Vacuum Reading
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Instructor
Joined: Feb 2011
Posts: 234
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Vacuum Reading
Just did a vacuum test from the quadrajet on 1975 350 4 speed. The reading was 17. Is that a good reading?
Carb was rebuit last year. Did not adjust air fuel screws. Was told by rebuild company that adjustment was not needed after rebuild.
Thanks
Carb was rebuit last year. Did not adjust air fuel screws. Was told by rebuild company that adjustment was not needed after rebuild.
Thanks
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
In general, 17" Hg is a pretty good reading; but that's not exactly what you need to do. And, the carb doesn't 'create' the vacuum level...that is done by the engine when it is running at low rpm (idle) with the throttle plates barely opened.
You need to measure your "baseline" vacuum level: that is the absolute maximum vacuum level YOUR engine will produce at idle. And that is done by removing/blocking all known connections to the carb and intake manifold that supply vacuum to other systems.
Remove all vacuum lines and block supply ports/hoses off so there can be no leakage paths. Leave dizzy advance can connected for the moment (so the engine will still fire and run smooth). Hook up vacuum gage to a port on the intake manifold so you can measure engine vacuum. Start engine and let it warm up. Now, take the vacuum level measurement and record it. If you feel that your engine timing and carb adjustments are as they need to be, the measurement you got will be your "baseline" vacuum level result. (If you want to experiment more, you could tweak your distributor timing and optimize your carb's idle mixture adjustments and take another reading to see if you can get that vacuum level even higher. But, it's not necessary.)
If this baseline vacuum level is significantly higher than the 17" Hg you got before (more than 1-2" Hg difference), then there is some vacuum leakage in your system. If you have more that 3" Hg difference, you should try to find the sources of vacuum leakage and resolve them.
A factory-new L-48 engine will have a baseline vacuum level around 20-21" Hg. Wear of piston rings and valves/valve seats will cause that value to lower, because of some minor leakages throughtout the engine. But, an L-48 engine which produces 15" Hg or more is still in decent shape. With more radical cams, that baseline vacuum level will drop, due to valve overlap losses at idle speeds.
You need to measure your "baseline" vacuum level: that is the absolute maximum vacuum level YOUR engine will produce at idle. And that is done by removing/blocking all known connections to the carb and intake manifold that supply vacuum to other systems.
Remove all vacuum lines and block supply ports/hoses off so there can be no leakage paths. Leave dizzy advance can connected for the moment (so the engine will still fire and run smooth). Hook up vacuum gage to a port on the intake manifold so you can measure engine vacuum. Start engine and let it warm up. Now, take the vacuum level measurement and record it. If you feel that your engine timing and carb adjustments are as they need to be, the measurement you got will be your "baseline" vacuum level result. (If you want to experiment more, you could tweak your distributor timing and optimize your carb's idle mixture adjustments and take another reading to see if you can get that vacuum level even higher. But, it's not necessary.)
If this baseline vacuum level is significantly higher than the 17" Hg you got before (more than 1-2" Hg difference), then there is some vacuum leakage in your system. If you have more that 3" Hg difference, you should try to find the sources of vacuum leakage and resolve them.
A factory-new L-48 engine will have a baseline vacuum level around 20-21" Hg. Wear of piston rings and valves/valve seats will cause that value to lower, because of some minor leakages throughtout the engine. But, an L-48 engine which produces 15" Hg or more is still in decent shape. With more radical cams, that baseline vacuum level will drop, due to valve overlap losses at idle speeds.
