TR4/4A PCV valve

[shoopal]

Hi all
Maybe someone can explain to me how the PCV valve operates on the TR4A. There is a diaphram which seems to block the plate and some kind of spring loaded plunger that sits in the hole going to the intake manifold. I just rebuilt the motor and when I turn it off there is a hissing noise that persists for about 10 seconds that sounds like a vacuum leak. Also I am having some trouble setting up the carbs. The Brits call it "Lumpy tickover"
I have seen a number of threads that suggest dumping the valve and replacing it with the old style (vented cap or vent pipe). Moss sells a complete PCV for $55 or so. What to do?

 

[KVH]

Are you sure that if you disconnect the PCV hoses, maybe one at a time, you still won't hear that noise? Your valve is clean and connected correctly?

Does your car also have a draft tube on the driver's side, or that plugged with a freeze plug?

Need more info re "Lumpy Tickover."

 

[shoopal]

Hi. The engine is set up to run on a stand so I can try to see if disconnecting the hoses alters the noise. No draft tube. I didn't have much chance to diagnose further but it seemed that the rear carb was the problem. Give me a few days and I'll let you know what I find. I also have a spare PCV setup from my donor car which may be in better shape. I also got a new diaphragm from Moss and I'll try that one as well
Thanks for the reply
Al

 

[Darrell_Walker]

I just rebuilt the motor and when I turn it off there is a hissing noise that persists for about 10 seconds that sounds like a vacuum leak.

Is the sound similar to what you hear when turning the engine over by hand? Just internal compressions/leakage/etc?

 

[shoopal]

It is a similiar sound. Pressure or vacuum leak. seems to be coming from the PCV valve area. I need to doa bit more checking. Maybe disconnect the hoses and block off the intake manifold side.

 

[texbears3]

The TR4A has a diaphram type PVC valve. The diaphram opens when there is enough vacuum from the intake manifold.
Not sure if you have the same PVC setup?

 

[poolboy]

I thought the diaphragm, reduced the flow into the manifold when the vacuum was too high, such as at idle and engine overrun.

 

[shoopal]

Hi Texbears3
I think that you may be on to something. Referencing the attached picture. What you don't see in the picture is that there is a spring loaded flapper check valve under the pin in the bore hole.
When the engine is not running, the spring keeps the pin up, closing the check valve.
When the vacuum negative pressure exceeds the blow-by pressure + the spring, the pin lifts up allowing the check valve to close. This permits the manifold to suck in the blow-by
However if the blow-by + spring is less than the vacuum, the pin goes down opening the check valve and permits free flow in either direction.
In my case. I have carefully rebuilt this motor and I doubt there is much crankcase pressure (the ring gaps have been set to .0015"), no taper in bores etc. The dipstick is fitted with double felts and a rubber ring and so is really sealed. My guess is that there is high negative pressure in the crankcase and when I shut the motor off, it releases back into PCV, past the opened check valve ond into the manifold and out the carbs causing the 10 sec. hiss that I mentioned before. During slow idle This vacuum leak would also lean out the mixture and cause the rough idle. So what to do? I have a spare from the donor car that I will look at. Blocking off the hoses to the PCV valve would also give more diagnostic info. I am going to try all this stuff tomorrow and I'll post the results.
Sorry, family business today (Wed.) I'll give it a go tomorrow.
(Added Wed Eve)
After inspecting the spare PCV valve, I noticed that the pin in one setup was significantly longer than the other. I am not sure what this means yet.
7/16 Noon. As I suspected, blocking off the intake manifold to the PCV valve eliminated the hiss on shutdown and allowed me to adjust the mixture on the rear carb so it idled smoothly. I am now going to rebuild the valve with the good parts and will proceed over the weekend.
There was absolutely no crankcase blow-by pressure. I suspect that when the PCV valve is reinstalled, I will have to richen out the back carb to compensate for the additional air leaking by the PCV valve but that will be determined.

 

[shoopal]

By the way, where did you get that valve cover cap?

 

[shoopal]

Where did you get that neat valve cover cap?

 

[3798j]

By the way, where did you get that valve cover cap?

I've seen them on late 60's "emission" GT6's, Vitesse's, and Spitfires.
Here's the one on my '68 GT6:

 

[martx-5]

 

[poolboy]

Thanks for posting that, Art. It illustrates the point I was trying to make in a prior post about the diaphragm in the Triumph version of a PCV valve