Engine Vacuum Across RPM range?

[ajkolar]

In trying to diagnose a recent issue I hooked up a vacuum guage to a port at the top of the intake manifold. I had approximately 18 hg at just above idle. As I increased throttle and got onto plane ~3000 rpm the vacuum had droped to ~ 9 hg, and as I approach WOT (4600 rpm) I also approach 0 hg (no vacuum). Is this normal? What kind of vacuum reading is typical at WOT. I have never read a vacuum gauge on an engine in use....

BTW, 1988 5.7 litre, Q-jet, Cobra drive.

The engine has always missed or hiccuped a little if I press the thottle all the way down, but I will run fine if I back it off less than a millimeter (and I still reach ~4600 rpm). Just wandering if the hiccup I am getting is related to hitting 0 hg (no vacuum). Really hasn't bothered me as I hardly ever operate the boat at WOT, but I would like to get to the bottom of it none the less....

 

[Maclin]

Re: Engine Vacuum Across RPM range?

That is completely normal. Vacuum is usually 12-16 at idle and 0 at WOT. If you zing the throttle and watch the gauge it will go from 12 down to zero then as the throttle is closed and the engine winds down it will jump to 25 then work it's way back down to 12-16. This is just the nature of most naturally aspirated internal combustion 4 cycle engines.

 

[Coors]

Re: Engine Vacuum Across RPM range?

Throttle plates may be over-center.
Not running, push throttle all the way forward, look down the carb, to see.
If so, make a stop, or remember the limit.
If you mean hiccup pushing down the throttle fast, that might be acellerator pump.

 

[ajkolar]

Re: Engine Vacuum Across RPM range?

You will have to forgive me, as carbs are probably what I know least about. What exactly am I looking for when looking down the carb with the throttle wide open? Looking down the primary or secondary positions? Thanks again!

 

[Coors]

Re: Engine Vacuum Across RPM range?

Both should not be over 90 degrees from horizontal.

 

[Coors]

Re: Engine Vacuum Across RPM range?

An addy; The primary is at 90 degrees, the secondaries can manually be pushed more than 90; there is an external linkage adjustment. Google

 

[ajkolar]

Re: Engine Vacuum Across RPM range?

Great, ill check it out this weekend. Thanks for the info!

 

[hullofalottatrouble]

Re: Engine Vacuum Across RPM range?

your 18 vacuum number is where a normal motor should be..

vacuum should be read at recommended idle RPM and after engine is warmed up.. is it 18 solid or 18 and swinging all over.. best to get a multimeter with RPM reading and use that to ensure your really at the recommended idle RPM.. dash gauges can less precise..

helpful link for reading the gauge..

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

 

[Bondo]

Re: Engine Vacuum Across RPM range?

Is this normal? What kind of vacuum reading is typical at WOT.

Ayuh,.......

When the Throttle Plates are Wide Open,.... There's nothing to draw a Vacuum Against,....
So a Gas motor will read 0....

That's Why you can't get a Vacuum reading on a Diesel motor,.....
There are No Throttle Plates.....