Timing Gun goes wonky with Vacuum Can connected
05-31-2012, 08:43 AM
#1
Racer
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Timing Gun goes wonky with Vacuum Can connected
1977 SB.
36 total, all-in with the vacuum advance disconnected. Timing Light works great up to this point.
Connect the Vacuum Advance hose and timing light can lo longer get a stable reading... the timing marks on the pulley start bouncing around all over the place.
** Is my vacuum advance extremely high at this point? Is this what is causing the "bouncing" of the timing marks? **
I have an adjustable vacuum advance so I can make any needed adjustments.
( I plowed through the threads but could not locate the "bouncing timing marks" thread )
36 total, all-in with the vacuum advance disconnected. Timing Light works great up to this point.
Connect the Vacuum Advance hose and timing light can lo longer get a stable reading... the timing marks on the pulley start bouncing around all over the place.
** Is my vacuum advance extremely high at this point? Is this what is causing the "bouncing" of the timing marks? **
I have an adjustable vacuum advance so I can make any needed adjustments.
( I plowed through the threads but could not locate the "bouncing timing marks" thread )
05-31-2012, 08:54 AM
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Had a 1976 L-82, 4-sp
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Royal Canadian Navy
Put a vacuum gauge on to check for full manifold vacuum and see if the needle remains steady or fluctuates.
05-31-2012, 08:58 AM
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05-31-2012, 09:01 AM
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05-31-2012, 09:05 AM
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05-31-2012, 09:06 AM
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If I understand correctly (and I am just your basic, driveway mechanic)... I should be at 36 total when "all in" without the vacuum advance?
And with the vacuum advance connected, I should be at less than 52 total?
Please correct me on this. I can easily set my timing and idle as desired.
05-31-2012, 09:10 AM
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05-31-2012, 09:39 AM
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05-31-2012, 09:52 AM
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05-31-2012, 02:40 PM
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Le Mans Master
If I understand correctly (and I am just your basic, driveway mechanic)... I should be at 36 total when "all in" without the vacuum advance?
And with the vacuum advance connected, I should be at less than 52 total?
Please correct me on this. I can easily set my timing and idle as desired.
And with the vacuum advance connected, I should be at less than 52 total?
Please correct me on this. I can easily set my timing and idle as desired.
05-31-2012, 02:50 PM
#13
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36 At IDLE only?
This is the thread I have been referring to:
http://forums.corvetteforum.com/c3-t...long-post.html
Next check total timing with the vacuum can connected. It should be 36º PLUS whatever amount your vac. can supplies. On my car it would be 36º + 16º from the can = 52º.
No matter what, you do NOT want to exceed 52º (total plus vacuum advance) or again your timing will be too high and detonation becomes an issue.
Now that you see your vacuum advance is working take the car out for another test drive. Again make sure you aren't hearing knocking or pinging plus feel carefully that the car isn’t giving you a surging, chugging, or jerking feeling. If it is your vacuum advance adding too much in over and above the centrifugal advance and needs to be lowered.
** Note: for more details on various vacuum cans, their specs, and requirements, read Lars paper Vac Adv Specs.pf that I listed early on.
Last edited by 81_vette!; 05-31-2012 at 02:56 PM.
05-31-2012, 03:38 PM
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05-31-2012, 08:07 PM
#15
Le Mans Master
Problem is this, for performance applications, vacuum advances are seldom used, so the engines are set up for about 36 deg. total not using the advance. But for a street engine that you plan to use a vacuum advance for part throttle road ability you must make concessions. I don't care what they say, 52 degrees total is too much and the engine will flutter and misfire when all of that advance is in. If you are going to use that vacuum advance, set it up for total 36 deg. with all in at 3000 or so RPM.
36 At IDLE only?
This is the thread I have been referring to:
http://forums.corvetteforum.com/c3-t...long-post.html
Next check total timing with the vacuum can connected. It should be 36º PLUS whatever amount your vac. can supplies. On my car it would be 36º + 16º from the can = 52º.
No matter what, you do NOT want to exceed 52º (total plus vacuum advance) or again your timing will be too high and detonation becomes an issue.
Now that you see your vacuum advance is working take the car out for another test drive. Again make sure you aren't hearing knocking or pinging plus feel carefully that the car isn’t giving you a surging, chugging, or jerking feeling. If it is your vacuum advance adding too much in over and above the centrifugal advance and needs to be lowered.
** Note: for more details on various vacuum cans, their specs, and requirements, read Lars paper Vac Adv Specs.pf that I listed early on.
This is the thread I have been referring to:
http://forums.corvetteforum.com/c3-t...long-post.html
Next check total timing with the vacuum can connected. It should be 36º PLUS whatever amount your vac. can supplies. On my car it would be 36º + 16º from the can = 52º.
No matter what, you do NOT want to exceed 52º (total plus vacuum advance) or again your timing will be too high and detonation becomes an issue.
Now that you see your vacuum advance is working take the car out for another test drive. Again make sure you aren't hearing knocking or pinging plus feel carefully that the car isn’t giving you a surging, chugging, or jerking feeling. If it is your vacuum advance adding too much in over and above the centrifugal advance and needs to be lowered.
** Note: for more details on various vacuum cans, their specs, and requirements, read Lars paper Vac Adv Specs.pf that I listed early on.
05-31-2012, 09:51 PM
#17
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