Author Topic: Chevy TBI 350 stumbles on acceleration  (Read 12006 times)

Offline rich weyand

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #15 on: April 20, 2014, 12:22:38 AM »
Rich, I'm confused by a "denser charge",  what does that mean?

At lower throttle settings, the intake manifold has significant vacuum as the engine keeps pumping air and the throttle plate is at least partially closed.  The pressure inside the manifold that feeds into the intake valve is less than atmospheric pressure.  This is a less dense charge, and ignites easily.

At higher throttle settings, the intake manifold is closer to atmospheric pressure because, even though the engine is pumping, the throttle plate is substantially open.  When the intake valve opens, this denser mixture enters the cylinder.  This denser charge is harder to ignite than the less dense charge that results from lower throttle settings.

When you lose power on high throttle settings, it may be because the ignition system is not providing hot enough spark to ignite the denser charge that results from less vacuum in the manifold, even though it can ignite the easier, less dense charge of lower throttle settings.  The ignition system is "on the edge" of not igniting the charge at lesser throttle settings, and at higher throttle settings it can't always ignite the charge and you have intermittent misses on one or more cylinders.  You might not notice such a miss at higher rpms that would be very apparent at idle.  That unburned charge from that miss is going to go into the hot manifold/header, and be followed up with still-burning gasses from another cylinder firing, and ignite in the manifold/header, giving you an after-fire (like a back-fire, but in the exhaust, not coming "back" through the carb).  Instead of a boom under the hood, you will get a boom from underneath the cab floor.

That's why I ask what kind of plugs and what kind of wires, and how old are they.  I think the AC Delco R45TS is the proper plug for all the 73-87 SBCs.  For wires, since those are already resistor plugs, you should run helical-wind (sometimes called spiral-wind) metal conductor wires.  Taylor has the reputation for good helical-wind metal-conductor wires.  I have a set on the way for mine (ordered them from Summit tonight for $60), given that I am experiencing an occasional after-fire when climbing some of the steep hills around here, with long pulls at high throttle settings.  I think my wires aren't giving me full spark any more.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline dvdswan

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #16 on: April 20, 2014, 01:04:02 PM »
that is a great explanation Rich.  thank you.

87becky did you by chance use the new dielectric grease on the ICM when you replaced it?

Offline 1979C20

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #17 on: April 20, 2014, 03:32:39 PM »
I hate to sound like a broken record, but on a Chevy TBI engine, the timing needs to be at ZERO. Having the timing advanced or retarded will cause problems, including hesitation, bogging or even misfire. Unplug the tan/black wire, time the engine to ZERO degrees and reconnect the tan/black wire.
1979 SCLB C20 Q-jet 350 SM465 14b F.F. 4.10 G80
1989 GMC Suburban V2500 TBI350 TH400 4in lift 35's 14b SF

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #18 on: April 20, 2014, 04:32:50 PM »
that is a great explanation Rich.  thank you.

87becky did you by chance use the new dielectric grease on the ICM when you replaced it?


I applied the special heat paste that was supplied with the new ICM but no dielectric grease. I'll be sure to apply some when I'm working on it next

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #19 on: April 20, 2014, 04:36:48 PM »
I hate to sound like a broken record, but on a Chevy TBI engine, the timing needs to be at ZERO. Having the timing advanced or retarded will cause problems, including hesitation, bogging or even misfire. Unplug the tan/black wire, time the engine to ZERO degrees and reconnect the tan/black wire.


I'll have to try that to see if it helps. But like I said earlier the timing wasn't perfect before and it ran fine for the past 2 years so I don't really see 2 degrees advanced making it do what it's doing but I'll try and see what happens.

Offline dvdswan

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #20 on: April 20, 2014, 05:42:06 PM »
that is a great explanation Rich.  thank you.

87becky did you by chance use the new dielectric grease on the ICM when you replaced it?


I applied the special heat paste that was supplied with the new ICM but no dielectric grease. I'll be sure to apply some when I'm working on it next

that's what I meant.  I thought it was dielectric grease of some sort.

Online bd

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #21 on: April 20, 2014, 06:50:41 PM »
As long as the base timing is between TDC and 6 BTDC, and the engine doesn't ping, timing isn't likely an issue.  There is no crank or cam position sensor on this motor, hence the timing map is merely offset by the amount of base timing.  If the engine becomes over advanced to the point of detonation, knock will cause EST to retard the timing and degrade engine output.

Does the SES light in the dash illuminate when the ignition is first switched on?  Are there any trouble codes stored?  Have you checked fuel pressure?  When was the last time the fuel filter was replaced?  Does the truck still have its catalytic converter?  Pull the plug wires off the distributor cap and coil and look up inside the boots.  Do any of them appear sooty or discolored?  Do the same thing at the spark plug ends.

If you think EGR is the problem, just unplug the vacuum line from the valve and go for a test drive.  Other than ping, does the hesitation/backfire disappear?

-------------------------------

Post a pic of the 'black wires' and where they exit the harness.  Elaborate on...

...They were spliced as if there were some type of connectors on them....  I don't wanna say they broke but more as pulled out of something cause they were spliced....

            ...Are the wires solid black or black with a white stripe?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 87becky

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Re: Chevy TBI 350 stumbles on acceleration
« Reply #22 on: April 22, 2014, 11:54:48 PM »
As long as the base timing is between TDC and 6 BTDC, and the engine doesn't ping, timing isn't likely an issue.  There is no crank or cam position sensor on this motor, hence the timing map is merely offset by the amount of base timing.  If the engine becomes over advanced to the point of detonation, knock will cause EST to retard the timing and degrade engine output.

Does the SES light in the dash illuminate when the ignition is first switched on?  Are there any trouble codes stored?  Have you checked fuel pressure?  When was the last time the fuel filter was replaced?  Does the truck still have its catalytic converter?  Pull the plug wires off the distributor cap and coil and look up inside the boots.  Do any of them appear sooty or discolored?  Do the same thing at the spark plug ends.



Ok, so today I got to get sometime with the truck and I found a few things. nothing major but good to know. SES light does come on when first switched on. No trouble codes stored. fuel pressure could be a possibility but I wanna say its not cause it can supply fuel up to 4000 rpm. Ive  reved it more in the past with the set up I have so its not likely fuel pressure but I will be testing that soon. fuel filter was replaced when I put this pump in about 6-8 months ago.  the wires are fine but it may be possible theyre breaking down inside but they are fairly new and ill be checking that tomorrow. I unhooked the egr line and nothing changed or made a difference. The two wires are solid black. As I was told solide black is just ground. so to day I put them together and grounded them out to the blok. before this my truck was running in an open loop with the O2 sensor running lean and O2 crossings weren't up where they should be. once I grounded these wires everything was normal and in perfect order. the truck is still doing what it has been doing. since I bout a ICM from autozone im going to a GM dealership to buy a good one. ill also be testing the module I have at the auto parts store to see what it does. I tried posting the pic for the wires but I cant get it to work