PCV issue

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1966273Valiant

1966273Valiant

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I have a 1966 Plymouth Valiant with an 318 dressed like a cuda 273. It has the cuda 273 valve covers with the stock breather on the drivers side and the PCV on the passanger side. It has always ran rough while idling. Have have chased this for a while. Installed allot of new ignition items and the carb is new and ajusted. I have the PCV connected to the front manifold vacuum on the carb. I got a wild hair to disconnect the PCV and plug the vacuum port on the carb. The car ran much better. The PCV valve moves freely and I dont think there are any leaks around the PCV. Any ideas?
 
plugging the pcv valve off makes your idle a small amount richer. adjust the idle richer
if its as rich as it can go then there is a issue with the carb idle curcuit,air leak some were
or the cam timing is way off. is the cam stock?
 
I agree with the wall. It's probably running lean and plugging the PCV valve makes it a little richer. How are you adjusting the carb? Adjusting it while in gear (with the parking brake on or wheels chocked) will give you the best/smoothest idle. Is that the factory 2 barrel carb? I've ran into a couple of them that I couldn't get rich enough at idle to idle smooth.
 
Thanks for the replys. It is an Edelbrock 1403 (500 CFM). I've ajusted the idle mixture screws with it idling around 600-700 RPM. I turned in each fuel/air mixture screw until bottomed and then ran each out until I got best vacuum. I'm not sure about the cam. Motor was just rebuilt prior to buying the car. He put solid lifters it it, so I hope it's a solid lifter cam! The cap that holds the PCV valve does not have a seal in the bottom of it. Would that cause enough of a vacuum leak to cause a problem? What about going to the next size jet and metering rod? Would that help the idle's lean condition? Thanks again for the help.
 
as long as the hose going to the valve is tight it should be fine. the metering rods and main jets have no affect on the idle unless you have the idle speed screw way out or should i say way in to maintain a idle. this happens when you timing at idle is very low or even a lean idle mixture. if your idle is adjusted to max vacuum and you open the mixture screws more does the engine slow down and drop vacuum or does it get to a point and just stay the same. if it stays the same then your idle circuity is just to lean. the typical amout of idle screw position is between 1.5 turn to 2.5 if its out more then that then your running into a problem. if its less then that then your off idle transfers slot may be too rich. timing and vacuum leaks are the two main reasons for a lean idle but a engine with a farely large rumpety cam will also do this.big cam carb are made to fix this on purpose with there metering intended to deal with the low vacuum that large cams give.the idle fuel orifice can be adjusted carefully on a edelbrock carb with a small drill. if your take out your booster nozzles there are two tubes, its the skinny tube with a small hole in it. the small hole is your fuel orifice. if this is your situation be very careful if you use this to fix it.
 
Like FormulaS340 said it may be the valve is bad, it's supposed to have a spring inside to act against the manifold vacuum so the valve doesn't just stay open all the time.
 
If you are running a solid cam check tappets are not too tight seeing that the engine has been rebuilt clearances might be out.If they are tight it will run rough at idle.
 
MOPEkidD-3 said:
Like FormulaS340 said it may be the valve is bad, it's supposed to have a spring inside to act against the manifold vacuum so the valve doesn't just stay open all the time.
Click to expand...

unless the pcv valve is missing parts it will only pass the same amount of air even if the spring is broken. the spring will close the valve at wide open throttle when the vacuum is very low but at idle there is enough vacuum to hold the pcv at max open. so even if you cant here it wiggle it wont effect the idle mixture unless parts are missing from it
 
I know i'm late here. Alot of good advice so far, but you said you adjusted the mixture screws to max vacuum.......just curious, what were your vacuum readings, may give a better idea of how much cam you actully have?
 
Thanks again for all the responses. I'm getting about 16 PSI of vacuum at idle. As far as the valve adjustment is concerned, I have new locking adjusters and have adjusted the valves to .010 for Intake and .018 for exhaust. The motor seems to running good in every regime other than idle.

If I do drill out the tubes in the carb, what size bit should I start with? Thanks again. Jon
 
Thanks for the info. Jon.

I'm gonna let the others speak to the carb. They'll know more when it comes to getting into the idle circuitry, etc.

