pvc
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=1478
Printed Date: November-22-2024 at 2:36pm
Topic: pvc
Posted By: Guests
Subject: pvc
Date Posted: April-19-2005 at 12:22am
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Curious what exactly the pvc valve does as it is related to vacum. How do you tell if it is bad? This is in my 1980 American skier with a 351 commander. Sorry guys I know this is a cc site but at least its the same motor. Thanks
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Replies:
Posted By: jimbo
Date Posted: April-19-2005 at 3:26am
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I'm by no means an expert but this is my understanding. The PCV should have a hose going to the flame arrestor. When air is being pull through the carb, it will also pull air through that hose, through the valve and from the crankcase. The idea is any volitile hydrocarbons will be sucked into the carb and burned by the engine. The way I've always undersood to tell if it's bad is to pull it out of the crankcase with the engine running and stick your finger over the hole. If it sucks it's ok. I've also heard you completely disconnect it and shake it. You should hear the valve rattle around. If the valve doesn't move freely, replace it. That's my understanding. Maybe somebody that knows what they're talking about can chime in.
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Posted By: David F
Date Posted: April-19-2005 at 12:50pm
Jimbo:
You have it about right.
Gordonw:
Just go ahead and replace the valve for peace of mind...they cost just about nothing.
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Posted By: captan1
Date Posted: April-19-2005 at 2:09pm
PCV hose connects to the carb PCV fitting or the PCV plate under the carb.
The breather hose goes to the flame arrestor and allows air to enter the engine to equalize pressures in the engine circulating internaly though the engine to the PCV valve.
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http://www.correctcraftfan.com/diaries/details.asp?ID=452&sort=&pagenum=1&yrstart=1978&yrend=1978 - 1978 Ski Nautique
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Posted By: 79nautique
Date Posted: April-19-2005 at 3:13pm
Jimbo is close he has the checking part correct but missed the connection location and function. It has to be hooked to a vaccuum source (i.e. carb base plate, carb port, or intake manifold port) the flame arrestor is not a vaccuum source. Also the PCV stands for positive crankcase ventilation and helps removed moisture from the crankcase. If their are hydrocarbons present in the crankcase it is not good because that means there is unburned fuel in the crankcase.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: jimbo
Date Posted: April-20-2005 at 12:48am
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I think y'all are right. On my Commandar engine, there are two hoses that go to the spark arrestor but now that I think about it, neither are for the PCV. One is attached to the valve cover breather and I can't remember where the other attaches. I'd check but I'm 200 hundred miles from home. But...this brings up another concdern on my part...my breather doesn't fit snugly on the valve cover. Is this a problem? should there be a gasket or something that should make it fit tighter? It's pretty loose.
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Posted By: Guests
Date Posted: April-20-2005 at 3:06pm
THANKS,
Yes,thanks for the input. Trying to figure out the rough idle problem. Runs great at anything but idle. loads up and dies. I'v replaced the carb, distributer mbi, all hoses, plugs, wires, coil, exaust flaps and everything else. Been going on for years. Taken it to a marine mechanic who says the motor is perfect and compression is good. I'm pretty much a mechanic myself but this one is killing me. If this thing stalls another season my wifes gonna divorce me! Not the power vale or cab ajustments either. Everything is new or has been replaced.
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Posted By: skyhawkflyer
Date Posted: April-20-2005 at 3:58pm
Holley's have a screw on the bottom of the baseplate that you can screw in ever so slightly to crack open your secondaries that will help engines with a rough idle. Depending on whether the secondaries are cracked or not cracked could be the problem.
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Posted By: skyhawkflyer
Date Posted: April-20-2005 at 4:00pm
Also, a bad pcv will show up as white oil sludge on the bottom of your oil cap and pcv valve, and possibly on the upper portion of your dipstick.
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Posted By: 79nautique
Date Posted: April-20-2005 at 4:06pm
Look for a vaccuum leak around the base of the carb or where the intake and heads meet. Take a can of carb cleaner, brake cleaner or even propane and spray around the base of the carb and listen for the RPM to pick up a little if it does then you have found the leak. Also double check the timing and maybe even give it two degrees more of advance of base timing.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: Guests
Date Posted: April-20-2005 at 6:07pm
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I replaced the intake manifold gasket a year ago and it has no leaks. Replaced the carb to base and base to intake gaskets. i'v checked for leaks with carb cleaner every where. Sent off the mallory mbi modules to make ssure they are functioning correctly also. Next to my buddys boat mine sounds like it has a frog in it's throat at idle. His sounds dry and clean. Mine has a slight vibration and just looks like it's working to run. Timing is right on and the advace is good. The weights are returning, the needle and seat is fine, power valve is good. Could the exaust be plugged? Do the manifolds fill with rust? I'v never had them off. The are the Commander alloy ones.
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Posted By: JEFF KOSTIS
Date Posted: May-05-2005 at 1:08am
As well as keeping the crankcase moisture free, I believe the main purpose of the PVC valve is to reduce engine emissions. The "blow by" in the crankcase gets returned back into the intake instead of going in the air. Some old GM Engines like our OMC 225 in the 74 Mustang has crankcase vents on each valve cover with NO PCV vales-just a elbow that looks like one. They go to the flame arrestor (no manifold vacuum-not as effective).
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MUSTANG COUNTRY
http://www.correctcraftfan.com/diaries/details.asp?ID=1074 - 1974 Mustang 17
http://www.correctcraftfan.com/diaries/details.asp?ID=1093 - 1976 Mustang 17
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Posted By: GottaSki
Date Posted: May-05-2005 at 9:28am
That shudder at idle is often a pluged idle circuit.
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole
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Posted By: David F
Date Posted: May-05-2005 at 12:43pm
Ok, I guess I will chime in.
Gordon:
Yes, the exhausts can get plugged. It is not uncommon for old rubber exhaust hose to collapse internally and significantly block the exhaust. Some say this is by design to snuff out an overheated engine before damage is done...I dunno, doubt it. But anyway, they can and do collapse internally. Usually you would notice this as loss of power but not necessarily idle quality...but who knows.
As far as PCV, my take is that you should temporarily disconnect this and plug the port at the carb to ensure this is not the cause of your idle problem. Many auto engines of old had the PCV plumbed to the air cleaner (i.e. spark arrestor on boats). The main purpose of the PCV is to allow blow-by gases to escape the engine thereby avoiding built up pressure that would cause seals to leak. And to allow unburned fuel and moisture a path out of the engine via evaporation.
IF the carb is as perfect as you say it is (I would borrow another carb to test), then I agree with 70nautique and I would play around with the timing. Forget the specified setting and set it by ear. Advance the timing until you achieve the highest rpm AND smoothest idle. Then back it off slightly to avoid pre-ignition. Test it under load. If it pings, then back it off slightly more.
Good luck.
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Posted By: nms1991
Date Posted: May-08-2005 at 6:05pm
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Idle quality is effected by mixure adjustment , engine not firing evenly, to low of an idle, improper timing, low voltage going to the ignition circuit. Ther first and easiest thing to do is adjust the idle mixture screws in or out. also check to see if your choke is fully openingwhen warm.But please do not time by ear seen to many engine failure due to timing by ear use a timing light to confirm timing. Also check to see how much vacumme the motor is pulling at idle, it should be 15 to 18 inches of mercury steady.
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