360 vacuum leak.

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Liams55belvedere

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I have a 77 360 with a Carter AFB style carb, headers, and performer intake, and I believe I have a vacuum leak from my breather caps/ PCV setup. Currently the passenger side has a breather AND the PCV valve and the drivers has a breather. I read that there's not supposed to be a breather on the side with the pcv so just to test if this was the source of my vacuum problems i took the pcv off and capped it and the carb port. When I ran the engine like this it ran extremely rough and died. When I connect the PCV it runs as good as usual. Im planning on getting new valve covers and just running one side PCV and one breather but I'm wondering if this is normal behavior? Does the carb just need to be retuned if I run no PCV?
 
Wrong thinking. Remove the breather from the side with the PCV. Then add a SEALED breather with a hose nipple on the opposite side and connect the nipple with a hose to the bottom of the air cleaner base. Like this diagram.
PCV.gif
 
Just went through this in another thread.
Breather blow by
And a properly connected and functioning PCV system does not cause a vacuum leak. For that, start looking around the intake manifold flanges, carb base gasket, and any crackedl or leaking vacuum hoses, or uncapped vacuum ports. A bad brake booster or check valve can also cause a vacuum leak.
And I'm kinda baffled by your last statement, that it runs better with the PVC hooked up, and how to go about retuning the carb to run without it... WHY??
 
Just went through this in another thread.
Breather blow by
And a properly connected and functioning PCV system does not cause a vacuum leak. For that, start looking around the intake manifold flanges, carb base gasket, and any crackedl or leaking vacuum hoses, or uncapped vacuum ports. A bad brake booster or check valve can also cause a vacuum leak.
And I'm kinda baffled by your last statement, that it runs better with the PVC hooked up, and how to go about retuning the carb to run without it... WHY??
Well I didn't want to run it like that but I was trying to see if my vacuum leak went away if I either (A) just ran two opposite breathers, or (B) ran it with pcv one side and breather on the other. I couldn't do option b because I had nothing to plug the breather hole with at the time. I'm just confused by the outcome because I've seen engines run with just the two breathers.
 
Well I didn't want to run it like that but I was trying to see if my vacuum leak went away if I either (A) just ran two opposite breathers, or (B) ran it with pcv one side and breather on the other. I couldn't do option b because I had nothing to plug the breather hole with at the time. I'm just confused by the outcome because I've seen engines run with just the two breathers.
Just because you've seen it, doesn't mean it's right. Have you tried a squirt bottle of water around the carburetor and intake gaskets? Don't use something flammable like brake cleaner or carburetor cleaner, because you're literally spraying gas on a fire.
 
If this is the intake you are using with an AFB or AVS style carburetor. Your vacuum leak will be at the carburetor base if you didn't use a metal separator plate and 2 gaskets


1732965421357.png



Here is what I used also acts as a heat dissipator
1732966039221.png
 
Ill try this out
All around the carburetor base, the intake gaskets and anywhere there's a vacuum hose plugged into something....like the intake, brake booster.....wherever. It's just water, so it won't hurt a thing, but it'll sure show up a vacuum leak. You'll know it if you hit one.
 
Well I didn't want to run it like that but I was trying to see if my vacuum leak went away if I either (A) just ran two opposite breathers, or (B) ran it with pcv one side and breather on the other. I couldn't do option b because I had nothing to plug the breather hole with at the time. I'm just confused by the outcome because I've seen engines run with just the two breathers.
The reason that the PCV and breather are on opposite sides is so that, as the PCV sucks up the blow-by fumes, Fresh air, comes in thru the breather on that opposite side. If the breather was on the same side, then the PCV would just suck the fresh air in and out, and the CC fumes would just stay where the are.
The thing is, if you already have a lot of blow-by, maybe the PCV cannot get rid of it all.
If you look at the picture that Rusty provided, you will see a breather hose going from the air-cleaner house to the breather. This provides clean filtered air to the breather under most circumstances. But when, like at WOT, the PCV system cannot keep up, then the excess blow-by, backs up the breather hose and into the filter housing, and the engine gobbles it up anyhow. By routing the breather hose to the clean side of the filter house, excess blow-by gasses do not oil-up your airfilter.

