The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

Mike_82_Shortbox · 08-13-2020, 02:55 PM

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?

08-13-2020, 02:55 PM	#1
Mike_82_Shortbox Registered User Join Date: Jan 2012 Location: Portland, OR Posts: 37	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.