Easy questions... vacuum lines & such Merc 5.7

[BuzzM]

While replacing the distributor cap on my 83 5.7 GM small block Merc model 260( 6817160 ); I noticed a 90 degree downward fitting that looks like a relief valve or vacuum fitting attached to the lower part of the cap. There has never been a hose attached on it since I've owned this boat. I also noticed an old vacuum line attached to a port on the front of the 4 barrel Rochester carb. It just lays off to the side, venting near the fuel /water seperator. Should either of these actually be attached to anything? Are they related?Old vacuum advance ditributors came to my mind, but I don't know.
I am finally past the major maintenance items on this engine and have time to really look at it. Thanks Jim

 

[bjcsc]

Re: Easy questions... vacuum lines & such Merc 5.7

I'm thinking that you're noticing the vent for the cap. The vent is one of the differences between a marine and auto cap. The vacuum line from the carb. is interesting. I didn't know marine carbs had vacuum ports...

 

[BuzzM]

Re: Easy questions... vacuum lines & such Merc 5.7

Thx for the quick assist. The cap vent explanation makes sense and I guess there is no reason to attach a hose to it. The carb line is attached to a small neck
center of the bottom on the front facing. It is just left of the idle mixture screw. The line just dangles down off to the side about 3 feet. Curious thing.

 

[bjcsc]

Re: Easy questions... vacuum lines & such Merc 5.7

Yeah, I doubt it is a vacuum line if it's open. But have no fear, one of the gurus will be along shortly and all will be clear...

 

[Bondo]

Re: Easy questions... vacuum lines & such Merc 5.7

As noted,.... The Elbow in the Cap is a Spark Suppression Vent,............
Sparks Can't turn corners.....
Simple,+ Effective......

I'd Guess the the Other Line you found, Should go to the Overflow/ Vent in the Fuel Pump.......
If there's a Failure in the Fuel Pump,.... That line is supposed to route the Spilled Gas to the Intake,........
Rather than the Bilge...........
It's Not a Vacuum Line,..... Boats Don't have Vacuum Lines......

 

[BuzzM]

Re: Easy questions... vacuum lines & such Merc 5.7

Thanks. Does the learning and $$$ ever stop with the
joy of boating?
Be safe.
thx again

 

[Don S]

Re: Easy questions... vacuum lines & such Merc 5.7

Hold on there, boats do have vacuum lines, and on your engine you should have a vacuum dashpot on the starboard side of the carb with a vacuum hose going to the carb. There is also a clear overflow hose from the fuel pump that goes into the carb on a fitting JUST below the top of the carb and the center section joint.
The vacuum line is a fitting about mid way or a little below the center line of the center housing.
Got a picture of the stray hose to nowhere?

 

[Bondo]

Re: Easy questions... vacuum lines & such Merc 5.7

on your engine you should have a vacuum dashpot on the starboard side of the carb with a vacuum hose going to the carb.

OK,....... I'm ready to Learn something..............

Got a Picture Don,..??..??........

 

[FreeBeeTony]

Re: Easy questions... vacuum lines & such Merc 5.7

Is the vacum line for the choke dashpot?

 

[Don S]

Re: Easy questions... vacuum lines & such Merc 5.7

Couldn't get the picture that showed both to scan so you could see it, it's a VERY dark photo, but I did copy from a newer manual with line drawings, unfortunatly it doesn't show both fittings at the same time. So here are a couple of pics to show the fittings.
Here shows the vacuum hose for the vacuum break for the choke, and it does show a plug where the sight tube fitting goes, but not the fitting. The second picture shows the sight tube fitting and hose.

 

[rodbolt]

Re: Easy questions... vacuum lines & such Merc 5.7

other than the fuel pump sight tube in the above picture the only other vacum line on a quadra jet is the 2" line for the secondary air valve pull off.
goes from just below the sight tube to the airvalve diaphram.
boats typically operate above an RPM range where there is much vacum, thats why they typically dont use vacum advance distributors and such.