To make your engine idle the best that it can; Try this;
Reset your Idle timing to in the window of 8 to 12 degrees, then;
reset the mixture screws to in the middle of their range, make sure the secondaries are fully closed but not sticking; then adjust the speed screw to provide the rest of the fuel from the transfers.
If the idle speed gets to be too fast, retard the timing. Repeat as often as necessary until you have your engine idling as smooth as is possible, at whatever idle speed it likes in the window of about 550>700 in gear, with the idle-timing ending up in the range of say 10 to 12 degrees. Then fine tune the Idle mixture, with a minor mixture screw adjustment.
If this is not willing to happen, THEN go look for a vacuum leak.
This will get you very close.

With a stock cam or a modest cam;
setting the idle-timing to a high number like more than 16>20*, is, in most cases, a sure fire way to screw up your low rpm Part-throttle fuel-delivery curve, and often introduces an off idle-stumble, that you might want to try and cure with accelerator pump-shot. For a street driven car, this is not a good idea. That extra gas will cost you at the pump, on every fill-up. And honestly, the sag is easily cured with properly working transfers. The first time your engine actually cares about Power-timing, is at convertor stall. There is no ideal Idle-Timing. Well there might be, but without EFI, it will be impossible to drive it off the line. This is because the number will climb to over 25 degrees, possibly as high as 35; but to get that, and to still idle in the window, the throttle will have to be nearly closed, and then, to get the gas, the mixture screws may have to be adjusted to the max. Ok now, just try to put it into gear, and drive off like that, It's Not gonna happen.
 
A working PCV valve should be closed at idle because the valve is sucked closed by high vacuum. It sounds to me that you have a valve that is stuck open, causing a significant vacuum leak at idle. A properly working valve should have no effect on the idle mixture.

That said, fixing your breather setup is a must, because a pcv valve on the same side as a breather does nothing at all. The breather is preventing the pcv from pulling air through the crankcase.
 
[1] No, the PCV is NOT closed at idle when working correctly. At idle, it has a small gap to let air pass.
[2] The breather arrangement is not perfect, but it would not cause your 'problem' & I wouldn't bother changing it.
[3] Your 'problem' could have a few causes:
- when you removed & blocked the PCV, you removed a SMALL amount of idle air that the carb needs to idle with. I would have expected a drop in idle rpm, but not the engine running very rough & stalling....which leads to the next possibility.
- the cam. If the cam is 'sizeable' & has low vac at idle, the PCV may not seat at idle..& stay open. So then, a large amount of air FOR IDLE is being supplied by the PCV & then when it is removed the engine stalls because it has lost a large amount of the idle air.
- what rpm does the engine idle at? Rough or smooth idle? Do you have the cam specs?
- measure the vac at idle.
 
[1] No, the PCV is NOT closed at idle when working correctly. At idle, it has a small gap to let air pass.
[2] The breather arrangement is not perfect, but it would not cause your 'problem' & I wouldn't bother changing it.
[3] Your 'problem' could have a few causes:
- when you removed & blocked the PCV, you removed a SMALL amount of idle air that the carb needs to idle with. I would have expected a drop in idle rpm, but not the engine running very rough & stalling....which leads to the next possibility.
- the cam. If the cam is 'sizeable' & has low vac at idle, the PCV may not seat at idle..& stay open. So then, a large amount of air FOR IDLE is being supplied by the PCV & then when it is removed the engine stalls because it has lost a large amount of the idle air.
- what rpm does the engine idle at? Rough or smooth idle? Do you have the cam specs?
- measure the vac at idle.
 
[1] No, the PCV is NOT closed at idle when working correctly. At idle, it has a small gap to let air pass.
[2] The breather arrangement is not perfect, but it would not cause your 'problem' & I wouldn't bother changing it.
[3] Your 'problem' could have a few causes:
- when you removed & blocked the PCV, you removed a SMALL amount of idle air that the carb needs to idle with. I would have expected a drop in idle rpm, but not the engine running very rough & stalling....which leads to the next possibility.
- the cam. If the cam is 'sizeable' & has low vac at idle, the PCV may not seat at idle..& stay open. So then, a large amount of air FOR IDLE is being supplied by the PCV & then when it is removed the engine stalls because it has lost a large amount of the idle air.
- what rpm does the engine idle at? Rough or smooth idle? Do you have the cam specs?
- measure the vac at idle.
 
