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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
I also don't have anyone to look at the tach and tell me when the motor is spinning at 4000 rpm while I'm back there working the throttle by hand in the engine compartment.
It needs to be up to 4000rpm because checking the ignition timing at 4000 rpm with the vacumm hoses disconnected from the distributor is only checking the total centrifical advance.
There is no more centrifical advance above 4000rpm because it is maxed out by then.
This is what I do and maybe it will work for you.

I sit in the car and rev it to 4000rpm and I listen and try to remember the sound of the motor like a musical note or pitch at 4000 rpm. Then I go back and rev the engine up by hand on the throttle butterfly linkage and listen for the same engine sound or pitch to the rpms while watching the timing light to read the igntion timing.
To make it easier you can rev it to 5000 rpms and the timing wil be the same as 4000 rpms because the centrifical advance is already maxed out and won't go any higher. So, if you're reving up it by ear like I do, go a little higher than 4000rpms to check maximum centrifical advance just to be sure. It won't hurt anything and the timing will be exactly the same.

As far as your distributor vacuum pot hose locations, on an '87 distributor the vacuum hose that has vacuum at idle should be on the vacuum retard pot like it was. Thats the one on the right closest to the distributor body.
You said you changed that hose to the other one which is the vacum advance and boost retard pot so you now have it wrong.

The vacuum retard line only has vacuum up until around 1500 rpms and then it goes away because the edge of the throttle butterfly changes in relation to the tiny hole in the throttle body bore above that point so it no longer has vacuum similar to intake manifold vacuum there.
It's purpose is to retard timing at idle and during closed throttle deceleration to lower exhaust emissions and heat up the catalytic converter. It does that because retarded ignition timing has higher exhaust temperature.

You should put the hoses back where they were because the way you have it now you no longer have vacuum advance to give you more power during acceleration and you may not have boost retard any more and that could easily damage the motor under boost.
Old 04-14-2013, 08:54 AM
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