So I'm guessing your 318 has a 2bbl on it?
I'm not gonna start guessing until I see the plugs; and while they are out, you might as well do a compression test. But,
And I'll bet you have too much idle timing.
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When you crank in the Idle/low-speed timing, looking for low-speed torque, it works. But at the expense of destroying the Transfer-Slot to Idle-mixture screw Synchronization.
Here's how this goes;
The extra timing causes the idle speed to go up.
In response, you close the throttle.
As a consequence, the throttle sits at the bottom of the Transfer slot, and the engine starves for idle-fuel.
And so, you add fuel from the Mixture screws, and all is right with the World ........ at idle.
>But this scenario introduces two consequences.
The First is an off-idle stumble/or hesitation, as the nearly closed Transfers, now running close to dry, try to bring fuel to the port. The response is slow and lazy, and so you feel the sag as the nose drops.
And the Second consequence, is that the too far open idle mixture screws, are now passing too much fuel at all times.
And so the plugs get sooty.
So then, in desperation, you drop the metering rods down into the jets in a misguided attempt to lean it out to prevent the plugs from fouling.
And then you get the popping at cruise.
Maybe you think the accelerator pump is bad so you mess with it, but it just can't cover the hole.
Or maybe you think the fuel level is low, so you crank it up some, but you don't really understand WET fuel-level, so who knows what you actually do.
And to think
this all started cuz somebody told you to run more Idle-timing.
So let me be the first to say, that if you did that, it was very likely, the wrong thing to do.
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If you want to experiment with looking for more low-rpm power, you do it with the rate of advance, and the Vacuum can. But you always have to follow, by limiting the WOT Power-Timing.
With a two-stage timing curve, it is easily possible for the distributor to add more mechanical timing than the engine wants. and
It is possible to modify the Advance-can to deliver up to 24 degrees.
Here's the thing;
Your engine cannot hardly ever get optimum idle timing, and it not be too much as soon as it reaches stall-speed.
I mean try it for yourself.
Just reach in there and advance the timing until the Idle-rpm no longer rises, then with the curb-idle screw, slow the engine back to idle. Then add or subtract Idle timing for max idle-rpm more than 600/less than 700. Keep sawing on the distributor until adding more timing fails to get more rpm.
NOW read the advance on the balancer.
My guess is it's gonna be more than 30/35 degrees.
Ok put it in gear and try to drive away.
I bet it stalls before it moves.
But if it moves, the VA and mechanical timing systems are gonna bring their additional advance in, and as soon as you hit stall-speed, it will detonate.
So that's what I mean.
Ok then, since you cannot run max Idle-timing anyway, and honestly once the engine is warmed up, the engine doesn't even care what it's idle timing is, why are people telling us to Crank it UP!
I'll tell you why.
This is a misguided sleazy way to get the timing up at stall speed, that's why. If you are able to give the engine the stall-timing it wants, then you will have an oh-so-wonderful, hard-charging, torque monster, at take off from zero mph. Sounds great right.
Wrong! At best, you'll pick up a few ftlbs, and the engine might be a lil more snappy, but so what, it's in neutral, and well below stall, so what.
You got a long uphill battle to make this work.
The better idea, is to
just go get/install a 2800 stall convertor, and NONE OF THIS is gonna matter, then put the timing back to stock, and limit the Power-Timing to not more than 36*.
I guarantee you'll have a ton of fun with that 2800 convertor.
But I digress.
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Here's what you do.
1) reset the mixture screws to 2.5 turns out.
2) Take the carb off, flip it over, and with the throttle-arm on the speed screw, adjust the throttle opening until the transfer slot is showing a little taller than wide.
3) flip it right side up. Adjust the float-level to spec.
4) adjust the M-rods to factory setting
5) plug ALL the little vacuum ports you may have, EXCEPT the one for ported spark.
6) reinstall the carb. Hook up the V-can to the spark-port.
7) install a known to be good PCV valve AND hose, and check the breather is working.
8) if you have a brake booster, hook it up and prove that it's not sucking air full time. If in doubt, clamp the line closed.
9) fill the float bowl and start her up. If your timing is higher than stock, the idle-rpm will be high;
DO NOT ADJUST THE SPEED SCREW!
Instead, take advance out, until the rpm drops into the window of~ 550 rpm in gear, +/- 50 rpm.
DO NOT ADJUST THE MIXTURE SCREWS.
10) check the power timing does not rise above 36* at 4000 rpm. If it does, this will need to be fixed, and until it is, do not run into that situation. Shut the engine off.
11) If you couldn't find a spec for the M-rods, and if yours are adjustable, then set them in the center of the slot.
