engine vacuum at idle??
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Junior User
Joined: Sep 2007
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From: medicine hat, ab
engine vacuum at idle??
HI Guys and Gals...
The motor (if youve been following the saga...lol) is a stock 51 Merc. flathead., no mods except the 6volt electric fuel pump.
Just put in points, condenser, coil, wires, cap and rotor. Gapped the points to .014 as per manual.
Have been reading the search and internet as how to use the vacuum gauge to set the timing. Twist distrib. until highest vacuum is achieved, back off 1 inch and tighten her down. Also read that total vacuum should be 22 degrees at 2500-3000 rpm.
Here is the thing. At idle, i am only at 14-15 inches. Some websites say a good running engine should be at 20. I turn the distrib. and can't get anymore vacuum.
Any help would be greatly appreciated.
Thx in advance...
Rick
The motor (if youve been following the saga...lol) is a stock 51 Merc. flathead., no mods except the 6volt electric fuel pump.
Just put in points, condenser, coil, wires, cap and rotor. Gapped the points to .014 as per manual.
Have been reading the search and internet as how to use the vacuum gauge to set the timing. Twist distrib. until highest vacuum is achieved, back off 1 inch and tighten her down. Also read that total vacuum should be 22 degrees at 2500-3000 rpm.
Here is the thing. At idle, i am only at 14-15 inches. Some websites say a good running engine should be at 20. I turn the distrib. and can't get anymore vacuum.
Any help would be greatly appreciated.
Thx in advance...
Rick
Fleet Owner
Joined: Jul 2004
Posts: 27,287
Likes: 1,052
From: NM
Total vacuum advance should be about 22 deg, under load, at 2,000+ RPM; you won't be able to duplicate that in the driveway. You would have to use a vacuum pump on the advance can. The workshop manual gives the amount of advance at different levels of vacuum.
The manifold vacuum quoted (20") is pretty high IMO, and is surely at sea level. What elevation are you? I am at 5500' and never see anywhere near that, I think 15" would be reasonable here. Besides setting the timing based on vacuum readings, usually you have to drive it and listen for pinging and make adjustments downwards.
The manifold vacuum quoted (20") is pretty high IMO, and is surely at sea level. What elevation are you? I am at 5500' and never see anywhere near that, I think 15" would be reasonable here. Besides setting the timing based on vacuum readings, usually you have to drive it and listen for pinging and make adjustments downwards.
Post Fiend
Joined: May 2008
Posts: 7,641
Likes: 21
From: Poway, Ca.
Hi Rick!
I used this method to confirm the timing on my 390. Come to find out that all the modern books had it set at 6 deg BTDC. They failed to mention that this was with the Thermactor (pollution control) installed (which I don't have). I had to find a pre 1967 book to get the real value at 12 Degrees (with an advance as far up as 17 for performance if desired). It made a TREMENDOUS difference in how that engine ran.
Anyway, the actual vacuum value when looking for correct timing is somewhat mute. What you are looking for is the HIGHEST vacuum you can get.
Now this also requires some other jimmying. You have to keep the idle at the right rpm, and you need to tweek your idle mixture while doing it as well. So it's actually a matter of fine tuning about three things in conjunction while setting timing.
When you are satisfied that you have done all that, THEN back the timing off to reduce the vacuum by 1 inch.
I'd check it with a light after to compare the running engine with the book value as well.
J!
I used this method to confirm the timing on my 390. Come to find out that all the modern books had it set at 6 deg BTDC. They failed to mention that this was with the Thermactor (pollution control) installed (which I don't have). I had to find a pre 1967 book to get the real value at 12 Degrees (with an advance as far up as 17 for performance if desired). It made a TREMENDOUS difference in how that engine ran.
Anyway, the actual vacuum value when looking for correct timing is somewhat mute. What you are looking for is the HIGHEST vacuum you can get.
Now this also requires some other jimmying. You have to keep the idle at the right rpm, and you need to tweek your idle mixture while doing it as well. So it's actually a matter of fine tuning about three things in conjunction while setting timing.
When you are satisfied that you have done all that, THEN back the timing off to reduce the vacuum by 1 inch.
I'd check it with a light after to compare the running engine with the book value as well.
J!
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