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Anonymous
Guest
I can't seem to get it to run smooth with the PCV hooked up to the adapter. Plug it and it runs great. I cleaned the valve (it's pretty new) and playing with idle and mixture.
Any ideas?
Any ideas?
I assume you are talking about running smooth at idle? Sounds like a bad PCV valve, because at idle it should be closed and not leaking any air into the adapter. There should be no change between the two configurations you are describing at idle. At higher vacuum levels, the PCV valve should open but you won't feel the vacuum leak it causes at those higher engine speeds.
Ronbo":zygfr3nd said:
So I've wondered in the past...what "specs" do you look at for the PCV valve to be correct? Just physical dimensions? The reason I ask is I switched from one that is specified for my engine to one that looks very similar but has a right angle stem on the top for the hose instead of pointing straight up.
It seems to be working fine but I just wondered if there are interenal differences like how much vacuum it takes to lift the valve, etc.
kspageddie
Well-known member
From what I remember from my old days of working the parts counter..There is a spring tension difference in the PCV valve. Normally, there are numbers stamped on the bottom of the PCV valve. (1,2,3 etc). This should note the difference in spring tension. If your new 90 degree PCV valve has the same spring as the old one then you should be just fine.
mustang6":1ww3b93q said:
I disagree on the pcv operation - pcv systems ARE "vacuum leaks", just restricted ones that are compensated for at the carb. At idle, it should be pulling quite a bit of vacuum on the crankcase, enough to feel with the palm of your hand if you put it over the oil fill hole. The pcv valve IS a restriction on the "vacuum leak", so if you pull the pcv valve out of the valve cover your idle will change because you're letting a lot more air into the intake manifold, changing the mixture.
A pcv valve is just a one-way spring-loaded checkvalve that will allow a restricted amount of air/fumes to be drawn out of the crankcase into the intake manifold, yet keep fuel/air mixture from going the opposite direction (manifold into crankcase) which could have explosive results.
Sounds like you've either got a leak in the pcv hose, or right at the grommit in the valve cover, or maybe your oil fill cap isn't sealing/filtering well. Most often a pcv valve will "fail" because it gets plugged/gunked up with all the fumes going through it, but sometimes the spring gets weak and it loses its ability to restrict the flow.
Either way, a new valve, hose, and grommit shouldn't be more than $10 altogether from your local partsmonkey. I changed from the "straight-up" one to a "right-angle" one recently with no problems - prolly the same p/n redfalken used...
Look over the PCV hose very carefully. My original hose which was shaped like an "S" and went from the PCV valve at the rear of the valve cover, between the valve cover and the carb base and then curved around to go on the nipple coming out of the carb spacer. It visually looked OK until I bent it a little and there were cracks all over it. I bought the PCV valve with the 90 degree stem and put on some new generic hose and it made a huge difference in performance.
I stand corrected, even at idle there is some vacuum loss to the PCV. But that is not the highest PCV flow condition (greatest vacuum leak). See the operation modes below:
PCV Valve Operation Modes
High Manifold Vacuum, Low Crankcase Pressure
This mode corresponds to the idle condition. The pressure differential presses the disk in the PCV against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold. This flow is kept to a small amount so as not to decrease manifold vacuum level.
High Manifold Vacuum, Moderate Crankcase Pressure
This mode corresponds to over-run (coasting in gear with the throttle closed). In over-run, manifold vacuum can exceed 20 in. Hg. Under these conditions, the disk in the PCV is pressed against the intake manifold side seat. A small amount of flow is passed through the metering slots. Blow-by is minimal as combustion is at a low level.
Low Manifold Vacuum, Moderate Crankcase Pressure
This mode corresponds to part-load conditions. When the pressure differential becomes greater than the spring tension against the disk then the disk floats between the intake manifold side seat and the crankcase side seat and a large flow of gasses can be passed through the valve. This is the normal operating position of the valve when cruising under light to moderate part-load conditions.
