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This is completely unrelated to V6 fbodies, but I know alot of you here are familiar with alot more in engines than just 3.4 and 3.8L V6's. Please take a moment and see if you can help me.
I have a '79 K10 Silverado LWB with a 350 engine and TH-350 trans. The engine has roughly 50k on the rebuild and is modified with an Edelbrock Performer Intake, Performer 600cfm carb and by the word of the previous owner, a Performer cam. It is also running an HEI ignition setup. The truck has had its share of both major and minor problems in the 2 years I've had it, and all but one I have been able to correct. That problem is an eratic timing issue. For starters, (with vacuum advance disconnected and plugged) when I set the timing at the standard 8-12 degrees BTDC of static advance, the truck will barely idle without the throttle half open. Because of this, the initial timing has to be set over 30deg BTDC (closer to 45 according to my dial-back timing light) to get a decent idle and drivable power. The timing seems to change with the engine temp too.... it seems to be over-retarded below operating temp, and then become over-advanced at operating temp -- to the point of pinging slightly unless I set the inital timing back so far it looses power over 2000 rpms. When it's warm and while watching with timing light, as I increase the revs from idle to 1300-1500rpms, the advance starts to fall back. It will retard over 35 degrees from where it is set. When I let off the throttle, the engine comes off the rev, stumbles and lopes for a while as the advance, over the course of 5 or so seconds, comes back up to where it was set. If I rev above 1500 rpms it starts to advance back up from where it fell back to as though it was set 35 degrees later than it was. This is hard to explain, but I hope you catch my drift.
I have changed every component related to ignition timing except the camshaft itself. From the timing chain, to the spark plugs, from the distributor, to the plug wires. The coil and HEI module were replaced with ACCEL components, as were the cap, & rotor. I have tried every spring & weight combo known to man for the mechanical advance curve. I thought for sure it was the timing chain because when I changed it, there was a serious amount of play. None of these changes have even slightly affected this eratic timing behavior. I've consulted numerous peple who are knowledgable of engines, spent hours on the phone with a mechanic friend, but have stumped every one of them. I have searched the internet up and down 5 times, and found nothing to help. I am at a loss on what to do short of pulling the engine, tearing it down and starting from scratch. I may just have to swallow my pride and spend a few hundred dollars and take it to a pro, but I'm hoping you can help before I do that.
One other thing that came to mind recently is that maybe the previous owner shifted the plug wire sequence 1 spark back. When I changed the plug wires, I just replaced them 1 by 1, and did not locate TDC and start from scratch. If this were true, it should start firing, going from Cyl #1 TDC, in this order: 1,8,4,3,6,5,7,2. If the wire sequence was started at 2 instead of 1, the firing sequence when the crank is at 1-TDC it would look like 2,1,8,4,3,6,5,7, and to compensate I would have to advance my distributor 45 degrees (360deg/8cyls) to compensate. This still doesn't account for the temperature related changes and the retardation of the timing around the 1500rpm mark, but it would explain the need for 40deg BTDC advance to even get a decent idle out of it. I'll check that this weekend and follow up.
Any help, info, input, opinions, and educated (or otherwise) guesses are welecomed and appreciated.
Thanks and regards...
-Mike
[ November 02, 2003: Message edited by: MTMike ]</p>
I have a '79 K10 Silverado LWB with a 350 engine and TH-350 trans. The engine has roughly 50k on the rebuild and is modified with an Edelbrock Performer Intake, Performer 600cfm carb and by the word of the previous owner, a Performer cam. It is also running an HEI ignition setup. The truck has had its share of both major and minor problems in the 2 years I've had it, and all but one I have been able to correct. That problem is an eratic timing issue. For starters, (with vacuum advance disconnected and plugged) when I set the timing at the standard 8-12 degrees BTDC of static advance, the truck will barely idle without the throttle half open. Because of this, the initial timing has to be set over 30deg BTDC (closer to 45 according to my dial-back timing light) to get a decent idle and drivable power. The timing seems to change with the engine temp too.... it seems to be over-retarded below operating temp, and then become over-advanced at operating temp -- to the point of pinging slightly unless I set the inital timing back so far it looses power over 2000 rpms. When it's warm and while watching with timing light, as I increase the revs from idle to 1300-1500rpms, the advance starts to fall back. It will retard over 35 degrees from where it is set. When I let off the throttle, the engine comes off the rev, stumbles and lopes for a while as the advance, over the course of 5 or so seconds, comes back up to where it was set. If I rev above 1500 rpms it starts to advance back up from where it fell back to as though it was set 35 degrees later than it was. This is hard to explain, but I hope you catch my drift.
I have changed every component related to ignition timing except the camshaft itself. From the timing chain, to the spark plugs, from the distributor, to the plug wires. The coil and HEI module were replaced with ACCEL components, as were the cap, & rotor. I have tried every spring & weight combo known to man for the mechanical advance curve. I thought for sure it was the timing chain because when I changed it, there was a serious amount of play. None of these changes have even slightly affected this eratic timing behavior. I've consulted numerous peple who are knowledgable of engines, spent hours on the phone with a mechanic friend, but have stumped every one of them. I have searched the internet up and down 5 times, and found nothing to help. I am at a loss on what to do short of pulling the engine, tearing it down and starting from scratch. I may just have to swallow my pride and spend a few hundred dollars and take it to a pro, but I'm hoping you can help before I do that.
One other thing that came to mind recently is that maybe the previous owner shifted the plug wire sequence 1 spark back. When I changed the plug wires, I just replaced them 1 by 1, and did not locate TDC and start from scratch. If this were true, it should start firing, going from Cyl #1 TDC, in this order: 1,8,4,3,6,5,7,2. If the wire sequence was started at 2 instead of 1, the firing sequence when the crank is at 1-TDC it would look like 2,1,8,4,3,6,5,7, and to compensate I would have to advance my distributor 45 degrees (360deg/8cyls) to compensate. This still doesn't account for the temperature related changes and the retardation of the timing around the 1500rpm mark, but it would explain the need for 40deg BTDC advance to even get a decent idle out of it. I'll check that this weekend and follow up.
Any help, info, input, opinions, and educated (or otherwise) guesses are welecomed and appreciated.
Thanks and regards...
-Mike
[ November 02, 2003: Message edited by: MTMike ]</p>
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