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Old 10-10-2006, 09:18 PM
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Low Vacuum Reading, Need advice

Just got my SBC 305 fired up and getting a low vacuum reading 6-7hg.
Checked for vacuum leak with the o carb cleaner trick and come up empty.
Timing is set @ 8 degrees btdc
Motor is basically a stock rebuild 1977 Chevy truck motor (rings, bearings, 3 angle valve job) and a few performance goodies like Crane RV type cam, Melling HV oil pump, Edelbrock EPS intake, Edelbrock 1406 carb, Accel Street billet distributor and a set of Patriot Roadster headers all stuffed in a 27 Ford Roadster body. Everything is new!! Holley Red Pump with regulator set @ 5psi
It seems to start and run great, but I smell it running rich and do not get much when adjusting the mixture screws
This board has helped me many times before by reading the posts so I thought it's time to post and collect opinions from the BEST BRAINS!!!!!!

Also I remember reading checking for vacuum leaks with propane. Any advice on this would be appreciated. Since I still feel strongly the Vacuum leak is the culprit.

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Old 10-11-2006, 09:08 AM
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Too rich or too lean a mixture will lower vacuum.

Try increasing the timing to 15 or 20 degrees, then reset the idle at 800 rpms, adjust idle mixture for best idle, re-measure the vacuum.

Double check, some how, to see if your vacuum gage is right.

Do a compression test. A low reading could indicate a timing chain installation problem.
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Old 10-11-2006, 09:32 AM
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If your idle mixture screws are ineffective you probably have the throttle open too far and you are running on the transition slot. With 6-7" of vacuum your metering rods are probably pulled out of the jets resulting in a rich mixture.

With a mild RV cam your intake vacuum should be 15" or better with a 350" motor.
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Old 10-11-2006, 11:31 AM
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I ran into a simalar problem years ago. Does this engine just have the little tube on the front cover to set the timing? Almost straight up at 12 o'clock. If it does are you putting the balancer mark in that hole? "IF" this is what you are doing that puts the timing 9.5 deg retarded. So if this is what you are doing the timing is way retarded and needs to be advanced, or set with the magnetic probe.
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Old 10-11-2006, 12:02 PM
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Slightly lower vacuum no can be expected during the ring break in period. If the engine is new. Get it close and drive it. You'll be readjusting all the time till one day or week, you realize it stops changing.

Slightly means a few No.
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Old 10-11-2006, 01:49 PM
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Low Vac

I did find out my vac gauge was off (70's vintage) 5 to 6hg. verified that by hooking up my Mightyvac to it. Ran off of the Mightyvac's gauge and was reading 11.5hg

I set initial timing to 12 btc which is the max mark on my good ol Mr Gasket timing tab and now have 13hg and got a little more response with the mixture screws. It starts and sounds great but the exhaust still smells rich. Wondering if it's the wrong timing tab???? 6 3/4 inch stock balancer
I appreciate the input.
Thanks
G

Last edited by glg97; 10-11-2006 at 06:04 PM.
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Old 10-12-2006, 01:43 PM
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Use TDC Stop to verify the tab is coerrct. I find they are often off a small amount.
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Old 10-12-2006, 03:15 PM
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Now, connect the vacuum advance to the manifold vacuum. Timing will increase to about 30 degrees at idle and idle speed will go up.

Then, turn the idle speed screw down and readjust idle mixture screws for best idle.

Then remeasure vacuum.

So now you have 13inhg of vacuum. Is that 13 + 5 (error) = 18, or is it 8 + 5 error = 13.
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