Very Low Manifold Vacuum, High Crankcase Pressure
This mode corresponds to heavy load to full-load conditions. Here, most of the pressure differential that opens the PCV valve comes from crankcase pressure. Under heavy load, blow-by gasses are at a maximum and the PCV valve is open to permit the gasses to be passed to the intake manifold for re-burning.
High Manifold Pressure
This mode corresponds to an intake backfire condition. Here, the high positive pressure in the manifold presses the disk tightly against the crankcase side seat, sealing the PCV valve and preventing flame propagation into the crankcase to prevent an explosion.
PCV Valve Operation Modes
High Manifold Vacuum, Low Crankcase Pressure
This mode corresponds to the idle condition. The pressure differential presses the disk in the PCV against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold. This flow is kept to a small amount so as not to decrease manifold vacuum level.
High Manifold Vacuum, Moderate Crankcase Pressure
This mode corresponds to over-run (coasting in gear with the throttle closed). In over-run, manifold vacuum can exceed 20 in. Hg. Under these conditions, the disk in the PCV is pressed against the intake manifold side seat. A small amount of flow is passed through the metering slots. Blow-by is minimal as combustion is at a low level.
Low Manifold Vacuum, Moderate Crankcase Pressure
This mode corresponds to part-load conditions. When the pressure differential becomes greater than the spring tension against the disk then the disk floats between the intake manifold side seat and the crankcase side seat and a large flow of gasses can be passed through the valve. This is the normal operating position of the valve when cruising under light to moderate part-load conditions.
Very Low Manifold Vacuum, High Crankcase Pressure
This mode corresponds to heavy load to full-load conditions. Here, most of the pressure differential that opens the PCV valve comes from crankcase pressure. Under heavy load, blow-by gasses are at a maximum and the PCV valve is open to permit the gasses to be passed to the intake manifold for re-burning.
High Manifold Pressure
This mode corresponds to an intake backfire condition. Here, the high positive pressure in the manifold presses the disk tightly against the crankcase side seat, sealing the PCV valve and preventing flame propagation into the crankcase to prevent an explosion.
rust collector
Active member
Just a thought...
Shouldn´t you use the pcv from the car the carb is from?
Since the carb is "adjusted/calibrated" for xx vacuum leak?
Shouldn´t you use the pcv from the car the carb is from?
Since the carb is "adjusted/calibrated" for xx vacuum leak?
A
Anonymous
Guest
I swapped valves and it's working a little better now. New hose, the filler cap is really a breather that draws filtered air into the crankcase from the air filter housing. It worked fine with the stock carb, so I think I either need a different pcv, or to adjust the carb to suit.
Doesn't matter, my carb from stovebolt is junk. Loose throttle shaft and stripped screw to the choke linkage that is causing it to bind. I have to get ahold of him and see if I can swap them out. I made some modifications to the throttle attachment point that aren't reversible. Testing will have to wait. If he has a new carb for $10 or $20 more like he did when I ordered mine, get it. It's not worth the hassle. I also didn't know these were old stock rebuilds. I was under the impression they were fresh and rebuilt at Stovebolt by someone who cares. Oh well, it'll have to wait.
Doesn't matter, my carb from stovebolt is junk. Loose throttle shaft and stripped screw to the choke linkage that is causing it to bind. I have to get ahold of him and see if I can swap them out. I made some modifications to the throttle attachment point that aren't reversible. Testing will have to wait. If he has a new carb for $10 or $20 more like he did when I ordered mine, get it. It's not worth the hassle. I also didn't know these were old stock rebuilds. I was under the impression they were fresh and rebuilt at Stovebolt by someone who cares. Oh well, it'll have to wait.
A
Anonymous
Guest
Rust collector had a good idea. I'll try getting a PCV from a car that came with this carb. I have a box of valves I bought at a yard sale. Quite a few different numbers, I'll try them all. The one on there now is a right angle one.
