Low RPM Drivability Issue / 1973 351C

[Hemikiller]

PCV is adding extra air into the intake, you need to increase your pump shot more to cover for the leaner mixture. 

 

[73inNH]

PCV is adding extra air into the intake, you need to increase your pump shot more to cover for the leaner mixture. 

I suspect you're right. Problem is, it's already on the top hole and I bent the rod to get a bigger shot. I'm pretty maxed out . . .

I'll continue to check for other air leaks. Maybe I'm missing something.

 

[Bentworker]

What is your idle mixture like?  You may try richening it.

 

[73inNH]

On the passenger side near the firewall, they're the large blue can and a small black coffee can. The small black can connect to a vacuum port on the manifold and the other side appears to go through the firewall into the car.

What is that? I don't detect a leak there with carburetor cleaner, but I'm wondering if I should disconnect that hose and plug the manifold port while I troubleshoot.

 

[Galucha]

I'd recommend running a PCV system over just having breathers. The pcv will actually pull out the combustion gases and carbon that builds up in the crankcase; whereas breathers only relieve the positive pressure built up by blowby.

Over time if the car is driven alot, that nasty stuff with get on all of your internal engine components and in your oil.

 

[boilermaster]

73inNH.

Bentworker makes a good point.

if you adjusted your idle mixture with the pcv disconnected and plugged and then added it to the mix you are going to be leaner than you were before, also what pcv valve are you using ? different pcv valves have different metering values.

I too like the ME wagner adjustable pcv valve, also 650 rpm idle speed seems a tad low (by todays standards) unless you have a totally stock engine and I have no clue how the transfer slots work on your particular carb.

 

[Hemikiller]

I suspect you're right. Problem is, it's already on the top hole and I bent the rod to get a bigger shot. I'm pretty maxed out . . .

I'll continue to check for other air leaks. Maybe I'm missing something.

Then you'll need to increase the size of the discharge nozzle to deliver a faster pump shot. You can drill them if you have the appropriate size drill bits, or get the kit from Edelbrock. 

https://www.summitracing.com/parts/edl-1475

 

[Bentworker]

On the passenger side near the firewall, they're the large blue can and a small black coffee can. The small black can connect to a vacuum port on the manifold and the other side appears to go through the firewall into the car.

What is that? I don't detect a leak there with carburetor cleaner, but I'm wondering if I should disconnect that hose and plug the manifold port while I troubleshoot.

Black can is a vacuum accumulator / check valve so you have steady vac to run your heater / ac actuators.  You could pull it off and plug it for curiosity.

 

[73inNH]

73inNH.

Bentworker makes a good point.

if you adjusted your idle mixture with the pcv disconnected and plugged and then added it to the mix you are going to be leaner than you were before, also what pcv valve are you using ? different pcv valves have different metering values.

I too like the ME wagner adjustable pcv valve, also 650 rpm idle speed seems a tad low (by todays standards) unless you have a totally stock engine and I have no clue how the transfer slots work on your particular carb.

Agreed. I'll reconnect the PCV this weekend and reset the idle air screws to see if it helps. It makes sense . . .

 

[73inNH]

I changed my spark plugs today. Here's a pic of the old ones.

Does this look a little lean?

 

[c9zx]

Hard to be precise looking at pictures but, the dark soot ring and perhaps the soot on the center electrode indicate a slightly rich condition at idle. Where the ground strap changes colors seems to indicate initial and total are right or very close to it. Might be a bit lean at WOT but you really need to look at the base of the porcelain, not the electrode end, to tell. You can buy a plug reading light, or cut the threaded portion of one plug off and look for a ring where the porcelain meets the steel body. I hope you get it sorted out. Chuck

 

[73inNH]

An update . . .

Thinking about the PCV and how when I connect it to the PCV port on the carb, the car bogs (running lean, likely), I decided to plumb the PCV into the air cleaner base. As I was looking at my setup, I remembered that the PCV was originally plumbed into the stock air cleaner housing, not the carb. When I switched to the Edelbrock 14" air cleaner, I plumbed it into the carb, which caused the bog.

So I bought an elbow and a grommet, drilled out a hole in the air cleaner base, and run a small hose from the base to the PCV in the valve cover. This worked. I now have the PCV connected, and the engine runs well. I still have a very small hesitation when mashing the throttle from low RPMs. I may buy the Edelbrock accelerator pump tuning kit to give it a bigger shot, but the problem is so small now that I'm in no rush. 

 

[boilermaster]

73inNH,

Just to be 100% clear here,

The OEM pcv setup on our cars were designed to have a vacuum side and a pressure side.

