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Low RPM Drivability Issue / 1973 351C


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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?

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My first guess is a vacuum leak, getting an intake installed on a Cleveland without any leaks can be a little tricky. Do you have any popping or backfiring when it doesn't want to run? My experience with Edelbrocks is that they have been too rich out of the box.

I would back the timing down 4°, see if that makes any difference. Then advance it 4° higher than it is now, see if that makes a difference. 

You might also try it without the vacuum hose connected to the vacuum advance, and plugged, see what effect that has.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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4 minutes ago, boilermaster said:

73inNH,

Did you have this low speed issue before the carb swap, or did this issue pop up because of the carb swap ?

I swapped the carb and intake together. It was not present with the old intake/carb. I have the old carb and could swap it to the new intake. I just didn't go there yet, figuring I'd get this nailed soon. But "soon" has turned into a week.  :/

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3 minutes ago, Don C said:

My first guess is a vacuum leak, getting an intake installed on a Cleveland without any leaks can be a little tricky. Do you have any popping or backfiring when it doesn't want to run? My experience with Edelbrocks is that they have been too rich out of the box.

I would back the timing down 4°, see if that makes any difference. Then advance it 4° higher than it is now, see if that makes a difference. 

You might also try it without the vacuum hose connected to the vacuum advance, and plugged, see what effect that has.

I have heard a slight backfire upon testing, but not significant or common. Maybe twice during my week of "playing".

I'll play with the timing tomorrow.

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73inNH,

did you adjust timing since the carb and manifold swap if not leave the timing alone.

suggest you get some carb spray and test for leaks and if none are found bolt your old carb back on (if possible) and see if the issue is gone, if it goes away, then you know you have a carb and (only a carb issue).

 

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15 minutes ago, boilermaster said:

73inNH,

did you adjust timing since the carb and manifold swap if not leave the timing alone.

suggest you get some carb spray and test for leaks and if none are found bolt your old carb back on (if possible) and see if the issue is gone, if it goes away, then you know you have a carb and (only a carb issue).

 

Timing was set at 16 before/after the swap.

Will test a few timing changes tomorrow, then check for leaks, then change plugs, then swap old carb back.

Thanks, all, for the help so far!

Edited by 73inNH
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My best guess is also a vacuum leak due to manifold install.  Start spraying brake/carb cleaner around the base of the manifold to see if you get any increase in idle.  Also try covering up the top of the carb with your hands or a rag, it should bog down or nearly stall out the engine.  It could also be a timing issue, but since you didn't have the issue prior and didn't mess with the timing, it's probably not a timing issue.

Out of curiosity, where in NH are you?

Jason (71 Mach 1, 351C 4V, 4 Spd. Toploader, Grabber Blue)

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15 hours ago, 73inNH said:

I have heard a slight backfire upon testing, but not significant or common. Maybe twice during my week of "playing".

I'll play with the timing tomorrow.

Backfiring can be an indication of a lean condition, which would point to a vacuum leak, and might just be one or two cylinders. Take a look at all of the plugs, see if some are lighter colored than the others.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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73in N.

I am not a Edelbrock/Carter guru by any means, strictly Holley spoken here, however,

I see you have your accelerator pump in the richest position and Clevelands Do like quite a bit of accelerator pump right now.

That being said, with the engine fully warmed and choke fully opened and at idle position (engine off to save eyebrows)

Have you checked to see if your pump nozzles are supplying fuel the very instant the throttle blades are moved off the idle position ?

Sounds very trivial, but this needs to happen to keep from a lean tip in regardless of how much total pump shot volume you might have.

You could even have a lean tip in with too much pump shot if your pump linkage is not right. costs you zero money and very little time to check this (unless you go the scorched eyebrows route)

Boilermaster

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24 minutes ago, boilermaster said:

73in N.

I am not a Edelbrock/Carter guru by any means, strictly Holley spoken here, however,

I see you have your accelerator pump in the richest position and Clevelands Do like quite a bit of accelerator pump right now.

That being said, with the engine fully warmed and choke fully opened and at idle position (engine off to save eyebrows)

Have you checked to see if your pump nozzles are supplying fuel the very instant the throttle blades are moved off the idle position ?

Sounds very trivial, but this needs to happen to keep from a lean tip in regardless of how much total pump shot volume you might have.

You could even have a lean tip in with too much pump shot if your pump linkage is not right. costs you zero money and very little time to check this (unless you go the scorched eyebrows route)

Boilermaster

Good timing on this one. . . :)

I haven't finished all the troubleshooting and wanted to do today, but one thing I did do was increase the accelerator pump shot. It was already on the top hole, which is the largest shot, however I remove the accelerator pump rod and bent it enough so that the shot would be larger. After a test run, that fixed almost all of the issues.

There is still a very minor bog right off idle, that I will still work on. Still verifying timing, and checking for vacuum leaks (I want this to be 100%). I'm also waiting for my new plugs to come in so I can replace them.

However, the car is now much more driveable.  :)

 

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Yes, it sounded like you were not getting enough of a pump shot from your accelerator pump. I am a Holley guy, so I am uncertain how the accelerator pump circuit works on the Carter carbs, but all carbs are more or less the same. Make sure that as soon as you touch the accelerator, the carb is pumping fuel in through discharge nozzle. On a Holley the accelerator pump arm can be adjusted loosely, and it will cause a delay form the time you press the accelerator to the time you get a pump shot, and it will cause the exact condition you have. I don't believe that the AVS has any adjustment like the Holley does. Can you get a bigger discharge nozzle for the AVS? 

I would try two things before I do anything else. First richen the idle screws and try it out, since you have a lean bog this may fix your issue. If that does not work, try to connect your distributor vacuum advance to direct instead of ported vacuum, this will increase you timing at idle and may also fix the issue, this is just a test, do not use the car with direct vacuum to the distributor. If the timing fixes the issue then you will need to increase your base timing, you balancer could be off and you think you have 16 degrees when in reality you have something else, like maybe 6. BTW unless you know exactly the centrifugal advance that your distributor is giving you, you should always time your engine for total advance with a timing light that has an advance dial. 

Edited by 71ProjectJunk
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Update . . . I fixed the bog issue by richening the accelerator pump. That worked great!

HOWEVER before that I disconnected the PCV, capped the port on the carb and installed a breather on both valve cover gaskets.

Now that things are running good, I wanted to put the PCV back. I purchased a new PCV valve, new hose and connected everything with hose clamps. This PCV valve has two nipples so I capped off the unused (small) one. I used the large front middle port on the Edelbrock. 

After reconnecting, the bog is back.  :/  So I removed the PCV again, capped the port on the carb, reinstalled the breather and the car runs great again.

So . . .

1. Do I need the PCV valve or are the dual breathers enough?

2. Any idea why the car bogs with the PCV connected and runs great without it?

 

Edited by 73inNH
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The PCV is basically a controlled vacuum leak designed to suck the crankcase vapors back into the engine for combustion.  It makes sense that you would have to tune differently with and without one.  If you like fiddling with stuff I suggest a ME Wagner adjustable PCV valve which is pretty slick.  Crankcase breathers are fine, but you are going to smell the funk of the engine while cruising.  

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2 minutes ago, Hemikiller said:

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.

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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.

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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.

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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.

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