Power valve issue?
Thread Starter
Instructor
Joined: Oct 2008
Posts: 154
Likes: 5
From: Colorado Springs
Power valve issue?
My car has been flooding out almost like its vapor locking. At times I have had gas shooting out the vents. I checked vacuum today and it was 9 in neutral and 5 with the clutch slightly engaged for a load. My power valve is a 65 and I realize I need to change that. Which number should I use to select a power valve. Would this be my overly rich problem?
This carb worked fine a 1200ft but since moving to Colorado (6600ft) it seems to be giving me fits. Does elevation effect vacuum?
This carb worked fine a 1200ft but since moving to Colorado (6600ft) it seems to be giving me fits. Does elevation effect vacuum?
Burning Brakes
Joined: May 2008
Posts: 860
Likes: 0
From: Stratford Ontario
I would try to get the vacum higher, but if you can't I would put in 3.5 power valve or valves depending weather carb has one or two. I would also think that the car would run richer at higher elevation because air is thinner.You may want to try leaning out jets and idle mixture first. Just my 2 cents worth.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Last year, when I went to the NCCC Convention at Colorado Springs with drag racing at Bandimere at nearly 6000 feet above sea level, I had to re-tune my carb from its previous setup at 500' - to 6000'. This involved cutting the fuel metering areas by 8%, advancing the spark by about 2 degrees, and changing the enrichment vacuum threshold by about 2" Hg. The latter means that if your 6.5" PV was fine, try a 4.5" PV. If you have a Quadrajet, use a softer power circuit spring.
Thread Starter
Instructor
Joined: Oct 2008
Posts: 154
Likes: 5
From: Colorado Springs
I would try to get the vacum higher, but if you can't I would put in 3.5 power valve or valves depending weather carb has one or two. I would also think that the car would run richer at higher elevation because air is thinner.You may want to try leaning out jets and idle mixture first. Just my 2 cents worth.
I was going to say 2 sizes smaller in number with your car in drive and your foot on the brake. Not sure how to do it in a standard.
Last year, when I went to the NCCC Convention at Colorado Springs with drag racing at Bandimere at nearly 6000 feet above sea level, I had to re-tune my carb from its previous setup at 500' - to 6000'. This involved cutting the fuel metering areas by 8%, advancing the spark by about 2 degrees, and changing the enrichment vacuum threshold by about 2" Hg. The latter means that if your 6.5" PV was fine, try a 4.5" PV. If you have a Quadrajet, use a softer power circuit spring.
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Fuel shooting out the vents is a Needle and seat issue or air in the fuel system, PV can't cause that issue. If its rich at idle you may have a bad PV, blown power valve.
the elevation change should cause it to be lean not rich.
need to figure out why the vacuum is so low at idle
Neal
the elevation change should cause it to be lean not rich.
need to figure out why the vacuum is so low at idle
Neal
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Nope. Less air at the higher elevation will cause it to be rich.
At 6,000 feet of elevation, the atmospheric pressure is only 24" Hg (~12 psia) unlike the pressure at sea level of 29.92" Hg (14.7 psia). Therefore a perfect vacuum at 6,000 feet can only be 24" Hg. There is no way to have a higher vacuum at that higher elevation.
As a rule of thumb, figure all engine vacuum will be 20% lower at 6,000' than at seal level.
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Nope. Less air at the higher elevation will cause it to be rich.
At 6,000 feet of elevation, the atmospheric pressure is only 24" Hg (~12 psia) unlike the pressure at sea level of 29.92" Hg (14.7 psia). Therefore a perfect vacuum at 6,000 feet can only be 24" Hg. There is no way to have a higher vacuum at that higher elevation.
As a rule of thumb, figure all engine vacuum will be 20% lower at 6,000' than at seal level.
20% would mean he had 11" at 1200' to get 9" at 6000', still sounds low unless he's running a good size cam which he could be didn't see any specs on the engine.
the elevation change can definitely cause a rich condition
Neal
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Thread Starter
Instructor
Joined: Oct 2008
Posts: 154
Likes: 5
From: Colorado Springs
Thanks for the info. I put a 4.5 PV in and it doesn't seem to smell as rich at an idle now. I'm still having my flooding problem though.
How does this lower vacuum at a higher elevation effect my vacuum secondaries. It seems like they're opening up late.
Here's the specs on my cam and it's in a 509BB. Any reason my vacuum would be this low?
http://www.compcams.com/Cam_Specs/Ca...?csid=445&sb=1
How does this lower vacuum at a higher elevation effect my vacuum secondaries. It seems like they're opening up late.
Here's the specs on my cam and it's in a 509BB. Any reason my vacuum would be this low?
http://www.compcams.com/Cam_Specs/Ca...?csid=445&sb=1
Last edited by 68sting; Jul 13, 2010 at 12:23 PM.
