my vacuum gauge reads 17 when cold but drops to 8 it warms up

[Fred Rawling]

I know that a steady reading of 8 indicates later valve timing.  What I do not understand is why it changes when the engine warms up.

 

I would like some more input before I start taking it apart.

[Spinneyhill]

Here is something that might help.Taken from http://www.gregsengine.com/gregs-engine-technical-articles.html

VacuumGaugeChart.pdf

Edited June 8, 2018 by Spinneyhill (see edit history)

[Gary W]

Hi Fred;

My '37 Special  (248) holds a steady 20 from start up and when warmed up.  If I goose it, the needle momentarily drops to about 5, then rapidly recovers back to the 20 mark.  Check your timing first.  Then make sure your manifolds or carburetor is not leaking any air.  I remember my friend John squirted some ATF along the manifold to check for leaks.  Good luck, I hope it is something easy!

 

[sligermachine]

I have always used carb-cleaner to spray around my manifold to look for leaks .A small jump in idle shows up if leaking. --kyle  

[Morgan Wright]

Maybe when the choke is closed (cold) the vacuum reads higher because the choke prevents air from going in there.

[carbking]

When the engine is cold, the choke should activate the fast idle circuit on the carburetor, causing a faster than normal idle. The higher RPM cold may be masking issues with rings or valves.

 

If the hot and cold idle RPM's are quite different, I believe a compression test is in order.

 

If the hot and cold idle RPM's are the same, I have no suggestions.

 

Jon.