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Old 06-28-2014, 01:35 PM   #1
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Default GM performance 350HO deluxe crate

Installed this in my '73 Blazer a month ago and still doesn't run correctly. Idles rough, bogs and stalls, etc. Have tried everything to no avail. With all ports plugged, am only getting 11 inHg. I live at 8,000 ft and have already re-jetted for the altitude. Any ideas or should I just start over? Thanks
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Old 06-28-2014, 02:18 PM   #2
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What'* your ignition timing look like? From my research the 350 HO has a base of 10 degrees BTDC and you need to advance one degree for every 1000 ft. over 4000 ft so that would put you at 14 degrees BTDC initial advance. Sounds like a lot I know, but might be worth a shot.

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Old 06-30-2014, 01:36 PM   #3
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i would try that first, 10 doesnt seem like that much they can take way more. base idle on a stock vortec is 20. you give it what it wants, dont worry about the books. what carb do you have?
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Old 06-30-2014, 11:53 PM   #4
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Thanks for your response. I'll try 14 as it is now set at 12. The carb is the Holley Street Avenger and I have already re-jetted to compensate for altitude but still runs lousy. Rough idle, slow acceleration, and actually stalls completely (3 times in less than a mile). Brand new engine and I live 200 miles from the nearest dealer, not that I could get it there anyway. Thanks again.
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Old 07-01-2014, 08:56 AM   #5
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set your timing where it runs good 14 16 18 whatever.

did you set the idle screws for max vacuum? mess with the squirters? primary jets are for cruising rpm and secondary jets are for WOT.

this is mostly for a 4150 but gives an idea of what needs done;
On a Holley, 1 jet size is an imperceptibly small step. 2 jet sizes is a tiny but noticeable step. 3 jet sizes is a standard adjustment. 4-5 jet sizes is a big step.

Look at your primary jets. Whatever size they are, buy some that are 2 sizes, 4 sizes, 5 sizes, and 6 sizes smaller; and 2 sizes larger. You'll be needing them.

First, set the fuel level. If it has sight plugs, set the primary one to where gas dribbles out while the engine is idling, when you just bump the fender; and set the sec one to where you have to jiggle the car pretty good to get it to come out.

Lean the primary jets (using the "step size" description above) until the car just barely starts to surge while cruising (55-60 mph) at a steady speed on a level highway and it starts going up a hill. Ignore all other misbehavior of any kind for the time being. Once you find that point, go back up 2 jet sizes.

Once you get it to do that, look at the primary side power valve. If it'* stock, it'* probably a 65. Stupid. Raise the value until the flat spot goes away when you're in high gear and driving along at about 35 mph (basically, as slow as you can possibly go in high gear), and you give it gas. It'll take a MUCH higher number power valve.... probably a 105 or a 125. If it starts giving you the burn-your-eyes idle, go back down one step.

When you get that right, set the idle speed right (800 RPM out of gear, say); then get the idle mixture right by turning each pimary idle screw in until the RPM drops, then back it back out 1/8 turn. Shut the motor off, and average the 2 screws (like, if one is 5/8 turn out, and one is 7/8 turn out, set them both to 3/4 turn out), and restart the motor, and check them again. Repeat until they're equal, or until you have determined that they absolutely CANNOT be made equal. In no case should they be more than 1/8 turn different.

Take the carb off, flip it over, and look at the transition slot. You want less than .050" of it exposed below the pri throtle blades. If more of it is exposed than that (it will be), open the sec throttles with their idle screw, 1/8 turn; put the carb back on; re-set the idle speed and mixture; then pop the carb back off and look at it. Repeat until you have between .030" and .050" of the transition slot showing.

At this point, the thing should run like a bat outta hell, on the primaries. The secondaries could be just about anywhere. Put a 65 power valve in them, if they don't already have that; and select jets for max mile mph.

At that point, you'll have your Holley set up better than about 95% of all other Holley owners. You'll be getting nearly the same gas mileage as you would with a Q-jet, and you'll be making 20% more power than most cars with the same cam & heads, and FI.
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