Intermittent carburetor backfiring on both sides

Tiny
CITYGUYUSA
  • MEMBER
  • 1968 CHEVROLET CAMARO
  • 7.4L
  • V8
  • 2WD
  • MANUAL
  • 123,000 MILES
Okay, I put the new valve covers on and I wanted to run the car since it's been a while. She started right up but I'm getting intermittent backfires on both sides.

I was told that I could skip the RTV on the engine side of the valve covers which makes it a lot nicer to be able to go back in if needed.

The spring tester for the valves was a bit too expensive for my tastes and so I thought this solution made some sense. I torqued them down to 70in/lbs as per the instructions that came with the covers.

I'd like to work through the rest of the list before the only thing left would be to RTV the valve covers. I also had to move the PCV to the other cover because of interference. I noted that it no longer seats completely. Maybe 1/2 way. I had to buy a push in filter which looks like a little lamp shade. Kind of looks ridiculous but it was all that would fit and was the right color. With all this air flow pushing out I wouldn't think being all that tight around the seals other than to not leak would cause much of a problem yet somewhere I read that having loose covers can cause it to misfire.

I also have 1/2 tank of gas that's been in there since last year of course it's got ethanol so water shouldn't be a huge issue other than watering it down so it doesn't ignite.

I did check the wires to the plugs to make sure they were seated. They all seem to be but whoever made the wires didn't always measure the best so some are taught.

I was having some backfiring issues after they upgraded my environmental controls (EC). They just seemed to go away although I was never happy with that answer or explanation. The only thing I could rationalize was that I had been buying ethanol free gas and the place I was buying it from had just got a new supplier coming up further from the south. The south likes to be a bit lax on regulations so who knows what they might have created and I put in my car but I haven't been back since and I had moved passed the misfires to just dieseling when I tried to shut it off. To be clear the dieseling also came after the replacement of the EC.

I also had a heat event after the EC was upgraded and before that my radiator had become a sieve and had to be replaced. I went from a 4 row b&c to a 2 row aluminum but the fan is a mere 16" and because the shroud is exacting and no more than 1" from the radiator and not more than 1.4" beyond the blades I'm wondering how much air is coming through vs my old mechanical setup where there was a shroud but it was loose on the fan side and it was more like a funnel than a wall. I think the older setup would tend to pull more air through from all corners of the radiator where the new electric fan would look to draw just from the center. The A/C condenser fan covers a large part of the front of the car. Where the older A/C condenser fan was off to the side and had room to blow through the more open shroud. The way it is now I would suspect the A/C condenser fan just hits the front of the new flat shroud and causes trouble with the main airflow.

Anyway when I brought the car home it was running awfully weird it was under full power output in a thunderstorm at night with the A/C defrosting my windows, lights on, etc and it was shutting down every time I stopped. Dieseling and misfiring if it wasn't mostly highway I probably would have called for a tow.
The only thing that the installer fessed up to was a loose wire on the fan controller and to read their review my whole car should be reworked from the ground up.
I certainly couldn't trust them anymore. When I had the chance to start looking through things the first things I did was checked the fans as they seemed to be running inversely. The condenser fan was running when the main should have been and vice-versa. When I disconnect the condenser fan I found out it was shorted across the relay from the power side to the control side and once I disconnected that the car was suddenly feeling much better. I still had the dieseling and had to be careful with overheating and some misfires but they weren't nearly as frequent as they are now. My car ran fine up until the radiator and EC replacements which were literally a few weeks apart. I wouldn't normally do things so close together especially such major systems but that's the way it came down and I didn't have much choice.

I'd like to know what order I should check things out for a carbureted car. As far as these misfires they're pretty strong so not something I want to allow to continue.

Maybe first is just getting some new gas added in to see if that's a problem?

