Hard to start, found problem. Now Why?

Had a problem after new rebuild on 72 340 engine. Block 0.030" over, 9.5 CR, hughes hyd cam, stock thermoquad, stock electronic distr. Got engine started quickly for the cam breakin and everything went well. Later when cold, tried to start and it would crank but wouldn't even pop. Went thru everything, had fuel and spark but not strong fire. Compression test showed all 8 at 175+ psi. Finally found volts from ignition switch was only 10v. Replaced ignition switch now 12v, but would only start with shot of starting fluid. Tried a proven ecu & distr too & check for vacuum leaks. Vacuum at idle 15 in-hg and timing 18*initial. Finally thought why did start for breakin? Only thing different was that I had a plug on the carb port for pcv during startup. Sure enough when I plugged that port, it will start on gasoline. So know the problem source.
But WHY won't start with pcv hooked up. Pcv seems good. Sure glad I had that plugged for first start. Would appreciate help to know why.

Pcv creates a controlled vacuum leak, too lean at idle, with pcv attached greater leak way too lean?

Is the pcv valve stuck? Shake it to make sure the valve is loose and moves freely. If not, then replace it.

Cleaned pcv valve, if blow backward it blocks flow. Is there diffrnt valves with restricted flow. I am going to put a restrictor in hose to limit, but that doesn't seem right either.

Check the PCV with pump vacuum gauge. They are not all equal and often you have to find the one that works for you Combo.

3406pk said:

Vacuum at idle 15 in-hg and timing 18*initial.

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That is a nice set-up you have, The PCV circuit is probably Ok.
My first suspect is the 18* initial timing. When you do this, to keep the idle speed in check, you close the throttle with the speed screw. Well that shuts off the low-speed system which is the transfer slots. So then the next thing you do is open up the mixture screws to get the thing to idle.
So now when you shut it off, the metering rods jump up. With a properly set up transfer slot exposure, the engine is now set up to start. There is not enough vacuum during cranking to pull the rods down, so the transfers will puke fuel right away and the engine springs to life. And immediately the Metering rods get sucked down, and away it goes.
But if your throttle blades are too far closed, instead of the transfers delivering fuel, air goes into the slots above the throttles, and exits below, but with either very little or no fuel in it, so you will have to give it several pumpshots to deliver starting fuel. Or; as in your case, it starts on starter fluid.
That's my take.

So, the fix is simple; just open the throttle blades a lil more
and if the idle speed is too high, then simply retard the timing.And reset the mixture screws.And make sure the Vacuum advance can is on the sparkport and NOT affecting the idle timing. The usual recommendation is to disconnect it until after the power-timing has been finalized.

If that don't get you back in action;
make sure the accelerator pump works, and then
check the float level, and make sure the liquid level is normal or ever so slightly higher than normal...... but NOT lower than normal.
Don't get hung up on idle-timing, let it be what the engine wants. Right now, IMO, it is telling you it wants more gas, which at start-up,without choke, is the transfers and idle ports. Like I said; if the warm idle-speed gets too high, just retard the timing.
After you get her starting nicely...... then later, you fix the power-timing.

If you inadvertently set the idle timing with the Vcan on manifold vacuum, then during cranking, the timing will be fully retarded, perhaps all of those 18* couldda been in the Vcan, and now all you got is zero advance or TDC timing. Don't do that. Now you got two problems.

Thanks for input AJ/FormS. All timing was done w VacAdvance disconnected and set to maximize vacuum from port on carb at idle of 800 rpm. The idle screws are turned out only 1 turn. The accel pump squirts fine. I will see what the metering rods do during start. I do know that at idle they are pulled dowm tight. I have tried start w throttle closed and partly open; neither made difference with pcv hooked up. I will try different pcv and also see what vacuum does during start. Thanks to you and others that replyed.

800Rpm, with idle vacuum of 15" points to a small cam, so that 800 is part of your problem right there.

Set your timing to 10 degrees, and 700rpm, or

Remove the carb, flip it over, keeping the throttle on the curb-idle screw, Then reset that idle screw to make the transfer slots look like a square. Flip it back up, install it and open the idle mixture screws to 2.5 turns. Leave that curb-idle screw alone! for now.
Start it up. Warm it up.
Set your idle speed with timing, to 700rpm with a cam 230 to 240*, to 650 with one between 230 and 220, to 600 with smaller than 220.
Do not exceed 14* Idle timing. Then shut it off.
Now see how it starts.
Wait a couple of hours and try again.
Depending on the size of your cam, you may need more idle-air bypass than the PCV can supply, to get the idle-speed up satisfactorily. After you get it ballparked,if it is still lean, then you can begin cranking up the speed 1/2 turn atta time, counting and recording the adjustments, but no more than 1.5 turns total, AND with each adjustment, you will have to revisit the idle-air bypass, the mixture screw adjustment, and the timing.