12) reset you accelerator Pump to factory spec. If you can't find a spec; adjust it so that it starts to move fuel, the very instant the throttle-valves move. You may have to pump it several times to fill the chamber. Then open the throttle all the way, and make sure the pump does not bottom and prevent the throttles from going WOT
13) make sure your gas pedal opens the throttle all the way. Check that your K-D is set right.
14) make sure the choke comes off and stays off.
15) make sure the air and fuel filters are serviceable.
16) Make sure the CC is filled to the correct oil-level, and make real sure the oil level is not so high as to cause the crank to be swimming in it.
17) take your gas cap off
18) air up your tires.
Ok start her up and warm her up.
Now you are ready for testing.
1) plumb in your vacuum gauge, and check that the needle is steady; and not so low as to suspect a jumped chain. Otherwise, No! I don't care about the number. The only thing I care about is that all eight cylinders are pulling about equally, and none of the valves are already burned up. If the needle has a steady drop, once every revolution, do a compression test. If the test shows one low cylinder, do a Leakdown test on it.
2) Having passed Test-1 above;
test two is to eliminate the tip-in sag.
With the engine idling in Neutral; put it into gear. If the car jumps and the transmission clangs, then the idle speed is too high. DO NOT ADJUST the speed-screw.
First check, what is your Idle-timing. If more than 12, you didn't listen to me, back it up to 8*. and try again.
If it still clangs, what is your Neutral idle speed?
If more than 700 in Neutral; First make sure the choke is not engaged. Second, make sure there is no air getting into the intake from any source but past the throttle blades and the that the previously proved and known to be good PCV, is in fact good. Also check for CC vacuum. Put your V-gauge on the dipstick, flip the PCV out of the cover and seal the hole. Then with the engine idling, take the breather off and seal it with your hand. It better NOT pull a vacuum! It should make pressure as blow-by from the rings fills the CC. Do not let the pressure rise over 3>4 psi, cuz it's gonna blow something out! Ok if no vacuum is found in the CC, then we know the SBM is not pulling air from the valley. Put it back right. then
With the speed screw, slow the engine down not more than ONE turn. If it now falls into the window of 550 in gear +/-50 rpm and only gains LESS THAN 100 when shifting to Neutral, whatever is clanging is gonna have to grin and bear it.
Now we can test for tip-in sag.
In Low, Very gently roll in some throttle. Repeat from a dead stop. If it willingly takes throttle, that's great. If it hesitates, that's bad. First, try three turns out on the mixture screws. If the sag goes away; put the screws back to 2.5 and instead, crank in a half a turn on the Speed screw, thus opening the Transfer a hair. If the sag goes away, we're done.
Remember, we are testing with a very slow throttle tip in. We don't actually want the Accelerator-pump confusing this test.
The Tip in sag has to be eliminated at all costs, with a warm engine, else it will be back, when the engine is cold.
If so far, the sag is still there, try raising the M-rods a hair atta time, and see what happens. Theoretically it won't change a thing.
If nothing seems to work;
I have assumed that your carb is clean and working as designed. If the Emulsion tubes are plugged up, or the air bleeds are, or the fuel level is low, or the mixture passages are dirty , or the PCV is sucking too hard, etcetera, it will make getting rid of the tip-in sag very difficult. That's why this is test #2. Cuz it's waaaaaaaaay more important than guys think.
So, if at this time the sag persists, install a new PCV and make darn sure the hose is serviceable. This hose, if you notice, has a thick-wall and is made specifically for this purpose. Replace only with similarly purposed hose. Same for the brake-booster hose.
Ok, I gave you the tools, and now I'm gonna leave this for you to chase after and move to the next test. The thing to always keep in mind is that what you are doing is balancing the quantity of fuel being discharged from the transfers with the quantity of fuel being discharged by the mixture screws, always favoring the Transfers. The mixture screws, set to 2.5 turns should always be enough. If the Transfers are not delivering properly atomized fuel, yur gonna have trouble, so then clean out your airbleeds. If the Transfers are lazy, it may be that the fuel level is too low. This has NOTHING to do with fuel pressure, unless the pump is so weak, or is sucking air. This circuit should NOT CARE about the M-rod adjustment., unless the rods are so deep as to restrict the jets which, I cannot imagine being possible, but I tried just in case.
3) Having got rid of the tip in sag; and if at the Neutral to Low shift , the rpm drop is less than 100, and the trans clanging is not objectionable, then we can move on.
Read your Idle-timing, so I can get an idea of where we are at. Factory is about 5*.
Ok lets roll!
Gently put the throttle down about 25% and keep it there. The throttle will be just past the top of the Transfers, and the sucked-down M-rod piston will be controlling the additional fuel required, by the clearance between the M-rod and the MJ. Obviously, if the engine runs out of fuel, then it will lay over before it reaches peak rpm, at that load setting and throttle opening. If it doesn't lay over, go back to about 2200 rpm, or just above stall, and repeat, this time with say a fixed 50% throttle. Continue to about 3500 rpm. If it doesn't lay over, you can bet she is NOT LEAN.