If you feel strongly that I need to RTV the valve cover to the engine than I'll do that. There would be some logic too that maybe I just don't want there to be? Of course if the engine could run long enough I would know if I had oil leaks. The PCV is like a check valve to let pressure out and that pressure is vented into the base of the carb for what purpose? The other side just has a baby air filter no check valve so air conceivable could go in both directions. But if the whole point of the push in air filter is to let pressure out why the PCV on the other side? Shouldn't both sides go through a PCV?

The misfires are intermittent. When I sit they seem worse then when I'm moving I have no answer for that. But I have a parking lot behind my home and I drove it down around the center to turn the car around and movement did seem to help although not a clue why. They come ever 1 - 3 seconds when I'm sitting and when I'm moving I have periods maybe 8 seconds with nothing. I didn't actually time it but it was noticeable.
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Friday, September 18th, 2020 AT 4:24 PM

102 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

You shouldn't need a PCV on both valve covers, and installing the valve covers with RTV isn't going to prevent a misfire.

As far as a misfire, it could be caused by a few things. Have you checked for vacuum leaks?

Take a look through this link and let me know if anything helps:

https://www.2carpros.com/articles/engine-backfires-while-running

Let me know.
Joe
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Saturday, September 19th, 2020 AT 5:41 PM
Tiny
CITYGUYUSA
  • MEMBER
I looked at that write-up earlier but it's written for a non-carbureted engines. I have no reason to suspect broken springs because I wasn't having these excessive backfires prior to removing the old valve covers.

The carb for all considerations is new and no one has messed at it or should have messed at it. They come preset.

I just had the plugs replaced before the radiator was replaced. After that the radiator was replaced and since I have temperature issues. I replaced the thermostat, they didn't flush out the system when they replaced the radiator so it was full of Stop Leak - I did 2 chemical flushes but still it can overheat during warm weather. Even at night when it's more humid I can overheat. I never got this resolved but I think it's a fan/radiator coverage problem. I didn't have time to resolve it because the Environmental Upgrade came along and added more trouble. I know they rewired something under the dash because they told me there was a line going across the dash that had various drops but I don't know what they dropped to and they didn't bother to look although why they would just take it up on themselves to rewire these without knowing what and why is very strange to me. How many other things did they do to make themselves happy? I have lights that aren't working that used to work. Then there's the short circuited relay they insist they didn't do and yet they should have found that.

The only thing I can think of that would have a large effect on 14:1 would be the new air cleaner. The old one wasn't dirty but it felt almost like a varnish on the pleats but getting more air would make it lean but I've run it sitting still without the air cleaner on it and never had a problem.
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Sunday, September 20th, 2020 AT 1:50 PM
Tiny
STEVE W.
  • EXPERT
Intermittent misfires with a carburetor on it could simply be a dirty carburetor. It doesn't take much dirt in a jet to lean it out and cause a miss. The PCV is a simple valve, one side goes to the intake so it has vacuum on it when the engine is running. The PCV system is there to pull any of the gasses in the crankcase out and burn them off. With any engine there will always be some blowby past the rings and as the oil gets hot it also gives off some volatiles. The system works by pulling air into the engine through the filter, that air goes through the engine, picking up those gasses and a bit of oil mist on it's way through. Those baffles you had an issue with before come into play and help remove the oil mist but let the mixed gasses out to be burnt. The slight vacuum that is applied is nullified by the air coming into the engine. If you were to install a second PCV valve the vacuum could build up and that can damage the seals as well as lead to increased oil consumption.
As to loose valve covers causing a misfire, not on your car, but it can happen on newer vehicles that run a more sealed system. Yours however is just one step up from the old draft tubes used on earlier engines. The only thing the valve covers do on that engine is keep oil in and dirt out. As long as they don't leak or hit anything underneath they are good to go.
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Sunday, September 20th, 2020 AT 5:33 PM
Tiny
CITYGUYUSA
  • MEMBER
I was reading the label on my carburetor cleaner it says do not use if you're having misfires. Are you suggesting that I do a dip style cleaning? Or did you have something else in mind? It looks worse than I expected having only about 2,000 miles on it.
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Wednesday, September 23rd, 2020 AT 3:06 PM
Tiny
STEVE W.
  • EXPERT
That looks to be running very rich. I spray them off first then pull them apart to clean out the bowl and jets.
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Wednesday, September 23rd, 2020 AT 3:16 PM
Tiny
CITYGUYUSA
  • MEMBER
I agree. The smoke burns your eyes and hangs him the air but this happened after the environmental upgrade. They said they didn't touch the carburetor and they had no reason to but I don't want to disassemble it to do it again because something somewhere is different. After it came back every time I tried to shut it off it dieseled which I presumed had to do with the subpar cooling. I might have the occasional backfire going down the street but nothing like it's doing now, every second.

