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Old July 17th, 2009, 11:15 PM   #1
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Default 350 misfire problem

I have about a 1980 chevy 350 with a 1 year old Edelbrock 4bbl carb (in a Jeep, of course). I don't notice uch of a misfire below 3000 rpms. If I hold it steady at about 3000+ rpms it misfires a little, at 4000+ rpms it is fires a lot (won't go over 4500-4700 rpms, starts to cough and misfire badly).

Done about everything I can with the carb. Replaced and gapped plugs. Set timing just low enough to avoid most/all spark knock

Any suggestions?
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Old July 18th, 2009, 02:19 AM   #2
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99% of miss fires are because of a bad ignition, get new cap and rotor, plugs and wires. If it still misses check pickup coil as well as the vacuum advance or mechanical advance. I bet $10 to your one it's something I just mentioned.
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Old July 18th, 2009, 07:15 AM   #3
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Thanks.
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Old July 18th, 2009, 11:10 AM   #4
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I have about a 1980 chevy 350 with a 1 year old Edelbrock 4bbl carb (in a Jeep, of course). I don't notice uch of a misfire below 3000 rpms. If I hold it steady at about 3000+ rpms it misfires a little, at 4000+ rpms it is fires a lot (won't go over 4500-4700 rpms, starts to cough and misfire badly).

Done about everything I can with the carb. Replaced and gapped plugs. Set timing just low enough to avoid most/all spark knock

Any suggestions?
To me this sounds like your cam is going out. if the missfire your getting is more of a "pop" sound and if its coming back out the carb.
the best way to check is pull the valve covers and watch the action of your rockers. the second best is to pull the air cleaner off and step on the gas in PARK with no load and see if it sounds like a full auto AK47 under the hood. if it wiped out the exhaust lobe on the cam it will fire the charge and shoot it back out the carb. "pop"pop"pop"pop"... very loudly will be heard when free reving if the cams gone.
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Old July 18th, 2009, 02:50 PM   #5
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To me this sounds like your cam is going out. if the missfire your getting is more of a "pop" sound and if its coming back out the carb.
I remember GM had this problem back in the day with cams going flat. Has the engine ever been apart as far as you know?
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Old July 18th, 2009, 09:15 PM   #6
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it was rebuilt about 2-3 years ago with a new cam. I may have not descrobed it accurately as a misfire: it is not a backfire. I initially thought it was a carb problem as I only experienced it at full throttle climbing dunes. The I reallized I only approach those RPMs on the dunes so I went out on a flat highway - in each gear it accellerates great until it hits 4000 RPMs and then stumbles - no noise just not firing right. As I pay more attendtion now, I can feel the misfire happening at a steady 3500 RPMs bt to a much lesser degree.
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Old July 18th, 2009, 09:44 PM   #7
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I had a engine that would float the valves around 4500, but there wasn't much of any noise. Was kinda nice, cheap rev limiter. It would just lay down at those rpm's.

Do you have a spare distributor you can try for giggles? I've seen stranger things happen to make a engine run like crap.
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Old July 19th, 2009, 03:45 PM   #8
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it was rebuilt about 2-3 years ago with a new cam. I may have not descrobed it accurately as a misfire: it is not a backfire. I initially thought it was a carb problem as I only experienced it at full throttle climbing dunes. The I reallized I only approach those RPMs on the dunes so I went out on a flat highway - in each gear it accellerates great until it hits 4000 RPMs and then stumbles - no noise just not firing right. As I pay more attendtion now, I can feel the misfire happening at a steady 3500 RPMs bt to a much lesser degree.
your discribing more of a fuel supply problem if its doing it under load at a certain set rpm. you either have a bad fuel pump or plugged filter.
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Old July 19th, 2009, 09:38 PM   #9
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Thanks for the help. Fuel filter is new this year and pump last year but I will check electric to pump to see if the connections are bad. Don't have a spare distributor, but I will try a new cap, rotor, wires and plugs and let you know what the results are.
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Old July 20th, 2009, 06:51 AM   #10
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well since I know better then to assume. I will ask you to change the fuel filter anyways. the idea being that I had a simular issue with my `77 K5 and it had rust in the tank. it was plugging filters at a rate of one a week. I finally added the "BIG" heavy truck filter with the spin on can to cure the problem. also take off your gas cap and run it. if the vent in the cap is bad or the wasps have built a nest in it the tank will pull a vacuum and won't get shit for fuel.
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