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Old 12-29-2010, 07:25 PM
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86 Firebird 305 Vacuum Leak???

Hey guys first time poster. I just bought a 86 firebird with a rebuilt 305 edelbrock intake and carb and I'm only pulling 10 inches of vacuum at around 700 rpm. I can't for the life of me find any leaks either. Any ideas? I have adjusted the valves and idle mixture, timing is set to 16 initial and 36 total. I was told a mild cam was put in and it doesn't seem to lope at all so I think it isn't much to expect at least 18" of vacuum if not more.

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Old 12-29-2010, 08:25 PM
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yep, sounds like you may have a leak. Look up vacuum leak - Tech Article WIKI on this site.
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Old 12-29-2010, 08:30 PM
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Quote:
Originally Posted by 406 bug
yep, sounds like you may have a leak. Look up vacuum leak - Tech Article WIKI on this site.
Thats just it I have taken every vacuum hose off and plugged it up and the vacuum stays the same. I am thinking if its a leak it must be intake manifold gaskets I have sprayed carb cleaner all over the intake and carb and idle is unaffected. If its a intake gasket it must be leaking underneath the intake.
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Old 12-29-2010, 08:42 PM
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Usually leaking intake gaskets show signs, hesitation in throttle response, smoke out the exhaust, that don`t mean there not leaking.
Is the vacuum advance connected to a full time manifold source?
16 degrees initial alone isn`t enough. The timing should be 12 degrees before top dead center with vac advance disconnected and plugged.
After set at 12 degrees, reconnect vac advance to a full time manifold source (one that pulls vacuum full time, including idle) recheck timing, it should now be around 20 - 24 degrees before top dead center.
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Old 12-29-2010, 08:43 PM
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Quote:
Originally Posted by jmabrao
Thats just it I have taken every vacuum hose off and plugged it up and the vacuum stays the same. I am thinking if its a leak it must be intake manifold gaskets I have sprayed carb cleaner all over the intake and carb and idle is unaffected. If its a intake gasket it must be leaking underneath the intake.

Did you try the method to determine an internal leak (WIKI)
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Old 12-29-2010, 08:46 PM
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Quote:
Originally Posted by DoubleVision
Usually leaking intake gaskets show signs, hesitation in throttle response, smoke out the exhaust, that don`t mean there not leaking.
Is the vacuum advance connected to a full time manifold source?
16 degrees initial alone isn`t enough. The timing should be 12 degrees before top dead center with vac advance disconnected and plugged.
After set at 12 degrees, reconnect vac advance to a full time manifold source (one that pulls vacuum full time, including idle) recheck timing, it should now be around 20 - 24 degrees before top dead center.
I set initial timing with vacuum advance plugged, Vacuum advance is also plugged into the ported not full time vacuum port. 406 bug I haven't tried that yet.
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Old 12-29-2010, 08:51 PM
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The low timing is one reason it has low vacuum. The other is likely due to the larger cam and small cubic inches. If the engine is stock it came with around 8.5:1 compression, add in a larger cam which bleeds off cylinder pressure at low rpm lowers it even more, so the engine wants more timing. Vac advance connected to a manifold source will also greatly improve low end power and throttle response.
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Old 12-29-2010, 08:57 PM
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It does hit a flat spot sometimes like right when you start putting the throttle down. I think I may need to richen the idle mixture a tad bit. And it really doesn't sound like it has all that radical of a cam in it.
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Old 12-29-2010, 09:08 PM
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If you would, just try my suggestion. You may not believe what I`m telling you and it`s not the first time it`s happened and it won`t be the last. But, to try something it won`t take you 5 minutes to do what do you have to lose? When the timing is set as I suggested, you`ll have to lower the idle speed due to the higher initial timing, due to the butterflies being closed more increases idle vacuum.
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Old 12-29-2010, 09:29 PM
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I will give it a shot thank you guys for your help.
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Old 01-03-2011, 12:52 AM
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Alright cannot find the leak at all, and I think the vacuum leak is leaning out mixture causing bog, and when cold backfires through the carb. I will be replacing the intake manifold gaskets tomorrow or the next day. Hopefully this will clear up the issue. I will let you guys know.

Joel
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