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Loss of Power During Accel/Popping Noise Under Hood

3976 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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Its either too much fuel into the cylinders or not enough.
You've heard old cars are simple, right?
That's exactly why I love them so much, they're relatively simple. With that being said I'm still learning and trying to figure out my way around it. As far as the fuel goes, how do I go about adjusting that? Is it just the screws on the carb or something more?
It sounds to me like your carb is going "lean" when you try to accelerate. Carbs have an accelerator pump that squirts extra fuel into the manifold when the gas pedal is pushed down. This extra squirt of gas helps the engine rev up. When the engine tries to accelerate without the extra fuel the lean condition causes the engine to backfire. The gassy smell, poor mileage and backfiring indicate that you need a complete carb overhaul.

Your balancer surely has timing marks on it and they are hidden by oil or paint. Go under the car with a wire brush, a can of carb cleaner and a ratchet with a 15/16" socket. Put the ratchet and socket on the bolt in the center of the crankshaft pulley. Clean the portion of the balancer you can see. If no timing marks appear rotate the crankshaft with the ratchet and continue to clean until you find the timing marks.
Hmm. I had a gut feeling it might be something with the carb. I just find it odd since it's brand new as of May 2021. It's an Autolite carb, unsure of the model off the top of my head. Something I forgot to mention, the car revs up fine in neutral or drive at a standstill. There is no popping, only when in gear and moving. When you say "overhaul" do you mean removing the carb, cleaning it, and reassembling it? Or replacing it with a new carb? As far as the timing marks go I'll check under the car later today and see what I can find.

I'll preface this by saying I'm no expert with these cars, but I had a similar issue when accelerating. My engine runs great when it is fully warmed up and the electric choke if fully open. Do you know a difference after driving around a while and fully up to temp?
No not really. The car pops and bangs from the second she hits the road to the second she pulls back in the driveway, no difference cold vs up to temp.

I had an old 289 once that popped and banged under more than half throttle. Timing chain was so worn it had skipped a tooth...
Funny you mention that, one of my dads friends mentioned (unrelated to my problem) that I should replace the chain. I'll take a look at it.
Sounds like fuel starvation; fuel filter, weak pump, plugged screen inside the tank. You could put an inline pressure gauge on it to verify.
A non vented fuel cap is possible but not likely.
The car has a brand new fuel filter, tank, and pump. Lines were replaced in some areas according to the P/O. Non-vented fuel cap? I know on airplanes we have vents to avoid creating a vacuum in the tank…is the cap responsible for doing so on my car?
My 289 did something very similar when I bought it. After fighting with jets and needles in the carburetor for a couple of weekends I replaced the points with a Pertronix unit, and the issue was completely resolved. I had set the dwell several times before and believe I had it set correctly.
I was considering purchasing that same unit not too long ago…I would buy it even if I had no problems, so maybe I’ll just do it now and see what happens.
Sounds like choke is not adjusted properly and the accel pump (squirter) is not giving enough gas.
Now that you mention it, I was monkeying around with the carb yesterday, kinda just trying to get a better idea of how it all connects. I lightly pushed on the throttle linkage and the choke plate slammed shut. I’m assuming this should be closed to start a cold engine? Maybe the choke is out of wack.
So I think I might’ve found a possible issue. My PCV system was totally installed backwards, in fact one valve cover had a 6 inch hose that wasn’t connected to anything.

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^This was on the passenger rear valve cover, not connected to anything, no PCV valve in it.
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^The driver side front valve cover had a cap & a PVC valve, with a hose running to the base of the carb. I replaced it with this chrome cap and 90 degree elbow. I’m running this to the air filter cover.

I ordered a new PCV valve to fit into the grommet on the passenger side valve cover, but the valve won’t fit easily. Any suggestions?
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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.
I appreciate the reply. So the hose from the carb wasn’t left open, it was run to a PCV valve which was stuck into the oil cap. The valve didn’t seat in the grommet properly, and wasn’t even completely in. I’m assuming this caused a vacuum leak? It needed to be addressed anyways so that’s what I’ve been doing for the last hour or so. I’m not convinced that it’s the timing chain either.
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This is what it looked like before. The hose connected to the PCV ran to the carb.
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For diagnostic purposes you could plug the fitting at the base of the carb and see if that helps your drivability issue. Won't hurt a thing but it may run a little rich. Just leave the pcv in wherever it is or leave it on the workbench if you want, it doesn't come into play with the port plugged.
I might go ahead and try that. At this point why not, it’s nothing but time. I’m not sure when I’ll get to it but I’ll let you know how it goes!
If the timing chain has jumped 1 tooth the engine will not run with the timing set at 10* or 12* BTC.
The timing chain in my '69 Mach 1 jumped 1 tooth causing the engine to die and I had to reset the timing to get it started again. A few days later it jumped another tooth and resetting the timing would not get it started. I was forced to replace the nylon tooth cam gear that had worn teeth.
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…
Gotcha. They’re all very dirty but they’re not overly fouled like you’d see with a rich mixture. They actually look pretty clean. Cylinders 3 and 6 have a white ground electrode. There isn’t that sandy buildup, just white color.
A weak ignition can also cause the symptoms you describe. When you accelerate you have a richer mixture and if you ignition is weak it will be unable to fire completely. Possible coil, coil or plug wire and your new points, cap, rotor are even a possibility.
Just usually an intake backfire is mostly symptomatic of a lean issue.
Yeah that makes sense to me. I was looking into all the Pertronix stuff as I’ve heard hood things about their products. I wasn’t going to replace anything since it was all new, but maybe it’s worth looking into. Is there an easy way to test whether the ignition is weak? At this point i’m open to trying just about anything, but I think I need to get some better tools to run some tests like someone mentioned I should.
You can hook up a timing light, tape the trigger down and tape it to your windshiield and go for a test drive. Watch the light and if it stops blinking or is erratic when the car is missing that is a clue. If it does it on a plug wire, then move the induction clamp to the coil wire and see if is does it there. That will help isolate the problem.
Wow. I would have never thought that would work. Thank you, I’ll try it out.
I plugged the port on the back of the carburetor and went for a drive. Started right up almost as easy as my 2019 Jetta. No stalling on the first accel, and no popping from the carb when I floor it. Runs like a Mustang should. There is still a slight hesitation but I’m guessing that’s due to my mixture and timing. Thank you all for the help and suggestions, I’ve learned quite a bit just by reading your responses. Happy driving :)
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