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 1960 - 1966 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 08-13-2020, 02:55 PM   #1
Mike_82_Shortbox
Registered User
 
Mike_82_Shortbox's Avatar
 
Join Date: Jan 2012
Location: Portland, OR
Posts: 37
Unhappy 1966 Inline 6 250 ci. Nightmare!

Hi, guys,

This week I'm helping my friend get his 1966 Chevy C20 long bed running so he can sell it.





I've never seen anything like the problem we are having with this seemingly simple motor. I wonder of you guys might take a moment to read about it, and reply with your guess as to the likely problem.

We started the motor and it idled rough and rich. So I started by checking if all cylinders were firing. I pulled the plug wires off the distributor cap one by one, and cylinders three and four, the two middle cylinders, made no difference. I could unplug one or the other, or both of them, even reverse the wires on the distributor cap, and it made no difference in the idle.

So we checked the compression and all cylinders had between 130 and 150 PSI, so I thought maybe the cam lobes on those cylinders were worn. We took the valve cover off measured the lifter deflection, and it was the same on all the intakes and all the exhausts.

We checked the plugs by rotating the spark plugs out of the two center cylinders when we had the plugs out for the compression check, and we replaced the spark plug wires, distributor cap, rotor, and finally the distributor assembly which included the points, condenser, and vacuum advance diaphragm. No difference. Then, since the accelerator pump was weak and the carburetor was leaking fuel from the bottom half, we put a rebuilt carburetor on it.

No difference.

The funny thing is on test driving, in second gear, under a load, in a very narrow RPM range, the engine will smooth out and start running on all six cylinders.

I'm at my wits end. I've never seen anything like this in my life. We basically replaced the entire ignition and fuel system, and the truck still runs and idles horrible. Any ideas?

Last edited by Mike_82_Shortbox; 08-13-2020 at 03:02 PM.
Mike_82_Shortbox is offline   Reply With Quote
Old 08-13-2020, 05:10 PM   #2
cwcarpenter98
Senior Member


 
cwcarpenter98's Avatar
 
Join Date: Jan 2018
Location: Hickory Flat, GA
Posts: 4,419
Re: 1966 Inline 6 250 ci. Nightmare!

Is the fuel pump ok?
__________________
Christian Carpenter

1963 C10 - Frankentruck 283, 3 speed with overdrive
Overdrive wiring here
1963-ish truck bed trailer - Half-Wit

1981 C10 - Penny 305, th350

1995 Dodge Dakota Sport

"I'll put it simple: if you're going hard enough left, you'll find yourself turning right." - Doc Hudson
cwcarpenter98 is offline   Reply With Quote
Old 08-13-2020, 05:38 PM   #3
cryowagon
Registered User
 
Join Date: Apr 2004
Location: Piedmont, OK
Posts: 44
Re: 1966 Inline 6 250 ci. Nightmare!

Check the carburetor. The old Rochester B's warp and fuel will run down inside and outside the carburetor. That will make it run very rich. I also got a brand new fuel pump from O'reilly that put out too much pressure. It forces the float needle open and floods the carburetor.
cryowagon is offline   Reply With Quote
Old 08-13-2020, 05:54 PM   #4
factorystock
Registered User
 
factorystock's Avatar
 
Join Date: Feb 2007
Location: west coast
Posts: 3,343
Re: 1966 Inline 6 250 ci. Nightmare!

I would check to see if there is a vacuum leak. Notorious for leaks around intake manifold gasket along head and or carb gasket..
factorystock is offline   Reply With Quote
Old 08-13-2020, 06:08 PM   #5
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 13,495
Re: 1966 Inline 6 250 ci. Nightmare!

Could be a timing problem.
What’s the initial set at?
Is vac pot still good (not blown)?
geezer#99 is offline   Reply With Quote
Old 08-13-2020, 07:57 PM   #6
Mike_82_Shortbox
Registered User
 
Mike_82_Shortbox's Avatar
 
Join Date: Jan 2012
Location: Portland, OR
Posts: 37
Re: 1966 Inline 6 250 ci. Nightmare!

Fuel pump is good, new filter. Carburetor is brand new, no leaks. Timing at 4° BTDC, new distributor has new pot. Have not checked the manifold. I guess we'll pull that off tomorrow.

Thanks, guys.
Mike_82_Shortbox is offline   Reply With Quote
Old 08-13-2020, 08:37 PM   #7
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 13,495
Re: 1966 Inline 6 250 ci. Nightmare!

Bump your initial up to 14 first.
See what that does.
geezer#99 is offline   Reply With Quote
Old 08-13-2020, 11:55 PM   #8
Desert dog
Registered User
 
Join Date: Aug 2020
Location: Tucson, AZ
Posts: 51
Re: 1966 Inline 6 250 ci. Nightmare!

Coil?
Desert dog is offline   Reply With Quote
Old 08-14-2020, 12:07 AM   #9
Warrens69GMC
Registered User
 
Warrens69GMC's Avatar
 
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,613
Re: 1966 Inline 6 250 ci. Nightmare!

Vacuum leak! Pic of engine?
Warrens69GMC is offline   Reply With Quote
Old 08-14-2020, 12:09 AM   #10
Warrens69GMC
Registered User
 
Warrens69GMC's Avatar
 
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,613
Re: 1966 Inline 6 250 ci. Nightmare!

See if you can borrow a HEI and feed it 12 volt and see if its the distributor
Warrens69GMC is offline   Reply With Quote
Old 08-14-2020, 12:35 AM   #11
Softpatch
Registered User
 
Softpatch's Avatar
 
Join Date: Sep 2013
Location: Vashon WA
Posts: 969
Re: 1966 Inline 6 250 ci. Nightmare!

