Fred Rawling Posted June 8, 2018 Share Posted June 8, 2018 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. Link to comment Share on other sites More sharing options...
Spinneyhill Posted June 8, 2018 Share Posted June 8, 2018 (edited) 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) Link to comment Share on other sites More sharing options...
Gary W Posted June 9, 2018 Share Posted June 9, 2018 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! Link to comment Share on other sites More sharing options...
sligermachine Posted June 9, 2018 Share Posted June 9, 2018 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 1 Link to comment Share on other sites More sharing options...
Morgan Wright Posted June 9, 2018 Share Posted June 9, 2018 Maybe when the choke is closed (cold) the vacuum reads higher because the choke prevents air from going in there. Link to comment Share on other sites More sharing options...
carbking Posted June 9, 2018 Share Posted June 9, 2018 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. 1 Link to comment Share on other sites More sharing options...
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