Running rich/stumbling after replacing spark plugs

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Chained_360

Chained_360

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I always like to see what the wiser people have to say before I start wrenching, so here I am asking for advice on where to start troubleshooting my '68 Barracuda. When I bought the car in 2015, I spend quite some time getting it back on the road. Aside from brakes, electrical, and fuel system fixes, I also threw on a new cap, rotor, and plugs, but I never checked the timing (probably what's causing my problem).

The car ran and drove fine for the entire summer of 2016, right up until about a couple weeks before I parked it for the winter in October. The last couple weeks I drove it, it ran progressively richer and richer. The exhaust smelled fine except for when it started and was revved, which would cause it to throw black smoke out the pipes. I figured it was because I didn't replace the #5 and #7 plugs due to the header blocking access to them. So I pulled the header back, threw two new plugs in, and started it up.

Naturally, the car ran much better and much stronger, but it started to smell like it was running rich. Fast forward another week, and the exhaust mwas reeking of gasoline. Another week later, and it was throwing thick black smoke out the pipes whenever I gave it gas (it would stumble when revved too). After spending the winter busy with school, I'm now working on the car again and trying to figure out what the deal is. Does anyone have any tips or advice as to where to start?
 
Need more info on the engine., carb, etc. Choke fully opening?
Double check you installed the correct plugs.
 
KosmicKuda said:
Need more info on the engine., carb, etc. Choke fully opening?
Double check you installed the correct plugs.
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Sorry about that. It's a mildly warmed over 360 with a brand new (as of 2016) Holley 650 CFM carburetor with vacuum secondaries and an electrical choke. The choke operates properly, but when I first start the motor I have to hold the choke open, otherwise the engine will smother itself with too much gas. I checked the gap on the plugs and they were in spec when I installed them.
 
Do you have 12 volts going to the choke. If you tap off the coil only getting about 8 volts. You can also adjust it to come off sooner with the outer ring on the choke canister. Make sure there are no leaks at the choke because vacuum also helps to open it up.
 
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tonysrt said:
Do you have 12 volts going to the choke. If you tap off the coil only getting about 8 volts. You can also adjust it to come off sooner with the outer ring on the choke canister. Make sure there are no leaks at the choke because vacuum also helps to open it up.
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The choke fully opens, but I still need to adjust it. It takes about 2-3 minutes to fully open, but I can check to see if it is getting 12 volts anyways. It's just that the choke has to be open for the engine to run somewhat well, but it still runs rich.

What I'm trying to see before I start working is if I should be looking for an ignition/timing issue, or a fuel delivery issue. The exhaust smells rich, and if I try to rev it in park, it stumbles and misses, and then throws black smoke out the pipes. What confuses me is that the problem really only got bad once I replaced the #5 and #7 plugs.
 
Is the vacuum operated choke pull-off working? That should open the choke some once engine starts.

Also the #5 and #7 plug wires could have swapped positions. That will make it run on 6 cylinders, unburned fuel out exhaust.
 
How about the power valve? Any chance you had a backfire through the carb?
 
Clean the needle and seat in the carb and check fuel filters, but first check what KitCarlson said as that is just stupid easy to get backwards.
It would be kind of ridiculous to readjust a choke to try and compensate for too much fuel.
 
KitCarlson said:
Is the vacuum operated choke pull-off working? That should open the choke some once engine starts.

Also the #5 and #7 plug wires could have swapped positions. That will make it run on 6 cylinders, unburned fuel out exhaust.
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I've made that mistake before, but that's not the case this time :)

Riff Raff said:
How about the power valve? Any chance you had a backfire through the carb?
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Now that would make a lot more sense, because yes, multiple times. When I first installed the new carburetor, it blew the vacuum advance line off and started chuffing through the carb. That was about 6 months before it started running rich and smoking though. Could that still be the culprit though?
 
Chained_360 said:
I've made that mistake before, but that's not the case this time :)


Now that would make a lot more sense, because yes, multiple times. When I first installed the new carburetor, it blew the vacuum advance line off and started chuffing through the carb. That was about 6 months before it started running rich and smoking though.
Chained_360 said:
I've made that mistake before, but that's not the case this time :)


Now that would make a lot more sense, because yes, multiple times. When I first installed the new carburetor, it blew the vacuum advance line off and started chuffing through the carb. That was about 6 months before it started running rich and smoking though. Could that still be the culprit though?
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Most definitely. Generally one backfire ruptures the diaphragm. Only cure is to replace it.
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Interesting. I thought it would have started running rich immediately after the backfire, not months later. Regardless, it would have taken me forever to guess that on my own, so thank all of you very much! It would be much more difficult for a young hooligan like me to pick up this hobby if it wasn't for this awesome community. I most likely won't be able to check the PV until tomorrow afternoon, but I will definitely post an update once I do.
 
Chained_360 said:
Interesting. I thought it would have started running rich immediately after the backfire, not months later. Regardless, it would have taken me forever to guess that on my own, so thank all of you very much! It would be much more difficult for a young hooligan like me to pick up this hobby if it wasn't for this awesome community. I most likely won't be able to
Chained_360 said:
Interesting. I thought it would have started running rich immediately after the backfire, not months later. Regardless, it would have taken me forever to guess that on my own, so thank all of you very much! It would be much more difficult for a young hooligan like me to pick up this hobby if it wasn't for this awesome community. I most likely won't be able to check the PV until tomorrow afternoon, but I will definitely post an update once I do.
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Hold the phone... I just reread your post. Newer Holley's have power valve blow out protection(a spring loaded checkball on the carb throttle plate). I'd double check all the obvious stuff and follow up with the suggestions from the other guys before you replace the PV.
check the PV until tomorrow afternoon, but I will definitely post an update once I do.
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A ruptured PV diaphragm will leak gas into the intake manifold beneath the throttle plates. You will never see it. The proof is in the way the engine behaves;just like yours. The constant rich smell at idle or even to 1/2 throttle, the black smoke, fouled plugs, and lousy throttle response.
 
Got a friend with another known good carb?
 
r
toolmanmike said:
Got a friend with another known good carb?
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I've got the old 650 sitting in my garage right now, it's been rebuilt, and it does have a new power valve I could rob off of it. It would probably be easier to just steal that than it would be to put the old carb back on
 
hum, I thought the newer Holleys had power valve blowout protection, seems my Holley has.
 
fishmens67 said:
hum, I thought the newer Holleys had power valve blowout protection, seems my Holley has.
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After 1992 they incorporated a check valve to prevent blown power valves. Go to 2:06 on the video below to see this valve and how to check if your PV is blown from an extreme backfire.
 
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So yesterday, it decided to rain prrtty much all day. And of course this weekend is finals weekend, so I unfortunately won't be able to get to checking the PV until next week. I may be able to check it this weekend sometime, but no promises :(
 
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