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1968 289 valve cover with PCV valve AND oil filler cap?

2629 Views 17 Replies 10 Participants Last post by  jjacks41
I’m kind of stumped. My passenger side valve cover has a PCV valve AND oil fill breather cap. Driver side just has oil fill breather cap. Is this setup ok, or is there a possible vacuum leak here? I’m having trouble smoothing my idle, even with spot on timing. Vacuum is on the low side, 6”, but no leaks at carb/manifold base or trans hose. 289 block, comp cam 268h, edelbrock 1405 carb, edelbrock performer intake. Motor runs incredibly strong, just a physically rough idle. Thanks in advance!
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Manifold vacuum will be unaffected by the number of breathers installed... there should be plenty of airflow through just one to "feed" the PCV. What IS possible is that airflow, following the rule of "path of least resistance" is being sucked in through the RH breather, down the length of the valve cover, and up into the PCV so gases in the rest of the engine aren't getting evacuated properly. I'd "cap" the breather on the RH side.

As far as your manifold vacuum reading is concerned, I would expect WAY more than 6"hg with that mild of a cam... more along the lines of 15"hg or more. Are you measuring vacuum at a manifold vacuum source? Also, at such a low vacuum your PCV is going to be wide open almost all the time as well. I also can't see where you have your PCV hose connected but, typically, the PCV port is at the FRONT of the Edelbrock carbs and brake booster port is at the rear. The PCV is the only vacuum component that FLOWS air so connecting it to a place where vacuum should be "static" can cause issues, as well.
 

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1968 Mustang Coupe-slightly built original block 289
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Discussion Starter · #3 ·
Thank you Bart!! I will cap off the driver's side cap filler for sure. Any specific way I should do that?

I totally agree about the vacuum. The pcv is going to the rear port on the Edelbrock 1405. I have read that can be used for brakes as well as PCV? That makes sense to plug into the front port of the carb. I can't believe I missed that, I'm definitely learning something new every day. When I bought the car over a year ago, the old carb had nothing in the front, and pcv plugged in the back. That's what I get for assuming it was connected properly.

Thank you again for your help!
 

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Is it the "XE" version of the 268 cam? Looks like you are running the "RPM" intake with a carb spacer so a lot of plenum and runner volume. Not surprised by the lo vacuum (if it is the XE) although you may be able to squeeze a bit more out of it.
 

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Cap the right breather, not the left. Make it pull vacuum from left to right, into the pan valve.

Sent from my SM-N960U using Tapatalk
RH side is the passenger side, that is what @Woodchuck meant. But the OP is looking at the engine and misunderstood.

Chris
 

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Discussion Starter · #8 ·
Is it the "XE" version of the 268 cam? Looks like you are running the "RPM" intake with a carb spacer so a lot of plenum and runner volume. Not surprised by the lo vacuum (if it is the XE) although you may be able to squeeze a bit more out of it.
As far as I know, it's the 268H. I've thought about taking off the spacer, but I was having some heat soak issues, trouble starting when hot. The spacer resolved that issue. I guess it wouldn't hurt to try without the spacer?
 

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I would go back through the carb setup to make sure the throttle plates are where they need to be and idle is set correctly as well. How did you test for vacuum leak around carb and intake?
 

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Discussion Starter · #13 ·
It looks like the small vacuum port above the PCV valve is open. If it is open put a cap on that. The little open port does not explain such low vacuum. I would leave the phenolic spacer. Where is the vacuum for the tranny coming from? Good luck
Thanks!! That little port is actually a solid piece, no hole. Trans vac is coming directly off intake manifold.
 

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Discussion Starter · #14 ·
I would go back through the carb setup to make sure the throttle plates are where they need to be and idle is set correctly as well. How did you test for vacuum leak around carb and intake?
Idle is right around 800 at idle, drops to 600 in drive. I just did the ol carb cleaner spray around the base of carb, no change in RPM at all. The crazy thing is the motor runs almost perfect, very strong throttle response, easy to start, never dies. It's just the dang idle is rough.
 

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Discussion Starter · #16 ·
What about just blocking that PCV line temporarily and seeing if that improves your idle and low vacuum. Then go about the suggested changes.
I have definitely thought about that. I'll throw that in the bag of tricks when I get home this afternoon to start her up.
 

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If this is the cam you are using:

I think the Vacuum is really low as well. The cam overlap calculator says its only 48deg which is not alot of overlap. I had a cam with 64deg of overlap and I had 12hg of vacuum. So something is amiss I think. What is your initial timing set to?
 
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Discussion Starter · #18 ·
If this is the cam you are using:

I think the Vacuum is really low as well. The cam overlap calculator says its only 48deg which is not alot of overlap. I had a cam with 64deg of overlap and I had 12hg of vacuum. So something is amiss I think. What is your initial timing set to?
Initial timing right now is set to 14 degrees. Anything below around 10-12 and it really starts to shake. 14 degrees is where I have the highest vacuum and smoothest idle. I'm still learning about this motor, but this is driving me crazy. It performs so well throughout the RPM range, just at idle is shaky and the low vacuum has got to be robbing me of some power. Is there any way my vacuum gauge is off? Its a pretty dang simple tool, hard to imagine.
 
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