Let cut to the chase.....
Whats the sale price .?
and does he have more pictures.?
.
.
.....sP
.
..
.
__________________
My Spelling is Not Incorrect...It's 'Creative'
.
.
1966 C-20 .....Swap 91 G-30(5.7 FI)/4L80E
http://67-72chevytrucks.com/vboard/s...d.php?t=789617
59 Viking Revival .. http://67-72chevytrucks.com/vboard/s...d.php?t=632341
Softpatch is offline   Reply With Quote
Old 08-14-2020, 03:01 PM   #12
hairball
Registered User
 
Join Date: Nov 2018
Location: Durango Colo.
Posts: 69
Re: 1966 Inline 6 250 ci. Nightmare!

Sounds like a vacuum leak. Take some Brake Clean and spray around the intake gasket and note any change in RPM.
hairball is offline   Reply With Quote
Old 08-18-2020, 12:31 AM   #13
Mike_82_Shortbox
Registered User
 
Mike_82_Shortbox's Avatar
 
Join Date: Jan 2012
Location: Portland, OR
Posts: 37
Re: 1966 Inline 6 250 ci. Nightmare!

He want's $5000 for the truck, but I think he'll take $4500. I keep telling him someone will buy the truck for what it can be, not what it is...

We haven't got back to working on it yet, but I'll try the spraying around the gasket, and the timing jump first. I'll see if he took some decent pictures for Craigslist...
Mike_82_Shortbox is offline   Reply With Quote
Old 08-18-2020, 08:30 AM   #14
Old Truck Man
Registered User
 
Old Truck Man's Avatar
 
Join Date: Feb 2014
Location: Pocahontas Arkansas
Posts: 684
Re: 1966 Inline 6 250 ci. Nightmare!

the stock points dist is marginal when new. convert to a HEI. pull all the plugs and put air pressure in the cyls with both valves closed. you might have a blown head gasket between two cyls. I would also suspect a vacuum leak. maybe at the intake manifold and cyl head?
Old Truck Man is offline   Reply With Quote
Old 08-20-2020, 12:33 AM   #15
1971 GMC SHORTY
Senior Member
 
1971 GMC SHORTY's Avatar
 
Join Date: Apr 2009
Location: SAN FERNANDO VALLE SOL CAL
Posts: 1,064
Re: 1966 Inline 6 250 ci. Nightmare!

WD40 Works good to find the Vacuum leak
__________________
1966 GMC K25 V6 351E Original Utility Truck
2000 C3500 Hot Rod Utility Truck
2003 ROAD KING 100th 124ci 6 Speed
145CI M/P Pro Street FXR
2002 FXDP
2007 2500HD DURAMAX. DD
Studio Teamsters / Mechanic
Retired GM Master Tech 20 Years
Retired Ford Tech 15 Years
1971 GMC SHORTY is offline   Reply With Quote
Old 08-20-2020, 12:47 AM   #16
Desert dog
Registered User
 
Join Date: Aug 2020
Location: Tucson, AZ
Posts: 51
Re: 1966 Inline 6 250 ci. Nightmare!

Is it possible the harmonic balancer slipped? I ran into this with a Chevy 4.3 V6 awhile back. We set TDC with a piston stop and found the timing mark to be way off. A new balancer and it fired right up.
Desert dog is offline   Reply With Quote
Old 08-20-2020, 12:49 AM   #17
AcampoDave
Registered User
 
AcampoDave's Avatar
 
Join Date: Feb 2013
Location: central California
Posts: 2,723
Re: 1966 Inline 6 250 ci. Nightmare!

Is there any brownish gray pukey looking condensation on the oil breather or bottom of the pcv valve?
AcampoDave is offline   Reply With Quote
Old 08-20-2020, 07:17 AM   #18
racerop
Registered User
 
racerop's Avatar
 
Join Date: Jul 2015
Location: Lindenwold, NJ
Posts: 94
Re: 1966 Inline 6 250 ci. Nightmare!

Since pulling the plug wires for cylinders
three and four revealed no change in
the idle, I would spend time looking for a vacuum
leak in the area
racerop is offline   Reply With Quote
Old 08-21-2020, 01:27 PM   #19
maxwoof
Registered User
 
Join Date: Mar 2010
Location: Edgewater NJ
Posts: 160
Re: 1966 Inline 6 250 ci. Nightmare!

Hi. I don’t see that you replaced the spark plugs. If that is the case, you should try new plugs. I’ve had similar symptoms and new plugs solved it. Also, once with a bad coil...same thing. Weak/intermittent spark can cause black smoke and give the appearance of a fuel problem. Hope this helps - good luck!
maxwoof is offline   Reply With Quote
Old 08-29-2020, 08:16 PM   #20
Mike_82_Shortbox
Registered User
 
Mike_82_Shortbox's Avatar
 
Join Date: Jan 2012
Location: Portland, OR
Posts: 37
Re: 1966 Inline 6 250 ci. Nightmare!

Thanks, guys. Haven't had time yet to look at the truck again, but these things are going on the list.
Mike_82_Shortbox is offline   Reply With Quote
Old 09-03-2020, 07:16 PM   #21
Corts60
Just here to tinker
 
Corts60's Avatar
 
Join Date: Jun 2008
Location: Holland, MI
Posts: 3,672
Re: 1966 Inline 6 250 ci. Nightmare!

Quote:
Originally Posted by factorystock View Post
I would check to see if there is a vacuum leak. Notorious for leaks around intake manifold gasket along head and or carb gasket..
x2
Corts60 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 07:03 AM.


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