The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1967 - 1972 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 07-17-2008, 10:57 PM   #1
Rippin-J
OurLadyofBlessedAccelerat ion
 
Rippin-J's Avatar
 
Join Date: Aug 2007
Location: Denver, CO
Posts: 148
Cool Help Needed - '68 c20 w/ 396

I've had my '68 now for over a year, and the guy I purchased it from "had the motor rebuilt" but then it sat parked in his shop for 5 years before I purchased it. It's never run great, but for the past 6 months or so is been SUPER rough, too sketchy to even rip around the block.

It'll turn over okay (after a jump..... alternator maybe?) and even idles OK for a few minutes. Once it warms up it begins to spit and sputter a bit, sometimes sounding like it's on the verge of dying. The left side (driver's side) seems to be the worse as it blows black smoke (not TONS) out of the pipe. Also, when I dump the throttle and throw open the carb, it is VERY late to respond and near dies.

First, the thought was carbs because the Quadrajunk that was on there wasn't even opening up with the throttle. Replaced it with a brand new Holley Street Avenger. That didn't fix it, but the motor gets more gas now.

The thought now is that the issue is something internal: compression, valves, gasket, piston (rings), etc etc. My major dilemma is that my network of 'gearhead' buddies are all in Colorado and I now live in Charlotte where no one in my network fiddles with or knows much about old cars. Oh, and I know very little about the diagnosis and repair of internal components.

So here's what I'm asking the forum for help on:

1) What novice level tests can I do to further diagnose the issues I am having.
2) Does anyone know anybody or is anyone in the great Charlotte area willing to help me learn the ropes.

It's a very clean truck and motor, so I'd hate to just scrap it and replace it with a rebuilt 350.... but am willing to if that seems like the best case scenario.

Anyways, you all have been very helpful in the past so I have no doubt that someone, somewhere will have some ideas.

Much appreciated,

Jordan
__________________
_______________________________________________

1968 Chevy C-20 w/ 396
1997 HD Sportster 1200 street tracker
1994 Chevy 1500 Silverado


Last edited by Rippin-J; 07-17-2008 at 11:02 PM. Reason: to make description of problem more thorough....
Rippin-J is offline   Reply With Quote
Old 07-17-2008, 11:46 PM   #2
kwmech
Senior Member
 
Join Date: Mar 2003
Location: Colfax-California
Posts: 8,630
Re: Help Needed - '68 c20 w/ 396

I'd go after a valve adjustment first, followed by a compression test.
kwmech is offline   Reply With Quote
Old 07-18-2008, 12:33 AM   #3
stllookn
Saving 1 truck at a time!
 
stllookn's Avatar
 
Join Date: Aug 2001
Location: Kent, WA
Posts: 6,465
Re: Help Needed - '68 c20 w/ 396

I agree with KW but would do the compression check first since it will be much easier with your level of experience. All you have to do is purchase a compression gauge from your local auto parts store and follow the instructions. Some of your problems could be timing as well as compression due to valves or poor break-in on the cam.
__________________
'68 C20 Longhorn 50th Anniversary 400/TH400
'68 C20 Longhorn 50th Anniversary 468/TH400w/buckets
'72 C20 Halfhorn (Longhorn w/o cab and front clip)
'69 Flxible Cruiser (look up ugly in the dictionary)
stllookn is offline   Reply With Quote
Old 07-18-2008, 01:41 PM   #4
BigDaddyDave
RIP BigDaddyDave ~7-23-09~ You will be missed~
 
BigDaddyDave's Avatar
 
Join Date: Aug 2007
Location: Concord NC
Posts: 1,361
Re: Help Needed - '68 c20 w/ 396

Have you tuned it up? I can tell when mine starts running rough that it is time. I spent a lot of money on my junk,then replaced the distributor and it made a world of difference.
__________________
BigDaddyDave's Truck: http://www.flickr.com/photos/daves71chevy/ | See it in Action!
BigDaddyDave is offline   Reply With Quote
Old 07-18-2008, 02:03 PM   #5
fun in dirt
Registered User
 
fun in dirt's Avatar
 
Join Date: Jan 2007
Location: Mississippi
Posts: 2,122
Re: Help Needed - '68 c20 w/ 396

