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Tune-up Specs |
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IB Sorgn
Supporter of TheAMCForum Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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Posted: May/11/2021 at 4:16pm |
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I decided to check a few ignition specs on my 390 over this past weekend and to my surprise I could not find a clear spec on ignition timing and spark plug gap. Is there no standard for these items? I live in Kentucky about 1000 feet above sea level and usually run 93 octane. However I was told that the compression ratio had been purposely reduced to afford use of regular gas, however, let me not digress. Currently the motor is running autolite 66 spark plugs which were gapped from 33 - 46 thousands. I re-gapped them all at 40. Plugs were a little black so I imagine the motor could be leaned out a bit. I have no idea if running a DUI distributor has any bearing on gap or timing. I should say that timing, without vacuum connected, looked to be a couple degrees advanced. I reset it to 6 or 7 degrees. Motor seems to run fine but once again I just pulled the gap and timing from settings I remember using to initially set up old BB mopars. Can anyone give me some good starting points? I even have an AMC shop manual that didn't specify these basic settings. Of course I'm getting old and can't see as well as I used too so I could have missed this info.
Appreciate the help I should mention the motor has long tube headers, probably 1 3/4", dual plane intake and a Edelbrock carburetor maybe in the neighborhood of 650-700cfm. Heads appear to be the dogleg style. |
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Madd X
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6913 |
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So the motor is not factory, why try to use factory specifications?
Best you can do is advance timing or lean out just a little at a time until you find hesitation or light detonation and then back off. Running a little rich is far better then running lean. You break things running lean.
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WesternRed
AMC Addicted Joined: Aug/03/2010 Location: Australia Status: Online Points: 5808 |
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FWIW the 70 TSM has:
Type 1 distributor 5 +/- 1 BTDC at 500 rpm with vacuum disconnected and 34 +/- 3.5 at 2000 rpm with vacuum connected Type 2 distributor TDC +/- 1 at 500 rpm with vacuum disconnected and 45 +/- 4 at 2000 rpm with vacuum connected
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I've finally given up drinking for good...........now I only drink for evil.
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IB Sorgn
Supporter of TheAMCForum Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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What is a type 1 or type 2 distributor? What is your source for timing settings? Would like to read.
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Madd X
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Sonic Silver
Supporter of TheAMCForum Joined: Nov/23/2011 Location: East Tennessee Status: Offline Points: 7969 |
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Sonic Silver
Supporter of TheAMCForum Joined: Nov/23/2011 Location: East Tennessee Status: Offline Points: 7969 |
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It is in the 1970 Technical Service Manual or TSM that Red mentioned.
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IB Sorgn
Supporter of TheAMCForum Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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That's what I'm trying to do is set to a base line. What do you mean the motor is not factory? Was not built at the factory yes but uses factory spec parts. As far as I know there are no exotic parts used that would impact a baseline setting.
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Madd X
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IB Sorgn
Supporter of TheAMCForum Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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American Motors 1969 Technical Service Manual
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Madd X
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Sonic Silver
Supporter of TheAMCForum Joined: Nov/23/2011 Location: East Tennessee Status: Offline Points: 7969 |
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ccowx
AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510 |
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Hi! There most certainly are items that would affect base line settings. You don't appear to know what your distributor is, whether it has been modified, what your compression is, etc. Those are the big ones, though the carb, intake, headers etc could have some effects too.
Here is what you should do. Get a timing light, or hire someone with one if it is not in your comfort zone. Find out three things. A) what is your total advance without the vacuum hoses connected, B) when does that come on and C) what is your resting timing setting? If you subtract C from A, you will have your total mechanical advance. You want it to have a total at full advance of around 34-38 for most engines. If your calculation of (A-C=?) is say 20 degrees, set your initial at 14 BTDC and your total will be about 34. Your numbers will be different, but you get the idea. The goal is to have it all in around 34, which is a fairly safe number. After that, simply advance it 2 degrees or so at a time and go for a test drive. If it doesn't knock and ping, try another 2 degrees, etc. If it knocks or runs on, back it up two degrees to your previous setting and stop. I would not bother going over a total of 38, at least without a knowledgeable person around, but I doubt you will anyway. Once you establish that, you have your best timing with what you have. Now all you need to do is get your timing at idle and you know where to set it to get the best overall timing. Your plug gap should be around .035", though depending on your ignition some would open them up a bit more. That is something to deal with later, .035 will work to get a baseline tune on it. If you do these things you should be able to run without hurting anything. You can assess what you have and see if there is a need for further fine tuning, but that will get you in the ballpark. It sounds like you have a mildly modified engine that may or may not have been set up by someone that knows what they are doing. Check everything and make sure the combination is running right, and they you can make a decision as to whether you need to start changing things. Chris
Edited by ccowx - May/11/2021 at 8:23pm |
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