New member introducing myself and Holley carb

[David]

I’m new to any automotive work but have wanted an older American vehicle for some time now. My strongest pull has been towards 60-66 Chevy and GMC and pulled the trigger on one this August.
Mostly original Oakland CA built ‘64 3/4 wide long GMC 305 (e? Or 305a?)Base model w factory power brake. Was parked in a neighborhood I pass through w a for sale sign in the window.
It runs pretty good as is but is running real rich. Was thinking I needed to tune the carburetor but I think I’ve got the wrong carburetor to start with.
Called Holley today for help id’ing which carb is in my truck. The numbers show I’ve got a 500 cfm model 4412.
Asked what he would recommend and he said a 350 cfm, he gave me a Holley part no. 0-08350,(I think I’d need a different one as that’s a electronic choke)
But the message was my engine couldn’t pull enough air through to get the right performance out of the 500 cfm. My fuel economy is not what it could be to say the least and this setup dosnt let me get enough air so it’s wanting to stall out on me from an idle.

Thanks for any advice,
David Cox
Portland, OR

[Funky61]

Hello David and Welcome!

The carb you have now is considered the upgrade for the V6.

Here is the link from the 6066 Jolly Page concerning that modification;



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Yours may just need to be jetted down as these do run rich out the box.

The 500cfm is the one most recommended. Mileage is typically around 12mpg(+/-) depending. Do you have dual exhaust? That really helps as well to wake up the engine too.

Let's have more

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[David]

Funky61 thanks! I haven’t gotten into it to see what jets they have in there yet. Came to me w dual exhaust already so that’s good. I’ll go through the article you linked and see what I can do. 12 mph(+/-) sounds real good I think I got closer to 5

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I’ll have some better pics in the future! I’m not sure why my cell phone pic is rotated 90 deg?

[FetchMeAPepsi]

Welcome David! You got any pics of your new truck? Holly told you wrong. You WANT that 500cfm model on there. That's the best version for your truck. Not numbers wise, but in real world experience. It's funny, but no one told the air it had to enter a carb in a nice orderly fashion like the math nerds command when they figure these things.

If you get it tuned up and acting right you'll be happy.

[lizziemeister'sV6]

A couple of other tricks you can do with GMC V6 with the 500 cfm Holley to help rid of the rich condition is first - advance your timing and second - burn a higher octane fuel 89 or 91 would do. Advancing your timing starts the burning sooner and with the slower burning higher octane fuel this provides a better burn cycle. There's no book setting on our modified GMC V6 engines once you alter the factory engineering specs - changing 1 item (carb) starts a snowball effect that takes time to conquer - there's a lot of experienced people on here to help with the puzzle. A better performance cam profile is a great help to get more air when you add more fuel in the mix. Welcome to the forum and Good Luck!

[LEWISMATKIN]

Welcome, David!!! Your 500 cfm Holley may be too rich. When I was driving my 1002, as long as I stayed between 55-58 mph, the WW Stromberg I used was getting around 15mpg. A number of factors need to be considered to get fuel mileage, like tire size, rear axle ratio, driving habits, and so forth. I got my best fuel mileage out of a truck I wrecked by running 7.00-15 6ply tires coupled with a factory 3.07 Spicer 44 rear axle, and 2" aluminum spacer sandwiching a 6-71 blower screen between the bottom 2 gaskets. I got around 17 1/2 mpg @ 2000 rpm out of the engine. A lot more than most of the advertised mpg of these engines.

[LEWISMATKIN]

remember, regular gas in 1965 was 92 octane and NO ETHANOL!!!!

[James]

I would love to see pictures of the truck.

On the carburetor I would check the float level, which is very easy to do. With the all warm up and idling. There is a plug on the right side of the fuel bowl. Unscrew and remove the plug. The proper fuel level should be just barely start to flow out of the bottom of the hole. If adjustment is need, loosen the screw (do not remove, only need to loosen about a turn) at the top front of the bowl then turn the nut. Turning the nut clockwise will raise the float level and counter-clockwise will lower it. Then tighten the screw. Then the idle mixture screw should be set approx. 1-1/2 turn out from full seated position. Idle may also need to be adjusted. Refer to Holley page for detail info.

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If you want an automatic choke you will need to buy the following:
De-choke linkage


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and electric choke (the picture in the instruction is on a 4 barrels Holley. The 2 barrels is setup the same way)


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While your at it buy a Holley Renew Kit so you can disassemble, clean, reassembled, and adjust for top performance(

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). This web page also has the disassembly instructions.



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[James]

Originally Posted by LEWISMATKIN

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remember, regular gas in 1965 was 92 octane and NO ETHANOL!!!!

And about 10 cents a gallon too.

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[David]

Hey thanks very much for the replies!

Sounds like I’m going to see how the 500cm will do for me for starters and go from there.
Step one was to identify which carb I have-done, and then find if it’s the part I should be using- which I’m hearing not a unanimous yes to but looks like a lot of folks say I should do good. So I’m going to try my hand at adjusting and see where that gets me.

I appreciate the advice! I’ll be referring to this thread and probably looking for good YouTube videos too to get it figured out. Will report back and let y’all know how it goes.

And more photos coming!

David Cox
Portland, OR
‘64 GMC 1500


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