Ignition timing question
Thread Starter
Burning Brakes
Joined: Oct 2006
Posts: 869
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From: Bakersfield, Ca.
Ignition timing question
Probably a dumb question, but I don't have a lot of expierence with timing lights. I checked the timing at 3000 rpm, as per the WSM, and the timing mark was where the red line is in the picture. Now, it call's for 23 degrees BTDC, but the picture shows right at 28 (if that is a 28). Should the timing mark be right befor the 28 or right on it as the picture shows? And just out of curiosity, is "O/T" TDC?
The reason I even decided to check timing, is every third (or so) time i stop at a light or for some other reason, the engine starts surging. Almost like it is going to die, but doesn't. The idle just seems low. Once I give it a little gas, or put it nuetral, it act's fine.
Thank you in advance for your input.
The reason I even decided to check timing, is every third (or so) time i stop at a light or for some other reason, the engine starts surging. Almost like it is going to die, but doesn't. The idle just seems low. Once I give it a little gas, or put it nuetral, it act's fine.
Thank you in advance for your input.
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From: Ball Ground, GA
Look carefully at the crankshaft dampener on your car, and see if it says 23 or 28 - I think that it will say 23.
Yes, 0|T is Top Dead Center (TDC) on #1 and #6 pistons. Remember that the piston comes to TDC twice in each cycle - TDC-Firing and TDC-Exhaust. The spark happens Before Top Dead Center Firing, not before Top Dead Center Exhaust. Likewise, the cam timing is based upon TDC-Firing, not TDC-Exhaust. The common way of showing ignition timing is degrees BTDC, with the assumption that you know that it is before the firing position, not the exhaust position.
You don't have to worry about that while using a timing light - just follow the manual instructions.
Yes, 0|T is Top Dead Center (TDC) on #1 and #6 pistons. Remember that the piston comes to TDC twice in each cycle - TDC-Firing and TDC-Exhaust. The spark happens Before Top Dead Center Firing, not before Top Dead Center Exhaust. Likewise, the cam timing is based upon TDC-Firing, not TDC-Exhaust. The common way of showing ignition timing is degrees BTDC, with the assumption that you know that it is before the firing position, not the exhaust position.
You don't have to worry about that while using a timing light - just follow the manual instructions.
Thread Starter
Burning Brakes
Joined: Oct 2006
Posts: 869
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From: Bakersfield, Ca.
Thanks Wally, great stuff. Actually it says 20 on the crankshaft dampener. I was looking at the wrong page in the WSM. What was pictured was for a '78 (I think). Evidently my WSM has a mix of several years. flipping a few more pages, I found the correct one for the '83. It matches the sticker on the inside of the hood which says 20 drgrees BTDC. Is it still worth adjusting for the amount it is off?
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Joined: Oct 2004
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From: K-town, Germany
I'd leave it where it's at--but that's just me. Quite a few with OBs advance the timing a little bit. Coincidently, your mark is almost exactly where I have mine marked on my 84 (assuming that would be your 20 degree mark). You can always adjust yours back to 20 and take it for a test drive and see what you think. Have fun with it!
/ Bruce
/ Bruce
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Joined: Sep 2004
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From: Stockton, CA
The timing spec for US 4.7L is 20' at 3000rpm.... Its also 28' at 5000rpm is the correct spec with vacuum removed.....
It is odd how "low" the timing is on the US 4.7L....since they have far less timing than any other 16V 928....most are around 36' at 5000rpm.....
The lemons racer is set to about 21-22' at 3000rpm....seems to run well at that point...
It is odd how "low" the timing is on the US 4.7L....since they have far less timing than any other 16V 928....most are around 36' at 5000rpm.....
The lemons racer is set to about 21-22' at 3000rpm....seems to run well at that point...
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Joined: Sep 2004
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From: Stockton, CA
I need to do some dyno tuning on the lemons racer to see if it responds to more timing.....