The vacuum side consists of a vacuum port at the carb (or spacer under the carb) for pcv vacuum

and pcv valve in the left rocker cover.

The pcv pressure side consists of  a grommet, elbows and an attachment to the air cleaner to allow the  pressure side to be drawn into the air cleaner to allow the system to flow.

If you don't allow the system to actually flow, all you are getting is a metered vacuum leak.

FWIW, If you are running stock valve covers with a twist on oil cap make sure the cap still has baffeling material in it.

If you are running aftermarket covers with press in covers you need to have a way to baffle the oil or you will end up with oil being sucked thru the pcv system and an oil consumption issue and possible engine damage.

I think you are doing this correctly, just want to be 100% thanks for understanding.

Boilermaster

 

[73inNH]

@boilermaster

My exact setup:

1. Run of the mill breather in passenger side valve cover.

2. PCV in driver side oil cap.

3. Hose from PCV to 14 round Edelbrock air cleaner.

A pic of my oil cap is attached.

Will this work well?

 

[mjseakan]

So I bought an elbow and a grommet,

I've been following this topic pretty closely because your problem sounds an awful a lot like mine: Bogging down when mashing down on the gas pedal.  I also have an Edelbrock Performer intake but with an Edelbrock 1406 carb. When I swapped out the intake and carb I plumbed the PCV valve into the carb like you did. Maybe this is my my issue too.

So, would you happen to still have the specs on the elbow and grommet you purchased? I'd like to see if that helps me out too.

Thanks,

Mike

 

[73inNH]

I've been following this topic pretty closely because your problem sounds an awful a lot like mine: Bogging down when mashing down on the gas pedal.  I also have an Edelbrock Performer intake but with an Edelbrock 1406 carb. When I swapped out the intake and carb I plumbed the PCV valve into the carb like you did. Maybe this is my my issue too.

So, would you happen to still have the specs on the elbow and grommet you purchased? I'd like to see if that helps me out too.

Thanks,

Mike

Mike,

Before spending any money, disconnect the PCV valve and plug the fitting on the carburetor and drive. If your problem is similar to mine your car will likely be substantially better after doing that. Then you can figure out next steps.

 

[mjseakan]

That makes good sense.  I'll give it a shot tonight if it stops raining.

I've been known to jump to conclusions a little too quickly.

Mike

 

[boilermaster]

73inNH,

IF your breather in the passenger side is open to the atmosphere , it has to overcome atmospheric pressure to flow.

If the breather is inside the air cleaner (as this system was stock) it uses the engines intake airflow to overcome that atmospheric pressure in order to work properly.

pressure and vacuum always trying to equalize, it's a funny thing.

Boilermaster  

 

[Don C]

@boilermaster

My exact setup:

1. Run of the mill breather in passenger side valve cover.

2. PCV in driver side oil cap.

3. Hose from PCV to 14 round Edelbrock air cleaner.

A pic of my oil cap is attached.

Will this work well?

There is not enough vacuum in the air cleaner (unless the filter element is plugged with dirt) to draw in the fumes from the crankcase. Both the breather and PCV are basically operating at atmospheric pressure. For it to function and evacuate the fumes the PCV valve must be plumbed into the intake. The fumes consist of moisture, unburned fuel and vapors from the engine oil, not something you want to stay in the crankcase and turn into corrosive goop.

The breather side of the system is plumbed into the air filter housing to provide a clean air source so that dust and dirt is not drawn into the crankcase.

The need for removing the fumes was recognized way back, and the original system was what was called a road draft tube to draw out the fumes and a breather cap that had some filtering built into it.

 

[73inNH]

There is not enough vacuum in the air cleaner (unless the filter element is plugged with dirt) to draw in the fumes from the crankcase. Both the breather and PCV are basically operating at atmospheric pressure. For it to function and evacuate the fumes the PCV valve must be plumbed into the intake. The fumes consist of moisture, unburned fuel and vapors from the engine oil, not something you want to stay in the crankcase and turn into corrosive goop.

The breather side of the system is plumbed into the air filter housing to provide a clean air source so that dust and dirt is not drawn into the crankcase.

The need for removing the fumes was recognized way back, and the original system was what was called a road draft tube to draw out the fumes and a breather cap that had some filtering built into it.

In essence, I want:

1. Driver's side PCV valve to PCV port on carb (which has given me problems, but I can revisit).

2. Passenger side breather replaced with hose to air cleaner.

That will give me a properly functioning PCV system, correct?