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Loss of Power During Accel/Popping Noise Under Hood

3975 Views 39 Replies 14 Participants Last post by  Milligan
I feel bad asking yet again another question here, but I am trying to learn and fix my issues without spending thousands at a mechanics shop every time I run into a problem that's over my head. So here it goes.

I bought my 1971 Mustang 302 C4 Coupe a few months ago with some known issues, mostly cosmetic but a few mechanical issues. The P/O did quite a bit of work to get the car running since it had sat in dry storage from 2007 to May of 2021. I'll put a list at the end of this post.

Now my current issue. The car usually cold starts pretty easily with only some minor hiccups every now and then. I give it three pumps of the gas, and then pump the gas while cranking and she starts right up and stays running so long as I give her a few mild revs. I didn't pump the gas while cranking until the colder weather came (I'm in CNY so it's pretty chilly in Oct.) and without doing so the car would sputter out within seconds. So now the car is started, I let it warm up for 2-3 minutes in the garage and then pull out of my driveway. My road has a slight downgrade so it usually isn't necessary to give it much gas pulling out of the driveway. I live on a short road connected to a larger main road, and I typically coast down my road until the stop sign. Once I pull away from the stop sign, I start accelerating and the car just sputters and I lose all power for a second, and it roars back to life. If I don't take my foot immediately off the gas, the engine shuts off. I've kinda got it down to a science at this point. The car usually doesn't do it again during my drive unless I'm idled at a light for an extended period of time.

So that's the first thing it's doing. The second issue (which I believe are related) is during acceleration. If I push the gas pedal maybe 1/4 of the way down, the car accelerates perfectly, but if I push it down more than 1/2, all hell breaks loose. I can feel the power stop reaching the rear wheels and the engine makes this terribly loud popping and banging noise. The only thing I can compare the sound to is pops and cracks from a tuner. Don't misunderstand though, these sounds are right from the engine bay, not exhaust. So I've now got my foot all the way down on the pedal, car is popping and banging, I release my foot back to around a 1/4 and it feels as if the car just found its groove. I really don't understand it.

So here's the work that myself or the P/O has done:

-Radiator (me)
-Thermostat (me)
-Starter Solenoid (me)
-Fuel tank and lines (P/O)
-Distributor cap and rotor (P/O)
-Ignition points and condensers (P/O)
-Ignition coil (P/O)

I would also like to note that I tore the mufflers off this car, but the issues I explained above presented themselves wayyy before the mufflers came off. Before you judge, I'm redoing the exhaust this winter so I figured I'd have some straight-pipe fun for a month or so...why not? P/O believes the carb is running rich and the timing is retarded. I have no marks on my harmonic balancer so I'm not in a position to check the timing with a gun. I can tell you that the distributor is rotated in the direction that advances the timing. As far as the carb mixture, the car smells very gassy and has terrible gas mileage. Something else to note is the PCV valve doesn't sit properly in the oil cap and might as well be non-existent. A new oil cap and valve is in the mail.

Thanks so much in advance, I apologize for such a long post. I'm just trying to figure this issue out and unfortunately it's way out of my league. As a 17 Y/O, I just don't have the experience to diagnose something like this. Happy driving :)
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You don‘t have to have a PCV. A lot of guys just run filters on those. I wouldn’t think that is your issue.

I know the PO tinkered with a lot of stuff so I wouldn’t check those things off the list. There are plenty of new parts that I’ve put on that take a lot of work to get running properly. Unfortunately with all the things that have been touched you really have to run down the entire system from fuel tank line to pump to filters to carb to timing to vacuum lines and such. Make a big list and just start checking stuff off.

What’s your vacuum pressure at idle (check when warm) and is it steady?
What is your cold idle?
What is your hot idle?
What is your initial timing?
When does your timing advance come on and how much advance does it give?
Does your choke close properly and then how long does it take to electronically open?
Check fuel pressure in the lines, should be 4-7psi.
Float set properly? Open it up and make sure there isn’t debris in there (If he replaced all the lines, typically you want to purge the lines with a gallon or so before hooking it to the carb.)

I’m betting one of those gets you on the right track.
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Good news! I’d recommend getting yourself a vacuum gauge, they are only around $15. A vacuum gauge can tell you stuff like that instantly and much more. They usually come with a little sheet to tell you what the different readings mean.

Watch Photograph Analog watch White Clock

The hole on the back being unplugged would have had your idle vacuum below 10 I’m sure which would have been the read out of the bottom left on that chart. Very handy to start with for troubleshooting!
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