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Old 11-07-2012, 12:35 PM
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350 misfire/backfire

Alright so i have a 350 engine, vortec heads, edelbrock intake, comp roller tip rockers, 650 edelbrock thunder series carb. Intial timing at 11 degrees.

Right now it idles terribly, backfires out the back some and wants to die if i dont let it warm up enough for driving. Also misfires at any engine speed(dash shaking) and im way down on power.

I replaced the cap, rotor, coil, control module and the plugs when the module went out a few weeks ago.

i adjusted the valves messed with the carb adjustments when misfire started and still nothing. also ran some seafoam through the tank and carb still nothing. iv been hella busy with work though so i dont know when exactly the problem started. The plugs were black so im thinking i might lean it out with a rod change but it has always run rich. even before the problem started just havent got around to getting new rods or springs for it.

any help is appreciated as this is very annoying

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Old 11-07-2012, 02:04 PM
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I adjusted the floats and it seems to be better. i still have bad idle. i hear a faint whisteling under the hood and gets louder when i give it gas so i think i have a vacumm leak. going to get some carb cleaner now to see if i can find the source
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Old 11-07-2012, 04:18 PM
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Quote:
Originally Posted by Stormtrooper View Post
Alright so i have a 350 engine, vortec heads, edelbrock intake, comp roller tip rockers, 650 edelbrock thunder series carb. Intial timing at 11 degrees.

Right now it idles terribly, backfires out the back some and wants to die if i dont let it warm up enough for driving. Also misfires at any engine speed(dash shaking) and im way down on power.

I replaced the cap, rotor, coil, control module and the plugs when the module went out a few weeks ago.

i adjusted the valves messed with the carb adjustments when misfire started and still nothing. also ran some seafoam through the tank and carb still nothing. iv been hella busy with work though so i dont know when exactly the problem started. The plugs were black so im thinking i might lean it out with a rod change but it has always run rich. even before the problem started just havent got around to getting new rods or springs for it.

any help is appreciated as this is very annoying
I need to understand when you say "backfire" followed by "out the back" I would read this as an afterfire out of the exhaust as opposed to a backfire out the intake (carb). The causes of each are mostly unique and the black plugs and rich running you observe would support rolling explosions out the exhaust. Lean on the other hand tends to be flaming, eye brow scorching explosions out the intake.

Different things can result in these symptoms as well for example a sparkplug that is too cold on the heat range scale will carbon up and cause the engine to run and act rich because of the miss or late fires that happen with this condition. Low compression will look like a rich condition for many of the same reasons as a carbon fouled sparkplug. If this is a Chevy one cannot rule out the cam and or lifters gone bad this will act much the same as the problems you're seeing. Late timing as well which could be ignition and or cam to crank. Certainly carb settings like too high a float level, too large a jet of too small of off calibration metering rod, too much fuel pump pressure can be contributors as well. A weak ignition from failing high voltage wires to the voltage going into the igniiton being too low such as running HEI with a 9 volt drop resistor made for a points ignition somewhere in the wiring to the distributor that wants a full 12 volts. Lets not forget choke adjustment and of course whether this has an air gap manifold. These are more things that can make an engine run rich or look as if it's rich.

A vacuum leak would add air which by definition would lean the mixture which would result in backfires through the intake or a lot worse. But here you'd be looking at hot, dry, white spark plug insulators unless the engine was so rich that it prevented the typical effects of an air leak into the intake system.

Something of shotgun approach to problem solving to be sure, but from here that's as good as it gets.

Bogie
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Old 11-07-2012, 04:29 PM
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Quote:
Originally Posted by Stormtrooper View Post
Alright so i have a 350 engine, vortec heads, edelbrock intake, comp roller tip rockers, 650 edelbrock thunder series carb. Intial timing at 11 degrees.

Right now it idles terribly, backfires out the back some and wants to die if i dont let it warm up enough for driving. Also misfires at any engine speed(dash shaking) and im way down on power.

