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351C Stumbling at low RPM
#1
Hello, picked up a 72 Fastback with a 351C 4V a few months back. Been working my way through an auto to 6 speed manual trans swap and am now struggling a bit to get the engine running right. The car starts and runs well once it warms up, but at low RPM it just falls flat. Once RPM gets a bit higher, its great and pulls hard, but off the line it's terrible to the point of being difficult to drive as it will sputter miss. 

It's running an Edelbrock intake and 1407 750 CFM carb with a Mallory HyFire 6A ignition and long tube headers. Went through the fuel system, new plugs, wires, coil, tried timing all over etc. It's all way better now than when I started working on it, but still has that dead spot that makes it a handful to drive. Accelerator pump seems to be working OK and changing to the position on the pump linkage doesn't seem to make a ton of difference. 

I don't have a lot of info on the engine. I was told it had a mild cam, but that was unclear as the previous actual owner was not available and the info was maybe 3rd hand. 

From the beginning it seemed like this might be too much carb for the car but I really don't know enough about this engine or the 351C to know if that's true or if that's what is causing this issue. I'm not in a position to just buy another carb to see if that might work right now, so hoping I can get this sorted out enough to make it more drive-able.  

Thanks for any advice.
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#2
Make sure the ignition is getting 12 volts. Often they are wired to the pink resistor wire and will only get 7 volts when the ignition isn't cranking.

What do your plugs look like and what is the condition of the wires and cap and rotor? A good ground strap to the engine block is also very helpful.

a large percentage of suspected carburetor problems are actually ignition problems.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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#3
Which intake are you running from Edelbrock? What is your timing set at with vacuum advance disconnected and plugged? (initial and all in and at what RPM)? What are you pulling for vacuum at idle? 750 CFM is not too much carb for a Cleveland, especially with 4V heads. Your stumble is most likely caused by a lean condition caused by the shot of fuel not getting into the engine quickly enough. Is your vac advance hooked up to ported or manifold vacuum?

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
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#4
Thanks and agreed on the possible ignition problems. That's where I spent most of my time. Replaced the plugs, wires, cap, rotor and coil. Old plugs were mostly OK with a couple that were borderline fouled. It still feels like ignition, but with everything seemingly OK, I have been wondering about the carb. I'll double check the voltage, never heard of that and I'll see about getting a better ground on the block.

Thanks
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#5
(12-18-2018, 01:50 PM)73pony Wrote: Which intake are you running from Edelbrock? What is your timing set at with vacuum advance disconnected and plugged? (initial and all in and at what RPM)? What are you pulling for vacuum at idle? 750 CFM is not too much carb for a Cleveland, especially with 4V heads. Your stumble is most likely caused by a lean condition caused by the shot of fuel not getting into the engine quickly enough. Is your vac advance hooked up to ported or manifold vacuum?

OK, that's good to hear that the 750 CFM should work. Looks like a Torquer 2 manifold. I'm running at about 10 degrees advanced attached to ported vacuum. I don't currently have a vacuum gauge but have ordered one. On the timing really just ended up there as that's where it seems to be running best and not sure really where it ought to be.
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#6
I would also check for vacuum leaks. Make sure all of the hoses are connected and not rotted. Make sure the brake booster holds vacuum and is not leaking. Also, make sure the PCV valve is functioning.

The intake manifold and carburetor gaskets are also potential places for vacuum leaks.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#7
Can't speak from experience with the Mallory ignition, but I can speak from experience with the stock Autolite distributor.
My first question is; what is your initial timing set at? These 4V engine seem to like about 14 to 16 degrees plus 20 degrees on the crank, i.e. 34-36 degrees total MAX, no vac applied. Mine has about 4-6 degrees V/A on top and idles and pulls like a train. I run the Pertronix Ignitor II and coil, full 12V.
Second question; are those earlier CC heads or open? That may depend on when the engine was actually built, but I don't know exactly when the change over took place. You may get an idea by taking the valve covers off and checking the date codes located between the springs. It may make a difference to the way it tunes I'm thinking.
Third question; Does it sit and idle okay and what rpm? For a manual it should be around 800.
 Mine has a Holley 670 S/A, but it stumbled at low rpm, running too rich. I took it to a tuner and he drilled a 3/32" hole in each primary plate, fixed the issue. I know some disagree with this method, but it worked, This was combined with reworking the distributor to achieve the 20* crank advance and 16* initial.
It gets frustrating for sure, so many possibilities!!
Good luck and let us know when you get it sorted.
Geoff.

I learn something new every day!
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#8
Initial timing all depends on your entire set up. I am running around 16 initial and 36 total (all in by 3000). Torker II is a single plane intake manifold. You may want to think about swapping out to a dual plane at some point as you will get better low end throttle response, but the stumble needs to be corrected first.

Where is your idle speed at currently?

I would try this to see if this helps as this seems to be a fairly common issue with the 1407 carbs. Not knowing what your compression is and how much centrifugal advance you have makes this a little more difficult though.

Try adding some advance to the initial timing as 10 is a bit on the low side. Maybe try 14 and check for pinging under load and adjust back if ping or go up by 1 degree at a time with the vacuum advance disconnected and plugged. Once you have that taken care of and the idle set where you need it, switch the vacuum advance from ported vacuum to manifold vacuum (this should increase the idle speed), adjust the idle speed back down to where you want it. First test to see if you are getting any ping under load, (adjust timing if necessary) if no ping then test to see if off idle stumble issue is fixed.

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
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#9
(12-18-2018, 02:29 PM)TheDude Wrote:
(12-18-2018, 01:50 PM)73pony Wrote: Which intake are you running from Edelbrock? What is your timing set at with vacuum advance disconnected and plugged? (initial and all in and at what RPM)? What are you pulling for vacuum at idle? 750 CFM is not too much carb for a Cleveland, especially with 4V heads. Your stumble is most likely caused by a lean condition caused by the shot of fuel not getting into the engine quickly enough. Is your vac advance hooked up to ported or manifold vacuum?

OK, that's good to hear that the 750 CFM should work. Looks like a Torquer 2 manifold. I'm running at about 10 degrees advanced attached to ported vacuum. I don't currently have a vacuum gauge but have ordered one. On the timing really just ended up there as that's where it seems to be running best and not sure really where it ought to be.

 Mine is connected to the TIMED port on the carb, Again others have differing opinions on this.

I learn something new every day!
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#10
Pretty much ditto what 73Pony said except for manifold vacuum, but then I don't know your carb.

I learn something new every day!
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