Low RPM Drivability Issue / 1973 351C

[Don C]

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?

Yes, that will give you a functioning system. Make sure the PCV valve is correct for your engine and that the valve inside of it moves. When you shake it up and down you'll hear it move. The valve is controlled by a calibrated spring. Under high manifold vacuum the valve is partially closed, limiting the amount of extra air entering the manifold and reducing the lean condition. Under acceleration, when the manifold vacuum is low, the valve opens further, this is also when the most bypass fumes enter the crankcase. If the engine backfires the valve closes to prevent it from entering the crankcase.

 

[73inNH]

Yes, that will give you a functioning system. Make sure the PCV valve is correct for your engine and that the valve inside of it moves. When you shake it up and down you'll hear it move. The valve is controlled by a calibrated spring. Under high manifold vacuum the valve is partially closed, limiting the amount of extra air entering the manifold and reducing the lean condition. Under acceleration, when the manifold vacuum is low, the valve opens further, this is also when the most bypass fumes enter the crankcase. If the engine backfires the valve closes to prevent it from entering the crankcase.

Got it.

OK, I'll work on the issues I have when the PCV is connected. It's fixable. . . . the extra air is leaning out the carb. I'll get it.

Thanks, Don and boilermaster.

 

[boilermaster]

73inNH,

DonC, as usual nailed it.

With just a breather on the passenger side rocker cover (open to atmosphere) and the pcv valve connected to the pcv port on your carb, you would have what is called an (open pcv system)

With the breather going to the carb, you will have a (closed pcv system) which is more efficient.

PCV valve selection is Critical, 

 

[73inNH]

73inNH,

DonC, as usual nailed it.

With just a breather on the passenger side rocker cover (open to atmosphere) and the pcv valve connected to the pcv port on your carb, you would have what is called an (open pcv system)

With the breather going to the carb, you will have a (closed pcv system) which is more efficient.

PCV valve selection is Critical, 

Awesome. You guys make life much easier. Thank you.

 

[trillizo_y_uno]

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

My guess is you need to swap out for stiffer springs.  Your secondaries are opening up before the secondary jets pop up and provide the fuel you need.

 

[boilermaster]

73inNH,

Motorcraft EV50 pcv valve comes to mind as a small orifice pcv valve for the Cleveland engines.

As for the bog, has correct fuel pressure been verified ?

Fairly easy with a Holley vacuum secondary carb when the secondary spring is too weak, don't know on the AVS if there is a way to Deactivate the secondary linkage or not.

 

[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.

Update . . . This worked. I reinstalled the driver side PCV and connected it to the proper (middle) port on the carburetor. The passenger side is a breather for now but I will connect that to the air cleaner tomorrow. I readjusted the idle air screws methodically with a vacuum gauge and it now runs great!

I still have a slight miss when I nail it hard, but that is likely an accelerator pump issue which I will deal with later.

 

[Bentworker]

Nice!  Glad to hear you are getting it sorted.

It is worth coming up with a tuning kit for the carburetor platform you run.  Takes the frustration out of it when you have all the bits.  
 

My Holley / Demon kit.

 

[c9zx]

I agree with Bentworker, it's good to have a few carb tuning parts on hand. Chuck

Inbox (3,143) - [email protected] - Gmail.pdf

 

[73inNH]

Update . . .

I bought a small hand drill set on Amazon and drilled out the accelerator pump one drill size larger. I have no idea what size that is, the drill bits are tiny. But I inserted the largest bit that I could into the accelerator pump jets, then drilled out to the next size up. It helped! But I think I could go one more size. I'll do that next time I have the carb apart.

To recap . . . here's what I've done to make the Edelbrock 1813 (800CFM) perform well on my 3514V

1. Set air screws as exact as possible using vacuum gauge (with PCV correctly functioning). I have my idle set around 850.

2. Drill out accelerator pump slightly larger.

3. Bend accelerator pump rod slightly to give a larger shot of gas.

4. Set the AVS secondary air flap to 2.25 turns closed (counterclockwise). Factory setting is 1.5 turns. It was opening way too early and leaning out the mixture. Adding 3/4 of a turn solved that. I might even try another 1/4 turn at some point.

5. Changed the rods 2 steps richer. NOTE: I'm not sure if this solved anything and might go back to stock at some point to test.

6. Changed the step up springs to silver (the stiffest ones).

I believe that items 1, 2, 3 and 4 had the largest impact on drivability.

Thanks to everyone for all of their help! 

 

[Hemikiller]

Sounds like you're making progress. FWIW, several of my 351C-4V engines required a #37 or #41 discharge nozzle. The stock Edelbrock comes with a #28. 

 

[hotrodjunkyard]

I'm having a drivability issue and I'm hoping for some help.

1973 Mustang 351 Cleveland 4V Q Code

The car runs great at higher RPM (2,500+?) but has very little power (maybe a bog?) down low. It's bad enough that it's difficult to get going in 1st gear without really slipping the clutch. Once RPMs get up, it runs really well. I can chirp the tires 1>2 and 2>3.

Background:

• Edelbrock 1813 AVS Thunder, 800CFM, Electric Choke (recently installed).
  
  
• Edelbrock Performer 4V Dual Plane Intake (recently installed).
  
  
• Vacuum advance using ported option on carb.
  
  
• 16 degrees initial timing with advance disconnected and plugged. I'm going by the markings on the harmonic balancer. I haven't verified the markings are correct, I'm assuming they are.
  
  
• Pertronix II distributor and Flamethrower II coil.
  
  
• 16 in HG at idle. Vacuum fluctuates between 16 and 15.5 (sometimes 16 - 15).
  
  
• Accelerator pump is on top-most hole.
  
  
• I set the air screws according to highest vacuum (I don't assume I didn't mess something up, but I did go through the procedure).
  
  
• I set the choke according to the manual. Idle is about 1,200 during fast idle.
  
  
• I changed the metering rods to 8% richer according to the manual (it didn't help).
  
  
• Idle speed is around 650 RPM.
  
  
• I have the carb mounted to a .5" phenolic spacer (order is gasket > phenolic spacer > gasket > carb).
  

This AVS carb has an adjustable secondary air "flap." I think I have it set a bit looser than factory. I'll reset it to factory, but I don't suspect it's the issue as I'm guessing the secondaries are not open when I have the issue. I did try spraying some carb cleaner around the carb, didn't notice a change in RPM or vacuum. I wasn't thorough, however.

I have the PCV port on the carb plugged and a breather in both valve covers.

I have new Autolite A25 plugs coming tomorrow. I'll change them when they arrive. Haven't changed wires yet and I don't know their age, but they look OK, visually.

The car reads 9,000 miles. Not sure if that's 109,000 or 209,000 etc. Timing chain could be stretched?

Again, the car runs great at higher RPMs. At low RPMs, it's a pretty doggy. To the point where it's tough to get going from 1st gear.

Thoughts?

Too large cfm carb and in my opinion sell that carb and find an old core of a Holley 735 vacuum secondary carb you will gain big horse power