New Weber Carb has no vacuum source for PCV


#1

I recently had a complete engine overhaul on this Tercel SR5 Wagon - my honey. Oh, yeah. The mechanic said that the old stock carb couldn’t be tuned perfectly but he was happy when I suggested I might buy a new Weber carb for it. I did that and he installed it. Oh man! What a difference. Two little problems, however. The hood wouldn’t close properly with the air filter as shipped so I had to buy a thinner filter. I now have about 1/2" clearance. Prob #2 - the stock carb had a vacuum port for positive crankcase ventilation and the Weber does not. There is a adapter plate shown in the instructions that goes between the carb and the manifold and it has a vacuum port labelled “Power Brake Fitting”. This adapter is not part of the kit for this car and if I got one it might have the air filter bumping the hood again. The power brake vacuum on this car is from a fitting in the rear area of the intake manifold. I could adapt into that circuit or there is another similar fitting in the front area of the intake manifold where a fitting might be installed for vacuum. What I am wondering is if it important for the PCV vacuum to be taken from a central location in the intake instead of taking it from one end or the other. Anyone?


#2

You can pretty much put it anywhere in the air intake upstream of the carb inlet. Maybe mount a fitting on the side of the air filter housing like many of the late 60’s through 70’s cars did. I wouldn’t tee into the brake vacuum supply.


#3

An unrestricted vent from the crankcase or valve cover to the air filter housing is essential but the air filter housing will not accumulate vacuum needed for a PCV valve.


#4

My fault that was the breather, the PCV was usually in the opposite valve cover of the older V-8’s. I sit corrected.


#5

You could drill and tap and install a new vacuum port in the manifold near the carb…I would resist tapping into the power brake line as it might affect brake performance…


#6

With great respect to Rod, I believe a fitting on the air filter box would work great. The only function of the crankcase ventilation system is to allow fumes to be ingested rather than vented to the outdoors. The fumes are generally under pressure relative to the airbox. If there’s insufficient pressure difference between the airbox and the space under the valve cover to open the PCV valve, it doesn’t matter. The PCV valve simply won’t open. It won’t affect operation at all. The vacuum port is really only a vent for the crankcase fumes under the valvecover anyway.

The intake doesn’t need the PCV valve open, it doesn’t need the fumes from under the valve cover. The PCV valve’s only function is to prevent a backfire from igniting potentially volatile fumes under the valvecover and in the crankcase. Were it not for environmental concerns, you could simply vent the crankcase/valvecover fumes to the outdoors, as was done in the old days. Note that I’m not suggesting doing this, I’m just pointing it out to illustrate my point that it won’t matter if there’s insufficient vacuum to open the PCV valve.


#7

If I simply vented the valve cover to the air filter box (upstream of the filter), I’d remove the PCV valve. I wouldn’t want the crankcase to pressurize and possibly blow oil past the seals. I’d also check the filter frequently.


#8

The reason they went to a vacuum source evacuation was to proactively DRAW the vapors out. This reduces varnish build up inside the engine and prolongs oil life by not allowing the corrosive fumes to linger inside the crankcase and settle onto the surfaces or into the oil. As mentioned a valve is used to prevent backfire from getting into the crankcase…


#9

The PCV METERS the amount of flow…It never really closes, At idle, high vacuum, small amount of blow-by, it meters a small flow so as not to upset the idle…At less manifold vacuum, it opens, providing the crankcase with more ventilation. Without it, crankcase ventilation becomes hit or miss and sludge buildup is certain…


#10

I agree with @Caddyman since I’ve done this a couple of times myself. Once for PCV and once for power brakes. It works great and it’s very easy and inexpensive to accomplish if you know how to use a tap.


#11

Wow, lots of responses. Thanks. I remembered my old Volvo B-18 had a fitting in the manifold near the carbs. Was thinking of doing that. Also lots of “stuff” was removed with this installation. There is now a fitting in the intake just before the number one cylinder that could be replaced by a larger vacuum take off, however I was concerned that doing that might affect #1 unequally. What say?