If you find yourself adding transfer slot fuel, and simultaneously subtracting mixture screw adjustment, then you are just trading one for the other. Go back to the beginning, that is why you recorded your adjustments.
If you failed to record your adjustments, you will have to ;

Remove the carb, flip it over, keeping the throttle on the curb-idle screw, Then reset that idle screw to make the transfer slots look like a square. Flip it back up, install it and open the idle mixture screws to 2.5 turns. Leave that curb-idle screw alone! for now.

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Start it up. Warm it up................................
At some point, it may be that you hit a wall, that at 14* idle-timing, the idle speed is just too slow. This is when you start bringing in by-pass air.Cuz the PCV is not supplying enough.
Also at this point, if you pull the PCV out of the hose, the engine should stall. But if you add just a lil air to that hose, in addition to the PCV, the rpm should go up. This is how you know to start adding idle-air. But don't add too much atta time cuz when you put fuel into that air,as tou are going to have to, the speed might go too high, leaving you with just timing retard to get it back down. And I know how much guys hate running small idle-timing numbers. As for me; I don't care, idle timing is just for setting the idle speed and quality. The more retard the better cuz the engine starts to proclaim it's size. Once you get it down to 500rpm in gear@ 5*, well even a 223* cam sounds pretty good down there.
Of course this entire story depends on the fact that the engine is not getting any air from anywhere, except where it is supposed to be getting air from. Same for fuel.

Got one of the Wagner adjustable PCV valves. It used to be that there were CORRECTLY ENGINEERED pcv valves for all cars but over the years the selection has been reduced almost to the point of a one size fits all situation. Even if you could get on that was made for your stock engine if you've changed you cam, intake, heads, carb etc, then even it's not right. This PCV is completely adjustable and has adjustments for both idle and cruise. It's not cheap but it is adjustable for YOUR car

High Performance PCV Valve Shootout – Flow Test Results – M/E Wagner Performance Products

3406pk said:

Thanks for input AJ/FormS. All timing was done w VacAdvance disconnected and set to maximize vacuum from port on carb at idle of 800 rpm. The idle screws are turned out only 1 turn. The accel pump squirts fine. I will see what the metering rods do during start. I do know that at idle they are pulled dowm tight. I have tried start w throttle closed and partly open; neither made difference with pcv hooked up. I will try different pcv and also see what vacuum does during start. Thanks to you and others that replyed.

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Did you ever find the magic PCV?

512Stroker said:

Did you ever find the magic PCV?

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I put on a new one that was specific to that year engine. I also had a restricted orfice in the hose, but it would not start when cold. I capped carb port and it started. Took car out for cruise today and it ran like a top. First time out in 8 years. So I guess that I can live with it. Thanks for checking.

3406pk said:

I put on a new one that was specific to that year engine. I also had a restricted orfice in the hose, but it would not start when cold. I capped carb port and it started. Took car out for cruise today and it ran like a top. First time out in 8 years. So I guess that I can live with it. Thanks for checking.

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So let me understand
You are not running a PCV at all?

That's right.

3406pk said:

That's right.

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How many oil leaks do you have?

512Stroker said:

How many oil leaks do you have?

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None

Yet;
how are you ventilating the blow-by gasses?
Typically;
If your car is calibrated for a PCV, but you don't run it;
then you will have to open the throttles further at idle to give the engine back the missing air. But this puts the throttles further up the transfer slots, so she goes rich. In compensation, you close the mixture screws, to get some semblance of idle quality. Then, it idles, but as soon as you tip the throttles in, it goes lean because of the missing idle fuel.
But, it is possible to get those back closer to synchronicity by advancing the timing. This works if you have a loose convertor. So this advancing speeds up the engine, and makes the low-speed operation a lil more powerful. So now you can lower the idle-speed, which shuts off the transfers, and in compensation, you can open the mixture screws back up.
But this will increase your low-speed power, because you have moved the point of max pressure closer to the best point of power delivery to the crank. Depending on the rest of the combo, this could be between; a great idea and a lousy idea.
Your timing might now be around 18/20* at idle. If you have a loose TC this will be fine. But with a lower stall TC, this might not be. Because you have 15/17 more degrees to feed it by ~3200 rpm, and if the engine has to work in that band, you might put her into detonation.
Or if you have a manual trans, the low-speed operation is gonna be jumpy as the powerful individual pulses hammer the crank.

But the real problem comes at WOT, when the blow-by gasses are the highest; they have to go somewhere. If you have not made provision for that, then they will find their own way out. And if that happens to be by blowing out the rear cam plug; you are gonna be one cranky person.

If you are sharp, you will have already seen,in this post, the reason for your hard-start, and why it went away after plugging the PCV port.