[1] No, the PCV is NOT closed at idle when working correctly. At idle, it has a small gap to let air pass.
[2] The breather arrangement is not perfect, but it would not cause your 'problem' & I wouldn't bother changing it.
[3] Your 'problem' could have a few causes:
- when you removed & blocked the PCV, you removed a SMALL amount of idle air that the carb needs to idle with. I would have expected a drop in idle rpm, but not the engine running very rough & stalling....which leads to the next possibility.
- the cam. If the cam is 'sizeable' & has low vac at idle, the PCV may not seat at idle..& stay open. So then, a large amount of air FOR IDLE is being supplied by the PCV & then when it is removed the engine stalls because it has lost a large amount of the idle air.
- what rpm does the engine idle at? Rough or smooth idle? Do you have the cam specs?
- measure the vac at idle.
You are correct, sir. Closed MORE at idle would be more accurate. The diagram I posted clearly says that.
 
[1] No, the PCV is NOT closed at idle when working correctly. At idle, it has a small gap to let air pass.
[2] The breather arrangement is not perfect, but it would not cause your 'problem' & I wouldn't bother changing it.
[3] Your 'problem' could have a few causes:
- when you removed & blocked the PCV, you removed a SMALL amount of idle air that the carb needs to idle with. I would have expected a drop in idle rpm, but not the engine running very rough & stalling....which leads to the next possibility.
- the cam. If the cam is 'sizeable' & has low vac at idle, the PCV may not seat at idle..& stay open. So then, a large amount of air FOR IDLE is being supplied by the PCV & then when it is removed the engine stalls because it has lost a large amount of the idle air.
- what rpm does the engine idle at? Rough or smooth idle? Do you have the cam specs?
- measure the vac at idle.
I don't know my cam by the reason I'm hunting for a vac leak is that I have idle in park all the way up at 1000 and In gear it drops to the 5 to 7 range depending on if it's warmed up
 
1977 engines were low compression. Not a good candidate for a performance cam.
Do what RRR said in post #8 to see if you have a vac leak; I doubt you have. Be careful when spraying around the throttle shafts because they have clearance & fluid will be sucked in & there will be a change in the engine note; this would be NORMAL, not evidence of a vac leak.
I suggest you try this & report back. I suspect the engine has 'a cam' in it & just needs tuning.
 
I don't know my cam by the reason I'm hunting for a vac leak is that I have idle in park all the way up at 1000 and In gear it drops to the 5 to 7 range depending on if it's warmed up
WHERE is your initial (idle) timing set? if it's too low, that could be your issue.
 
1977 engines were low compression. Not a good candidate for a performance cam.
Do what RRR said in post #8 to see if you have a vac leak; I doubt you have. Be careful when spraying around the throttle shafts because they have clearance & fluid will be sucked in & there will be a change in the engine note; this would be NORMAL, not evidence of a vac leak.
I suggest you try this & report back. I suspect the engine has 'a cam' in it & just needs tuning.
My block is a 77 but I really don't know how much of my engine is in its original configuration. It's in a 55 Plymouth with lots of custom fab all over. From what I can gather I believe a lot of the parts used probably came off of a Dodge truck. I don't know if any of that is helpful information but I'll be back with the results tomorrow hopefully.
 
The symptoms could be due to a 'hot cam' [ which would require more timing at idle ]. But the possibility of a vac leak needs to be checked out first to avoid going down rabbit holes.....
 
The symptoms could be due to a 'hot cam' [ which would require more timing at idle ]. But the possibility of a vac leak needs to be checked out first to avoid going down rabbit holes.....
I sprayed water all around the carb and the intake and the idle didn't fluctuate. I will be replacing the valve covers soon to eliminate the extra breather and then Im going to try redoing the timing and carb adjustments.
 
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