Bad Positive Crankcase Vent Valve?

[TomG]

HISTORY: I have a 2008 MMI rebuilt A-4. The boat sat on the hard for nearly two years prior to my purchasing her. While bringing the boat back to life, I did several upgrades including adding the Indigo PCV valve kit to eliminate any blow-by in the cabin. Other than an unrelated "water in the fuel" issue last month, the A-4 has performed beautifully. One of my latest upgrades was to add a tachometer, in addition to upgrading all the old gauges. I always thought that the engine idled high, but without the tach, I wasn't sure. After installing the tach, I discovered that the engine wouldn't idle below about 1,100 rpms. I ran my symptoms by the forum here and received very good input. I thought I had narrowed the issue down to a clogged idle port or ports. Last weekend, I was very fortunate to have dinner with Don, Bill, and Ed at Hanley's "raft up" in C-Town. I mentioned my symptoms to the group and we discussed various causes. I decided that I would rebuild the carb with new jets, gaskets, and give it a good thorough cleaning. The rebuild kit arrived from Ken at MMI in less than 24 hours (how do they do that!?) and today I was able to rebuild and replace the carb.

Feeling certain this rebuild had solved my problem, I fired her up and tried to get her to idle lower than 1,100 rpm. No es bueno.

I then remember Don suggesting that I take the PCV out of the equation to eliminate that as the source of the problem. Oh, that I had done that first!

With the PCV out and the line blocked, the A-4, she ran like a Singer Sewing Machine. A wonderfully smooth idle at 750 to 800 rpms was like music to my ears.

With the PCV valve back in and the opening covered with my finger, she runs perfectly. With the PCV open, she will not idle.

QUESTION: Can a PCV valve be bad? Is there a way to check?

I'll shoot Tom at Indigo an email to see what he knows, but I thought I'd run this by the forum for your input and experiences.

Thanks,

[Dave Neptune]

on a limb here

Tom, first thing I would do is check to see if you indeed have the correct valve or borrow one from someone in your area to check with. If the vavle is for a larger displacement it will allow more air in making it more difficult for a smaller engine. And if it is faulty it will be doing the same thing by not closing down enough for idle stability.
Another thing to add is has your butterfly in the carb been removed for cleaning? If so or if it has even been loosened the butterfly is not an easy thing to get re-seated in the proper spot so the throttle can actually "close" all the way with the idle stop screw backed all the way out. If it doesn't seat properly first it won't close enough and the gap that sucks the fuel through the idle ports may not be oriented properly which will disrupt fuel draw at idle.
It is normal for an engines idle to be turned down (and the mixture richened a tiny bit for the additional air) to compensate for the positive drawing of air into the manifold that is un-metered by the carb. This bit of air needs to be corrected for by additional fuel and a backing off of the idle speed.

Dave Neptune

[chiron]

I just checked my installation instructions for the PCV valve and the idle mixture screw should be turned 3/4 rotation out from its original setting of 1 1/2 turns out from fully seated to compensate for the added air. Just as Dave has stated.

[sailhog]

I installed an Indigo PCV valve two weeks ago, and I'm having the EXACT same problem.... The idle was all messed up when I ran her for the first time. I'm currently 1.5 turns off the seat with the mix adjustment, but will correct to .75....

Good luck, Tom!

[Bold Rascal]

Good to know

Looking at installing a PCV set up as well. Let us know how things turn out, or "1/2 turn in" as the case may be.

[Dave Neptune]

A correction

When installing a PCV system there is a bit of tweeking necessary to compensate for the additional air. As you are adding "un-metered" air flow to the manifold two things need to happen for a smooth stable idle. First the idle speed will need to be decreased with the idle stop screw to compensate for the "un-metered" air being added AND the idle mixture screw (an air valve on the A-4 carbs) will need to be richened a bit which is closing down the screw (clockwise) to decrease the air and increase to fuel being drawn for idle. You will still need to adjust the idle by fine tuning with the mixture screw once warmed up!!

I have been using the PCV for many years and my engine will idle a 500 just fine although I set it at 700 just because.

Dave Neptune

[chiron]

correction accepted

Dave is correct. The coffee had not kicked in when I was reading the PCV valve instalation instructions this morning. The directions read "

"Rotate the idle mixture Adjustment screw (found on aft end of carburetor just below the flange which mates the aluminum spacer plate) clockwise (in) until it is seated fully closed (probably about 1 1/2 turns). Now rotate the screw counterclockwise (out) 3/4 turn out from its fully seated position. This should be a good starting point".

Sorry for the confusion. If only I could configure my alarm clock with and IV drip of caffeine.

[sailhog]

Dave is right.... Just got back from the boat. Dialed the idle mixture to .75 off the seat, and idles much better. Still need to dial back on the throttle setting to lower the rpms....

Boat smells great!

