The Pantera Place
"Your de Tomaso Connection"
Replacing the Holley 600 with 650
By Mike Dailey
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I replaced my Holley 600 with a new 600 a few years ago because some of the galley plugs started weeping fuel. The new carb had a slight stumble off idle when the engine was loaded. I tried a bunch of different accelerator pump adjustments and was never able to completely solve the stumble problem. My next step would have been changing out the pump nozzle to a larger size but they are a bit pricy. I decided to buy a new Holley 600, but when I got it home, I discovered that Holley eliminated the external float height adjustments. Not a good thing for a Pantera because of the level mounting of the engine and the carb being tilted forward. I decided to go with a 650, part number 0-80783C. I could have bought a 750 for less money but I think the 650 is better matched to my engine modifications.
View of the Holley 600. This is a picture of the one that was weeping fuel. Note that the fuel inlet is on the driver side of the car and how well the braded fuel line and metal fuel filter fit beside the throttle linkage.
Be sure to read the Holley carb installation instructions that come with the carb and follow them carefully.
One of the things that always concerned me about the 650 and larger Holley carbs is the passenger side fuel inlets and the fuel manifold required. I've see some pretty ugly fuel manifold installations and in some cases the fuel line hanging around the back of the engine. I was determined to figure out a clean and safe way to route the fuel line and use my metal fuel filter.
The first few fuel manifolds I looked at were too long causing them to stick out and cause fuel line routing problems. After more research I discovered that Holley makes their own manifold that fits nicely beside the carb and the inlet is close to the carb. With this fuel manifold I was able to simply route the fuel line in front of the distributor and the fuel filter fits nicely in front of the carb. The Holley part number for the 650 manifold is 34-150. I found that using a small amount silicon grease on the manifold fittings and the flared ends of the tubes made torquing the fittings much easer.
I had to bend the front manifold tube up slightly so the manifold tube would clear two intake manifold bolts. Be very carful doing this and do not let the pipe bend right at the carb fitting. I also replaced the rubber fuel line with the best quality I could find because it was a few years old.
After installing the carb I adjusted the float levels and set the mixture adjustment.
One of the many things to be careful with is the throttle plate closed idle positron as it is related to the idle transfer slots. If your Pantera has a high performance longer duration cam it is possible to get the throttle plates open too far and then the transfer slots will not provide the fuel needed as the engine goes off idle. This will cause a stumble off idle.
The image on the left shows the transfer slot on an old Holley 600 fully exposed with the throttle plate open part way. The image on the right shows the throttle plate in the idle position but the slot is too far exposed. In the closed throttle position the slot should look like a square. Holley tech support recommends a max of .020 that you can expose on the primaries with the plates closed and .015 on the secondary's. The Holley website spec for the primaries is .025 to .030. If you have a big cam sometimes the secondary throttle plate may need to be opened to get a fast enough idle. There is an adjustment screw on the bottom of the carb plate to adjust the secondary plates.
To use the Holley carb on the Pantera, a couple of special brackets are required to modify the throttle cable position and an arm for the carb. They were already on my car when I bought it and I believe the parts can be purchased through Panterra vendors like Pantera Performance in Colorado. .
This view shows the arm that bolts to the carb for the ball joint connection. The cable shown was purchased from Pantera Performance. Note that there are two throttle return springs for backup.
View of the cable mounting bracket that moves the cable position forward so that full throttle can be achieved. The cable is retained to the mount with a truearc clip.
I discovered that float bowl vents are a little taller than the 600 requiring a taller air filter.
The taller filter required some adjusting work on the back part of the engine screen to clear the air filter top. I bowed it up to give about 1/4" clearance with the filter top. Even with the bowing the screen lines up pretty well with the deck lid.
After trying a number of filters, I ended up using a Purolator A60102 (shown above) that fits OK and is about 1/2" taller than the filter I was using on the 600. Purolator A60102 is used on the 69 - 73 GTO and other GM cars. As can be seen in the above image it fits intothe bottom of the air filer base very well, but because of the lip size on the cover, the cover does not fit down around the top edge of the filter.
The stumble off idle is gone and it feels like it pulls a bit stronger. The plugs have a nice brown color so it looks like the jetting is pretty close.
If you have problems with the installation Holley provides an excellent email tech support service.
Update: The 650 worked great until the fall cool weather and then it developed a stumble off idle under load. I sent it back to Holley to have it checked out and they returned it as flow tested OK. I reinstalled it and had the same problem. I was not able to correct the stumble. I reinstalled the original 600 and the problem went away.