engine vacuumn
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4wd High
Joined: Aug 2023
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From: Fortuna
engine vacuumn
Hi, I recently rebuilt my 390FE engine. When I take a vacuum reading, I get 20 lbs. Is this too high? I've always thought 16 to 17 psi was normal. I've got initial timing set at 8 degrees, should I retard the timing or leave it alone? Thanks.
Hotshot
Joined: Feb 2004
Posts: 10,841
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From: The Shenandoah Valley
You do not see 20 pounds of vacuum. You might see 20 inches of vacuum, it's not the same. The highest vacuum you'll see is just under 30 inches, it is limited by the atmospheric pressure existing at that location. There is more but you can find it online.
Assuming you see 20 inches, just the slightest change in throttle will change it. If it's idling smoothly at under or at 800 rpm and the vacuum gauge is rock steady at 20 inches, that's good. If it spark knocks on the road, you likely have the timing too high and need to retard it, but expect idle vacuum readings to dip too.
Assuming you see 20 inches, just the slightest change in throttle will change it. If it's idling smoothly at under or at 800 rpm and the vacuum gauge is rock steady at 20 inches, that's good. If it spark knocks on the road, you likely have the timing too high and need to retard it, but expect idle vacuum readings to dip too.
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4wd High
Joined: Aug 2023
Posts: 9
Likes: 2
From: Fortuna
engine vacumn
Thanks for the info, it's idling at about 650 rpm with the timing at abour 8 degree and I only live a couple hundred feet above sea level
. Thanks also for educating me on inches rather than psi, I should know the difference.
. Thanks also for educating me on inches rather than psi, I should know the difference.
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