What if someone purposely did something? It's been vandalized twice for money; trying to steal the stereo and speakers and some loose change. Then one night someone bent the antenna. Something in my gas? It didn't start this until I put the covers back on which makes no sense.

What if I adjust the carburetor and see if I can get rid of the misfires? Then spray the carburetor?
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Thursday, September 24th, 2020 AT 3:01 PM
Tiny
STEVE W.
  • EXPERT
Running rich can cause it to diesel and create some nasty exhaust. Pull the PCV out and see if it runs better, It could be stuck open and sucking in oil vapor a lot.
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Thursday, September 24th, 2020 AT 4:19 PM
Tiny
CITYGUYUSA
  • MEMBER
Aren't you supposed to shake it. It does shake.

Dieseling was just at shutoff.

These misfires are new. But what would lead to running so rich suddenly?
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Thursday, September 24th, 2020 AT 7:08 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
The idea that it is running rich is likely the cause of all problems you mentioned. Could someone have put something in the tank? Yep, but it doesn't sound like it.

As Steve mentioned, the carb should be taken apart and cleaned. When it's apart, confirm that the float drop is within spec. When Steve suggested removal of the PCV, he is thinking allowing more air to the engine will help lean the air/fuel mixture. Thus, it should run better. Try that and see what happens.

Joe
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Thursday, September 24th, 2020 AT 7:15 PM
Tiny
CITYGUYUSA
  • MEMBER
PCV has a nice suction out or in no difference.
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Friday, September 25th, 2020 AT 3:20 PM
Tiny
STEVE W.
  • EXPERT
Is it the correct one for that engine? When at an idle there shouldn't be much air going through the PCV, it should almost stop flowing at that high vacuum. Then at part throttle cruise it should allow vacuum to flow. The entire reason for the PCV is to regulate that airflow. If the flow is too high it will pull oil in and cause carbon deposits and other issues that will only be made worse by a rich condition.
For a lot more in depth info on the valves job:
http://www.agcoauto.com/content/news/p2_articleid/197
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Friday, September 25th, 2020 AT 5:27 PM
Tiny
CITYGUYUSA
  • MEMBER
I don't Know if the PCV is set up properly or if it's the correct one. I have never changed it but please keep in mind these backfires are new.

My PCV is connected by hose to the carb base and it no longer fits deep down to the seat I'm assuming is the baffles preventing it from seating. But the suction should keep the PCV in place. The other valve cover has a push in lamp shade.

I was thinking that on my 1974 Camaro the PCV went to the valve cover and the hose went to a plastic sponge inside the air cleaner which had a scroughy sponge inside the air cleaner case on the outer wall held with a u-clip on the wall. I don't have that setup on the 1968 Camaro.
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Saturday, September 26th, 2020 AT 3:17 AM
Tiny
CITYGUYUSA
  • MEMBER
I should change the title since the misfires are in the tailpipes somewhere while the dieseling happens in the carburetor.
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Saturday, September 26th, 2020 AT 3:19 AM
Tiny
CITYGUYUSA
  • MEMBER
But again neither of these were ever a problem until the radiator and the EC upgrade.
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Saturday, September 26th, 2020 AT 3:21 AM
Tiny
STEVE W.
  • EXPERT
The radiator shouldn't change anything for misfires. As for the environmental upgrades, do you know exactly what they did or changed?
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Saturday, September 26th, 2020 AT 7:01 AM
Tiny
CITYGUYUSA
  • MEMBER
I know what they were supposed to do but like I said prior they did things that they shouldn't have done like rewiring multiple circuits that they didn't even know what they did and they wired a short across the new evaporator fan relay from the power side to the control side.

