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94 Chevy 350. Back fire through TBI

Discussion in 'GM Powertrain' started by James4012, Feb 23, 2013.

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  1. James4012

    James4012 Member

    Okay guys. I'll go ahead and list what I've not done. I've not replaced the crank and pistons. Other than that. Everything from the Camshaft to the TBI itself is new. Brand new. Coil, plugs, dizzy, fuel relay, fuel pump, filters, cam, lifters, cam bearings, push rods, spark plugs, all plugs for the wiring harness. Fuel line, fuses, Throws no codes, shows no check engine lights. When I'm driving, around the range of 40MPH in 4th gear the motor will back fire through the TBI every now and then. I hit 50MPH, throw it into 5th and it will do it literally every 3 Mississippi seconds without fail. I changed the valves first, then bought entire brand new heads not rebuilt, still did it. Replaced cam shaft/lifters and push rods next due to believing it had a worn lobe. Did nothing to help. After that I basicly restored the old rust bucket with replacing every single little thing that has anything to do with the way the motor runs. And still does it. I love this truck. Truly do. But I'm at the point I'm about to take it to the beach, put it in 1st and 4 low, throw a brick on the gas and drop the clutch and tuck and roll. Then enjoy watching it go for a very deep swim. I've dumped enough money into it that I could have just bought a crate 350 and been done with it. Only thing I could possibly think of is taking off the converter because it might be clogged *though seein as it's hollowed out, I don't see why it would be* Help me please, before I start to think it's getting thirsty.
  2. dpeter

    dpeter Rockstar 4 Years 500 Posts

    I cannot remember the exact details but another member had a sympton like yours and eventually found a screw that had come loose in his distributer and was bouncing around and causing a missfire. Check the mechanical and vaccume advance mechanisms along with the timing. Thinking more about it, definitely check your timing or just retard it some and see if it improves.
  3. James4012

    James4012 Member

    I've checked the dizzy multiple times for cracks, and loose hardware. I just pulled it yesterday when I installed a new camshaft. The truck hasn't went more than 20 miles in the past 3 months because I'm tired of it back firing. It doesn't loose power when it back fires, it keeps on going without any hesitation, runs great with plenty of power. Just back fires through the TBI intermittently. Also, forgot to mention. Durring heavy acceleration, it doesn't do it at all. Only in normal driving conditions above 35MPH. I know the dizzy is good, pulled it apart to check it. Seeing as it's only about a month old, it really ain't been used. I retard'd the dizzy yesterday, but that was only because it was idling to low to accelerate properly. Didn't think of the vaccume advance though.

    - - - Updated - - -

    Just swapped out the vaccume advance, didn't do nothing.
  4. dpeter

    dpeter Rockstar 4 Years 500 Posts

    As a last thought, at least from me, I have been cautioned about running plug wires to close together without crossing them at least once because if you do then there is a chance that the firing wire will induce a current in the other one to fire its plug. I have never had a problem but in theory it is possible. Heavy loads require a healthier spark than light loads so maybe a correlation? Please let us know what the problem was when you find it.
  5. James4012

    James4012 Member

    I've fought with this truck probably since about September of last year. When I bought it, I got it for $1000, guy said it had clogged injectors on the TBI and needed a clutch, the pedal was jumping and could hear noise while in motion. He threw in a spare tranny and a brand new transfer case *was still in box it was shipped in, un opened* After I got it home, I noticed that 6 bolts on the transfer case to the tranny were all but backed out. Tightened up, and figured "hell lets prime it and see if it runs." did so and it did but died. I poured a couple gallons in the tanks and behold...it ran. After awhile I was playing around, removing the dizzy cap and I always number my plugs. 1, second I reached for 8, traced it back. Found it was plug 6. I had drove it about 800 miles by then and that's when it started back firing out of the TBI. Now I've done everything to this truck. Entire new dizzy, coil, plug wires and plugs was the first thing. It's just escalated from their. I ran a compression test on all 8 cylinders. They are all sitting at 135 and holding. SB350 compression should be 150 with an allowance of 20 so that checks out fine. Not done it since the replacement of the heads and cam though. Their's about 7 things that I know of for sure that make it do this. All have been addressed. I'm thinking of just throwing a under powered 305 in it and being done with it.
    Last edited: Feb 25, 2013
  6. dpeter

    dpeter Rockstar 4 Years 500 Posts

    OK, it was not my last thought. There is an aftermarket distributer cap that will allow you to put the plug wires in order of the cylinders. All right side plugs on right side of cap and left hand plugs on left side but still maintains the correct firing order via routing internally in the cap. I cannot remember how to tell them apart or if you can without using an ohm meter. Go ahead, ask me how I know!

    Last edited: Feb 25, 2013
  7. James4012

    James4012 Member

    Na I know the firing order perfectly, and never had a problem with setting the time. I went ahead and cut a hole in the convertor to see if it's clogged, but we got like 55MPH winds here right now and I don't want to take a chance of my rusted ass bed flying off down the road. I cranked it a min ago and one of the damn rocker arms is tapping again. I installed the cam and let it break it in for over 30 mins *more than required* opened em up and readjusted them again. But seems one backed off. Idleing rough wanting to die. It was seizing up bad on my way home in first and second gears when heavy accelerating. *sigh* if not one thing, it's always another.
  8. geo1

    geo1 Member 1 Year 100 Posts

    i know im late on this one,just looking to see if you got an answer to the issue.once i had lifter trouble,too tight,didnt prime properly i guess.went thru 2 times,once backing them off,2nd final adjustment.
  9. James4012

    James4012 Member

    No hasn't been resolved yet. The lifters and push rods were all replaced when I installed a new camshaft 3 weeks ago. I set them, let it run for about 20-30 mins and readjusted after they broke it. One of them back off again after a day of driving but I adjusted it again. They fine, but even when I first drove it after installing it still back fired. I've checked timing and shaft orientation since then and they are on the money. Replaced a few sensor again in hopes of something being faulty but no dice. It does do it much less. Instead of popping every 3 seconds in 5th gear it does it randomly. Sometimes will go half a mile at 60MPH before it does it. I've got one last thing to try and if that doesn't fix it, then it's getting the crappy 305.
  10. geo1

    geo1 Member 1 Year 100 Posts

    many chevys ago,i had a carbed monte,same issues,tore it down,found nothing wrong,took a bit of thought,swapped some parts and wound up being alternator,cant remember all thoughts but do remember car kept starting and running,so alt was not even concidered in begining,just a thought for you,possible module in distributer?good luck,keep me informed

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