Mercarb 2BBL vacuum hose?

[komodo_goods]

Hello,
Thank you for all the help thus far in getting my boat correct.
I have the carb off to clean it, and this vacuum line was capped. Should it be, and if not, where should this be connected? It was capped on my 5.7

 

[komodo_goods]

 

[Mohawkmtrs]

Normally connected to the fuel pump with a see-through hose (NOT the fuel line).
This hose allows the skipper to view the condition of the fuel pump diaphram. If you see fuel in the hose, you have a ruptured diaphram.

 

[flashback]

I've never seen the fuel pump rupture line connected to a vacuum port. it usually just dumps into the air horn. you probably just want to leave it capped..

 

[1960 Starflite]

May be a replacement carb. Some boats may have vacuum gauges.

 

[Bondo]

Ayuh,...... Ditto, flashback,....

 

[dubs283]

Definitely not a fuel pump diaphragm rupture hose connection. It is a vacuum port most likely tied into a pcv valve in one of the rocker arm covers

For years and years mercruiser just vented the covers on gm based engines to the flame arrestor but in the late 90's/early 2000's decided to add a pcv valve and vacuum line to the carb base. WAG would be for emissions related/necessary for epa standards. Another component they added was an egr line from one exhaust manifold to the intake for epa compliance

 

[komodo_goods]

OK, thanks, fellas. It looks like I will keep it capped. FYSA, I already have the valve cover hoses running to the flame arrestor.

 

[flashback]

Be sure you route the hose from the fuel pump to the carb. Don't want raw fuel in the bilge if the pump fails.

 

[komodo_goods]

Be sure you route the hose from the fuel pump to the carb. Don't want raw fuel in the bilge if the pump fails.

Currently there is only the feed hose from the pump to the carb. The overflow fitting has a set screw in the carb. Is there another hose fitting you are talking about?

 

[Bondo]

Currently there is only the feed hose from the pump to the carb. The overflow fitting has a set screw in the carb. Is there another hose fitting you are talking about?

Ayuh,... Do you have a mechanical, or electric fuel pump,..??

 

[Scott06]

Only mechanical fuel pumps use the diaphragm rupture line. If you have a mid 90 s and up motor with electric fuel pump you won’t have one

 

[tank1949]

I've never seen the fuel pump rupture line connected to a vacuum port. it usually just dumps into the air horn. you probably just want to leave it capped..

Might be for neatness.

 

[komodo_goods]

Ayuh,... Do you have a mechanical, or electric fuel pump,..??

I have a mechanical

 

[Bondo]

I have a mechanical

Ayuh,..... Then there should be a tygon tube from the pump's vent, to the top of the carb, or the flame arrester, to dump gas from a ruptured diaphragm, into the throat of the carb, not the vacuum port at the base,.....

 

[Scott06]

I have a mechanical

Is this a pipe plug (circled) in your photo? that I believe is where I had one on my Mercarb. Mine had a 90 degree elbow threaded on one end and hose nipple on other. like this fitting in amazon link -

Amazon.com

I assume this is off a different engine originally?

 

[komodo_goods]

Is this a pipe plug (circled) in your photo? that I believe is where I had one on my Mercarb. Mine had a 90 degree elbow threaded on one end and hose nipple on other. like this fitting in amazon link -

Amazon.com

I assume this is off a different engine originally?

Scott06: that is a set screw that is in place and not installed by me. this is a different engine from original. Based on the casting number of the block its a 1988-1994 SBC and the boat is a 1998. I'm also assuming that the carb is from the original 1998 5.7 which probably had an electric fuel pump and the reason for the setscrew in place of the diaphragm nipple.

 

[komodo_goods]

Ayuh,..... Then there should be a tygon tube from the pump's vent, to the top of the carb, or the flame arrester, to dump gas from a ruptured diaphragm, into the throat of the carb, not the vacuum port at the base,.....

ok gotcha. So what then is the lower nipple that is capped on my carb for? Would it have been used on a 1998 or newer 5.7 but not an older 5.7 (1988-1994) model which I now have in my boat?

 

[flashback]

The base of the carb has a standard vacuum port, it's normally not used in the marine world so it's capped. In an auto it could be used for spark advance. Or maybe the windshield wiper.

 

[komodo_goods]

The base of the carb has a standard vacuum port, it's normally not used in the marine world so it's capped. In an auto it could be used for spark advance. Or maybe the windshield wiper.

ok gotcha on the lower nipple, staying capped.