Vacuum 14" Hg at idle (1 Viewer)

[togntime1]

F1.5 with fresh rebuilt carb. I have timing set at approximately 10 degrees advance, and valve tolerances seem ok. I'm at sea level. No vacuum leaks. Is this vacuum reading acceptable or a sign of other problems? Anything I can do other than adjusting carb to increase the vacuum? Thanks!

 

[Curt]

18hg would be a good goal. A really good engine at sea level with 10* of advance should see close to 20hg. Oh with idle at 650

 

[tls3601]

@Curt would a lower than 20 number be acceptable at a mile high? 5300 ft. Or should the vacuum pressure he higher than 20 at a mile high. My 2F that runs excellent and was rebuilt in 2013 typically runs at 16-17 at idle 650rpm at 5300ft. Thanks for your input, Tom

 

[Pighead]

Anything I can do other than adjusting carb to increase the vacuum? Thanks!

Yes. Advance your timing while watching the vacuum. Many people (like me) set timing by vacuum more than a light. Advance timing until best vacuum then back off a little, more if you start pinging under acceleration.

 

[Pighead]

would a lower than 20 number be acceptable at a mile high?

I am not Curt, But, in my experience your vacuum numbers will decrease with higher altitudes. Lower atmospheric pressure=less ability to generate vacuum. I only pull about 15"Hg at idle at 5300ft altitude, but my engine is 10 years older than yours...

 

[Dizzy]

The amount of crusted oil that is on top of my intake valves is silly. That is going to be hard to clean without pulling the head. I had an engine in storage which seemed to clean-up with a year of exposure to Marvel Mystery oil dripping down the valve stem - it fell off in huge chunks upon disassembly.

I got my valve stem seals renewed - no more burning oil at startup, but, I can't imagine how the old crusty oil wouldn't restrict air flow, and reduce vacuum. I tightened up my valve lash and got better vacuum - if nothing else, to account for wear between the valve and the arm that is about a .003" step that prevents accurate lash settings.

Are your plugs clean? Do you have compression numbers?

 

[Curt]

@Curt would a lower than 20 number be acceptable at a mile high? 5300 ft. Or should the vacuum pressure he higher than 20 at a mile high. My 2F that runs excellent and was rebuilt in 2013 typically runs at 16-17 at idle 650rpm at 5300ft. Thanks for your input, Tom

The higher the altitude the lower the Hg. Your numbers are fine, at 3600ft i get about 18Hg. You can compensate a bit with more advance. If it runs good leave it and drive.

 

[middlecalf]

Yes. Advance your timing while watching the vacuum. Many people (like me) set timing by vacuum more than a light. Advance timing until best vacuum then back off a little, more if you start pinging under acceleration.

Regardless of how advance you are, and not pinging on acceleration? I’ve gone to 30-deg advance with vacuum still increasing and no pinging on acceleration.

 

[Pighead]

Regardless of how advance you are, and not pinging on acceleration? I’ve gone to 30-deg advance with vacuum still increasing and no pinging on acceleration.

Yes. Good. Drive it around for a while 30* BTDC and see what you think. Maybe, just for fun, see where your vac peaks with more advance, at a slow idle.