So reset your M-rod adjuster, dropping the rods about a sixteenth and repeat. Keep dropping the rods test after test until it runs out of gas, then put a little back. .
To do this test, I fabricated an adjustable throttle stop, that I could feel as I opened the throttle, so it always opened to the same place.
After 50% and with the pedal slammed down, the spring under the M-rod carrier will have boosted the rods to max travel, and now the MJs are working.
It's important to understand that none of the other systems ever turn off, so at WOT, they are all pumping gas into the engine. At this point, your exhaust system comes into play; in as much as, it better be able to handle the quantity of hot expanding gasses coming out of the chambers.
Let's say they can, and your exhaust system is running at less than 2>3psi. That's all good.
So now, we're testing the MJs.
Without an AFR;
If they are too small, here's how to tell, run it up at WOT in first gear to 3000. Then with the car still accelerating, lift off the gas just a hair. If the car seems to surge ahead, then it was getting too much air, and you need a bigger MJ. Stand on it again, and lift a hair this time at 3500. If the car immediately surges ahead, then it wants more MJ, again. Repeat at 4000. and if it surges ahead again, then still lean. So fix it
Rich is harder to tell. Therefore I always start lean and sneak up on it.
Chances are, that if you are at less than 1000 ft elevation, your 1968 carb will be pretty close.
Ok moving on;
Cruise AFR
4) Depending on your cruise-rpm, and your engine condition, and how willing your car is to roll, will determine how far open your throttle will be at say 65 mph. The throttles will be up on the transfers for sure which are pulling from the mainwells thru the clearance between the M-rods and the MJ. IDK how far open your throttle will be, but I'm pretty sure the MJs will be flowing at least a little. You can thus, adjust your cruising AFR, using the M-rod height. But before you change it, you gotta figure out how to get back to where it is, as testing continues. And if you mark it, then whatever mark you make, it's gotta be permanent, and repeatable.
IIRC I used a dial-caliper, and WROTE the measurement down.
And so, if you move it, this will alter your other driving characteristics, which you just spent several hours setting up. So easy does it.
With fresh plugs and a high-powered ignition system, You can lean the cruise circuit way out, until the engine complains by surging or it takes a lotta throttle to achieve the desired cruising speed.
I suggest you not go that lean on account of the exhaust valves will burn, and the engine may run hot. The additional economy does not pay for the valve job, etcetera. But, my 318 cars have survived quite well at 20 mpgUS.
Now remember; if you change the height of the M-rods, you will have to go back and check for a sag, accelerating at Part Throttle. Where you end up, will depend on your MPG target, and willingness to put up with a less than stellar Part-Throttle, lol.
5) Now we need to talk about timing again.
For cruising, your engine wants a lot of timing, right up until it is one degree too much. The actual amount varies engine by engine, so I can't give you a number except to give you a window that I know works.
This amount depends on your cruise rpm, your car weight, the elevation at which you are traveling, the Cylinder pressure, the fuel quality, the inlet air temperature, the coolant temperature, your convertor slip, oh man so many things. My guess is that your 1968 engine, in excellent condition, running 2.76 gears, running about 65= 2400rpm, and slightly lean, is gonna ask for at least 44*, but may like up to 56*.
Chances are that your distributor, with just 8* initial, cannot deliver more than 30*, at 2400rpm.. This will kill your fuel economy. You have to modify your Vcan to the max to get anywhere near what the engine wants, but if you do, you gotta watch for detonation at part throttle.
I modded my Vcan to get 22 degrees, Another fella here on FABO modded his to 24*, and I don't think they can do any better than that. Lets say you set your Idle-Timing to 8* and your mechanical runs .78* per 100rpm beginning at 900 rpm. Then, by 2400 it will have brought 12 degrees to the 8= 20. Add to that 22 from the Vcan, and I get 42 total cruise-timing, getting closer. But Ima thinking a solid 10 degrees not enough to be optimum. If you spend a lot of time on the hiway, then this needs to be worked on. If only the occasional trip is made, then I'd call it done.
If you change your cruising speed, the max timing will change. For instance, if you installed 2.45 gears, then 65= ~2100, 300 less than before. so right away you lose over 2* from the mechanical system, but with the Vcan set to 22 degrees, the total of 40* may now be too much.
Only you can figure out what your combo wants.
Never give it any more than what it wants, unless you are prepared for the consequences, which can range from having to run better gas, to freshening the heads, to losing an engine to detonation.
I'm not gonna buy you a new engine if you get over-zealous, lol.
Ok so,
Happy HotRodding.
And no, FABO guys, I'm not sorry this post is so long. You can only wish you were retired and could spend all day typing. Put me on ignore already.
