ported or unported
Thread Starter
Pro
Joined: Oct 2005
Posts: 581
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From: Boynton Beach Florida
ported or unported
I am using the ported vacuum for the advance now running 10 btdc when I switch to nonported the engine turns over like its has to much advance, Its a 81 with a crate motor, 350/300hp. I have one question they say at full advance it should be about 36 degrees btdc. if you use ported vacuum it stays at 36 degrees , so why use a vacuum advance..
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Is the computer still hooked up? If so, it may be doing something stupid as the engine is cranking, but I doubt it.
Are you sure you're getting the correct readings? "Where" you have the vacuum advance hooked up should have no affect on how the engine cranks. The timing mark may have moved on the balancer. Believe me this has fooled many over the years.
Make sure that the distributer hasn't fallen apart inside and that everything that is supposed to be there is. Make sure the end play is not excessive.
I would make sure that the timing marks are correct. Then, set initial advance to 16 degrees with vacuum advance disconnected. Now, change the advance weight springs to the lightest ones in the timing advance re-curve kit. You did get a re-curve kit didn't you? With the vacuum advance disconnected, run the engine speed up until the mechanical advance stops advancing. Record these two readings. Hopefully they are around 36 degrees at something less than 2800 RPM.
If maximum advance is more than 36 you may need to back off on the initial advance to bring it in line. If the point at which maximum advance is reached is too low (say 2000 RPM) you will need to use heavier springs to get this number up to around 2600 RPM.
You can reconnect the vacuum advance anywhere you want really. The engine "should" run better with it on manifold though.
BigBlockk
Later.....
Are you sure you're getting the correct readings? "Where" you have the vacuum advance hooked up should have no affect on how the engine cranks. The timing mark may have moved on the balancer. Believe me this has fooled many over the years.
Make sure that the distributer hasn't fallen apart inside and that everything that is supposed to be there is. Make sure the end play is not excessive.
I would make sure that the timing marks are correct. Then, set initial advance to 16 degrees with vacuum advance disconnected. Now, change the advance weight springs to the lightest ones in the timing advance re-curve kit. You did get a re-curve kit didn't you? With the vacuum advance disconnected, run the engine speed up until the mechanical advance stops advancing. Record these two readings. Hopefully they are around 36 degrees at something less than 2800 RPM.
If maximum advance is more than 36 you may need to back off on the initial advance to bring it in line. If the point at which maximum advance is reached is too low (say 2000 RPM) you will need to use heavier springs to get this number up to around 2600 RPM.
You can reconnect the vacuum advance anywhere you want really. The engine "should" run better with it on manifold though.
BigBlockk
Later.....
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