But, right now, I gotta go to work.

Years ago, engines did not have PCVs. Instead, they had road-draft tubes. An Engine would have a tube leading from the top of the engine down to about 6 inches from the road, and slash-cut, so that the air passing underneath the chassis, would draw out the blow-by as the car moved down the road. Idling at a stop,, they would sit there huffing the smoke into the air and stink up your clothes, and interior.
The point is, that the crankcases were still ventilated.
And the carbs were not calibrated for PCV.
So the conclusion is that your engine does not absolutely need a PCV; but it absolutely does need to be ventilated.

Now;
if you have a carb calibrated for a PCV, such as yours, and it has a starting problem that goes away when you plug the port, you only need to think about it to quickly conclude that your engine wants; a lil less air, or a lil more fuel, during cranking. When you plugged the port, you forced the engine to pull harder on the transfers, and on the idle-discharge ports. So it got the fuel it needed. There are better ways to meet the engines needs during cranking than plugging the PCV port.
The PCV is just a spring-loaded, variable-flow, device,with 4 modes of operation;
1) it has a fixed minimum flow, in response to a high vacuum
2) it has a fixed maximum flow in response to little to no vacuum
3) in between those two it has a variable flow, as calibrated by the spring fighting vacuum
4) it has anti-backfire protection to prevent the crankcase from exploding if there is a backfire in the intake with the throttles closed.

During cranking, there is no manifold vacuum, so the valve will be at maximum flow. No big deal for a carb that is calibrated for it, on a healthy unmodified engine, running the factory timing....... because the throttle will be in the right place for the atmosphere to push fuel out the float bowl, up the idle wells, over the top where it spills out the transfers; to be mixed with the air that is pushing past the throttles. Simultaneously, additional fuel will be discharged from the idle ports, and Air from the PCV system completes the picture.
This assumes three things;
1) the float bowl is connected to atmosphere via the bowl vent, and
2) there is fuel in the float bowl, at the correct level.
3) that the throttles are sufficiently open

That # 3 is the first thing that gets screwed up when somebody cranks up the timing. More timing increased the engines efficiency, so the idle-speed goes up. So in response, you back out the speed screw to slow the engine down. But now the fuel supply from the transfers is wrong, so in response, you open the mixture screws. Now it idles at a reasonable rpm.
But your engine doesn't idle all day, so when you put it into gear,this loads up the engine, so she slows down maybe a hundred rpm, maybe 150. and this slows the airflow past the nearly closed theottles, the fuel stops flowing, the mixture screws are maxed out... so she stalls.
So what do you do? Well heaven forbid you put the timing back; no, you increase the speed screw setting, which restores the fuel flow, but now the neutral idle is ~150 higher, so when you put it into gear; BANG, the U-joint take a hit. And air flow again slows down past the throttle blades, but at least id doesn't stall. But it sure pulls hard while waiting at a red-light.
So what do you do? Well heaven forbid you should retard the timing a few degrees; no, you go buy a hi-stall TC, cuz you're getting tired of the constant banging into gear, and the constant pull.

Now lets complicate that with a plugged PCV port shall we?
Now, Unless you have a big cam of say 230* @.050 or more, or for whatever reason your idle vacuum is so low as to put the PCV in or near WOT mode, your engine wants that missing PCV air. You can tune the idle to run without it. But as soon as you tip in the throttles, the engine will go rich. Once on the mains, she may not noticr the missing air, cuz you can downjet in compensation. But at Part Throttle, while on the transfers, the engine will be rich all the time, and you may wonder why your little engine gets such lousy fuel mileage, in spite of the crazy timing you keep throwing at it. Heaven forbid you should put the timing back to stock.
The timing your distributor controls is your power-timing. It is only correct at WOT; And only if you have made it so.
All iron-headed gas engines will want about the same ignition timing after ~3500 rpm. I mean give or take a couple of degrees. But the problem is below that. Every combo is different. But the one thing that is constant is that no matter what timing curve you work out below 3500rpm, it will only be right at WOT, and only if you make it so.
The rest of the time, your V-can is in charge.
Except at idle, because the spark-port is dead.
At idle, the transfer exposure under the throttle blade is in charge. Set that right, and use the timing to set the idle speed, and twiddle the mixture screws for best idle and freedom of tip-in hesitation.
If your mixture screws end up to far open, then increase the transfer fuel, and back up the mixtures. If the idle-speed rises too far, retard the timing.
If your mixture screws end up too far closed, reduce the transfer fuel, and open the mixture screws. If the idlespeed is too slow, increase the timing.
Of course this assumes the PCV is working, and in the low-flow mode. and that the doggone bowl-vent is open. and the doggone fuel level is correct.
Ok bed-time now,lol.

But if you have a manual trans, that's a whole nuther story,lol.