[TomG]

Thanks for all the info and suggestions. A little more info on my problems: The PCV valve is brand new from Indigo. It's a model 2088. With the PCV valve in the system, I can't get the engine to idle at all below 1,000-1,100 rpms. IF I cover the PCV valve with my finger, it idles normally. With the PCV valve completely out of the system (and the hose closed off), it idles normally (7-800 rpm). To get the engine to idle - even at 1,100 rpm - with the PCV valve installed, I have to close the idle mixture valve all the way (i.e., full rich). If I back the idle mixture off full rich, I can get maybe a quarter to a half turn before she really starts to labor... like a big vacuum leak.

Bottom line, she runs perfectly without the PCV valve. I can't get it to idle smoothly below 1,100 rpm in any way with the valve installed. I'm thinking bad PCV valve, but that seems so remote.

I think I'll stop by Autozone on the way home (when I get back home) and get a new 2088 PCV valve and try that. I still haven't heard anything from Indigo.

[ndutton]

**Blasphemy Alert**

My comment goes against the grain as it seems 99% favor the PCV system.

Why? Do you have excessive blowby that cannot be managed by the stock slash tube? Isn't such blowby an indication of internal wear (rings, valve guides)?

Why? Why does the stock slash tube work fine on so many engines for decades? All of a sudden we need a PCV system??

The PCV system by its design is an intentionally installed vacuum leak. A properly sized PCV valve attempts to calibrate the leak according to manifold vacuum but it still introduces unmetered air into the combustion mix thereby affecting the carburetor's ability to deliver a proper fuel/air mixture.

I see the necessity of a PCV system as a stopgap measure in anticipation of future repairs/rebuilding. Sure, if you need it, go for it but know it comes with mixture and carburetor adjustment consequences. If we had computers, injectors and MAP sensors we wouldn't have an issue but we don't, we have carburetors.

Okay, blaspheming is done. You may return to your regularly scheduled programming.

[dvd]

Alternative to PCV

Instead of a pcv valve which I have and may be causing a vacuum leak, why cant you just extend the original hose set up out the back of the boat. This would alleviate the need for a pcv system because any noxious blowby would just be sent out the back.

DVD

[Ball Racing]

If you leave the pcv valve off the engine, but still connected to the manifold, it "is" a leak.
But,,,,, sealed into place on the block, it can only pull as much air as the block has in it.
If the engine has any blow by, it has a pressurized base, and would "supercharge' the manifolds vacuum, and not make it actually lose any.
What lets in some extra air would be the oil cap which is vented on the bottom side, and would allow more of a vacuum leak.
Most cars have a sealed oil fill cap, and if you remove it with the engine running, you will hear the rpms drop, you can place you hand over the opening, and feel the vacuum, and hear the rpms come back up.
A vacuum placed on the base of a motor make for better ring seal.
Because the more pressure on the bottom side of the rings, the harder it is to seal them pushing from the top of the rings.
The more pressure in the base, the more pressure the piston has to force it's way into.

In short, you can't apply pressure on both sides of the piston and rings and expect full performance from each firing of the cylinder.

Kinda like having a the tide always running against you.
or wind coming at your sail from two ways at once.

[TomG]

Instead of a pcv valve which I have and may be causing a vacuum leak, why cant you just extend the original hose set up out the back of the boat. This would alleviate the need for a pcv system because any noxious blowby would just be sent out the back.

DVD

All that stuff is going to go somewhere. If it just goes out the back, then you'll look like a diesel. I think the value of the PCV setup is it not only helps keep the cabin cleaner, but all those gases pulled out are now sent through the combustion process.

[edwardc]

Instead of a pcv valve which I have and may be causing a vacuum leak, why cant you just extend the original hose set up out the back of the boat. This would alleviate the need for a pcv system because any noxious blowby would just be sent out the back.

DVD

It's a nice idea. Unfortunately, the blowby gasses will come out of the engine anywhere there's a vent into the block, like for instance the oil fill cap, and not just the breather hose. By putting a suction on the breather hose, the PCV valve hookup draws air IN through all those vents, and sucks all the smoke (unless you've got a LOT of blowby) out through the breather hose.

[ndutton]

Well yes but . . .

All internal combustion engines have some blowby. What I was trying to say was they've been managed very well by the original slash tube arrangement for decades. If the slash tube isn't getting the job done anymore, why?? It did when the engine was younger and the fit of the parts was tighter.

My engine is 34 years old, appears to have been well maintained by the P.O. and has no evidence of serious repair or disassembly. I have no cabin atmospheric issues with the slash tube.

So Tom, my questions are:

• Is your blowby serious enough that the standard arrangement is no longer sufficient (and what does that tell you)?
• Or are you using the PCV system because it has been recommended and seems to be a better way to go even with resultant carburetor adjustment issues?