?bad gas? Drain lines, tank & all. Just a thought if it's been sitting with same gas for over a month or so, especially as crappy as gas is now. I spent $100+ on mine one time, when it was acting like that, & it was just the gas.
__________________
'72 C to K20 "Cannibalizer" http://67-72chevytrucks.com/vboard/s...32#post8653432
'85 K30 "Big Nassy"
[B]'85 C10 [B] "Corpse"
'01 2500 Suburban ...party wagon & tow truck
'06 Silverado SS...DD
'06 C1500 WT...DD
'07 Classic C1500 WT...DD
Built & sold cause can't keep 'em all:
'72 Blazer "The Bucket" http://67-72chevytrucks.com/vboard/s...d.php?t=433190
'55 Chevy straight axle gasser https://www.trifive.com/forums/showthread.php?t=173938
'69 Pro Street Chevelle http://www.chevelles.com/forums/showthread.php?t=332541
'86 M1031 CUCV http://67-72chevytrucks.com/vboard/s...d.php?t=553619
fun in dirt is offline   Reply With Quote
Old 07-18-2008, 02:08 PM   #6
Rippin-J
OurLadyofBlessedAccelerat ion
 
Rippin-J's Avatar
 
Join Date: Aug 2007
Location: Denver, CO
Posts: 148
Re: Help Needed - '68 c20 w/ 396

You know, I hate to admit, but I've heard the term 'tune up' used my whole life and I can't say that I know exactly what that entails.

When you say tune up, what all does that include?
timing, distributor, wires, oil, ?

Any chance you feel like making a trip down I-85 to help out? I'm just off the interstate!

I think the tune up sounds like a good first step, followed up by compression test and then valve adjustments. My assumption is that tune would eliminate a bunch of other possibilities, then the compression would confirm others, and finally a valve adjustment would be the last case scenario.
__________________
_______________________________________________

1968 Chevy C-20 w/ 396
1997 HD Sportster 1200 street tracker
1994 Chevy 1500 Silverado

Rippin-J is offline   Reply With Quote
Old 07-18-2008, 02:10 PM   #7
Rippin-J
OurLadyofBlessedAccelerat ion
 
Rippin-J's Avatar
 
Join Date: Aug 2007
Location: Denver, CO
Posts: 148
Re: Help Needed - '68 c20 w/ 396

Gas is another good point to check... the gas that's in there is at least 3 months old now.
__________________
_______________________________________________

1968 Chevy C-20 w/ 396
1997 HD Sportster 1200 street tracker
1994 Chevy 1500 Silverado

Rippin-J is offline   Reply With Quote
Old 07-18-2008, 02:53 PM   #8
Longhorn321
Size Matters
 
Longhorn321's Avatar
 
Join Date: Aug 2007
Location: Lyman, SC
Posts: 9,189
Re: Help Needed - '68 c20 w/ 396

Have you tried a new set of plugs? I would start there if you haven't tried that already and then look at the timing, compression and cylinder leakage, and then the valve adjustment as stated above. The timing and valve adjustment would be more suspect to me due to the fact that you said the motor had been rebuilt and had not run great since you have had it...who knows if the PO knew what he was doing.
__________________
"Reading the owners manual is only for quitters." Jason Harper, Bloomberg News.

...WOOOO HOOOO!!!!

Last edited by Longhorn321; 07-18-2008 at 02:54 PM.
Longhorn321 is offline   Reply With Quote
Old 07-18-2008, 02:56 PM   #9
Rippin-J
OurLadyofBlessedAccelerat ion
 
Rippin-J's Avatar
 
Join Date: Aug 2007
Location: Denver, CO
Posts: 148
Re: Help Needed - '68 c20 w/ 396

yep, new plugs for sure. replaced those with the carb in April.
__________________
_______________________________________________

1968 Chevy C-20 w/ 396
1997 HD Sportster 1200 street tracker
1994 Chevy 1500 Silverado


Last edited by Rippin-J; 07-18-2008 at 02:59 PM.
Rippin-J is offline   Reply With Quote
Old 07-18-2008, 03:30 PM   #10
kwmech
Senior Member
 
Join Date: Mar 2003
Location: Colfax-California
Posts: 8,630
Re: Help Needed - '68 c20 w/ 396

Quote:
Originally Posted by stllookn View Post
..................................................................................... Some of your problems could be timing as well as compression due to valves or poor break-in on the cam.


Hence the reason for the valve adj first----if the valves are too tight then the compression is gonna be a false reading
kwmech is offline   Reply With Quote
Old 07-18-2008, 03:33 PM   #11
Longhorn321
Size Matters
 
Longhorn321's Avatar
 
Join Date: Aug 2007
Location: Lyman, SC
Posts: 9,189
Re: Help Needed - '68 c20 w/ 396

Quote:
Originally Posted by kwmech View Post
Hence the reason for the valve adj first----if the valves are too tight then the compression is gonna be a false reading
Tru dat! I actually had that same condition...missing on 3 cylinders and compression was horrible. Adjusted the valves and she ran like a top.
__________________
"Reading the owners manual is only for quitters." Jason Harper, Bloomberg News.

...WOOOO HOOOO!!!!
Longhorn321 is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 10:59 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com