Author Topic: Lost much of my power, shuttering and shaking - K20, SBC 350  (Read 3127 times)

Offline 1980K20

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Lost much of my power, shuttering and shaking - K20, SBC 350
« on: August 25, 2022, 09:37:19 PM »
Ok, starting over with the startup note.  I meandered about on this, but crossing fingers have a handle on it.

1980 chevy, '91 SBC 350 block with mild roller CAM, Vortec heads, Holley Sniper EFI with fuel pump in the gas tank.  Transmission TH-350 with NP205 transfer case.  Everything was rebuilt 600 miles ago.

After the first 100 mile drive including nice highway speeds, I filled her up with gas and as I drove home, and it was then driving awful.  To be fair, it could have begun to act poorly before I got gas, nice even roads to the station so no hitting the gas hard.  Symptoms:
  • Truck shook and shuttered when you hit the gas anything but very light tap
  • Struggled very hard going up a steep hill into the neighborhood, 20 MPH was it.
  • Wouldn't downshift
  • Idled fine, revving up in park sounded just fine.  Problem only happens when in gear.
  • Hold the brake, step on the gas, feels like it wants to go, let go of the brake, not even enough power to make the tires spin

After initial investigation, discovered low on transmission fluid.  Being in a glass half-empty mood, assumed that i just trashed the transmission.  Naaaah, it's just fine.  No brunt smelling fluid.  Added fluid and it runs just fine.  (I will get it changed in November when it's in the shop).  I drove around a bunch running experiments with the following results...
  • Occasional backfire, sounds like thru exhaust, when you stomp on it
  • When it's cold, stomp on it, backfires and stalls
  • People comment that it smells rich
  • Missing under load

Read on the internet that many times missing under load is electrical.  Discovered the plugs unevenly carboned.  #7 is the worst, completely black.  #3 was clear as can be.  Must be electrical, right?  Put in some new plugs, no change.  Checked the cap, everything looked fine, but i accidently broke a piece off the cap when trying to get that back hook unhooked, so on went a new cap, no change.

It was suggested that perhaps the O2 sensor went bad, is reading lean, and dumping fuel.  Did some reading on the internet which says that can be a problem.  Discovered a bad exhaust header outlet gasket that had failed in front of the O2 sensor.  New gasket, new O2 sensor.  Ran better and different, but still no power.  Maybe the old O2 sensor is still good and slightly better running was due to the new gasket.

Took it to a local shop specializes in setting up carbs and efi, he discovered that the tables were all whacked given the problem with the gasket.  He fixed it all up, spent a lot of time cleaning things up, fixed some things.  Dyno reading much, much smoother than previously.  But it only had 195 lbs-ft torque.  Something really off.  One run had the lbs-ft 207, but then back to 195.  Mystery.  Engine builder dyno'ed it at 400 lbs-ft.  Guy says you lose a good 30% through the drivetrain, which would make my reading around 250, still far off.  Found throttle didn't go to max, cause of kickdown issue.  Of note: the missing still happened, but it was an interaction with the EFI and vacuum advance.  Removed the vacuum advance and missing disappeared.  Timing set to 16 initial, 36 max.  Guy suggested a fresh set of plugs.  New plugs, still runs poorly.  #7 carboned like crazy and wet too!  Gee, they've all got some wetness to it.  I don't think it's oil.

Called the engine builder.  Out of business.  No luck in mudville.

More reading on the internet.  The internal fuel regulator can fail on Holley Sniper EFI.  Usually goes from the 58.5 psi setting to 100 psi.  Ran a test, mine was around 85, shaking 82-87.  Now, new fuel regulator on order.

Oh yeah, aftermarket parking brake won't stay on anymore.  Can an O2 sensor cause that? :-)  This is certainly the journey of fix the startup issues.  It was a ground up restoration, I'm learning along the way, fixing what needs to be fixed.  As you can tell, there are a lot of little things being corrected along the way.

It's another week to get the part.  Whew!

What other suggestions do you all have?
« Last Edit: September 19, 2022, 10:23:54 PM by 1980K20 »
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1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline VileZambonie

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Re: I think I messed up my TH350
« Reply #1 on: August 26, 2022, 04:09:52 AM »
If you are talking about the vacuum modulator, it should be connected to manifold vacuum, not ported vacuum.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline 1980K20

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Re: I think I messed up my TH350
« Reply #2 on: August 26, 2022, 04:53:14 PM »
If you are talking about the vacuum modulator, it should be connected to manifold vacuum, not ported vacuum.
Alright, I’ll check it. I’ve read that the wrong connection can cause some sort of trouble.


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--------------------------------------
1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline Geezer77GMC

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Re: I think I messed up my TH350
« Reply #3 on: August 26, 2022, 06:35:41 PM »
Old hot rod trick from 70's, disconnect vacuum to the modulator, shift points are much higher and more firm. No or very low vacuum signals the 350th valve body that the throttle is wide open (floored gas pedal). Made the tires chirp when up-shifting.
It won't hurt your transmission, as it causes full apply pressure to the next gear clutch(s) quicker, less overlap during the gear change. Vacuum line to trans modulator should properly be manifold vacuum only.

Offline 1980K20

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Re: I think I messed up my TH350
« Reply #4 on: August 28, 2022, 11:55:10 PM »
Checked out, was on straight manifold vacuum.  Strange, last two times I ran it, idle was super low. Set it up to 850 rpm, purrs nicely.

