Just rebuilt the carb - PCV valve question

Just rebuilt the carb on the '67 108 with 283 V-8. It's a Rochester "2 Jet", with what looks like a very early serial number (700xxxxx). Since the serial number doesn't show up in the service manual I figure it's not the stock carb. The throat in the bowl assembly is stamped 1-3/16, and the stock carb is supposed to be 1-7/16. Anyway, I used a 2G rebuild kit for a '67 Chevelle 283.

I notice that this motor is 1/2 way set up for closed positive crankcase ventilation. That is, the part that takes fresh air from the air cleaner is still attached, but the intake from the oil filler tube is plugged shut at the filler tube and at the carb base. I'd like to restore the positive crankcase ventilation. I looked at some PCV valves at the auto parts store and I think some of them could be made to fit. My question is, will a generic PCV valve work, or is there something special about a closed positive crankcase ventilation system PCV valve that I need to worry about?

Thanks in advance.

OK, thanks.

This is a V8. According to the service manual, it is a closed positive crankcase ventilation system. It should have a hose running from a fitting on the oil filler tube to the bottom of the carb (manifold vacuum) with a valve of some sort in between. I assume this is a PCV valve but that is not what the manual calls it.

The closed part is because instead of a vented oil filler cap, there is another hose leading from the crankcase to the air filter.

The way it is supposed to work is that clean air from the air filter is supplied to the crankcase. Then air is sucked out of the crankcase via the fitting on the extended oil filler tube, into the base of the carb (manifold) for incineration in the cylinders.

The problem is that only half the system is current in place. That is the hose running from the crankcase to the air filter. There are fittings on the oil filler tube (which is about six inches tall above the intake manifold) and at the base of the carb, but these are plugged off with pipe thread type plugs. So there is no valve and really no positive ventilation going on.

What I want to do is to remove the plugs and run a hose from the oil filler tube to the base of the carb, with a PCV valve in between.

My question is: will a generic PCV valve work ok in this setup, or do I need something special in view of the "closed" nature of this setup. All the closed part means is that instead of a vented oil filler cap, the system takes clean air from inside the air cleaner.

I prefer the closed setup because it's neater and also because the motor has a long oil filler tube with a non-ventable cap setup. I also think the closed system will result in less fumes escaping the crankcase when the motor is off. But I just want to make sure I'm not setting up something dangerous when I stick a generic PCV in there.

My 68 has a PCV, and the valve cover to air cleaner hose, but I don’t know if it has a Spark Arrestor.

I spoke to a mechanic friend of mine this weekend about removing the air cleaner hose. He explained that the PVC keeps pressure from building inside the block while the engine is at idle. But when accelerating the valve cover to air cleaner hose is what keeps pressure from building inside the block. The reason is to keep the pressure inside the block from blowing out seals.

He said that I could get away with replacing the valve cover to air cleaner hose with just a filter on top of the valve cover. It would be fine since it would relieve pressure from inside the block, that having it put a vacuum on the engine by using the air cleaner is a little over the top. Since using the vacuum from the air cleaner doesn’t hurt the engine performance in any way, why not leave it? It’s really just a cosmetic decision.

So, if I understand all that correctly, the piece flopping around inside the PVC valve blocks the vacuum when the engine is accelerating?

Actually, according to the 1967 Chevy Truck/Van service manual, both the closed and open systems have valves. The difference is the makeup air in an open system comes from a vented oil filler cap. The makeup air in a closed system comes from a hose that connects to the air cleaner before the carb but after the air filter.

There should be little to no negative pressure in the closed system's makeup air circuit, unless the air filter is badly blocked. The most pressure would come from the lower hose - usually connected to a pcv valve at the valve cover, or in this case, at the oil filler tube - which in turn is connected to the base of the carb, effectively the same as the intake manifold.

A spark arrestor in the makeup air is probably a good idea. This particular motor has been running with just the makeup air tube, which I suppose could function to relieve crankcase pressure. But if there is a lot of crankcase pressure then I suspect there are more serious problems with a motor.

I do not know if this particular motor is original to the vehicle. It is the right size V8 for that year - 283 - but it shows signs of having been out of the vehicle at some point and given a somewhat crude blue spray paint job (overspray on some external tubing, for example). I'm told it was rebuilt about 10 years ago.

Part of the problem I'm having is that the local shops either don't list the van that far back, or list similar engined Chevy's as having open systems.

I'll have to try the local Napa parts store to see if they can provide the proper PCV valve and maybe a spark arrestor as well.

Don I got a setup like yours on my 64, But its got a 250 out of a Nova that came from CA??? Also the 65 from NY got the same setup(or close) but I was told that it had a 250 out of a pickup in it??

OK, good idea. The spark arrestor isn't currently visible but I suppose it's probably buried under the distributor at the rear of the engine - the one area I couldn't really get to when I cleaned the top of the motor.

The fresh air hose is huge. Haven't measure it, but I'm sure it's at least 1".

