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Help with motor vacuum

Fred's Vettes

Well-known member
Joined
Jun 26, 2002
Messages
141
Location
Rochester,NY USA
Corvette
1969 Blue/Black 1969 Green/Green Project
I am having a problem trouble shooting a rapid fluctuating needle between 13" and 16"Hg. The vacuum tends to smooth out on acceleration but is very erratic at idle.I'm taking the reading directly from the manifold port and find no gasket leaks. Do valve guides sound like the problem ? Any ideas will be appreciated. Thanks, Fred
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
Have you adjusted the idle mixture screws for highest steady vacuum, or are you just reporting what you saw when you connected the gauge without adjusting them?
 

Fred's Vettes

Well-known member
Joined
Jun 26, 2002
Messages
141
Location
Rochester,NY USA
Corvette
1969 Blue/Black 1969 Green/Green Project
Hi, the carb is adjusted and still have a very erratic needle between 15Hg and 12Hg. I'm leaning toward a valve leaking. Haven't done a leakdown test yet. If you have any ideas let me know, Fred
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
I am having a problem trouble shooting a rapid fluctuating needle between 13" and 16"Hg. The vacuum tends to smooth out on acceleration but is very erratic at idle.I'm taking the reading directly from the manifold port and find no gasket leaks. Do valve guides sound like the problem ? Any ideas will be appreciated. Thanks, Fred

Fred,

When you say that you find no gasket leaks, did you mean that you checked the intake manifold gaskets for leaks? If not, then use WD-40 (while using the little red straw to minimize the mess). Spray at the joint between the intake edge where it contacts the cylinder heads. If the idle speeds-up or smooths out, then you have a leak. Carburetor cleaner works well also.

Now, it is important to check the spark plugs, for if they're fouled, then you'll misread symptoms, and it will send you on a goose chase. You don't need to put new ones in, but if they're fouled, then you must clean them out with a wire brush. While you're at it check the gaps.

A bad Idle fuel/air mixture adjustment can cause this. Q-Jet carburetors usually require between 2 2/3 turns out from bottoming to 3 1/3 turns out INITIALLY. Holleys usually require an initial of 1- 1/2 turns.

Sometimes old and cracked-up ignition wires will cause this as well. The number of misses increases dramatically. As you loose and gain cylinders in every cycle, it gives the appearance of a rough idle or surging.

On stock advance timing cannisters, the advance is set during manufacture. I've heard of defective cannisters causing this. If it is a after market cannister, the neutral setting is about 7 turns in after having back-out the setting screw. You will need a 3/32" Allen wrench for this.

A leak-down test would be good at this point if none of the above fixed the problem.

GerryLP:cool
 

black_81_vette

Well-known member
Joined
Dec 1, 2002
Messages
295
Location
wisconsin
Corvette
black 81 corvette 4-speed
valves adjusted too tight? check to make sure the cam is good too and no bent parts.
 

wishuwerehere82

Well-known member
Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
I'm leaning toward either mis-fireing ignition, or too tight lifter adjustment. In any case your vacuum should be up around 18" at idle.
Let me know if you need an extra eye on the problem. I'm just a half hour away. And I have a cutaway valve cover for doing lifter adjustments.
 

jdp6000

Well-known member
Joined
Aug 23, 2004
Messages
646
Location
Aurora, Ontario, Canada
Corvette
1982 CROSSFIRE
I'm leaning toward either mis-fireing ignition, or too tight lifter adjustment. In any case your vacuum should be up around 18" at idle.
Let me know if you need an extra eye on the problem. I'm just a half hour away. And I have a cutaway valve cover for doing lifter adjustments.

Pete,

Can you post a picture of those cutaways? I need to make a set. Or can they be bought that way???

Jim
 

bossvette

Gone but not forgotten
Joined
May 19, 2003
Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
just use an old valve cover, these are rather crude but have served me for years ;LOL
 

Fred's Vettes

Well-known member
Joined
Jun 26, 2002
Messages
141
Location
Rochester,NY USA
Corvette
1969 Blue/Black 1969 Green/Green Project
Thanks for all the suggestions guys. I have double checked the intake for leaks and it is fine. The carb adjustment is at the highest idle vacuum and rpm. I'm replacing the plug wires as we speak and the plugs are new with no miles on them. This is on a year long frame off project and just another bump in a long road. I have a cut out valve cover with the clips also ,so will do another adjustment when the wires are done. Thanks again, Fred
 

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