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Discussion Starter #1
What are the causes of a low vacuum reading? I am in the process of tuning my motor and did a vacuum check. It does not pull more that 7-8Hg of vacuum at idle. I have the motor idling around 800 rpm in neutral. It seems to idle bit rougher than i expected but not bad. The vacuum reading is not very steady and seems to bounce around a bit. I was considering hooking up my vac advance to help with the idle. The specs on the Vac can state that it will pull 10* with 15lbs of vacuum. Since I am no where near 15 in of vacuum, Its not going to help any. I have timing set to 15 initial, 36 total. Carb is a 750 Holley HP VS, 70 front 74 rear, 6.5 PV. I've checked for vacuum leaks and haven't found any. Shouldn't I be pulling more vacuum? I'm very new at tuning and have read through some previous posts on the subject but the low vacuum concerns me.
thanks,
Greg
 

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If you've adjusted your idle richness mixture for highest vacuum I'd suggest looking for a bad plug or plug wire causing the rough idle. As far as low vacuum check your rocker adjustment. Comp says half turn after zero lash. Too much preload will reduce your vacuum. I have a comp roller [email protected] with 1.5's and made 11" and re-adjusted the rockers to a half turn and gained about 2". making 13+" now. Your 1.6 ratio rockers have increased your duration a few degs more than what is shown in your sig so don't be looking for more than 8-10" at idle. You can also get an adjustable vacuum canister that you can stick an alan wrench in and adjust the amount of vacuum needed for it to operate...
 

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Discussion Starter #4
I set the rockers to 1/2 turn past zero lash when I was setting up the valvetrain. It is idling a bit rich and it does load up the plugs some. I'm thinking of getting the crane adjustable vacuum can so that I can lower the point at which the vacuum advance comes in. I was just wondering if the vacuum seemed low for my combination and if there could be some other cause. Do you think that adding the 10* at idle via the vacuum advance will help smooth it out at all? Will running 25* at idle and part throttle be a problem?
 

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cool site thanks


[ 05-20-2004, 07:47 AM: Message edited by: click ]
 

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I fixed that link Jim...

Greg we all have different hand strength, that makes finding zero lash with hyd lifters harder than we think using the spin the pushrod method. It's easy to over tighten them and still spin the pushrod. I've found rattling the pushrod as I tighten the rocker works better for me, start with the rocker loose and lift the pushrod up and down as you tighten the rocker nut, stop tightening when the rattle stops. I don't know what method you use but for me it made a big difference.

I'm no cam expert but our cam profiles are close enough for me to think you should be seeing 10" at 700 - 800 rpm idle. Check plug gap, a bad wire, bad plug, timing, and carb adjustment. Was your cam installed straight up? Also you might try some 1.5:1 rockers to see how the ratio change effected things...
 

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what does the vacuam guage do when you rev it up, say a steady 2000 rpm... If it's not rock solid, you'll be havin a bad cylinder (do a compression test) still could be ignition (1 cylinder not firing)

Adj. the valves with the motor running.... I know it's messy, but it'll wash off. Loosen one til u hear it knockin loud... tighten till it stops knockin, then a lil bit more. watch and make sure the pushrod's still spinnin. Any pushrods ain't spiinin now, way too tight. do one side at a time to minimize the mess... then gunk it and hose it down.
 

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Discussion Starter #8
Vacuum stays steady at 2000 rpm at about 20 inches. When adjusting the carb, the idle mixture screws don't seem to impact the vacuum reading at all. I started at about 1/4 turn out on all four corners. At about 800 rpm my vacuum reads about 6-7 inches. When I turn the mix screws out, the idles goes up a bit, as does the vacuum. When i turn the fast idle back down, the vacuum drops back down to where it was. Am i doing something wrong? I'll try adjusting the valves again this week.
 

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You NEED the vacuum advance at idle, but the stock can won't help you due to your low vacuum; stock cans need 15" Hg. to fully deploy and stabilize idle. Get an Echlin VC-1810 can at NAPA ($10.00) and install it; they provide the same 15* advance as the stock can, but are fully-deployed at only 8" Hg.
 
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