Topic: Power Valve Roulette
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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Gold MemberI have been checking out a few things in my quest to see if I can make my LT1 run better. I seem to be running rich. The eyes-watering-when-you're-sitting-in-the-car kind of rich. I have adjusted the idle mixture screws as suggested by Holley in their book on 4150/4160s. I thought maybe the power valve might be a problem if it was open more than it should be.
My understanding is that the LT1 engine produces less vacuum in general because of solid-lifters and cam (?). The power valve that came with the car when I bought it two years ago, was a two stage (4.5/6.5). When I hook a vacuum gauge at idle, I get a wildly fluctuating reading between 5 and 7. Is this too low? What can cause this?
Anyway, I assume that at speed, the vacuum would be even lower, so a power valve in the 3.5 or 4.5 range might be more appropriate(?)
Any thoughts? Anyone know what vacuum a '71 LT1 is supposed to pull? Am I completely out of whack; should I just seek professional help and not try to be Joe Mechanic?
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Hello, Your getting a Vacume reading of 5 to 7 ??? If your getting Less than 15 Inch's of Vacume then Your probably having Problems with You Power Brake Booster as well providing You have one. If Not Your Probably having Problems with Your Gauge and or Reading. Your probably not going to replace any one Part on the Carb and see a Huge Improvment this is where a Dyno Tune and somebody who knows a Holley Carb inside and out Pays off. Richard.....
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I agree...if you're getting vac. readings at idle of only 5-7inches, you got a problem somewhere other than the carb. Check all vacuum lines for cracks, or looseness. Chances are, the headlight system will be the biggest leak.
A decent engine should draw at least 10-15 inches of vac. at idle. This is probably why the power valve is dumping fuel.
A decent engine should draw at least 10-15 inches of vac. at idle. This is probably why the power valve is dumping fuel.
Joel Adams
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If I am checking vacuum directly off of the intake manifold, the only possible leak, I would think, would be the hose coming directly off of the intake manifold, which seems OK. I'm thinking a valve not closing, or the valve lash being off (?)
I agree with Joel and Richard, anything less than 10-15 is a problem. I've just redone my bottom end and I'm getting 17 on average at an idle of 500 rpm with all the vacuum items hooked up. I'm measuring at the vacuum take off at the bottom of my holley ( the large pipe). My 600cmf with vac secondaries works just fine and the power valve opens just at the right time. I think you must have a leak somewhere as the guys have said.
If your valve lash is off to the point that the valves aren't closing properly your engine would not be running all that well. If it's the other way and they are not opening enough you'd have the same problem. If you are running solid lifters you would know in a instant if the lash (tappet adjustment) is out, if they are hydraulic they should adjust themselves correctly as long as you have set them up correctly at install.
Are you absolutely certain your power valve is good? One good pop-back thru the carb can ruin one. Pull it, clean it then see if you can suck air thru it, if you can it's shot. Luckily they're relatively cheap!
None of that explains away your low idle vacuum though..... If it is fluctuating much you could very well have a bent or burnt intake valve which would also cause firebelches thru the carb.
I love my Q-JET!
DaveAutom8r 2007-04-06 09:41:42
None of that explains away your low idle vacuum though..... If it is fluctuating much you could very well have a bent or burnt intake valve which would also cause firebelches thru the carb.
I love my Q-JET!
Dave
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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