Looks by your vacuum readings that you have a fairly mild cam. My only concern is the lash settings may be a little tight. Especially the intake side. 67gt also spoke to this. I would loosen them up a hair and see if it helps. Wont hurt anything except maybe a little noisier. Maybe .016 int. & .022 ext.? You could probably even go a little farther but i would start there and see how she does.

Oh yea.....Nice Valiant!
 
I would try a new valve and re check all the adjustments. The pcv is a big vacuum leak and the carb has to be adjusted accordingly. Disconnect and plug the vacuum advance when you're checking things too. A advance diaphragm leak on an older distributor can caused some rough running. Vacuum choke pull off's can cause problems too. My 273 hasn't ran right since I overhauled it 2 years ago. I've tried 3 different distributors, 2 different carbs and a bunch of different checks and adjustments. I finally figured out what it was. I put in too cold of spark plug. It's worth a check. tmm
 
Rick, I failed to mention that I adjusted the valves while they were cold. I took the stock 273 numbers (.013 and .021) and went .003 less. Do you still think I should loosen them up? Also, I have a PCV valve on the way. Thanks!
 
1966273Valiant said:
Rick, I failed to mention that I adjusted the valves while they were cold. I took the stock 273 numbers (.013 and .021) and went .003 less. Do you still think I should loosen them up? Also, I have a PCV valve on the way. Thanks!
Click to expand...

I think you adjusted the lash spec .003 the wrong way. All the metal expands when it heats up ( steel more than cast iron ).
 
1966273Valiant said:
Rick, I failed to mention that I adjusted the valves while they were cold. I took the stock 273 numbers (.013 and .021) and went .003 less. Do you still think I should loosen them up? Also, I have a PCV valve on the way. Thanks!
Click to expand...

Jon, that will be good to try another PCV valve. When i worked at a Gm dealer ( please don't beat me up :violent1:) it was 25 yrs. ago...lol, aftermarket PCV's caused issues alot of the time and replacing them with stock Ac parts cured the problem. To this day i still dont know why? Many were Fram's.

Redfish is correct, you should add a few thou to hot specs when adjusting cold. I know it's a pain, but when you can get around to it i would reajust to a looser setting. jmo.
 
I agree that the lash on the intake side might be too tight. If you set it at .010" cold you may be hanging valves open when it gets hot. That'll sure make it idle rough.

btw; vacuum is measured in inches, not psi. Total opposite. It's just terminology and most guys would know what you mean anyway so it's not a huge deal. Just sayin
 
1966273Valiant said:
I re-adjusted the valves .016 Intake and .024 exhaust. Didn't seem to make any difference. Still waiting on the PCV valve.
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Sorry to here the results wern't positive. Props to the guys who thought it wouldn't help.

Seriously, not trying to cover my rearend, but i still believe your better off with your valve clearence where it is now then before. Fingers crossed for'ya on the PCV valve.
 
I do appreciate the advice on the valve clearence. I did have the lash to small. I'm hoping the PCV will make a difference as well. Thanks again.
 
1966273Valiant said:
Thanks again for all the responses.

If I do drill out the tubes in the carb, what size bit should I start with? Thanks again. Jon
Click to expand...

i could nt tell you what size. need to take your apart measure it and move to the next size up. but i will look and see what i have on one of mine just for a comparison. the size is different for different carbs. air bleeds affect it cam, emissions.

on the pcv valve i dont know how the factory decide which one to use but they are specific to model and manufacture. but even then the differences are small. its basically a sized air leak. so i would think that one for a slant would leak less air then a v8 but this is a guess on my part
 
1966273Valiant said:
I do appreciate the advice on the valve clearence. I did have the lash to small. I'm hoping the PCV will make a difference as well. Thanks again.
Click to expand...

I re-read this thread and I see you have a 318 with a solid lifter cam and of course adjustable rockers. Valve lash does some goofy things with these engines. I have used different specs for my 273 and it runs best when I adjust them on the loose side of spec. The motor is pretty noisy when I start it cold and quiets down a bit at operating temp but here is still a bit of clatter even when warm. It seems to like it that way. It is crisper and revs easier. tmm
 
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