Vacum secondary?

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  #1  
Old 02-23-04, 04:00 PM
johng chevy
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Question Vacum secondary?

I just installed a edelbrock 600cfm carburater on a 86 chevy 350.The engine has headers,aluminum high rise manifold with spacer,700 trans and maybe cam?My question is should i put the vacum on full or part time?(the old carb was a holly)
 
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  #2  
Old 02-23-04, 04:49 PM
Desi501's Avatar
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I don't quite understand what you are asking. There are no options.
There are 2 kinds of secondarys
1 has a fuel squirter in the rear and can open instantly because of the squirt of fuel that primes the secondary and eliminates the bog.
2 has no fuel squirter and has to open slowly so as not to bog the engine dowm with a sudden loss of vacuum and no fuel. The vacuum gives it a timed, slow opening and smooth acceleration.
 
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Old 02-23-04, 05:15 PM
johng chevy
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The carb itself has 2 ports.1 is timed vacuum port(distributor vacuum advance port for emission-controlled engine)and the 2nd is manifold vacuum port(dis vac adv for non-emission engine).My engine is older(non-emission) thats why i was wondering which port to use?(PS: i have HEI distributor) thanks!!!
 
  #4  
Old 02-23-04, 06:45 PM
mike from nj
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you might want to look on the underhood vacuum diagram and see where the original hose was hooked up to. does your distributor even have a vac advance?

you could try either port, and see which runs better. if it pings, you have too much timing





let us know
 
  #5  
Old 02-23-04, 07:18 PM
johng chevy
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My distributor has vac advance and i'll try both. Thanks a lot!!!
 
  #6  
Old 02-23-04, 08:59 PM
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You did not say if the instructions said you could connect to either one. The ported (no vacuum at idle) might not be the best choice. Vacuum secondaries always run off straight manifold (non ported) vacuum as far as I know.

As Mike said you can try them both and see which works best.
 
  #7  
Old 02-24-04, 08:35 PM
mike from nj
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car nut, i think you might have it backwards. i would think you would want the vacuum secondaries to open progressively as the throttle opens. if you tapped into manifold vacuum--you would have zero inches of vac at wide open throttle, yet ported would gradually increase with the throttle opening---which is what i think is best. on holleys, this passage is internal to the carb though, i think, so there would be no choice. i never dealt with the type carb he has, so i don't really know. this poster didn't make his question too clear, i still don't know what he wants to hook up to those vac ports---the secondaries or the distributor? that's why i said to do like the diagram says.

we'll see
 
  #8  
Old 02-25-04, 05:17 AM
johng chevy
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The carb is vacuum secondaries and i want to hook up to the distributor.The diagram tells me to pull the vacuum off the old carb and feel when it's on idle but i already took the carb off??
 
  #9  
Old 02-25-04, 05:34 AM
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i'm a little confused here...

...we're talking about two things here...

first...vacuum secondaries...all vacuum secondaries that i've seen and worked on use manifold pressure to keep the secondary air valve closed...when you go to WOT manifold pressure increases (vacuum goes down) and the dashpot that holds the air valves closed begins to release allowing air to enter at a gradual rate thru the secondary side of the carb...can't have them opening too fast or it'll bog. so...hook your secondaries up to manifold vacuum

second...manifold or port vacuum for distributor vacuum advance...whatever way it was originally designed to work (unless it's an aftermarket distributor) is how it should be. factory applications went both ways, so looking at the underhood diagram (if it still has one) or in an old tune up book may help you here. lots of advance at idle means cooler running at idle but may give you burst detonation when first opening the throttle. i agree that you may have to try it both ways to see which way it runs best but make sure you set the base timing to the correct spec for that engine to start with and then don't mess with it.
 
  #10  
Old 02-25-04, 06:01 AM
johng chevy
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What i was looking for was the vacuum "advance" on a 86 chevy.It snowed last night so i can't try it on the road but i'm going to get a book and look it up! thanks!!!
 
  #11  
Old 02-25-04, 08:59 AM
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apples...

...and pomegranites here.

the 86 chevrolet truck with a 5.7 came with ESC (electronic Spark Control) no vacuum advance.

you're at the trial and error stage now since there is no specs to base from. you can buy all sorts of aftermarket springs and weights to tweek your advance curve for centrifugal advance. i don't know of many choices as far as vacuum advances for chevrolets but you might do some research on the web or call a local speed shop. i'd try it connected to manifold pressure first and then try port and see which one is best
 
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