When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
My 1965 original engine 300 h.p. car seems to have been changed slightly in this regard according to the info below. It has a rubber hose from the oil fill tube routed along the passenger side of the carb (held in place by what looks like an original clamp) and goes into the bottom of the rear of the carb and there is no PCV valve in this hose anywhere. The original twin snorkel air cleaner is connected to the draft tube hole fitting and there is no PCV valve there and it does not look like it had enough room for one. My confusion is in this statement "engines with Carter WCFB or AFB carburetors continued to use a PCV valve and fitting at the back of the baseplate" Is he referring to the base plate of the carb or the air cleaner? If he means the carb base plate then I guess I need a PCV valve in that hose.
Also, what is the correct PCV valve for this application?
The below info is from:CRANKCASE VENTILATION AND PCV 101 John Hinckley I have bolded info that I believe is pertinent to my situation.
The 1963 “closed” ventilation system, used on all engines, used the oil fill tube as the “intake” side of the system, to admit outside air into the crankcase, and used a hose from the hole in the back of the block (previously used for the road draft tube) to the base of the carburetor as the “exhaust” side of the system.At the “intake” side, a hose carried outside air from a fitting on the “clean” side of the bottom front of the air cleaner to a fitting on the oil fill tube. At the “exhaust” side, the hose from the vent hole in the back of the block had a PCV valve where it connected to the carburetor base. The valve was exposed to full manifold vacuum, which “pulled” outside air through the crankcase, carrying the hot vapors into the intake manifold to be burned along with the intake charge. The PCV valve is essentially a spring-loaded one-way check valve, which only allows flow through it in one direction under certain vacuum conditions.The “closed positive” ventilation system used engine vacuum to force a continuous flow of fresh air through the crankcase, and burned the vapors as part of the normal combustion process instead of exhausting them directly into the atmosphere. This finally created a controlled ventilation system to reduce sludge formation, and reduced hydrocarbon emissions into the atmosphere.Changes For 1964: Continuing development indicated that the system’s efficiency was improved by reversing the ventilation airflow so the “intake” side was through the vent hole in the back of the block, and the “exhaust” side was through the oil filler tube to the carburetor.
The “intake” side used a steel adapter and large-diameter vent pipe at the vent hole in the back of the block, connected by a large rubber hose to an elbow in the bottom of the air cleaner, so clean air was drawn from the air cleaner into the back of the block. Inside the air cleaner, a steel fine mesh screen was pressed over the stub on the elbow as a “flame arrestor”; this prevented a carburetor backfire from propagating through the vent tube into the crankcase, which could cause an explosion. The same type of flame arrestor technology is used for the air cleaner element on inboard marine engines so a carburetor backfire won’t ignite bilge fumes.The “exhaust side” of the system was moved to the oil fill tube, which had a fitting for a hose to a manifold vacuum source at the carburetor; engines with Carter WCFB or AFB carburetors continued to use a PCV valve and fitting at the back of the baseplate. The 365hp engine used a Holley carburetor, with a slightly different arrangement; the hose from the oil fill tube connected to a 90° fitting on the driver’s side of the carburetor with an internal .090” restrictor orifice instead of a PCV valve. The orifice in the fitting was supposed to be cleaned with a fine wire at each tune-up interval; if the orifice became plugged, the crankcase ventilation system ceased to operate, and crankcase vapors would be driven up the rear vent tube into the air cleaner. The 1964 system design continued unchanged into 1965 for small-block engines.Fuel-injected engines used the same basic design (“intake” from the air cleaner, and “exhaust” from the oil fill tube to the intake plenum), although the plumbing was different.
A 1965, 300HP used an AFB with a metered orifice fitting in the back of the carb. that was connected to the oil fill tube with a hose.
If the statement you highlighted is referring to the 1965 model year, it is incorrect.
A 1965, 300HP used an AFB with a metered orifice fitting in the back of the carb. that was connected to the oil fill tube with a hose.
If the statement you highlighted is referring to the 1965 model year, it is incorrect.
According to John Hinckley's article what you are showing is correct for a 350/365 h.p. engines
It is correct for 350/365 hp. All '65 SB's used a metered orifice instead variable valve including the 250/300 hp engines.
I think BB was the same but I'm not sure..
If that is the case then you are saying John Hinckley's article is wrong? If you read what he has written it says for the 1965 250/300 h.p. cars they use the same system as the 1964 which used a PCV valve. Corvette Central shows this to be the correct one for 1964. According to the article ONLY the 350/365 h.p. cars used the orfice fitting. https://www.corvettecentral.com/c2-6...e%26count%3d18
As noted above, no PCV valve on the '65 300hp motor, or any other '65 Corvette SB. Below shows you the correct fittings. Notice the rounded off elbow. Not the easiest to find.
