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Old 10-13-2018, 06:52 PM   #1
shp4man
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Tune up, 1960's style!

My truck:


Is a 250 (yes 250) not OEM. Today, I decided to tune it for max power. (something like 155 HP) , found a decent Rochester B carb at a swap meet a while ago, but seemed like it just didn't climb hills as well as the the old leaking junker that was on it when I bought it.
Turns out, it was jetted for a smaller engine, maybe even a 140 four banger for an early Nova. So removed the 58 jet and replaced it with the 71 (San Diego is at sea level) from the old carb and added about 3 degrees of spark advance...big difference! Runs like a raped ape! Well, for an old 6 cylinder truck.. Won't be taking on any Vettes with it, but more power is always better.
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Old 10-14-2018, 08:50 PM   #2
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Re: Tune up, 1960's style!

The 1970 250 CID GM inline 6 is rated at 155HP and 235 FT LBS torque. With torque being the key word here. Oh ya, the thing will pull up hills like hell. At 35MPH, high gear, it accelerates up steep hills so hard, the old worn out bearings in the transmission start making noise. It's kind of cool, actually.
God, I love that old truck.
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Old 10-14-2018, 09:02 PM   #3
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Re: Tune up, 1960's style!

Set the timing at 8 degrees before, that's 4 marks on the timing degree tab, or 1 mark under the "A" on the indicator with the vacuum advance unplugged.
I'm thinking the "A" stands for advance.
Check the operation of the vacuum advance and the centrifugal advance with the timing light.
Vacuum is way faster, but with the vac advance unplugged, timing should also advance about 20 degrees when revving the engine past about 3500 RPM. That's the centrifugal advance.
Makes a big power difference.
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Old 10-15-2018, 08:44 AM   #4
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Re: Tune up, 1960's style!

Wow! 8 degrees! Imagine what it would be like at 14. Should run there no problem.

When I saw the title of your thread I thought you were gonna tell about the old dishonest mechanic method of doing a tuneup.
2 ounces of water trickled into the carb!
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Old 10-15-2018, 11:36 AM   #5
shp4man
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Re: Tune up, 1960's style!

Quote:
Originally Posted by geezer#99 View Post
Wow! 8 degrees! Imagine what it would be like at 14. Should run there no problem.

When I saw the title of your thread I thought you were gonna tell about the old dishonest mechanic method of doing a tuneup.
2 ounces of water trickled into the carb!
Should probably do that, too. Get some of the carbon off the top of the pistons.
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