Road Draft Tube

[kilkm]

I have a '62 Electra Convertible with the 401 engine. I've noticed a few times while curising in the summertime with the top down, exhause fumes, fairly strong at times. The only source of these fumes I can think of is the draft tube since the car has a new exhaust system. I've seen a custom valley cover from Mooneyes for Buick Nailheads that does not appear to be set up for a draft tube. The car has chrome valve covers on it with breather caps on both sides but no place for a PCV valve. I'm wondering if I replaced the valley cover with one not set up for a draft tube, would the duel breather caps provide adequate crankcase ventilation, or would I need valve covers set up for a PCV valve? The engine is already equipped with a modern Edelbrock performer carb. Anyone have any thoughts on this, I've never owned an engine with one of these draft tubes.

[NTX5467]

In some of the early (circa 1967) Chevy V-8s, there was an adapter housing of sorts that bolted onto the "hole" where the draft tube mounted and then had a rubber tube going to an inline pcv valve. Seems like this was originally California-only. It was the precursor of the "modern" pcv system that later evolved.

What will come out of the draft tube is crankcase vapors, not exhaust fumes. Two different smells! The crankcase vapors will (by memory) be more oil-related whereas exhaust fumes will be more hydrocarbon-related.

If you plugged up the draft tube with no other way for the crankcase vapors to escape, the engine would become more sludged-up internally and the vapors that used to go out the draft tube would now exit from the valve cover breathers--not pretty.

I found an SAE paper that GM did on crankcase ventilation in the early 1960s. They had two security cars (Chevy 6 cylinder full-size sedans) that were the "test vehicles". One had draft tube crankcase ventilation and the other one had a pcv system (where varying amounts of pcv valve flow were also tested). These cars, by their use, seldom exceeded 30mph. The slower speeds, combined with periods of idling and lower-than-optimum engine temperatures resulted in a good deal of crankcase "moisture" that ended up as sludge inside of the motor (with the normal road draft tube). When the similar vehicle's engine, but with a pcv system, was torn down for inspection, internal sludge was markedly less. When the baseline ventilation flow specs were doubled, sludge decreased even more.

If the road draft tube, itself, becomes clogged, it will result in the same things you'd get if you did away with it and didn't put some sort of pcv system in its place. The "time honored" way to clean it out was to remove the draft tube and put it on the open shop (cement) floor and burn the deposits out with an open torch. Sometimes, the deposits would catch fire (even more theatrics!) and then smolder a while (as the tube cooled off enough to use something to hold it for inspection). I don't know that anybody ever got one fully open again, but at least enough for it to work as designed at highway speeds.

You could usually tell a motor was "worn" when there was oil vapor (i.e., smoke) coming out of the tube at idle, possibly with some wafting through the valve cover breather or "slots". Worn rings/cylinder walls resulted in "blow-by" that had to go somewhere.

The Chevy adapter was a conical metal stamping, with a (5/16"?) through bolt and a hose connection for the pcv/hose assembly. I suspect these road draft tubes were all the same diameter and hold-down method, so it's possible the Chevy item would work on a Buick or other GM engine. Probably the best place to find something of this nature would be the Corvette restoration vendors.

Another alternative would be to use the Moon cover (referenced) and cut a hole in it for a pcv vave and grommet and then hook it to the pcv tube on the carb you have. Usually, if a carb has the pcv valve hose hookup, the fuel metering curve is calibrated to have that additional air flow into the motor. KEY THING is for the valve to have some sort of baffle under it to keep oil splash from being sucked into the system.

I seem to recall some aftermarket oil filler caps that looked like a push-on breather cap, but were sealed on the bottom, and had a pcv valve that popped into a rubber grommet on their topside. Looked kind of jicky, but it worked.

I also came across a chrome breather cap, which would push into a rubber grommet (ala mid-80s small block Chevy V-8) and had a pcv valve internal with the cap assembly. Again, it looked just like the companion chrome breather cap but had the pcv valve inside it. I don't recall the brand, but it was a full-line selection of chrome engine accessories and such at AutoZone. Still, it would need the baffle under it so it didn't suck too much raw oil into the system.

Hope this helps,

NTX5467

[kilkm]

Thanks for all the detailed info, I appreciate it. I know the later model nailhead engines eliminated the draft tube and used the PCV system. Could I just use a valley cover not equipped with a draft tube and valve covers off a newer engine that are set up for a PCV valve and run the line to the Carb. or are the older engines that were set up with the draft tube specificaly designed to vent the crankcase from the rear of the engine through the draft tube? Seems the easiest solution would to be use the valley and valve covers off of a newer engine.

[NTX5467]

Over the years, as time progressed from the first pcv systems to the later versions, they have also changed sides with the pcv valve itself (in the valve covers). I don't suspect that the direction of flow through the crankcase might have a real bearing on things, just that it flows at a certain volume.

