Tune Up Assistance NEEDED!!!!
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,713
Likes: 31
From: Shreveport LA
LA Events Coordinator
Tune Up Assistance NEEDED!!!!
Ok...here's the deal....I have a 75 Convertible with an HEI ignition (Mallory) and a new Holley 650 Street Avenger and recently I have replaced all the spark plugs.
I have a mild cam so I initially set the timing to 6 degrees BTC and the car pings on acceleration...so that tells me I'm advanced too much... I reset the timing to 4 degrees...still has the pinging (not as bad) and I cannot set the RPMs...it idles (in park) at 700 then it jumps to 1100 then it drops to 500...all in about 1 minute. When I "goose" the engine a couple of times, the RPM stay at 1000. I check the timing....and it now show 12 degrees.... WHAT THE HECK IS GOING ON????? A vacuum leak? I checked all the hoses for leaks (all seem to be ok)....my lights work perfectly....The only thing I am going to replace is the console Heater/AC control...the left wheel won't go all the way to the top....I understand there are some tiny vacuum lines in the control unit...do you think that the heater/AC controls not working may be affecting the engines timing???
Any help will be GREATLY apprectiated!!!!...I am at witts end!!!! I have spent the better part of today trying to set the timing......WITHOUT ANY LUCK!!! 
Decided to check with you guys....
I know, I know...."Just because I'm cussing, doesn't mean I'm not having fun!"
I have a mild cam so I initially set the timing to 6 degrees BTC and the car pings on acceleration...so that tells me I'm advanced too much... I reset the timing to 4 degrees...still has the pinging (not as bad) and I cannot set the RPMs...it idles (in park) at 700 then it jumps to 1100 then it drops to 500...all in about 1 minute. When I "goose" the engine a couple of times, the RPM stay at 1000. I check the timing....and it now show 12 degrees.... WHAT THE HECK IS GOING ON????? A vacuum leak? I checked all the hoses for leaks (all seem to be ok)....my lights work perfectly....The only thing I am going to replace is the console Heater/AC control...the left wheel won't go all the way to the top....I understand there are some tiny vacuum lines in the control unit...do you think that the heater/AC controls not working may be affecting the engines timing???
Any help will be GREATLY apprectiated!!!!...I am at witts end!!!! I have spent the better part of today trying to set the timing......WITHOUT ANY LUCK!!! Decided to check with you guys....I know, I know...."Just because I'm cussing, doesn't mean I'm not having fun!"
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
most of us are running much higher timing numbers then those.....what kind of vacuum are you getting and what vacuum cannister are you running and is it ported or unported? i don't think you should be getting pinging....what octane are you running and what is your CR?
edit: you know is there any evidence of your harmonic balancer having slipped????
edit: you know is there any evidence of your harmonic balancer having slipped????
Last edited by bobs77vet; Mar 4, 2006 at 08:12 PM.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
the vacuum controls for the heater have nothing to do with this ....you may have a vacuum cannister that is not working correctly. is this an auto or a 4spd? i think you have a vacuum cannister that is hunting and changing the timing on you .....i also think your harmonic balance might have slipped...i can't think of any reason for you to be getting ping unless you have a really CR at the timing setting you "think" you are getting
edit: you know not for nothing here but ....my fan clutch crapped out at exactly the time i was setting my timing and i mistakenly thought that i had detonation....you may want to look at the fan clutch
edit: you know not for nothing here but ....my fan clutch crapped out at exactly the time i was setting my timing and i mistakenly thought that i had detonation....you may want to look at the fan clutch
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
Lean Idle Condition
Sounds like a lean idle condition. Make sure the mixture screws are set according to the manual and go from there. Also, you need to find a base line for your timimg. Set your timing with the vacuum advance plugged. Try 8 degrees at 700 RPM and then use full manifold vacuum. Your idle speed will go up, just adjust down the idle. Start with heavy mechanical advance springs so the advance comes in slowly and go from there. Good luck, you will get there.
Racer
Joined: Aug 2005
Posts: 435
Likes: 2
From: Boise Idaho
I have 20* at idle and 36* with full centrifical advance
Assuming the balance and pointer are accurate.
Assuming the cheap Harbor freight Advance timing light is close.
77L82
Stongly recommend Lars Timing paper
Assuming the balance and pointer are accurate.
Assuming the cheap Harbor freight Advance timing light is close.
77L82
Stongly recommend Lars Timing paper
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,713
Likes: 31
From: Shreveport LA
LA Events Coordinator
Originally Posted by bobs77vet
most of us are running much higher timing numbers then those.....what kind of vacuum are you getting and what vacuum cannister are you running and is it ported or unported? i don't think you should be getting pinging....what octane are you running and what is your CR?
edit: you know is there any evidence of your harmonic balancer having slipped????
edit: you know is there any evidence of your harmonic balancer having slipped????
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,713
Likes: 31
From: Shreveport LA
LA Events Coordinator
Originally Posted by TexSon
You may want to check the vacuum advance. It might be or is going bad. Or it might just be the vacuum advance line.
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Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,713
Likes: 31
From: Shreveport LA
LA Events Coordinator
Originally Posted by Jeff_Keryk
Sounds like a lean idle condition. Make sure the mixture screws are set according to the manual and go from there. Also, you need to find a base line for your timimg. Set your timing with the vacuum advance plugged. Try 8 degrees at 700 RPM and then use full manifold vacuum. Your idle speed will go up, just adjust down the idle. Start with heavy mechanical advance springs so the advance comes in slowly and go from there. Good luck, you will get there.
I will also check the lean idle condition in the morning....by the way...what do you mean by "Try 8 degrees at 700 RPM and then use full manifold vacuum." I understand the 8 degrees and the 700 RPM but what do you mean by use full manifold vacuum?
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
Carbs usually have 2 vacuum ports; you will use 1 for the vacuum advance hose. Earlier engines used full manifold vacuum (ported below the throttle plates) while smog engines used ported vacuum. Ported vacuum is low (or non-existant) at idle because the port is above the throttle blades. When the throttle blades open slightly, atmospheric pressure forces air through the throttle body (ported vacuum) which activates vacuum advance. Smog engines were designed to run hot. Full manifold vacuum will help with idle to off-idle transition and at cruise. Chevys like a lot of timing, just make sure your carb is not tuned too lean. There are many opinions on timing, it is best to experiment and see what your engine likes.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
CR is compression ratio.....my post should have said unless "you have a really high compression Ratio engine".
you should be able to get this right, something is askew but we can help you figure it out.
you should be able to get this right, something is askew but we can help you figure it out.
