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351 cleveland hesitation and slow.


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I have a 1973 Mach 1 with a 351 cleveland and c6 auto. I just completed a Restomod on the car after having it for almost 20 years. The car started life as a 2-barrel but I changed to a 4-barrell with the correct 73 4 barrel heads, I know its yucky open chambers. Anyway with this new motor refresh it was bored .030, Edelbrock Performer 4 barrel intake, a mild cam, and a Holley 670 cfm carb. So the issue is that the car starts and runs but is very draggy or slow with hardly any responsiveness in the throttle till 2000 rpm. Once its there the motor responds better and really responds at 3000 rpm, problem is that's 80 mph which is hard to find a road in the Ozarks to drive it on at that speed, LOL! So this is my first Mustang build and is my 30 year National Guard retirement present, from wife. I'm not really sure how it is suppose to respond but it really seems like there is something wrong. Any thoughts on what to try would be greatly appreciated. Thanks!

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There are two possibilities for 4V heads used in '73, they both have open chambers (75.4cc) but one has small valves (2.04/1.66") and the other has large valves (2.19/1.71"). Do you know what the casting numbers are?

That said, the open chamber heads still have the potential for making good power on pump gas and mild cams.

Do you have the specs on the cam? Was it degreed when it was installed? If it is retarded you will have less bottom end and more top end power.

The other potential issue is in the rear end gearing. Your car probably came with a rear end ratio of around 3.00:1. Even a mild cam is likely to want a lower ratio, in the 3.50:1 range. Going with a little lower ratio in the rear end will make a big difference in seat of the pants feel on acceleration.

Edit: And of course, the carburetor can have a huge effect on power. You may be too lean until the secondary throttle plates begin to open. The Clevelands like 12° to 16° initial ignition timing. What is your timing set at?

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

--Albert Einstein

 

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As a beginner carburetor tuner myself I have a few suggestions.  Buy a vacuum gauge if you don't have one.  Buy a timing light where you can advance the strobe and it has an advance display.  Get an exhaust shop to weld in a bung for an O2 sensor, and install an Air Fuel Ratio gauge (temp install is fine, mine plugs into the cigarette lighter).  Having those tools at your disposal will make your tuning experience far less frustrating.  

My car had some drivability issues, and with the help of some tools and knowledgeable folks it now rips, and has manners.

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6 hours ago, 73 SGM Mach said:

I have a 1973 Mach 1 with a 351 cleveland and c6 auto. I just completed a Restomod on the car after having it for almost 20 years. The car started life as a 2-barrel but I changed to a 4-barrell with the correct 73 4 barrel heads, I know its yucky open chambers. Anyway with this new motor refresh it was bored .030, Edelbrock Performer 4 barrel intake, a mild cam, and a Holley 670 cfm carb. So the issue is that the car starts and runs but is very draggy or slow with hardly any responsiveness in the throttle till 2000 rpm. Once its there the motor responds better and really responds at 3000 rpm, problem is that's 80 mph which is hard to find a road in the Ozarks to drive it on at that speed, LOL! So this is my first Mustang build and is my 30 year National Guard retirement present, from wife. I'm not really sure how it is suppose to respond but it really seems like there is something wrong. Any thoughts on what to try would be greatly appreciated. Thanks!

Look into ignition stuff, and timing. What do you have installed in that motor? Points? Aftermarket stuff? How did you set the timing? Bunch of great useful info on this site for that too.

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What are you comparing it to? The C6 takes quite a bit of power to drive, the likely 3.00:1 gear ratio will make it feel sluggish, carb tuning can play a big role as well as cam timing. 

I always start with asking what is your realistic plan for the car? Cruise nights, Road trips, Track days, Drag racing? This will make the biggest impact on how you refine the car. If you plan road trips you do not want a 4.11:1 rear gear with a C6. 

Start with your goal in mind and make changes from there. There is a ton of amazing advice on this site and we'll try not to steer you wrong. 

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[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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Could be a lot of things. A mildly cammed 351C should be really snappy on the low end, regardless of what's attached to it.

Start with the basics, especially your initial timing and the timing curve. The factory specs and timing curve are no good for a performance setup. You can safely run 10° ~ 16° initial as long as you don't have issues with the starter turning it over when it's hot. A 2V headed engine will want in the neighborhood of 36° total. With a 3.00 gear, I'd want that all in around 2500rpm. 

For vacuum advance, which will help with throttle response and efficiency, I prefer ported vacuum. It'll bring in extra timing right when you open the throttle, which helps get a car with highway gears off the line. You'll want to make sure you don't have an excessive amount of VA, it'll cause pinging on hills and such. 

Also, let us know more about your setu, it'll help with specific recommendations. 