It might have been that crankshaft rotation direction might have had some bearing on which side got the valve and which side got the air breather (intake). Just like the road draft tube being at the back of the motor (the best place for it to be in a lower pressure area and out of the way, although some inline 6 cylinders had it midway of their cylinder block) and later pcv valves being at the front of the valley cover (on the same engine family).

Enjoy!

NTX5467

[kilkm]

So, do you believe then that removing the draft tube ventilation system and replacing it with the PCV system from a later model (replacing valley and valve covers) provide adequate ventilation for this model engine? What would you do to your car? As I bought this car a couple of years ago from the Volo auto museum North of Chicago, who had bought the car at an Auction in Dallas, TX. I don't know anything of it's history other than the car came from Oklahoma and I am the 3rd owner. It's in beautiful condition and was told that it was restored in 2000. The motor was highly detailed, block freshly painted and new freeze plugs inserted, but I have noticed some smoke (blow-by) coming out the draft tube at idle. This leads me to believe that maybe the motor wasn't overhauled (odometer showed 88,000). But this old nailhead runs sweet and strong. Some performance upgrades had been made to the motor, Edelbrock performer carb, Petronix module in the distributor with an Accel super coil and a Carter electric fuel pump. I'm happy with the performance of the engine, it's just that annoying odor in the car at times from the draft tube. For clarification, what would YOU do if this was your car?

[Smartin]

I have to ask - what does the PCV system have to do with exhaust fumes? You either have an exhaust leak at or near the manifold, or possibly the exhaust crossover on the intake manifold is rusting out, causing a small leak.

The smoke coming from the tube is not exhaust gas. It is from the hot oil, or if it's only coming out when the car is running, it could be minor blow-by at the piston rings.

[NTX5467]

Considering that there is an optimum flow level of the pcv system, which a pcv valve for the nailheads that came with such would certainly have, upgrading to the later model parts would be the easiest thing to do . . . I suspect.

The Edelbrock carb is probably there as the orig had problems and the Edelbrock AFB is basically a re-run of the orginal Carter AFB carb, but in a much more universal orientation (think Chevy 350 V-8). The metering is also similarly "generic", so it might be a little off (usually "rich") for your application. There is a manual you can download and print on the Edelbrock website that detains the fine-tuning of the carb for particular applications. It's pretty much "cut and try", unfortunately.

Not only is the carb metered by jets and rods (as the Quadrajet is), but the phasing of the mixture curve is accomplished by a variety of springs under the power pistons (one on each side of the primary side of the carb). The primary metering rods are "two step" with "power" and "economy" diameters and the metering rod springs determine when the power mixture cuts. Therefore, in addition to the blow-by from the crankcase, being downwind of the car could also result in a "hydrocarbon" smell when it's running. A smell that might be aggravated a little by the fact the pcv passage is blocked off (hopefully) and plugged. The pcv valve is a calibrated "vacuum leak" to the metering activities of the carb, but are calibrated for in the metering specs of any carb that is set up for the pcv valve.

As mentioned, what comes out the draft tube is resulting from "blow-by" or compression gas leakage from the compression rings as the engine runs. Some is considered "normal" with a higher-mileage engine, but if it's pumping like an exhaust pipe, that's too much.

As the car is said to sit so much, it could just be that the rings have become a little "happy" with where they are and need to be limbered up a little with more driving. On the other hand, when those cars were built (ANY car back then), 100,000 miles was usually considered to be the useful life of a production engine before it needed to be "overhauled" (which could range from new rings, bearings, gaskets, and such all the way to a complete professional engine rebuild job).

For general principles, you might go over the idle mixture adjustment on the carb to see if getting it "optimized" at max intake manifold vacuum might help with the hydrocarbon smell (if that's what it is and where it's coming from). Then double check the basic ignition timing to see where it is (by factory recommended proceedures). You might also pull out a spark plug or two (after the engine has run at full operating temp for about 30 minutes of driving at freeway speeds, for example) after the engine cools down. If the center ceramic insulator is tending to be toward fluffy black rather than tan or "natural", then the mixture curve is toward the rich end of things. "Natural" is desired.

None of these diagnostic items are really difficult or involved, but will require some basic tools and expertise to do. What you're smelling could be a combination of things, including the proximity of the add-on a/c drain tube to the road draft tube and lack of sealing around the drain tube to the floorpan.

Hope this might help . . .

NTX5467

[bebop138]

I know I am getting in on this late----I took a plug that fit in the old draft tube opening with a 90 degree PVC valve and a short hose up to the back of the carb. This way I didn`t have to change valve covers or valley cover---easy fix. Good Luck!!

[palbuick]

Have you checked the bottom of the intake manifold. For years the intake had a freeze plug at center of intake. over the years this plug would rot away, causing an exhaust leak, and some fumes. Try to get a hand under the intake and feel around for a freeze plug about 2 1/2 in diam. if you can reach and feel a rusty hole this may be your problem.

Jim Schilf

palbuick