and IABs.
Furthermore, with mega Idle-timing comes a major increase in low-rpm Power. This leads to a hard N/P shift into gear with a typical stall convertor. But worse is that it makes the low-rpm throttle application quite sensitive; that is to say, tiny amounts of throttle application are immediately felt as acceleration; sort of like an on/off switch. If you have a manual transmission, this is exceptionally aggravating as the car gets jumpy. And, of course, the starter doesn't like it either.
IMO, and
for a streeter,
there is no good reason to run mega-Idle-Timing; it creates more problems than it solves. For that reason, do I recommend setting the Idle-Timing AFTER the T-Slots exposure is set; because it will automatically cure most of your problems.
Your low speed fuel-delivery circuit is comprised of some combination of the mixture screws and the Transfer Slots. You can vary the proportion of each, by experimentation, to whatever gets rid of the tip-in sag, while simultaneously setting the off-idle AFR leaner and closer to 14.7.. To help with that, what I do is this; set the rpm to 2000/2400 with whatever fast idle step it takes. then adjust the mixture screws for lean best rpm. They should fall into the range of about 1/2 to 1.0 turns out on a Holley, and about 2.5 turns out on just about everything else. Then flip out the fast idle.
Now, leave the mixture screw alone. If your idle wants more or less fuel, you get it from either the T-slots by the curb-idle screw, or the IABs by swapping/drilling. If the Idle speed gets out of whack, simply change the timing to get it back.
Here's the deal;
In my 11/1 367engine combo I have run three different cams, from the 292/292/108 Mopar, to the Hughes 223/230/110, to the current 230/237/110 Hughes, and I can tell you that ALL THREE of them will idle all day at 5* advance, without overheating, without a tip-in sag or hesitation, all of them pulled themselves around the parking lot with a manual trans, down to 550 rpm, and one of them got me 32mpg on a certain all-day test run. What I'm saying is this;
IMO, there is NO good reason to run mega Idle-timing, except perhaps for emission reasons. But yes, with an auto trans, you can usually get away with it ...... especially if the car has a hi-stall in it.
Let the T-slot exposure be your guide.
Adding Idle-Air bypass, allows you to close the throttle back down, to get Slot exposure down, to get the Idle AFR back in line, to get rid of the Tail-pipe burns your eyes stink. With too much Idle Air, the T-slot exposure will become too small again at a decent Idle-speed; SO; easy on the bypass-Air. You can experiment with Bypass air in/into the PCV circuit. If you do NOT have a 4-corner idle system, then I highly recommend NOT to introduce this bypass air by cracking the Secondaries. In my experience this just makes the rear cylinders lean.
My best advice is to set the T-Slot exposure first. This will make/force the other parameters to fall into line.
Once the amount of bypass air if any, is established, I drill the throttle valves. One hole in each Primary. On the front side, between the T-slots and the Idle discharge ports, but no closer that 1/8th inch from the edge. The bigger the cam, the more air it will need.
My 292 Mopar/
[email protected] cam required two holes total, each one being 7/64= .109inch
My
[email protected] required NO holes.
My
[email protected] cam also has no holes, but I think it wants some, just not enough for me to get to it.
Happy HotRodding.
BTW
1) once the Idle-timing is set, you may have to revisit your PowerTiming and re-engineer the Timing curve. So do not run WOT until you get the PowerTiming back in line. If you later change your Idle-Timing, you will again have to re-engineer the PowerTiming. So then, again, you gotta get the Idle bugged out first.
2) And that means also, that the coolant temp has to remain rock-solid. You cannot tune the idle with the coolant temp wandering around. If you later change the minimum cooling system temp, you may have to fiddle with the Idle again, which may lead to changes in the Power-Timing..... again.
3) if your cooling fan is blasting air across your float-bowl vent, that is sure to cause trouble, do what you gotta do to stabilize the air-pressure in the float bowl.
4) if your intake manifold is too cold at idle, the Idle AFR will go lean as the fuel condenses on the floors. Then rich when it tears off and re-enters the airstream, then lean if the fuel particles are too big to catch fire in the allotted combustion time.... To help solve this, I run a coolant temp of 207*F in my AirGap equipped, alloy-headed, 11/1-367.. Yeah I know, everybody says I could get more power at 160; Ask me if I care, on account of she already goes 93 in the Eighth.
For a streeter, hi-rpm Power ain't everything. Nor is low-rpm torque everything. The most fun I have is in Second gear(manual trans), below 35mph. The rpm at 35 in mine is 3000 rpm. It takes torque here, to get the tires spinning in Second, with just a foot-stomp, then it's game-on; a lil left turn, a lil right turn, steering with the rear tires, a lil sliding here, a lil there, zip the tach up to 7000, then compression braking back to 35mph, reload, and take the next shot. Whoot-whoot! Then get outta town. lol.