Sputtering under load - Vintage Mustang Forums
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post #1 of 9 (permalink) Old 09-10-2019, 09:08 PM Thread Starter
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Sputtering under load

I managed to destroy another speedometer cable on my '68 fastback last night so I dusted the cobwebs off the '69 to take to work today.

She fired up great and everything was normal. Once warmed up, I mashed the skinny pedal going onto to the freeway ramp. Shortly after, I noticed surging at freeway speeds. When I pressed the accelerator pedal, the engine would sputter- it almost felt like there was a rev limiter prevent the RPM from increasing. It revs fine in neutral, but sputters and refuses to accelerate when there's a load such as cruising on the freeway.

The last thing I did to it was replace the 20 gallon tank with a 22 gallon tank, everything seemed fine after the swap from what I recall.

I *think* it's ignition related.

MSD ready to run distributor
MSD Blaster SS coil (the little square one)
Ford Racing plug wires
NGK spark plugs
Both the coil and distributor have been wired to receive 12 volts.

I haven't done anything to start diagnosing yet besides revving it at idle.

I'm thinking to start with a new cap and rotor? Anyone have this issue?
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post #2 of 9 (permalink) Old 09-10-2019, 09:14 PM
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Sounds to me like it’s running out of fuel.

Like maybe the pump isn’t pushing enough or float level is too low.
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1970 Mustang coupe
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post #3 of 9 (permalink) Old 09-10-2019, 09:51 PM
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How long did it sit with ethanol gas in it?
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post #4 of 9 (permalink) Old 09-10-2019, 10:48 PM
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Could be a hairline crack in the dist cap.
I had about 1/4 tank of ethanol gas sitting in my car in 2015 when I replaced the engine, took the chassis apart and rebuilt it, and swapped the C4 out for a T5. I finished the work in Feb. 2017 and the car started right up with the gas that was in the tank along with a bottle of Stabil fuel rejuvenator. Been driving it ever since.
Point being: I've heard a lot of horror stories about ethanol fuel issues clogging things up. I've owned and have been driving and storing my car since 2004 using ethanol (10%) fuel and have never had an issue.
Fuel stabilizer is a good thing though.
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69 Convertible, 351W, T5, E-Brock Performer 351-W intake, Performer Plus cam, Holley 4160, power assist, dual exhaust, 4-wheel manual drums.
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post #5 of 9 (permalink) Old 09-11-2019, 02:44 AM
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Sounds like the power valve on the carb, possibly the vacuum advance on the distributor not working well when the car needs vacuum for the carb. Auto transmission?

My 1st car...
'66 Tahoe Turquoise/ Aqua coupe
•289 / 4100 •C4 Auto •Disc Brakes
•Dual Exhaust •Quick Manual Steering
•Rally Pac •Console •Deluxe Belts
•LOTS of Rotunda accessories

Older son's 1st car...
'66 Emberglo / Parchment deluxe coupe
•289 / 2100 •C4 Auto
•Dealer A/C • Console
•Dual Exhaust • Power Steering

Younger son's 1st car...
'66 Nightmist / Blue & white deluxe coupe
•289 / 2100 •C4 Auto
•Factory A/C •Console

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post #6 of 9 (permalink) Old 09-11-2019, 10:09 AM
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Was fine until you "goosed it" then started? Going with junk in the carb. Not too often but, occasionally, I've been able to "unjunk" a carb with some carburetor cleaner aerosol, then a "giant suck" performed by opening the throttle wide to about 5,000 rpm and dropping a wet towel over the carb air horn. The high suction and fuel enrichment sometimes works the blockage loose. Start back up and shoot again with some carb clean and see how it is. If not better, time for a "kit".


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- Alfred E. Neuman

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post #7 of 9 (permalink) Old 09-11-2019, 01:14 PM
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Before you go too crazy, double check the float levels in the carb. Simple things first.

Nothing worthwhile is ever quick, cheap or easy, those that can't do, complain

71 Mach 1

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post #8 of 9 (permalink) Old 09-12-2019, 01:13 AM Thread Starter
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Thank you all for the suggestions. Messing with the float levels didn’t change anything. It seems to do it at slow speeds too once load is applied. I’m going to start with spark plugs and a fresh cap/rotor.

I’ve heard of weird behavior with cracked MSD’s caps, we’ll see what happens!

1969 Convertible field find (450hp spaghetti noodle)
1968 Fastback (428 and a 4 speed)


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post #9 of 9 (permalink) Old 09-12-2019, 02:03 AM
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If you have a fuel pressure gauge tee it in the line at the carb, place it where you can watch and go for a drive. If the pressure drops under acceleration you could have a marginal fuel pump, plugged filter or other restriction.
If the miss is a consistant sputter like you describe I would agree on ignition being a good guess too. Lean fuel will often also have a intake pop or backfire.

Last edited by Matter; 09-12-2019 at 02:07 AM.
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