total advance question.
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Senior User
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From: Northern Virginia
total advance question.
I'm sorta new to these FE motors.I have a 65 250 camper special.I had a bud over to work on the carb, Holly 650.While he was here we verified the TDC to time it and check the length of stroke to see if it was a 352 or a 390.Its a 390 and overall is in great shape.The carb is a bit big for the motor but it really does run good so I am not tempted to spend much time on it for now.My question is when we had the light on it and the motor rpms up under no load the total advance was 49 degrees.He said that seemed like too much and from doing a search on this forum he is correct.How is this adjusted?I have Pertronix on the truck and the dizzy seems to be the stock one.Thank you in advance..
Joe
Joe
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Joined: Mar 2004
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From: Union, Washington
Disconnect the vacuum advance. The total mechanical advance should be no more than 38° (32° to 36° is safer). With no load the vacuum advance pulls the timing around farther, but the mechanical total is the one that counts.
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Senior User
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From: Northern Virginia
Makes sense to me.Thank you.What you are saying is that the total advance vacuum + mechanical can be over 38 degrees?I realize that under load the vacuum wont be advancing it as much but is there a way to check the total of the two under load or is that not an issue?
Joe
Joe
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From: Montana
With engine under load, throttle plates are open a fair amount so vacuum has dropped alot. Thus the vacuum advance will be NONE, but the distributor shaft will be turning lots of revs so mechanical advance will be full ON.
Bear is correct: Have vacuum advance hose connected, with engine revved up the timing will be reading only the mechanical advance per my comments above. The total timing should be 32-36 degrees. More efficient combustion will require less total timing, so don't be fooled by someone telling you the engine needs over 36 degrees or so. With electronic ignition you set the timing once and pretty much forget it since there are no points to wear thus timing does not change.
Bear is correct: Have vacuum advance hose connected, with engine revved up the timing will be reading only the mechanical advance per my comments above. The total timing should be 32-36 degrees. More efficient combustion will require less total timing, so don't be fooled by someone telling you the engine needs over 36 degrees or so. With electronic ignition you set the timing once and pretty much forget it since there are no points to wear thus timing does not change.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Under load can be checked, but you have to have a dyno and that will be expensive. I check total mechanical at whatever RPM it Max's out at, then plug in the vacuum and see what that adds to it. Under load will always be less than no load.
I do know of away to check how much the vacuum advance is cranking in when you are loaded and heading down the road, but it is complicated and requires a vacuum Gage, tach, hand vacuum pump and recording the results of different test runs. It's a PIA and unless you really have to know not worth the effort. As long as the engine doesn't ping under load or overheat, it really isn't an issue. My vacuum adds 14° max to my timing which is set at 34° at 2800 rpm.
I do know of away to check how much the vacuum advance is cranking in when you are loaded and heading down the road, but it is complicated and requires a vacuum Gage, tach, hand vacuum pump and recording the results of different test runs. It's a PIA and unless you really have to know not worth the effort. As long as the engine doesn't ping under load or overheat, it really isn't an issue. My vacuum adds 14° max to my timing which is set at 34° at 2800 rpm.
