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1971 302 C4 Coupe - Light Green Metallic
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Discussion Starter · #1 ·
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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Its either too much fuel into the cylinders or not enough.
You've heard old cars are simple, right?
 

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1971 302 C4 Coupe - Light Green Metallic
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Discussion Starter · #3 ·
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?
 

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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.
 

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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?
 
1966 coupe and 1970 sportsroof
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1971 302 C4 Coupe - Light Green Metallic
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Discussion Starter · #7 ·
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.
 

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1966 Coupe-289-3sp-No A/C
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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?
best and easiest way to adjust carb is with Vacuum gauge, super easy and gauges are cheap, works good for timing also, google and see video's
 

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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.
 

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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.
 

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Discussion Starter · #11 ·
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?
 

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1971 302 C4 Coupe - Light Green Metallic
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Discussion Starter · #12 ·
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.
 

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Sounds like choke is not adjusted properly and the accel pump (squirter) is not giving enough gas.
 

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Discussion Starter · #14 ·
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.
 

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The procedure for starting a cold engine is to depress the accelerator pedal to the floor ONCE, and release. This sets the choke and gives the engine a "squirt" of fuel. If the engine doesn't start right up and idle at 1,400-1,500 rpm while coming up to temperature then those choke needs attention. There is also a de-choke piston inside the thermostatic housing on the carburetor that frequently gets stuck. Its purpose is to crack the choke open slightly once the engine starts.

Cold weather driveability issues were addressed by Ford by having a heated air stove attached to the RH exhaust manifold and a tube that connected it to the snorkel of the air cleaner. Cold air is dense and requires more fuel to maintain a proper mixture. Lean fuel mixtures are typically the cause of backfiring through the carburetor on sudden throttle openings.
 

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You just described what one of my Chargers did after a situation caused by old gas. I changed my carb with a spare and it's back to running great again.
 

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Discussion Starter · #17 ·
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.

Motor vehicle Automotive tire Electrical wiring Automotive wheel system Rim

^This was on the passenger rear valve cover, not connected to anything, no PCV valve in it.
Automotive tire Hood Bottle Motor vehicle Automotive lighting

^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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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.
 

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1965 200 White: 1965 289 Black: 1966 289 Green
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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.
 
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