#12

I’d be disinclined to put the PCV port anywhere that would allow the venting to only one cylinder. It would run the risk of imbalance in the metering. If it’s a later model with EFI the O2 sensor will be tweaking the metering based on an average of the output of all 5 cylinders and if one cylinder is unbalanced in its intake the result might be skewed by the one cylinder and not optimum for any of the cylinders. It it’s an early Tercel it might not affect the other cylinders, but it would make that one cylinder lean. That could cause a hot cylinder and all the potential damage that can do.

I still think the airbox just inside the air filter would work. The crankcase will always be at a higher pressure than the airbox, and as already stated the PCV valve never totally seals the vent line anyway. A hole, a tap, a fitting, and you’re home free.


#13

@artkennedyclearwirenet‌, you shouldn’t need to worry about cylinder balance on #1. The vacuum draw will be slight enough not to cause an imbalance. If there is a port now empty, it sure beats cutting into the manifold to put another in.


#14

With a free flowing vent from the valve cover to the air filter housing installed there should be no need for a PCV valve. And I recall European cars calling their diaphragm type PCV valves anti backfire valves.


#15

The PCV provides fairly high flow at cruising speed and under acceleration…If you funnel all these crankcase vapors into one cylinder, you might upset cylinder balance, especially at idle…But then again it might not…try that solution and see…Check that spark plug now and then, compare it to the other plugs…If they look even it should be okay…The effect of the PCV input is not critical or destructive so it’s not a big worry…


#16

Your OEM carb did NOT have a provision for the PCV, that was on a plate under the carb and you mechanic should have left it on the manifold when he installed the Weber. You need to get back to him and get this plate before he looses it because you cannot get a replacement.

I have done this weber conversion on an 86 Tercel 4wd wagon and there are a number of write up on this for this engine. There is a web site dedicated to this car if you are interested. They have a lot of good information for you.

http://www.tercel4wd.com/viewtopic.php?f=7&t=1112


#17

Well I remember when they started PCV valves on cars and it was a little thing that made a great improvement,personally I would run the line to the airbox and put a little horsehair or fiber filter on the side between the housing and filter element and in some cases it does make a difference were you get your vacuum(that was drummed into my psyche a good while back.an interesting note though ,most modern diesels still use road draft tubes(anybody want to hazard a guess as to why?)-Kevin
PS-good deal on that Weber{what were the biggest improvements you noticed?}


#18

Diesels have no manifold vacuum to tap into…In turbo-diesels, the manifold is pressurized… But a road-draft tube? I don’t think so…I have a small diesel engine that vents its crankcase back into the air intake downstream of the air cleaner…


#19

Thanks to everyone. Lots of information - some disagreement. Let’s work on it here.

First, the reason for being so particular. I really like this car.
I also have an '86 - same model. I call them TT-1 and TT-2.
I bought TT-2 to drive while my man was doing an overhaul on TT-1.
TT-1 is the one with the new Weber and incomplete PCV.
I’m only keeping TT-2 that until TT-1 is perfect.
The engine is “new” and I don’t want to do it again in 50K because it is all sludged up.
Therefore just having a vent from the valve cover to the air box is not enough.
I have attached a scan from the manual showing the correct circuit.
BTW, these particular cars never had EFI.

I have all the stuff that was removed in a box so nothing is getting lost.
I have examined the setup on TT-2 to confirm the layout of the stock PCV system.
There are two PCV fittings on the valve cover.
The aft one goes to the air box, the forward one to a port on the throttle assembly under the carb. (Isn’t that part of the carb?)

@keith, I think the plate you mention is ( at least on this vehicle ) the throttle assembly.
The new carb has its own throttles and a much larger throat. I don’t believe they are compatible.
Anyway the kit I received from Redline had instructions that showed a plate between the carb and the manifold adapter which was designated “PCV/Power Brake Fitting”.
They say it isn’t needed in this application and indeed there might not be enough room under the hood for the additional height - not sure.
Thanks for the referral to tercel4wd. I will go there to research further.

@Caddyman, I had thought of doing that spark plug check thing. If I have to go that way that is what I will do.


#20

Ooops! Spoke too soon. @keith, just back from tercel4wd. My mistake, is that the fuel heater plate?