If that is the case then you are saying John Hinckley's article is wrong? If you read what he has written it says for the 1965 250/300 h.p. cars they use the same system as the 1964 which used a PCV valve. Corvette Central shows this to be the correct one for 1964. According to the article ONLY the 350/365 h.p. cars used the orfice fitting. https://www.corvettecentral.com/c2-6...e%26count%3d18
As noted above, no PCV valve on the '65 300hp motor, or any other '65 Corvette SB. Below shows you the correct fittings. Notice the rounded off elbow. Not the easiest to find.
The big block used a metered orifice as well but in a brass elbow. I've never seen the rounded off fitting like you show. The one I pictured above is the original fitting that also matches all the pics. in the NCRS Judging Guide. There must have been more than one supplier.
If that is the case then you are saying John Hinckley's article is wrong? If you read what he has written it says for the 1965 250/300 h.p. cars they use the same system as the 1964 which used a PCV valve. Corvette Central shows this to be the correct one for 1964. According to the article ONLY the 350/365 h.p. cars used the orfice fitting. https://www.corvettecentral.com/c2-6...e%26count%3d18
64's used a PCV valve. 65's didn't. The article is wrong.
The big block used a metered orifice as well but in a brass elbow. I've never seen the rounded off fitting like you show. The one I pictured above is the original fitting that also matches all the pics. in the NCRS Judging Guide. There must have been more than one supplier.
Something interesting about that rounded fitting is the "GM" stamp on it. But I agree the squared corner fitting is what the judges would expect based upon its widest usage.
Well, I consider John Hinckley as a pretty trusted source but anyone can make a mistake.
My problem is that Corvette Central shows a PCV valve for a 64 (article says 65 remained the same from 64) Eckler’s shows nothing, Paragon shows nothing and LIC Supply agrees with you guys.
My car is an original 76K mile car with no evidence that the carb has been rebuilt and has a oil fill tube with a hose nipple and a two piece elbow fitting at the back of the carb. The elbow fitting hole is small with about an 1/8" diameter so I am assuming this is the "orfice"? Mine does not have the rounded elbow. was this a running change or was it like that throughout the production year?
I plan to upgrade my engine with a 350 h.p. carb, intake and cam at some later time and have purchased an intake and oil fill tube from a 1966 350 h.p. car. That chrome oil fill tube has the threaded fitting for the PCV valve. I think I will use that now so I can put a PCV valve in as I have heard that the orfice fitting is subject to clogging often unless what I have is OK. I am just trying to determine what is correct/incorrect with my current set up.
The orifice on my '65 has never been cleaned since 1972 and I assume it is still open as the PVC system works overall.
OK, good to know but that does not answer either of my questions.
The elbow fitting hole is small with about an 1/8" diameter so I am assuming this is the "orfice"? Mine does not have the rounded elbow. was this a running change or was it like that throughout the production year?
OK, good to know but that does not answer either of my questions.
The elbow fitting hole is small with about an 1/8" diameter so I am assuming this is the "orfice"? Mine does not have the rounded elbow. was this a running change or was it like that throughout the production year?
"Orfice" is spelled "orfifice".
I can't answer the question on the overall shape of the orifice fitting but I think mine is like the photo Snaky posted. I seldom ever raise the hood.
On another note regarding all of this. I plan to upgrade my engine with a 350 h.p. carb, intake and cam at some later time and have purchased an intake and oil fill tube from a 1966 350 h.p. car. That chrome oil fill tube has the threaded fitting for the PCV valve. If I leave the existing fitting at the back of the carb with the small hole will it be too restrictive with the PCV valve also?
If you upgrade the carburetor to the 350hp Holley the AFB fitting is not relevant, the pcv valve screws into the oil fill tube and connects to the throttle body on the Holley. The engine breathes back in thought the connection in the rear of the air cleaner to the rear engine block.
If you upgrade the carburetor to the 350hp Holley the AFB fitting is not relevant, the pcv valve screws into the oil fill tube and connects to the throttle body on the Holley. The engine breathes back in thought the connection in the rear of the air cleaner to the rear engine block.
Thanks for the info.
Waste of $$ IMO, the 300hp is a nice engine.
Yes it is a nice engine. To each his own but I have always like the additional power.
Yes it is a nice engine. To each his own but I have always like the additional power.
The 300 is a nice engine. Just like a little ole lady is like a nice person.
The 350 makes the Corvette run/sound like a Corvette should but you could get nearly the same results just by changing the cam. The Holley, aluminum intake and air cleaner are mostly eye candy, not that much more SOP power..
Put a performance curve in the distributor.
Tbarb is telling you right about the hose routing on the '66 carburetor. Don't use a valve at the back of the carb.
To answer your "orifice" size question, I think John gave the right diameter on the hole size.
The '65 AIM shows 3853720 "Connector" as the PCV hose fitting for the snorkel motors. Digging around, found a 1966 parts catalog which lists 3853720 described as "Connector with 7/64" restrictor at hose end".
It was eventually replaced by 555214, which does NOT have restrictive orifice.
07-20-2018, 03:42 PM
1965 300 h.p. PCV system questions
