Can vacuum leak increase idle speed?

[rpuchowicz]

I was using a vacuum gauge to aid in adjusting the idle mixture screw on a 1987 MCM 200 today.

When I momentary popped the vacuum gauge off and introduced unmetered air into the engine the rpms rose sharply.

Is this normal or is it sympthomtic of some other problem.

The engine runs fine except that his has low vacuum at idol (wanders between 13-15 mm/hg).

 

[a70eliminator]

Re: Can vacuum leak increase idle speed?

I would say yes that is normal for the rpm to increase with the vacuum line disconnected. The 13hg is on the low side. idle speed and timing will effect the readings, the wandering could be many things from worn dist. shaft (points ign) to overly rich mixture and fouled plugs.

 

[gostephen]

Re: Can vacuum leak increase idle speed?

yes this is normal especialy if ur running a little rich what altitude are you at vacuum drops about 1 for every 1500ft if i remember right, also check your plugs to make shure there all fireing then maby check ur timing

 

[Zackman]

Re: Can vacuum leak increase idle speed?

Normal, even at sea level... or slightly below in my case

 

[rpuchowicz]

Re: Can vacuum leak increase idle speed?

It?s good to here that this is normal.

I am sea level here in CT. I have been trying to correct this low wandering vacuum problem since the end of last season.

The engine has good compression; plugs, wires, rotor, cap, and coil, all new; fuel pump - new; timing set at 8 degrees; carb - rebuilt once and rebuild again (to make sure I did not mess up the first rebuild); lower intake manifold gasket - replaced; and no detectable vacuum leaks.

The boat seams to run fine otherwise. Starts up within a couple of cranks.

I did notice that when I bump up the timing a bit the vacuum increases.

I am going to keep wrenching on it tomorrow but I am running out of things to try.