Working the kinks out I suppose


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--------------------------------------
1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline 1980K20

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Re: I think I messed up my TH350
« Reply #5 on: September 03, 2022, 01:03:32 AM »
Thank goodness, transmission guy says it’s all good.

However engine is the problem.  Realize now it’s missing under load, probably the el cheapo spark plugs, one is very fouled.

 Stopped by a place, got appointment for tuning.  Problems with idle too, Snyper efi , may be bad o2 sensor also.

Whew, not so bad I’m crossing fingers


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1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline JohnnyPopper

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Re: I think I messed up my TH350
« Reply #6 on: September 03, 2022, 01:40:41 PM »
Engine issues aside, running that low on trans fluid probably stressed it, with some burning, degradation to the clutch discs.

I would service it with new filter and fluid as a protective measure.

If you keep them clean they last a long time  8)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline 1980K20

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Re: I think I messed up my TH350
« Reply #7 on: September 05, 2022, 09:49:10 PM »
Will do Johnny, great suggestion!  Appointment in November.

Working on the next problem.  Cylinder number 7 isn’t fitting well.  New plugs in half to start with, no fix.  Put on a new wire, expect it to be the same

 1,3,5,7.  Yowsers,  that’s bad.



The idle timing without vacuum was about 18, I dropped it to 14.4.  The truck sounds awful when you have the brake on and hit the gas.  Sounds fine in idle.

So there’s multiple issues, but what would cause just one plug to completely foul up?


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1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline bd

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Re: I think I messed up my TH350
« Reply #8 on: September 06, 2022, 08:47:20 AM »
It appears that cylinder #7 isn't firing at all.  Remove and inspect the spark plugs on the even bank.  Verify the firing order.  Run a wet/dry compression test on all cylinders.  Closely inspect the distributor cap for cracks and carbon tracking and the condition of the carbon button.  Inspect the rotor for heat stress and perforations through the rotor directly above the distributor shaft.  Measure the resistance of the spark plug wires and look closely up inside the wire boots for greenish discoloration and/or black soot.  Make sure the ground strap is in place under the coil cover.  At any time, did you disassemble the distributor and remove the shaft?
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 1980K20

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Re: I think I messed up my TH350
« Reply #9 on: September 07, 2022, 12:19:22 AM »
Running Rich all around, I looked up on line, found exhaust leaks can cause this.  Boom, bad header to exhaust gasket, cheap one, just in front of o2 sensor.  Will get a new gasket and two plugs to replace the two I’ve horribly fouled.



Here’s the bad exhaust gasket. It was loose and broke apart in my fingers. Lots of carbon.



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1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline 1980K20

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Re: I think I messed up my TH350
« Reply #10 on: September 11, 2022, 01:14:21 AM »
New exhaust collector flange installed, still runs poorly.  Checked compression, all good 150-155 wet.

I’ll put in a new cap tomorrow, I broke a little piece of trying to get the hook in place.  Ordered a new o2 sensor since that could cause these symptoms.

Found the idle setting was too low causing the servo not to kick in, so at least I fixed two things.


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--------------------------------------
1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline bd

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Re: I think I messed up my TH350
« Reply #11 on: September 11, 2022, 10:39:40 AM »
...At any time, did you disassemble the distributor and remove the shaft?
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 1980K20

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Re: I think I messed up my TH350
« Reply #12 on: September 12, 2022, 12:11:33 AM »
...At any time, did you disassemble the distributor and remove the shaft?
Oh no.

I did put on a new cap today.  New exhaust collector gasket, but I think the damage is done. New o2 sensor on order.


Sent from my iPhone using Tapatalk
--------------------------------------
1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline 1980K20

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Re: I think I messed up my TH350
« Reply #13 on: September 15, 2022, 12:38:21 AM »
Bad o2 sensor!  Still doesn’t run like before, but behaved much differently and better. Took her out for a four mile drive, ran 2nd gear to keep the rpm’s up above 3,000 and try to get the efi to start relearning.  Ran a bit better when I got home.  Now gas gauge says I’m running out. Guzzle, guzzle. Looking forward to getting it running right and seeing where I stand in mpg.

Appointment Saturday at carb connection, should get it straight.  I’ll wait until I get there for new plugs.

Thanks again for all the advice!


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--------------------------------------
1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline 1980K20

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Re: Too much fuel goose chase
« Reply #14 on: September 18, 2022, 10:13:05 PM »
Changed title of this, it’s really been a goose chase.

Took the truck to carb connection Saturday.  They fixed some things…
1) cleaned up tables from the bad o2 sensor
2) idle timing 16, disconnected vacuum advance since it was fighting the efi and causing some miss
3) added an extension for the accelerator and now throttle is smooth as silk. Also fixed that it was just a touch off from full throttle
4) readjusted kickdown

It ran better but stop no get up and go.  New set of plugs, also fouled.

Discovered that intake manifold is a dual plane. He has recent experience with an sbc383 stroker that they got another 40 HP.

Did some more reading on the internet, found a bad regulator can cause this issue.  Put pressure gauge on, when 85 psi, not 58.5.

FYI he thinks the 180F thermostat is perfect.  He thinks a 195F will get another 0.2-0.3 MPG, not much.  Once this is all fixed, will see where it all stands.

Pictures of high pressure, plugs along with number seven from fresh set of plugs if I can get taptatalk to cooperate.

Regulator on order, will update you next week.
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1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350