Author Topic: '77 K20 Running Rough  (Read 627 times)

Offline ehjorten

  • Senior Member
  • ****
  • Posts: 1077
'77 K20 Running Rough
« on: September 23, 2020, 08:36:47 AM »
So Sunday I was driving my '77 K20 and after I drove it for a while, it all of a sudden started running rough; like stumbling and down on power. Felt like it was missing pretty bad! I could pedal it really easy and it would run fine, but if I started climbing a hill or putting it under some load, it was difficult to get it to accelerate and if I put my foot into the throttle, it would just bog-out and start slowing down! Now if I came to a stop for an amount of time like at a light, or just pull off on the side of the road and look under the hood, then it would pull away practically without issue, but would soon start acting up again!

This truck has a 350 with a 4160 Holley on it and HEI ignition.

Yesterday, I decided to put new plugs in it, because it does leak a little past the valve seals and will eventually build-up that brown, crusty deposit on the plugs. I thought it was starting to miss a little at times, so probably time to just do that. I did that and took it for a drive. It drove fine for probably about 8 miles and then it started doing the exact same thing!

I did pull the cap on the HEI and inspected for carbon tracing and the like. All looked good, but the contact button probably needs to be replaced. It is looking fairly worn. I think it could still be the coil, but the last work I did on the engine was to rebuild the carburetor bowls. One thing I noticed on the front bowl is that the accelerator pump check ball had an issue with the little metal strap that holds it in. When I took it apart the metal strap was laying in the bowl. It went back in, but it felt kind of loose. This was probably 3 months ago! I think that would cause the accelerator to NOT pump enough fuel because the check ball would just fall down and NOT prevent the accelerator squirt from pushing back into the fuel bowl. I have never blown a power valve, so I don't know if that would cause it? When I looked into the barrels, it just seemed like it was putting a lot of fuel into the engine when you rolled into the throttle.

It just seems like it is a fueling issue, but could be the coil is breaking down? It just happened all of a sudden. It doesn't act like a vacuum leak, but I do think there is one or two vacuum plugs I should check.

Thoughts?

Thanks,
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 5778
Re: '77 K20 Running Rough
« Reply #1 on: September 23, 2020, 10:58:19 AM »
First, you need to determine whether the symptoms are fuel or spark related.  A worn carbon button is probably insignificant unless it is burned back from arcing.  Use an HEI spark tester...

to verify a consistent, healthy blue spark.  Take a fuel sample and inspect for clarity and layering.  Perform a full fuel pump test for draw, pressure delivery and volume.

I agree that a problem in the carburetor accelerator pump circuit would manifest as a stumble or severe lag with initial throttle opening, but would then recover assuming the hesitation was not so extreme as to kill the engine.  Whereas a blown power valve diaphragm would flood the engine at idle.  If you suspect excessive main nozzle flow and the carburetor has float bowl windows, check the float levels.  Otherwise, check the floats for fuel loading/saturation.

How worn are the old spark plugs?
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 ehjorten

  • Senior Member
  • ****
  • Posts: 1077
Re: '77 K20 Running Rough
« Reply #2 on: September 23, 2020, 11:27:20 AM »
I have an adjustable gap, HEI spark plug tester. I also have an old Fluke 98 Automotive ScopeMeter, but I am missing the inductive pickup and secondary pickup cables that I have meant to see if I can re-purchase. I am not entirely sure I know how to test the secondary on an HEI. The spark plugs looked decent sans for a few cylinders that had some deposits built up. I replaced them all anyways, but they all looked fairly tan. I do need to check the float levels, but it was odd that it just happened all of a sudden.
« Last Edit: September 23, 2020, 11:53:03 AM by ehjorten »
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 5778
Re: '77 K20 Running Rough
« Reply #3 on: September 23, 2020, 12:05:05 PM »
Your spark tester should be suitable.  Expect 5/8" - 3/4" arc length for factory HEI w/o CD addon ignition.  Fluke leads should be readily available, just Google Fluke 98 leads.  Prices vary widely. 

