Need some help on this. I have a 68 400, original engine, T400. Had an overheating issue, which I solved with an aluminum radiator and vacuum plumbing adjustments. In the process, my PCV hose moved from the intake to the Holley carb. I now have oil pushing out the dipstick (AC car) onto the head and exhaust. I tried putting a breather on the passenger side valve cover (M/T aftermarket), and that resulted in rough idle. So, I have two questions. Is it possible that the carb port is not pulling enough vacuum to properly operate the PCV valve? It is the correct larger port on the choke side of the carb and the PCV valve is new, but I have not tested the port with a vacuum gauge. If I put the breather back on, can I solve the rough idle by simply adjusting the idle mix screws? I'm not good with timing adjustments, so trying to avoid that. Otherwise, it's a trip to the shop.
I could try moving the PCV hose back to the intake, but that would mean Tee-ing off my transmission vacuum or power brakes. The Holly carb does not have a vacuum port on the back side. Appreciate any guidance. Thanks!
Last edited by KCfirebird; 08/03/1910:08 AM.
68 400 convertible - recent purchase
Used to own: 69 400 convertible - Winward Blue 69 350 convertible - Verdoro Green 69 350 coupe - Matador Red
First thing to check would be vacuum at the suspect port at idle. If you have no vacuum until you start to go OFF idle, then yes you should move it to manifold vacuum.
Teeing it into another shouldn't be an issue.
Can you snap a pic of the location of the hose on the Holley?
what the heck kind of dip stick is that??? never saw one on any Pontiac
in a non stock config the PCV valve should be in the center under the carb and both valve covers should have breathers, they don't get set up like other engines, like in the valve cover
oil should not be coming out of your dipstick or anywhere, something is very wrong
if someone extended the tube on the dipstick but used the stock stick your crankcase is way over filled, way over which might cause oil to get pushed out
That is a stock dipstick for cars with AC. It is routed that way to be accessible when the compressor is attached. I'd have to change valve covers to have breathers on both sides. I added on breather to the passenger side, which resulted in a rough idle. Thanks!
68 400 convertible - recent purchase
Used to own: 69 400 convertible - Winward Blue 69 350 convertible - Verdoro Green 69 350 coupe - Matador Red
Still haven't told us whether you have vacuum at that port at idle or not. Also, the drivers side valve cover is the oil fill cap location and the passenger side is either a breather or the crankcase vent tube that went to a stock air cleaner (68' and 69' anyway).
Is the PVC valve in the valley pan?; Is the PCV system fresh air supply hooked up at the air cleaner?
One needs to have air-in and air-out to ventilate the crankcase positively. With a non-positive system, like we all had in the old days, the crankcase just vented through the oil filler tube and usually had a breather tube running down beside the block to drain any oil, moisture and smoke in the crankcase fumes.
A PCV system needs an air-in supply port and a positive air-out source. On most of our FGFs the air-in was at the air cleaner. There is a pipe sticking out of the air cleaner with a filter inside the cleaner case, a tube is attached to the pipe and goes to one of the rocker covers to feed air into the crankcase. To positively exhaust the air from the crankcase the exhaust tube was connected to a vacuum source which fed the air, fumes and moisture into the intake to be burned along with the air/fuel charge. (see the beautiful attached diagram made at great sacrifice and expense) The PCV valve was usually attached at the valley pan and the connected hose hooked up to the manifold vacuum at the carburetor or the intake manifold. The idle mixture was adjusted to compensate for the extra air entering the intake through the PCV system.
When you added a breather to the rocker cover the idle got rough? That would suggest to me the air-in source at the air cleaner may be plugged and the extra air entering the added breather and exiting into the intake is causing a lean mixture. You would have to adjust the idle mixture and check the air-in source.
If the PCV system worked well before you moved the air-out to the carburetor from the manifold, would suggest to me the vacuum port the system is now connected to is not sufficient to evacuate the crankcase correctly, I would suggest removing the PCV valve hose from the carburetor and teeing it into the manifold vacuum port.
The PCV system doesn't necessarily have to be connected in the manner it was when it came from factory, but does have to have filtered air-in and PCV valve controlled vacuum sourced air-out. I have mine set up with a filtered breather on both rocker covers, the PCV valve in the valley pan with the exhaust hose hooked up to a manifold vacuum port on the carburetor. Don't have a good pic but one that shows the cheapo rocker vents and the line from the valley to the carb. They make filtered vents for the push in and the twist in rocker cover caps.
