Vacuum Gauge Testing
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
I've been a fan of these for engine tuning and diagnostics and always recommend them to others for troubleshooting. Am wondering though if maybe I am overlooking the correct way to interpret the results.
At sea level the manual sez the 292 should pull a steady 18" or more at idle. But it also sez the correct ignition timing is 6°. There's no way it will draw 18" or better with the timing at 6°, is this an indication of a problem?
This thing currently idles at a very steady 20" but, that's with the initial advance well beyond stock initial ignition timing. The manual also says that ignition timing can be advanced over the "normal" setting, up to 5°, for better performance and fuel economy. That would be 11° BTDC by my math. Could probably get it in the zone there. Running 17° right now. Haven't checked in a while, but again know for certain that it's nowhere near pulling 18" at the stock base timing spec.
One of the reasons I am chewing over this, is because it was running pig rich for so long before we got a handle on things. Rings can get washed out.
At sea level the manual sez the 292 should pull a steady 18" or more at idle. But it also sez the correct ignition timing is 6°. There's no way it will draw 18" or better with the timing at 6°, is this an indication of a problem?
This thing currently idles at a very steady 20" but, that's with the initial advance well beyond stock initial ignition timing. The manual also says that ignition timing can be advanced over the "normal" setting, up to 5°, for better performance and fuel economy. That would be 11° BTDC by my math. Could probably get it in the zone there. Running 17° right now. Haven't checked in a while, but again know for certain that it's nowhere near pulling 18" at the stock base timing spec.
One of the reasons I am chewing over this, is because it was running pig rich for so long before we got a handle on things. Rings can get washed out.
Logistics Pro
Joined: Mar 2005
Posts: 3,771
Likes: 80
From: tujunga, calif
I don't recall checking the vacuum on a stock engine at 6º, I've only had one stock engine and that was in 1965 
. I would more concerned about setting the initial at 17º, that plus your total mechanical advance seems like way to much. I set mine at 12º plus 24º mechanical advance is 36º and I wouldn't feel safe running it any higher.
Running a little rich is safer then too lean. If the engine is reaching correct operating temperature ( I use a 180º thermostat ) the fuel should atomize, it won't be raw and I don't think it will wash the rings
. I would more concerned about setting the initial at 17º, that plus your total mechanical advance seems like way to much. I set mine at 12º plus 24º mechanical advance is 36º and I wouldn't feel safe running it any higher.Running a little rich is safer then too lean. If the engine is reaching correct operating temperature ( I use a 180º thermostat ) the fuel should atomize, it won't be raw and I don't think it will wash the rings
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Yeah I limited the mechanical advance inside the distributor to about 20°, it's around 36° total all in when vac advance disconnected.
Rich mixture generally is better, but the plugs were black. I didn't know how to tune an engine. Installed a hotter plug, of course that didn't do anything. Yesterday did a compression check, engine cold. Spec in manual is 130-170 when warmed up. What do you think?
1. 135
2. 145
3. 135
4. 130
5. 139
6. 140
7. 135
8. 140
Rich mixture generally is better, but the plugs were black. I didn't know how to tune an engine. Installed a hotter plug, of course that didn't do anything. Yesterday did a compression check, engine cold. Spec in manual is 130-170 when warmed up. What do you think?
1. 135
2. 145
3. 135
4. 130
5. 139
6. 140
7. 135
8. 140
Cargo Master
Joined: Apr 2003
Posts: 2,389
Likes: 58
Is the engine stock? Recently rebuilt or old?
The compression numbers are a little low, but ok. They do not necessarily indicate a problem.
Some things that will affect idle vacuum are idle mixture, valve lash, retarded cam timing (different than ignition timing and not as easy to adjust), vacuum leaks.
The compression numbers are a little low, but ok. They do not necessarily indicate a problem.
Some things that will affect idle vacuum are idle mixture, valve lash, retarded cam timing (different than ignition timing and not as easy to adjust), vacuum leaks.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Rebuilt about 25K miles ago. Bone stock. I paid someone to do this, not sure what they all did for my hard earned money though. But it doesn't smoke or leak, so I think I did pretty good ha ha
I read compression will kick up about 20 points typically, between cold and hot, so figured these were good numbers.
Wondered about the cam timing, is there a way to tell how its clocked or whatever without disassembling?
I read compression will kick up about 20 points typically, between cold and hot, so figured these were good numbers.
Wondered about the cam timing, is there a way to tell how its clocked or whatever without disassembling?
Cargo Master
Joined: Apr 2003
Posts: 2,389
Likes: 58
Yeah, you can check it using the same method you might to degree the cam during assembly. Minimal disassembly and special tools will be required to do that, though.
I wouldn't bother with it if it's generally running well.
I wouldn't bother with it if it's generally running well.
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