Diagnosing the low vacuum..

[Blackhauk90]

Alright so as the title reads, my boat engine is running with a really low vacuum. It's a mercruiser mag 350 with a rochester carb, and est ignition. I assumed... that it was just an intake leak causing the low vacuum, causing my engine not to idle without feathering with the throttle.

I changed my intake gasket along with my carb gasket.. Still the problem persisted. Then, i rebuild the carburetor cleaned it all up real nice, still no change with how it idled. One thing that made me wonder is i get an occasional puff of smoke out the carb right before it dies when trying to idle. The timing is set right where it's suppose to 10 degrees.. Is there some sort of explanation of this odd running condition?

 

[180shabah]

Re: Diagnosing the low vacuum..

It is low, but is the needle steady, does it wander up and down, flutter rapidly, does it have an occasional "tick" upwards.........

 

[Blackhauk90]

Re: Diagnosing the low vacuum..

Sorry i forgot to note that. It wanders up and down between about 5-10 in-hg

 

[180shabah]

Re: Diagnosing the low vacuum..

Sounds like multiple issues. Need to eliminate a few things and see if we can get a better baseline.

Is the timing set properly?
Idle mixture adjusted properly?
Any intake leaks?

See if we can get rid of some of the free stuff, before we worry about valves, valve guides etc.

Do you know the history of this engine?
Have you also done any compression or leakdown testing on it?

 

[jtybt]

Re: Diagnosing the low vacuum..

Exactly how low are your readings? You sure you're taking vacuum from the correct port?

Best place is the one between the carb and dist on the intake manifold.


This is the best place to learn readings...and there are 2 pages.


http://www.users.bigpond.com/ergoff/vac1.htm

 

[Bifflefan]

Re: Diagnosing the low vacuum..

Id just like to add,
Make sure the balancer has not slipped causeing the timing to read correct but its not.
Verify TDC and check the pointer/marker to be on the 0 mark.

 

[Blackhauk90]

Re: Diagnosing the low vacuum..

Well, i tried taking the vacuum from a different place, turns out i get different results. While attempting to idle it still reads fairly low, (5-10) but once i rev it up and let the rpms start to fall, the needle drops to 0 then comes back to 5-10..

 

[jtybt]

Re: Diagnosing the low vacuum..

Unless you're taking readings from the correct vacuum port, your vacuum readings don't mean anything.

Did you read the link I put up?