Technically, they were supposed to replace the old Vintage A/C with a newer one. That should have been replacing the controls, the inside heater box (condenser and a heater core), replace the plumbing, use the existing compressor, replace the evaporator and the A/C fan.

It shouldn't have done anything but what I got back barely got home. Dieseling ever time I tried to shut it off. The relay short was causing severe issues every time I'd come to a stop I stalled. There were neon lights that showed through the grill that no longer work, a switch that used to light up only when it was on is constantly lit. The fan controller no longer seems to retain settings and there's probably more but I was trying to focus on the overheating but now I'm being forced to deal with the misfires.
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Saturday, September 26th, 2020 AT 8:01 PM
Tiny
STEVE W.
  • EXPERT
It sounds like they did some damage. Doing an A/C conversion isn't that hard and shouldn't require any rewiring to make it work, only adding a couple of relays to make the pressure switches work and possibly adding a condenser fan control. It sounds like you may need to reverse what they did to get to a good starting point to work from since these issues started after they did their work. It could be that they tapped into the wrong circuit or crossed some wires that are making these issues show up. If it was here I would likely go over the wiring to see what they added or altered. Then disconnect everything from the engine that isn't needed and basically use the car as an engine test stand to be sure the engine itself is running okay. Being a 68 it should be a simple engine to work with. What ignition system do you have in it? Original would have points, has it been changed to HEI or something else?
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Saturday, September 26th, 2020 AT 11:34 PM
Tiny
CITYGUYUSA
  • MEMBER
Pertronix 2 I believe. I get what you are saying just pull the fuses and see how things are running. I was heading down that road which was how I found the shorted relay. I can't undo what they did but I can disconnect it.

I tried getting a couple places to check through the wiring but they wouldn't touch it. No one wants to do anything complex anymore that's why they needed computers to tell them what to do.

I have to say this is getting bigger and bigger.

My 2007 Mustang doesn't appear to have any way to check the transmission fluid or even a way to change it so I asked the last time I was at the dealership. Not a clue if it's even serviceable anymore, they just don't know.
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Sunday, September 27th, 2020 AT 10:37 PM
Tiny
STEVE W.
  • EXPERT
Sounds like you need to find a different shop that likes older cars.
The 2007 uses a different method to check the fluid but it's still serviceable.
Pertronix makes a good system. I misspoke as well, that would have had the early electronic ignition from the factory. Yes, I'm thinking, go after getting the engine running on it's own and then add on the electronics to see what is wrong. From the factory that car is very basic really, the schematic seems complicated but that is because they used ground returns for most items due to the plastic body so where a Chevelle of that era had one wire to a light your car has two.
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Monday, September 28th, 2020 AT 4:20 AM
Tiny
CITYGUYUSA
  • MEMBER
I think I may have confused you by bringing up my 2007 Mustang.

This is about not being able to get anyone to look at my 1968 Camaro after the questionable environmental upgrade. No plastic here other than some Bondo maybe.

The 2007 Mustang was just about why most won't work on the classic cars. They don't know how. If the computer doesn't tell them what's wrong they don't know. They don't even know if the AT transmission fluid is serviceable or how to know if it's full, etc.

When it was knew I bought the Pony floor mats which use clips to keep them in place. When parts called me to tell me they had arrived the car was in their shop for something and I asked them to install them. That apparently meant throw them on top of the old carpet ones.
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Monday, September 28th, 2020 AT 12:06 PM

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