Idle vacuum and power valve!!!!

[americanmusc1e]

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

[americanmusc1e]

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!!!!

[ZZ4Blazer]

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.

[mongoose]

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.

[454HO]

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

[Alex1]

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.

[toms84ss]

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.

[68 C-10 KID]

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.

[americanmusc1e]

Originally Posted by toms84ss

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.

actually, I was thinking the same thing, I might swap it out for a modified Q-jet or a 750 holley