It is connected to the top of the air horn, via a spacer that's about 2" tall and resides between the air cleaner and the carb air horn.

I'm assuming the spark arrestor is towards the rear of the motor where the fresh air tube connects to the crankcase fitting. I just haven't gotten in there to figure out exactly what it's attached to back there. I kinda sorta don't want to pull the distributor (it's a combo coil/distributor/HEI unit) until at least I figure out where the current timing is at.

Doni,

Thanks.

Do you know where I might be able to purchase a similar in-line flame arrestor?

The return air line from the carb horn to the crankcase connects not to the valve cover on this 283 V8, but to an older style fitting at the rear of the motor, more or less under the distributor. I cleaned that area up and pulled the fitting. It doesn't have any screen inside although I could probably cut one out of some copper or stainless steel screen I have lying around here and make do with that for the time being. But I'm a bit concerned that given its relatively low position that the screen will clog up with oil faster than an inline one closer to the air cleaner. Also, it's sort of a hassle getting to the fitting at the rear of the motor - had to disconnect the brace that the throttle linkage attaches to, and then remove the top of the distributor plus the vacuum advance unit in order to get at it.

I did some searching on-line for in-line flame arrestors and came up with nada so far. But they can't be all that uncommon, can they?

Well, since this motor came to me with no flame arrestor.... I have no idea what it's supposed to look like. But the one in the picture looks good. The rest of the hose/fitting looks sort of like what's on the 283, but of course being a V8 and the fact that the fitting is on the top rear of the crankcase, the bends are a bit different.

The internal fins are probably what arrest the sparks without getting clogged with oil as much as a mesh or screen might. Just thinking.

I just got through putting the air return, distributor, and throttle linkage back together. I wound up sawing off the fitting on the air return pipe that used to hold the coil. It kept on shorting out on a relay (starter?) that is mounted high up on the doghouse by the distributor, as I tried to fit the air return pipe back onto the motor. It sort of goes from starboard to port at the rear of the motor under the distributor and the veers forward towards the air cleaner, and then ends where the big red rubber hose takes over.

I'm wondering if a muffler for an old lawnmower might work. Or maybe I could fabricate my own out of whatever.

In any case, I notice that the return air fitting at the rear of the motor was probably a source of oil leakage. The rubber gasket under it was a bit malformed. So I put a big o-ring in there instead, and also added a copper washer under the head of the bolt that holds it in place. Will see if that stems that particular point of ooze.

When is a PCV not PC?

Well, now I know why a previous owner removed the PCV valve....

The PCV valves that Napa and Fram sell for this vehicle are back-asswards. They have a very nice 1/2 inch male pipe thread on one end, which fits very nicely into the very nice 1/2 inch female pipe thread lug welded onto the side of the extended oil filler tube. Only one problem: mounted in this fashion, the PCV's valving is completely ineffective. Wrong direction, you see.

I imagine the engine would run lousy with one of these supposedly correct PCV valves installed in the stock position. It'd be off the spring all the time unless there was a backfire, in which case it would probably not block the backfire completely. There would be no metering of the air, so the results would be erratic - like a major air leak under the carb.

I imagine a previous owner put in one of these valves, got disgusted by the results, and remove it and plugged off the fittings.

I thought about mounting the PCV in the opposite direction, at the carb thottle plate, but the problem there is that it will be at the lowest point before the carb and more likely to clog up. Also, the throttle plate female thread is 5/8, not 1/2". I could use a reducer but there would still be the problem that it's not the right location to begin with.

So I bought a generic PCV valve with hose barb fittings on both ends - one is 3/8", the other is 1/2". I'll get proper hose barb fittings for the oil filler tube and the back of the carb. I was going to have to do that for the carb end of the line, anyway.

These PCV valves might be correct for the 283 motor with open crankcase ventilation, don't know.

Arrested Flame

The saga of the missing flame arrestor goes on. Most, no, all, parts counter guys don't know what one is, to begin with. I got a local Napa shop to find an entry for one on their system, but "there is no data for this selection". In other words, no corresponding part to be had.

As I explain to one parts guy, I'm intent upon getting this addressed because of the closed nature of the PCV system, which makes a crankcase fire resulting from a backfire more likely, but also because the engine sits inside the passenger compartment and an engine fire or "grenading" would be much more dangerous than in a hooded vehicle.

I have a couple of ideas for making my own flame arrestor... taking from the information I've gleaned here and on the internet. The finned nature of Don's flame arrestor, combined with some info on the web, led me to conclude that the primary function of a proper flame arrestor is to remove as much thermal energy from a backfire as possible. The internal fins would no doubt accomplish this, as well as physically dampening the flame, without using a fine mesh that is more likely to get clogged. So this gives me an idea for an effective but high flow rate flame arrestor I can make from some pipe fittings and assorted materials.

Doni,

Thanks a bunch! PM with address info on its way...