Author Topic: Chevy TBI 350 stumbles on acceleration  (Read 12332 times)

Offline 87becky

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Chevy TBI 350 stumbles on acceleration
« on: April 19, 2014, 12:15:48 AM »
Hey all. I've posted before about electrical and wiring and figured it out with help to the diagrams on this forum and from all you here that helped. We'll I'm back with a new problem that I haven't had before. All started when my ICM(ignition control module) went out while wheeling. Got it running a few time but died and wouldn't do anything at all. Got it home and looked for any wires that could have shorted anything out.  I pulled up my main harness that runs behind the distributor and as I did this it was hard to pull up. I got it up and I get two black wires that were spliced as if they went to something.  I thought they got stuck so it shorted something out. Didn't do anything. Tried grounding them out on the fire wall and put them together and nothing. No power in the wires either. Got it home and changed the module from my old distributor and fired up as it always did. Bought a new module to be safe and timed it at 2 degrees advanced. Other friends with the same motor run 4 degrees advanced but mine was set this way temporarily. My issue I'm having is when I accelerate to around 2500 rpm or give it a quick rev it stutters, coughs,misfires and sometimes gives a backfire. I'm questioning it's timing maybe but not likely. I'm now down to it being the erg and that's what those two black wires were. One wire measures 74 ohms resistance to a ground and the other wire gives me -0.01 ohms resistance. I've tried grounding the wire with a lot of resistance but still does nothing. I need major help with this. I'll be testing my erg in a few days cause I'm not home now but if anyone can help please do. Thanks

Offline 1979C20

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #1 on: April 19, 2014, 01:16:06 AM »
On a TBI motor with Electronic Spark Timing, you unplug the Tan/black stripe wire next to the brake booster to disable the EST and time the motor to 0. Then plug the EST wire back in. If a TBI motor is not at 0 you have problems with acceleration because the computer takes over advance/retard of timing from 0. When you set it to anything other than 0 you confuse the system.
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Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #2 on: April 19, 2014, 10:25:22 AM »
I did unplug the black/tan wire to time it and shut the truck off and fired it back up and is fine. My guessing is the two black wires i pulled from behind the motor is causing this issue. Possible that they control my erg solenoid and not letting it operate properly and that's where I get it stumbling and what not on acceleration

Offline bake74

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #3 on: April 19, 2014, 11:08:13 AM »
     Did you trace the 2 black wires to see where they go and what they are for ?  In the tech section there are the manuals for our trucks along with the wiring diagrams if you do not have them.
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Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #4 on: April 19, 2014, 11:33:03 AM »
I haven't had much time to trace them all the way to where they go. I've searched the tech section for the wires but not much luck. My former high school auto teacher said they sound like they're ground wires that go on the back of the block. Tried grounding them out and nothing changed. I also don't see wires that ground to the block in the tech section that can relate to these wires

Offline dvdswan

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #5 on: April 19, 2014, 11:43:53 AM »
just remember grounds need to be clean; no rust, paint, or corrosion.  any of those will add resistance and affect how voltage passes through the circuit.  something that requires 12vdc +/- .5 and say corrosion causes a .7 vdc drop the circuit either won't work or operate erratically.

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #6 on: April 19, 2014, 11:48:08 AM »
I made them ground on clean metal. Even wire brush it to be sure it's good and it was. But again I'm not sure if they are suppose to ground out or if they go to something else. No power to them even with key on so I assume they should be grounded

Offline dvdswan

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #7 on: April 19, 2014, 11:58:15 AM »
did you by chance try the timing a 4* like your friends vehicles?

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #8 on: April 19, 2014, 12:02:28 PM »
No I haven't done that yet. I've always been told TBI motors are picky with timing that if it's not perfect it won't run right. But funny thing is I timed it without unplugging the black/tan wire two years ago and has ran fine everywhere it's been. So I can but can't say 2 degrees off is going to make it stumble on acceleration like it's doing.

Offline dvdswan

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #9 on: April 19, 2014, 12:09:50 PM »
so the 2 black wires, do you know what there were connected to on at least one end?  you said they are in the main harness behind the distro, did they go to the distro?  did you break the wires when you pulled them up?  what year are you working with?

Offline rich weyand

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #10 on: April 19, 2014, 12:11:08 PM »
What kind of plugs and wires are you running?  How old are they?

When you open the throttle, the engine gets a denser charge.  Denser charge is harder to ignite.

Sounds like something in the ignition is on the edge.  When you give it a denser charge, it can't ignite it.
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Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #11 on: April 19, 2014, 12:14:41 PM »
No I don't know what they were connected to. They were spliced as if there were some type of connectors on them but no clue as to what. They didn't go to the dist. that I could see. It don't wanna say they broke but more as pulled out of something cause they were spliced. It's a 1987 r30 TBI 350 to be exact.

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #12 on: April 19, 2014, 12:17:47 PM »
What kind of plugs and wires are you running?  How old are they?

When you open the throttle, the engine gets a denser charge.  Denser charge is harder to ignite.

Sounds like something in the ignition is on the edge.  When you give it a denser charge, it can't ignite it.


The plugs and wires are about a year and half old. New dist cap too I put on cause the cap I had looked burned inside on the terminals. What could be on edge? Could dense charge mean something with the erg not functioning properly?

Offline dvdswan

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #13 on: April 19, 2014, 12:20:30 PM »
you said you grounded them to the firewall, did you try grounding to the engine block?

Rich, I'm confused by a "denser charge",  what does that mean?

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #14 on: April 19, 2014, 03:34:43 PM »
you said you grounded them to the firewall, did you try grounding to the engine block?

Rich, I'm confused by a "denser charge",  what does that mean?

I grounded the wire with 74 ohms resistance to the block and left the other wire alone. Still had the same problem