I replaced the cap, rotor, coil, control module and the plugs when the module went out a few weeks ago.

i adjusted the valves messed with the carb adjustments when misfire started and still nothing. also ran some seafoam through the tank and carb still nothing. iv been hella busy with work though so i dont know when exactly the problem started. The plugs were black so im thinking i might lean it out with a rod change but it has always run rich. even before the problem started just havent got around to getting new rods or springs for it.

any help is appreciated as this is very annoying
It might be a case of the timing being off. Check to see if the distributor is loose in the engine and has let the timing wander.

Be sure plug wires for #5 and #7 weren't accidentally swapped. Check at night w/the engine running to see if there's arcing between the wires and a ground or between the wires themselves.
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Old 11-09-2012, 05:54 AM
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Yes I ment afterfire. I fixed the vacuum leak. The cheap gaskets that came with the carb spacer were leaking. I made 2 new ones out of a sheet of gaskets material. Also found a few spark plugs that weren't very tight. I could almost turn them by hand. (my bad) adjusted floats because they were way off. It's now running alot better. Actually drivable now. Idles alot better

Still a slight miss in one of the driver side cylinders. Misses at idle and light throttle. Once rpms get higher it stops missing and I can feel the power difference when it kicks in. Im guessing 1800-2400 rpm it kicks back in (I have no tach)

The wires are about a month old. I'll have to check tonight to check for arching to headers. I'll check on the drop resister thing. Thanks
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Old 11-09-2012, 07:53 AM
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350 misfire/backfire

Here is an article on hotrodding a HEI distributor. http://www.crankshaftcoalition.com/w...EI_distributor. It also has tests you can run to check the pickup, coil, etc. You can also check the resistance in your sparkplug wires with an OHM meter.
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Old 11-09-2012, 10:26 AM
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Drive some today and I have a few more symptoms. It afterfires off idle with a load. But when I punched it on the way home it backfired out the carb. I'm still down on power. I think im going to adjust my valves on the driver side. And adjust the secondary's to get then to kick in soooner. May solf carb backfire problem. I just don't know witch way Is witch on the adjustment screw.
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Old 11-09-2012, 11:11 AM
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sounds like your distributor is a tooth off and you are compensating by turning the distributor untill it runs ok. sometimes you can get themn to run whene there a tooth off. ive been told its not possible to run a tooth off but by golly i made it work before lol...low power and unexplainable popping and rolling exhaust flameing while trying to use power. the idle sounded mean and the car would start right up and go.
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Old 11-09-2012, 11:40 AM
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Originally Posted by Stormtrooper View Post
Drive some today and I have a few more symptoms. It afterfires off idle with a load. But when I punched it on the way home it backfired out the carb. I'm still down on power. I think im going to adjust my valves on the driver side. And adjust the secondary's to get then to kick in soooner. May solf carb backfire problem. I just don't know witch way Is witch on the adjustment screw.
These could aslo be symptomatic of the time set wearing out, timing chain stretch and gear tooth wear. As these parts wear the cam starts to fall retarded to the crank's position. The engine looses power, always needs the timing readjusted, tends to back and after fire. A seeming miss that goes away at cruise but is present at idle and WOT can indicate the cam has a wiped lobe/lifter. These are failures that send you adjusting everything else but seemingly you can never get the engine to run right. An engine when everything is operating properly doesn't require constant attention unless it is a really high strung motor with a lot of cam timing and compression. Those type engines loose a lot of power from tiny wear issues so you're always making adjustments. But a regular production engine should go many tens of thousands of miles without much if any attention. So when they start needing constant adjustment it's usually a sign that big things inside are no longer correct and it's getting time for serious repair or replacement work.

Bogie
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Old 11-09-2012, 07:03 PM
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Move the wires to a different cylinder and make sure they are pushed down all the way. If the miss moves replace the wires. No effect replace the plugs and check the cap and rotor for wear. Rule out all the easy stuff first. You should be able to feel the push rod and tell if its the same clearence as all the others usally just by turning it and moving it up and down.
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Old 11-09-2012, 08:34 PM
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about a month ago i was fighting with the exact same problem, but it was fixed by jetting my carb pretty far down, changing the rods and playing with the timing a little bit. i had the afterburn at the exhaust and backfires at the carb just like you....usually the ones at the carb mean you need to tune that thing or a spark wire is misplaced and the misses were all caused by fouled out plugs
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