The oddity of "suddenness" suggests ignition (e.g., ICM, PUC, coil) but such leaps of assumption too often lead into blind alleys.  So do your best to identify ignition vs fuel at the onset of your diagnosis and keep us posted.
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 TexasRed

  • Junior Member
  • ***
  • Posts: 656
Re: '77 K20 Running Rough
« Reply #4 on: September 23, 2020, 04:16:26 PM »
You may be able to find a place that tests ignition modules. If the module shows bad, I would replace the coil. The internal coils have a tendency to take out the module. A simple resistance check at cold (ie non operating temperature) won't tell you if the coil is good. A stock one will work best.

Offline DanMcG

  • Registered Users
  • *
  • Posts: 119
Re: '77 K20 Running Rough
« Reply #5 on: September 24, 2020, 04:27:03 AM »
A few of those symptoms sound like a pluged fuel tank vent. Other then that I got nothin.

Offline ehjorten

  • Senior Member
  • ****
  • Posts: 1077
Re: '77 K20 Running Rough
« Reply #6 on: September 27, 2020, 03:11:14 PM »
Took the coil out and ran a DMM across the primary and the secondary. Coil seemed fine. 0.6 Ohms Primary resistance and about 8,500 Ohms Secondary. Replaced the brush and rubber insulator for the coil. Checked the resistance on the pickup coil and couldn't seem to get a reading. I believe I should get something like 500-1,500 Ohms resistance. Seems it is open circuit?! Wondering how it could run like that?! So I think my pickup coil has open circuited.  Checked spark after that with a HEI spark plug tester and it was firing, but not consistently, so I think I found my culprit!

I also checked float levels and they were just fine!
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline TexasRed

  • Junior Member
  • ***
  • Posts: 656
Re: '77 K20 Running Rough
« Reply #7 on: September 29, 2020, 12:09:32 AM »
make sure the connections on the pickup coil are okay. Sometimes they push out of the connector really easy (may need to push them into the module individually). The coil can check good at room temperature but be bad at operating temps.

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 5778
Re: '77 K20 Running Rough
« Reply #8 on: September 29, 2020, 09:45:51 AM »
Erik, following is an excerpt from the Technical Pages article, Distributor components testing.

"Pick-Up Coil (PUC):
The PUC can only be tested for hard (non-intermittent) failures.  For the following tests, remove the distributor cap and rotor and carefully disconnect the PUC from the ICM.  Measure the resistance between either PUC lead and the metal frame of the PUC (ground).  The ohmmeter should indicate infinite resistance (open circuit).  Measure the resistance of the PUC with the ohmmeter connected to both PUC leads simultaneously and apply vacuum to the vacuum advance can while watching the meter.  If the distributor does not have a vacuum advance, gently wiggle and lightly tug on the PUC leads while performing the resistance test.  PUC measured resistance should remain constant (unchanging) within the range of 500 - 1,500 ohms.  If the PUC fails any of the resistance measurements it is defective and should be replaced.

Intermittent failures of a PUC are generally caused by temperature associated expansion and movement of the fine wire that comprises the coil winding, or flexing and internal breakage of the PUC leads that connect to the ICM, which result in temporary opens or shorts as the wire moves.  Typical symptoms are random misfire and/or dying of a warm engine that clears up once the engine cools or is cold.  Tests for intermittent PUC failures are generally unreliable."


The PUC may have a broken lead or filament that makes/breaks connection with flexing, causing an intermittent open.
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 ehjorten

  • Senior Member
  • ****
  • Posts: 1077
Re: '77 K20 Running Rough
« Reply #9 on: September 29, 2020, 04:51:42 PM »
That is exactly what I did BD. I measured an open circuit on the pickup coil and didn't measure anything to ground. I contemplated rebuilding the distributor, but noticed the advance weights were pretty loose on the pins and kinda flopped around. Also, the bushings were of unknown condition, so I just bought an AC Delco stock reman unit to save me the hassle. I have never serviced the HEI other than cap, coil, rotor since I have owned it, which has been about 22 years.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline ehjorten

  • Senior Member
  • ****
  • Posts: 1077
Re: '77 K20 Running Rough
« Reply #10 on: October 13, 2020, 09:28:57 AM »
Update on the issue...it was the fuel pump! Not sure what was going on with it, it didn't seem to leak either through the weep hole, or it didn't look like it was leaking into the engine. Maybe something happened to one of the internal valves. Anyways...it seems to be running fine now. After I replaced the distributor when I found I couldn't get a reading on the pickup coil, I drove it and all seemed good, until I started climbing a hill and the same problems presented themselves again.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6