What is the model of your Holley? Some of them had a pipe thread plug at the back of the base plate that can be removed and a pipe thread nipple be turned in for a vacuum source.
there may also be a pipe thread plug in the intake behind the carburetor, if so it could be removed and a nipple installed to power your brakes.
Will test the vacuum at the port tonight. There are aftermarket valve covers on the engine at the moment. Oil filler cap is on the passenger side, no opening on the driver's side. Engine was dry as a bone when the PCV was hooked up to manifold vacuum, so I believe the culprit is the carb port. I'll test to make sure.
68 400 convertible - recent purchase
Used to own: 69 400 convertible - Winward Blue 69 350 convertible - Verdoro Green 69 350 coupe - Matador Red
PCV is in the valley pan. I have an aftermarket style air cleaner similar to yours pictured above. Oil filler cap on the passenger side valve cover. No opening on the drivers side. I had the breather installed in place of the oil fill cap which resulted in the rough idle. I'll first try re-routing the PCV hose back to the manifold if the carb port is insufficient and there are no other available ports. If that doesn't work, I'll put the breather cap back on and adjust the mixture screws.
I'll report back on any changes. Thanks!
68 400 convertible - recent purchase
Used to own: 69 400 convertible - Winward Blue 69 350 convertible - Verdoro Green 69 350 coupe - Matador Red
How was the crankcase vented before you replaced the rocker cover filler cap with the breather? If there was no air-in source, then you put the breather on the rocker cover that would partially explain the rough idle. Going from no fresh air-in to a breather would act like a vacuum bleed/leak.
Looks like there is a pipe thread plug right under the hose that connects the brake booster to the carb manifold vacuum port, might be a bear to remove though.
Looks like you have a Holley 4175 spread bore vacuum secondary carb. There should be a power brake vacuum port at the rear of the base plate. These are sometimes plugged with a pipe thread plug. There may be some without but worth a look. Looks to me you have the pcv hooked to stepped vacuum. The PCV should be connected to a 3/8" vacuum port, such as the one you have the brake hooked up to. I'd take that carb off and check for a hidden brake vacuum port at the rear, if there is definitely not one, then tee off the front one you have the brake hose attached to or pull a plug in the manifold and install a nipple for the brake or PCV hose.
Last edited by Bluebird428; 08/05/1903:56 PM. Reason: Brain malfunction
Correct, before the breather there was no other air inlet. Just through the carb. And, your right about the rear vacuum outlet. I found the plug at the rear of the Holley. I plan on installing a new port at the back and connecting that to the power brakes, then running the PCV hose to the main large port on the front of the carb. Hopefully that solves the issue. Wondering if I should still put the breather on for additional air in/out and adjust the mixture screws to compensate for the rough idle?
68 400 convertible - recent purchase
Used to own: 69 400 convertible - Winward Blue 69 350 convertible - Verdoro Green 69 350 coupe - Matador Red
KC, If it were mine, I would leave the breather on the rocker cover as long as it has a filter or screen in it to filter the air entering the system. Without it all your doing is creating a vacuum in the case.
The BOSS at the back of the Holley with the plug in is sometimes called the Gulp Valve port, it was used on cars with an Air Injector Reactor emissions control system, also known as "smog pump". They were mandatory for California cars starting back sometime in the mid Sixties. Can't quite tell but it doesn't look like yours has that. If you take the slotted head plug out you should be able to install a 1/8"NPT nipple in there and connect your brake booster with a 3/8" hose. If the plug has straight threads you may have to tap pipe threads in or use a straight thread nipple with a shoulder to get a tight fit. Then as you suggest, connect the PCV to the front port and re-adjust the idle mixture and speed screws.
Just going by my rather alarmingly fading memory, I think it's a 1/8 NPT but I've been wrong before. Maybe take the plug to the parts store when you buy the nipple just to insure you get the correct size.
So you have your vacuum advance hooked up to the port in the front metering block?
You can go another route. You can purchase aftermarket open element air cleaners with hose nipples on the underside similar to the OEM setup in Al's drawing. The one that was on my car happen to have the provisions for 2 of them. This turned out to be perfect when my new 455 had such crankcase pressure that it was forcing oil past my rear main seal (the BOP Viton one-piece) before the rings seated.
So my setup was the valley pan PCV connected to the largest carb nipple and then aftermarket filtered breathers on both the driver's and passenger's valve covers with hoses running to the underside of the air cleaner. It worked well. I did have to buy several types of breathers from Summit to find ones with the largest hose connections.
In your case you could vent one side to the air cleaner and perhaps have just a breather on the other to provide the inflow.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI