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need manifold vacumm 

 

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 Post subject: need manifold vacumm
Posted: Sat Feb 23, 2008 12:20 pm 
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Hi, was wondering if anyone might be able to offer some ideas to achieve intake manifold vacumm. Its a 351 Clevo, 4 spd, holley carb, HEI dizzy. Thanx.
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Posted: Sat Feb 23, 2008 2:36 pm 
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How much vacuum do you have now???? What are the specs of the cam??

 

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Posted: Sat Feb 23, 2008 5:33 pm 
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If you have a problem with little or no vacuum I suggest you get some aerostart and spray around manifolds and other joins that are below carby including carby base. If there is a leak revs will increase. It may be you are measuring engine vacuum at a point higher than a leak.
Also you need to check timing. Remove spark plug and get piston #1 to the top and check that the balancer is actually readin TDC or 0 degrees if you cant see mark turn motor by hand again 360 degrees and check again. It may have been timed on a harmonic balancer where the outer ring has slipped.
Next and most involved is to check valve timing in case it was set up wrong or the chain has jumped a tooth. Cross this bridge if you come to it but all these things will affect engine vacuum start with the siple and work up.
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Posted: Sun Feb 24, 2008 9:41 pm 
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Thanx for the info; timing at moment sitting on 6 degs. Have manifold vacumm at "T" intersection for brake servo at rear of manifold...engine stalls when hose removed. As for manifold vacumm from carby to dizzy..absolutely zero, has no effect on engine speed when hose removed. No leaks at base of carby, Holley 600 vac sec. Engine has a slightly lumpy cam, unsure of specs as it was already installed in engine when I got hold of it.
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Posted: Sun Feb 24, 2008 10:51 pm 
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is your main issue the brakes?
maybe it needs a new one way valve on the booster
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Posted: Mon Feb 25, 2008 8:36 am 
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A big cam will reduce your manifold vacuum noticeably and usually the idle would be increased from 750 to 950 on a really big cam and also they run more advance. try running 8 degrees and see if it pings and if not run it at that and bring your idle to 850 for a start. If a big cam is no dialled in it can be retarded in the valve timing as well. Who fitted the cam? Was it dialled in using a multi keyed set of gears. Either way the bigger the cam the more revs you will need to launch it but once goin hang on..
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Posted: Mon Feb 25, 2008 9:31 pm 
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Blstik wrote:
Thanx for the info; timing at moment sitting on 6 degs. Have manifold vacumm at "T" intersection for brake servo at rear of manifold...engine stalls when hose removed. As for manifold vacumm from carby to dizzy..absolutely zero, has no effect on engine speed when hose removed. No leaks at base of carby, Holley 600 vac sec. Engine has a slightly lumpy cam, unsure of specs as it was already installed in engine when I got hold of it.



you shouldn't have vacuum at that port when the engine is idling.
with the engine idling remove the hose on the side of the metering block.
put your finger on the barb and then slowly crack the throttle. you will then feel the cacuum. the reason is that the vacuum for your dissy IS NOT direct manifold vacuum, it is ported through the carb just above the primary throttle plate. (and i honestly can't believe how many people and more so how many mechanics don't know this)

 

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Posted: Mon Feb 25, 2008 10:08 pm 
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on some motors it actually helps to have the dizzy connected to manifold vacuum instead of ported vacuum, especially with big cams it can help them idle better. Thats why i raised the question, it may be an advantage running the vac advance to manifold vac in this case to increase the amount of off throttle vacuum.

Regardless of cam size, there should be sufficient vacuum for brakes. if theres not, then it may be the one way valve on the booster, in extreme cases you may need to fit a vacuum tank.

Were runnning a cam in the vicinity of 260* @ 050 on a cleveland with sufficient vacuum to run the brakes.
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Posted: Mon Feb 25, 2008 11:42 pm 
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Blstik wrote:
Have manifold vacumm at "T" intersection for brake servo at rear of manifold...engine stalls when hose removed.


This is where your vacuum should be measured anyway or directly at the manifold. your reading at idle at that point should be around 18 to 20
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Posted: Tue Feb 26, 2008 11:05 pm 
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66 coupe wrote:
on some motors it actually helps to have the dizzy connected to manifold vacuum instead of ported vacuum, especially with big cams it can help them idle better.


until you hit the throttle and the timing retards by about 10 degrees and the thing falls on it's face.

if the dizzy is set up right there would never be a need to have that sort of setup. if the engine idle better like that, the dizzy needs recurving so you can run more base timing with out over advance at revs.

 

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Posted: Tue Feb 26, 2008 11:13 pm 
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the base timing remains the same, so it will be no more gutless than it already was.
some motors you cant get aftermarket dizzies, so its a little trick you can use to help them run better.
Vacuum advance wont over advance at revs under load, its only going to advance at part to no throttle.
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Posted: Thu Feb 28, 2008 9:14 pm 
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66 coupe wrote:
the base timing remains the same, so it will be no more gutless than it already was.
some motors you cant get aftermarket dizzies, so its a little trick you can use to help them run better.
Vacuum advance wont over advance at revs under load, its only going to advance at part to no throttle.




drugs??
the engine will fall on face when you crack the throttle because it will loose vacuum to the dizzy and retard about 10 degrees. causing an anoying flat spot,

you can recurve any dizzy, it's just that factory one take a little longer.

IF an engine likes it when you put vacuum to dizzy at idle, it's telling you it wants more base timing. so you modify the dizzy so you can set the base timing at at 15-20 degree instead of 5 to 10 yet still still have 32 odd degrees all in.
if you set a stock dizzy at 15-20 it WILL over advance at high revs

then you hook up you vacuum advance in the normal way so you can still retain some sort economy while cruising.


it's just a little trick to set your ignition the right way

 

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Posted: Thu Feb 28, 2008 9:36 pm 
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[quote=
you can recurve any dizzy, it's just that factory one take a little longer.

IF an engine likes it when you put vacuum to dizzy at idle, it's telling you it wants more base timing. so you modify the dizzy so you can set the base timing at at 15-20 degree instead of 5 to 10 yet still still have 32 odd degrees all in.
if you set a stock dizzy at 15-20 it WILL over advance at high revs[/quote]

I'm with you on re-curving. On my coupe I rebuilt the motor for LPG with custom cam and 11.3:1 comp (and s*@t load of other goodies and tricks)and I re-curved the dizzy I was running 18deg at idle at total advance to 30deg and let me tell you it went like a cut snake from the moment it was fired up. Mate of mine had a similar setup with stock dizzy and they were chalk and cheese down low.
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