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Loss of Power During Accel/Popping Noise Under Hood

3994 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)
-Carburetor(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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It doesn't matter what side the pvc valve is on, and the hose from the pcv goes to the base of the carb as it was. The other side can be open as it was but is more desirable to have the hose on that side go to the air cleaner so it is drawing in filtered air.
So I agree that the PCV may be the issue, but its not likely. Like Matter stated the PCV ports/locations can be left open, the engine will run the same. This is not true for the port on the carb spacer that the PCV hose connects to. If this port is left open you will have a vacuum leak which will cause the engine to run rough. Was the port on the carb spacer plugged or left open?

There are many possible causes for the issues you are experiencing. Likewise a lot of good information has been listed above. Be sure to diagnose the issue first. Do NOT throw parts at the problem hoping for a solution, this just causes aggravation. Additionally when the time comes only change one part at a time this way you know for sure what fixed the problem. This will allow you to expand your knowledge and will make diagnosing issues easier in the future.

The chances of this being your timing chain are very unlikely. Perform a compression test first. Yes a replacement timing chain is cheaper than a compression tester but like I stated above knowledge is power. Additionally what happens the next time you have engine troubles?

Keep in mind a timing chain replacement requires new oil, coolant, gaskets, and a harmonic balancer puller at the minimum assuming you have basic hand tools. This all adds up and the more you spend the more aggravating the hobby becomes...

Just food for thought.
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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.
Follow Milliagan’s recommendations above this will provide yourself with more information to diagnose the problem.
I’m sure that my chain is worn but I don’t think it’s skipping teeth. My engine starts up like a charm 3/4 times and doesn’t have an issue until heavy acceleration.
I agree if your engine has ever been rebuilt there is a good chance the nylon timing gears have been replaced therefore timing chain skipping teeth is a very rare problem.
I agree with awhtx the nylon gear design wasn’t the beat idea and many have had timing chain slipping issues.

My point is run as many teats as you can first and learn what each one of them is telling you. Id take a look at your spark plugs as well and check to make sure your points are operating as desired.
I’m not sure if the engine has been rebuilt before or not. I can send the P/O a text and ask him. Points were replaced in May, I will check them anyways and make sure they’re set properly. I changed the spark plugs and plug wires a few weeks ago. I gapped them to .035 and double checked before installation…those should be fine. Removed the nasty Bosch platinum plugs and put Autolite coppers in, someone mentioned copper is best for these older engines since that’s what they were originally made with.
I agree that the autolites are fine and I’m not telling you to re gap the plugs. Im telling you to read the plugs to see if the engine is running rich or lean…
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