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Discussion Starter #1
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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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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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?
 

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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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Discussion Starter #8
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!
 

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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.
 
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