 

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Ok the valves are the big ones, (2.19/1.71"). The rear-end is the factory 2.75 gears. The cam specs are pictured below. I will say I don't believe this problem is related to the rear-end specifically. The carb seems to be loading up. When you first take off there is hesitation like a flat spot so much so its trying to die. Then it will pickup a little and the more speed/rpm you get it seems to come out of it. Also sometimes driving down the road below 4 mph the carb wants to load up and or will backfire through the carb. We sat the initial timing to 8 degrees but I see we need to up that. Also friend who helped me build the car says he wonders if it's a tooth out on the dizzy but it's his first Ford build also. The cam we are unsure if it was retarded on installation and the guy who did it is not available to ask, Covid issues. So we are kinda stuck. As always thanks in advance.

20211022_153341.jpg

20210913_171301.jpg

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73SGM Mach,

Sounds like you are in need of some accelerator pump tuning.

Would you please post your Holley carbs list number so we can see what pump nozzle your carb came with ?

You might end up having to buy a new larger nozzle, there is more to tuning a Holley pump but it is pretty common to need a larger nozzle on a Cleveland 4 V.

 

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I would say more initial timing, it really woke my engine up. I am at 12-14 initial and 26 degrees centrifugal (38-40 total) with 2V open chamber heads. I have a 213,213 cam.

my car used to go down the highway at 60 between 2400 and 2500 rpm with 2.75 gears and a FMX 1:1 third gear.

I used to have 6 degrees initial and 36 degrees centrifugal advance and the engine was very sluggish to 3000 rpm.

you can recurve a ford distributor by following directions below or call Parkland Performance to get a custom curve.

http://www.reincarnation-automotive.com/Duraspark_distributor_recurve_instructions_index.html

I recommend using manifold vacuum advance to keep your engine cool at idle.

 

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That is indeed a mild cam, and shouldn't have any problem pulling with the rear end you have. 

As long as you can turn the distributor body and cap so the rotor points to number 1 terminal when the engine is at top dead center on the compression stroke it doesn't make any difference what the tooth position is in relation to the cam gear. If the distributor shaft teeth are rotated too far off the distributor vacuum advance can will prevent the correct rotor/cap alignment. 

If your timing chain were off one tooth that would be a huge issue.

Start with the ignition timing, that is the easiest. If that doesn't help then start adjusting the carburetor. Check what the accelerator pump cam is set at. You'll need to see what your vacuum is at idle, to get an idea of what power valve your carburetor needs. Do you hear any popping or backfiring when you try to accelerate? You'll need to make sure you don't have any vacuum leaks, check all of the vacuum hoses and PCV valve and hose.

Edited by Don C

 

 

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

--Albert Einstein

 

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You also have a Holley SA 670 carb, which is pretty difficult to tune properly. Those carbs usually require a 11/32” (I think that’s the size) hole drilled in the primary butterflies. You will also need to make sure there’s not to much of the transfer slots exposed. I know the one of our members, Stanglover can guide you through this if you’re still having issues after confirming your ignition settings. He went through this with his car and he has it running well. But with 2.75 rear gears, it won’t be really quick off the the line.

John - 72 Q Code

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Ok, that cam is mild and should be fairly good down low when tuned properly. Did you use flat top pistons, or the original Ford dished pieces?

The factory 71-73 351CJ, which had a 206/220 @ .050" duration cam used 16° BTDC as base timing. Your 8° initial is too low. Advance the timing to around 15°, and make sure she still starts without issue when hot. This will make a huge difference in your throttle response. You may have to limit your total timing back to the 36° range. 

Your backfire on acceleration is almost definitely a lean condition caused by insufficient accelerator pump shot. You will need to increase the nozzle size and probably change to a more aggressive pump cam. Rolling at 40mph in top gear is only 1500-ish with the 2.75s, so don't expect a lot. 

Another thing to consider, the 351CJ automatics had a high stall torque converter from the factory, about 2800rpm. The 351-2V had a 1500-1700 stall converter, which will definitely cause sluggish response on the low end. 

 

 

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On 10/23/2021 at 9:33 AM, jpaz said:

You also have a Holley SA 670 carb, which is pretty difficult to tune properly. Those carbs usually require a 11/32” (I think that’s the size) hole drilled in the primary butterflies. You will also need to make sure there’s not to much of the transfer slots exposed. I know the one of our members, Stanglover can guide you through this if you’re still having issues after confirming your ignition settings. He went through this with his car and he has it running well. But with 2.75 rear gears, it won’t be really quick off the the line.

 Yeah John, 11/32" hole is a tad on the big side! You meant 3/32" drilled center and in front of the shaft on each plate. That is what my tuning "guy" did on mine. I also increased the pump nozzle (squirter) from a 31 to a 35. Transfer slot should be "square" i.e. about .020" exposed. On mine (71 M code) with a mild cam, I found the original pump cam was good. I played with others, but ended up back at the original. Vacuum is 17" but I kept the power valve at 6.5. It did not like the 8.5 that the vacuum number would indicate. 

@73 SGM MachTiming on mine is 14 deg. initial, 20 crank (10L slot) plus 4-6 vacuum. Search my threads on timing that worked for me. Your 73 may be different with open chambers.

Geoff.

 I learn something new every day!

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