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06-21-2009, 05:25 PM | #1 |
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Join Date: Jan 2009
Location: Austin, TX
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Idle vacuum and power valve!!!!
Ok I just put a new 388 in my '70 C10,
it has a XE 268 cam, and a holley 600 vacuum secondary carburetor right now it's pulling about 5 inches of vacuum at 600rpm idle, it has the stock 65 power valve in it and the power valve is open at idle. what is a good Idle rpm for this cam (automatic transmission)??? what is a good ignition timing setting (at idle w/vacuum advance disconnected) for this engine???? do i need to change out the power valve to a smaller one??? or am I doing something wrong???? here are the specs: home-ported vortec heads weiand stealth vortec manifold holley 600 vacuum secondary (4160) flowtech 1-5/8 headers with full dual exhaust scat 3.75 stroke cast crankshaft eagle H-beam rods .060 over hyperecutec pistons comp cams XE 268 cam (part# 12-242-2) stock HEI distributer
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-- Josh Instagram: @americanmusc1e OLD SKOOL-- 1970 C10. 454/Th400/3.07 posi Build Thread FARM TRUCK----1949 Chevrolet 3800 Power Wagon Hauler Build Thread 1999 4wd OBS Tahoe - daily. DM me if you can't see photos i posted |
06-21-2009, 08:43 PM | #2 |
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Re: Idle vacuum and power valve!!!!
OK, I pulled the front bowl and metering block off and looked at the power valve, then I put it all back together, now the carburetor is working better. its pulling 13.5-14 inches of vacuum now.
i still need to know what the ignition timing should be. and I upped the Idle a little bit, that helped!!!!
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-- Josh Instagram: @americanmusc1e OLD SKOOL-- 1970 C10. 454/Th400/3.07 posi Build Thread FARM TRUCK----1949 Chevrolet 3800 Power Wagon Hauler Build Thread 1999 4wd OBS Tahoe - daily. DM me if you can't see photos i posted |
07-01-2009, 02:50 AM | #3 |
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Re: Idle vacuum and power valve!!!!
That sounds better. Theres no way you should have had only 5" of vacuum with that mild of a cam in a 388.
I read on the holley website, to use a PV that equalled half your idle vacuum (motor warm, truck in gear). EX, your 14" of vac, would start with a 7 PV. Id also start around 10-12 degrees timing at idle, and go from there. Tune little by little, increasing it a degree or two. Too much and it will be a bear to start and very hard on the starter. I prolly wouldnt go more than 16 degrees at idle, and around 34 degrees total. |
07-01-2009, 02:57 AM | #4 |
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Re: Idle vacuum and power valve!!!!
i;d run around 10 degrees at idle... i'd also just get rid of the vac. advance run run straight mech. get it all in under 3K.. and stick the idle at w/e it idles at... 800, 600... w/e lets it run smooth and doesn't stall...
remember to do all these adjustments with the car in gear too. not in park.
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-'73 c/10 - Low with Go - will be complete... probably never. -'90 2wd Blazer - well... soon enough anyways. -'84 SWB - the daily gas guzzler. |
07-01-2009, 05:46 PM | #5 |
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Re: Idle vacuum and power valve!!!!
If you have a vacuum gauge, just keep advancing the timing until you get the highest vacuum reading at 600rpm. It will probably be around 16-18°. Then you need to limit the mechanical advance to 36-38° total. If this is a street driven truck, then I would still run the vacuum advance with straight manifold vacuum, but you will probably need to limit it to 10-12°. Every engine is different, so you need to play with it to find what your combo likes best. The vacuum gauge and a tach will help you tune it.
Have you read the timing FAQ? - http://67-72chevytrucks.com/vboard/s...d.php?t=290498
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07-01-2009, 06:06 PM | #6 |
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Re: Idle vacuum and power valve!!!!
There are two schools of thought for PVs (i've noticed). One is 1/2 of your idle vacuum (in drive) and the other is 2-3 inches less than your idle vacuum in drive.
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07-05-2009, 05:50 PM | #7 |
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Location: Arizona
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Re: Idle vacuum and power valve!!!!
That carb is a little small for a 388 isnt it?It would be OK for cruising around. But your gonna starve that motor when you go WOT.
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07-09-2009, 10:36 AM | #8 |
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Location: Austin, TX
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Re: Idle vacuum and power valve!!!!
actually, I was thinking the same thing, I might swap it out for a modified Q-jet or a 750 holley
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-- Josh Instagram: @americanmusc1e OLD SKOOL-- 1970 C10. 454/Th400/3.07 posi Build Thread FARM TRUCK----1949 Chevrolet 3800 Power Wagon Hauler Build Thread 1999 4wd OBS Tahoe - daily. DM me if you can't see photos i posted |
07-09-2009, 01:39 AM | #9 |
Mean Green
Join Date: Sep 2007
Location: Portland, OR
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Re: Idle vacuum and power valve!!!!
I'm not sure if you got this all fixed.
When it comes to v-tech heads you do not want to run any more than 37* of total timing. So adjust your total and your initial will be set. When it comes to hot engines on the street a good carb tune is its foundation for street driving. I always make sure engine temp is to normal, timing set, Drop it in gear and that is your Vac. Take your vac reading and go down 2. For better tuning I always run a 4 corner Idle circuit. I play w/ all 4 corners till i get the most and steady vac reading. You do not want the needle bouncing all over. I start at about 1 1/4" turns out and go from there. good luck and happy tuning. |
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