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GMC V6 and V12 Engines Engine repair and rebuilding

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Old July 11th, 2017, 02:19 AM
wcstory wcstory is online now
Join Date: Jan 2015
Location: Mashpee, Ma
Truck: 1966 2503
Age: 50
Posts: 58
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Default Holley 500 cfm conversion on a 1966 GMC 305E

I’m almost complete with the Holley change over. I've been picking away at it but the work schedule keeps getting in the way.

I bought a Holley 2bl carburetor to replace the original single barrel. I am going to be swapping the SM420 granny low 4 speed for a 700R4 so I also decided to swap out the rod linkage throttle assemble for a '72 & up throttle assembly. I'll need the cable assembly to add the TV cable for the 700R4. If I was just swapping the original carburetor for a Holley it would be quicker.

First step I pulled the pedal, linkage and carb - they'll be appearing in the parts forum

I removed the manifold and mounted the Mr Gasket adapter. I scribed the manifold to open it up to match the adapter. I then used a carbide bur to grind out the opening to match. A slow process I need access to a miller

With the manifold re-mounted and the Holley seated atop I started on the accelerator cable

The firewall on my GMC was different from pedal and cable swap I found posted. Not sure if this is a Chevy vs GMC issue.

I took a grinder with cutting wheel and removed the old rod pivot mounting attachments. I will weld in a patch when the transmission is out.

I mounted kickdown bracket 20-121, Holley 20-38 throttle stud and Holley 20-95 Throttle and TV cable bracket.

2017-07-09_08-32-44 by wcstory, on Flickr

The throttle bracket I bought was about an inch too close to the throttle stud. I had to cut the end flat and weld on a one inch extension.

2017-07-09_08-33-07 by wcstory, on Flickr

2017-07-09_08-32-12 by wcstory, on Flickr

Drill a .500” hole in the firewall directly in line with the Throttle bracket. My firewall had a stamped flat spot location directly in line. I then used a square file to dress the edges of the hole to accept the square snap in end of the throttle cable. It was apparent the throttle cable I bought was going to be waaayyy too long. After asking on here I found the ends can be removed and the cable cut. I snipped off the metal stops crimped onto the wire rope throttle cable. Remove the firewall end and mark for length. Pull the cable out of the sheath and cut to length. Place the cable back in and put the firewall end back on.

2017-07-09_08-33-35 by wcstory, on Flickr

Take the ’72 and up pedal assembly and cut the upper bracket off leaving the plastic pedal mount base and metal cover with 3 bolts. I found where the pedal seemed correct and marked a hole. I used rivet nut setter to place a ¼-20 rivet nut on firewall. With the assembly mounted I checked fit and feel. I drilled the other two holes and put a rivet nut in. Of course they ended being a little crooked just to **** me off.

2017-07-09_08-31-50 by wcstory, on Flickr

2017-07-09_08-30-54 by wcstory, on Flickr

The upper end of the throttle arm needs to be bent to the right to line up with the throttle cable coming through the firewall. It took a bunch of bend, fit and check trips between the bench vise and the truck. A whole bunch.

2017-07-09_08-29-23 by wcstory, on Flickr

2017-07-09_08-29-54 by wcstory, on Flickr

I clamped the cable and tried the throttle travel and pedal location. Once I’m certain of the location I will crimp a proper stop and solder it in place.

2017-07-09_08-31-26 by wcstory, on Flickr
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