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  #1  
Old Jul 31st, 10, 09:17 AM
rrobson1977 rrobson1977 is offline

Rich
 
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Default vacuum bouncing around on zz3283

Well the day came yesterday, finally fired up my new zz383 after a long restoration. Everything so far so good. I have one issue that I dont know if I'm chasing my tail on and need some help from persons with experience (as I am far from a mechanic, learn everything from the books and this forum). My vacuum is hunting around a little you can here the engine responding to it as well. I have attached a video link for you guys to look at.



so far, set timing to the recommend 32 deg at 4000 rpm
at first fire up this hunting was present then,
set idle mixture to the highest vacuum
set bowl floats
went over spark plugs and wires to make sure they look good
cant find any apparent vacuum leaks
still got the issue.
I dont know if this cam just makes for a sloppy idle or not. any thoughts would be greatly appreciated.


Engine stats: GMPP engine zz383 with all the accessories recommended.
Camshaft: ............. Hydraulic roller tappet
Lift: .................... .509 intake, .528 exhaust
Duration:................222 intake, 230 exhaust @.050 tappet lift
Centerline: ............ 107 ATDC intake, 117 BTDC exhaust

carb statsw Holley 750 cfm vacuum secondaries
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  #2  
Old Jul 31st, 10, 09:29 AM
Nashville Beth's Avatar
Nashville Beth Nashville Beth is offline
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Bethanne
 
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Default Re: vacuum bouncing around on zz3283

Idle seems too rough for that cam but I could be wrong.

Vacuum advance hooked up to manifold vacuum ?

What distributor are you running?
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  #3  
Old Jul 31st, 10, 12:15 PM
Nantooch Nantooch is offline
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Brandon
 
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Default Re: vacuum bouncing around on zz3283

Considering the lift, I think the bounce your experiencing would be normal. The needle would not sit still do to the overlap. Given that, as you'll notice the needle only deflects about 1 hg off of where it seems to like to sit and then it's to the higher and not the lower. If there was a bigger bounce as in dropping 2-3 hg then I'd worry about a valve being overly tight and not seating. Granted this isn't to say that you don't have a vacuum leak somewhere as you didn't mention how far out you had adjusted the idle screw from just touching the ramp to get it to the idle at present, or how many turns the mixture screws are turned out from closed. The motor does sound healthy though so congrats for that.
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  #4  
Old Jul 31st, 10, 02:49 PM
Nashville Beth's Avatar
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Default Re: vacuum bouncing around on zz3283

I think the vacuum is way low.

I remembered a test of a ZZ4 350 and it had a slightly bigger cam 230/234 and still pulled 15.5" at idle and the 383 has the benefit of 9% more displacement.

Here is that test:http://www.hotrod.com/techarticles/e..._camshaft.html

Was the vacuum advance hooked up?
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  #5  
Old Jul 31st, 10, 09:23 PM
Fred Mertz Fred Mertz is offline
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Default Re: vacuum bouncing around on zz3283

My ZZ383 has a shaky idle vac.. A about 16-18 inches. I consider it normal. Years ago I had a high lift Crane Hyd. cam in a "MO" 302. It had 6-8 inches of shaky idle. That motor had NO bottom end.
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  #6  
Old Aug 1st, 10, 04:15 AM
Nashville Beth's Avatar
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Default Re: vacuum bouncing around on zz3283

Quote:
Originally Posted by Fred Mertz View Post
My ZZ383 has a shaky idle vac.. A about 16-18 inches. I consider it normal.
Hi Dave,

Were you on "I Love Lucy?" I loved the "Fred Mertz" character.

The problem I see is the vac is only ~12". Shouldn't it be up around 16-17"?
This is not a big cam.
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  #7  
Old Aug 1st, 10, 05:47 AM
srode srode is offline
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Steve
 
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Default Re: vacuum bouncing around on zz3283

I have 13 inches of vac at 950 RPM with my 408 10:1 and Comp Cams XR276HR which has about the same specs as your cam - seems normal for the amount of vac, can't say anything about the flutter, mine's not in the car right now so I can't check to compare.
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  #8  
Old Aug 2nd, 10, 03:52 PM
JohnZ JohnZ is offline
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Default Re: vacuum bouncing around on zz3283

Quote:
Originally Posted by rrobson1977 View Post
My vacuum is hunting around a little you can here the engine responding to it as well. I have attached a video link for you guys to look at.
What rpm was it idling at in the video?

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  #9  
Old Aug 2nd, 10, 03:58 PM
john68 john68 is offline
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Default Re: vacuum bouncing around on zz3283

Are you sure your valves are adjusted correctlly, a valve too tight can cause that to.
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  #10  
Old Aug 3rd, 10, 01:40 PM
zdld17 zdld17 is offline
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Default Re: vacuum bouncing around on zz3283

Quote:
Originally Posted by john68 View Post
Are you sure your valves are adjusted correctlly, a valve too tight can cause that to.
What I was thinking. I just put together a GM zz383 short block with the factory cam. Seemed smooth behind a 700R
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  #11  
Old Aug 3rd, 10, 06:08 PM
Nashville Beth's Avatar
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Default Re: vacuum bouncing around on zz3283

Hey !!

Earth to Rich...... we want an answer ??
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  #12  
Old Aug 4th, 10, 11:32 AM
68RS-SS 68RS-SS is offline
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Bill
 
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Default Re: vacuum bouncing around on zz3283

My ZZ383 idles significantly smoother in comparison. I get a very stable 12"hg at 800 rpm idel but attribute the lower end of spectrum due to being at 5,500' above sea lvl. I don't trust GMPP to get the valve adjustment correct (mine was all screwy from factory and 50% of the studs were actually loose - some quality control huh?). You'd best go through valve adjustement procedure to be certain.
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  #13  
Old Aug 4th, 10, 07:24 PM
fastercar68 fastercar68 is offline
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Ozzie
 
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Default Re: vacuum bouncing around on zz3283

I bought that exact same engine (AFR heads vs. vortecs) and have it in front of a 700r4. Mine idles at 800rpm and pulls a steady 16" of vac.

The last time I had bouncing vacuum was my previous engine (327) and we think either a lobe was going flat, or a valve was misadjusted.
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  #14  
Old Aug 4th, 10, 07:49 PM
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Default Re: vacuum bouncing around on zz3283

My 2 cents to add.
Carb to intake gasket. Paper thin or thick? If paper thin, replace with thicker gasket.
Re-adjust rockers the good ole fashioned way one at a time, engine running and warmed up, loosen till clack starts, then re-tighten till clack stops, then tighten 1/2 turn more. Lock it.
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  #15  
Old Aug 4th, 10, 09:43 PM
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ZZ430DropTop67RS ZZ430DropTop67RS is offline
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Don
 
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Default Re: vacuum bouncing around on zz3283

Quote:
Originally Posted by 68RS-SS View Post
My ZZ383 idles significantly smoother in comparison. I get a very stable 12"hg at 800 rpm idel but attribute the lower end of spectrum due to being at 5,500' above sea lvl. I don't trust GMPP to get the valve adjustment correct (mine was all screwy from factory and 50% of the studs were actually loose - some quality control huh?). You'd best go through valve adjustement procedure to be certain.
Exactly.

I have seen the same thing. I no longer trust GMPP either, I won't get into it now but I have my reasons.

It's not normal, try adjusting the valves.
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