Low vacuum normal with new intake?
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Freshman User
Joined: Mar 2003
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From: Plainwell, Mich.
Low vacuum normal with new intake?
I bought a new longblock '85 5.8L 4v HO for my truck.With it I installed a new Edelbrock Performer cam and intake as well as a set of headers.I need low end torque for towing.My question is,at an idle there is only 10lbs of vacuum.Shouldn't it be higher than that?It runs good but when ya stick yer foot in it and it's above 3000 RPM the exhaust sounds like it cuts in and out.I do have the EGR valve closed off if it matters.(actually all the emissions garbage is gone)Maybe the carb needs to be re jetted with the changes I've made?Are there jets in the back 2 barrels or is it just the main jets in the front that need changed?I just don't know and am open for suggestions.
Thread Starter
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Freshman User
Joined: Mar 2003
Posts: 43
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From: Plainwell, Mich.
Low vacuum normal with new intake?
The sprockets were dot to dot with Edelbrocks timing gears like it's suppose to be.The carb is the stock holley 4 barrel.I'm thinkin' the carb needs rejetted but do I just do the main jets or is there jets in the secondaries also.The carb is the 4180 or 4190 I'm not sure.I was told before that the EGR valve being disconnected could cause problems with the mixture.I'm not sure what it even does.
Maybe just pull the plugs and try to figure out if it's runnin' rich or lean and go from there.Gonna have to run it more and see what kinda mileage I get.Just seems the vacuum should be higher at idle than it is.
Maybe just pull the plugs and try to figure out if it's runnin' rich or lean and go from there.Gonna have to run it more and see what kinda mileage I get.Just seems the vacuum should be higher at idle than it is.
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Joined: Jan 2003
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From: Still under the hood in O
Low vacuum normal with new intake?
It definitely should be higher. I would leave the secondaries for now until you can dial in the primaries. Have you tried adjusting the timing with the vacuum gauge attached?
I would lean out the primary needles until you get a lean pop when revved. Then richen it just enough until you don't. Do one side at a time.
Also double check to see if you have a vacuum leak.
After you have it as close as you can, pull the plugs to clean them. Then run it with that tune for a while. Then pull the plugs again to evaluate to see if you need to change the jet size.
I would lean out the primary needles until you get a lean pop when revved. Then richen it just enough until you don't. Do one side at a time.
Also double check to see if you have a vacuum leak.
After you have it as close as you can, pull the plugs to clean them. Then run it with that tune for a while. Then pull the plugs again to evaluate to see if you need to change the jet size.
