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#1 |
Registered User
Join Date: Jan 2012
Location: Portland, OR
Posts: 38
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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. |
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#2 |
Senior Member
![]() ![]() ![]() Join Date: Jan 2018
Location: Hickory Flat, GA
Posts: 4,458
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Re: 1966 Inline 6 250 ci. Nightmare!
Is the fuel pump ok?
__________________
Christian Carpenter 1963 C10 - Frankentruck 283, Muncie 3 speed with overdrive Overdrive wiring here1963-ish truck bed trailer - Half-Wit 1981 C10 - Penny 305, th350 --> Soon to be 350, Saginaw 4 speed 1995 Dodge Dakota Sport "I'll put it simple: if you're going hard enough left, you'll find yourself turning right." - Doc Hudson |
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#3 |
Registered User
Join Date: Apr 2004
Location: Piedmont, OK
Posts: 44
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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.
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#4 |
Registered User
Join Date: Feb 2007
Location: west coast
Posts: 3,369
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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..
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#5 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,600
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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)? |
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#6 |
Registered User
Join Date: Jan 2012
Location: Portland, OR
Posts: 38
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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. |
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#7 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,600
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Re: 1966 Inline 6 250 ci. Nightmare!
Bump your initial up to 14 first.
See what that does. |
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#8 |
Registered User
Join Date: Aug 2020
Location: Tucson, AZ
Posts: 51
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Re: 1966 Inline 6 250 ci. Nightmare!
Coil?
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#9 |
Registered User
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,612
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Re: 1966 Inline 6 250 ci. Nightmare!
Vacuum leak! Pic of engine?
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#10 |
Registered User
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,612
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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
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#11 |
Registered User
Join Date: Sep 2013
Location: Vashon WA
Posts: 969
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Re: 1966 Inline 6 250 ci. Nightmare!
Let cut to the chase.....
Whats the sale price .? and does he have more pictures.? . . .....sP . .. .
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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 |
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#12 |
Registered User
Join Date: Nov 2018
Location: Durango Colo.
Posts: 74
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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.
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#13 |
Registered User
Join Date: Jan 2012
Location: Portland, OR
Posts: 38
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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... |
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#14 |
Registered User
Join Date: Feb 2014
Location: Pocahontas Arkansas
Posts: 684
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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?
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#15 |
Senior Member
![]() Join Date: Apr 2009
Location: SAN FERNANDO VALLE SOL CAL
Posts: 1,064
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Re: 1966 Inline 6 250 ci. Nightmare!
WD40 Works good to find the Vacuum leak
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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 |
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#16 |
Registered User
Join Date: Aug 2020
Location: Tucson, AZ
Posts: 51
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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.
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#17 |
Registered User
Join Date: Feb 2013
Location: central California
Posts: 2,735
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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?
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#18 |
Registered User
Join Date: Jul 2015
Location: Lindenwold, NJ
Posts: 94
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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 |
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#19 |
Registered User
Join Date: Mar 2010
Location: Edgewater NJ
Posts: 159
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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!
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#20 |
Registered User
Join Date: Jan 2012
Location: Portland, OR
Posts: 38
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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.
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#21 |
Just here to tinker
![]() Join Date: Jun 2008
Location: Holland, MI
Posts: 3,692
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Re: 1966 Inline 6 250 ci. Nightmare!
x2
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