1974 4 cylinder 140 hp - distributor vacuum capped

[fishrdan]

Pretty sure that it's the original fuel pump too.

Looks like the fuel pump has been swapped out to me, not painted black, clean, and the sight tube fitting being on the pump. Looks just like the updated replacement pump I put on my 72 140HP a couple years back.

Pull the cap off the fuel pump's sight tube fitting and see how it runs (if fuel comes out you have a bad pump). Better yet, plumb it to the carb's air horn as suggested earlier so it's plumbed correctly, then see how it runs. If the pump's vent can't breathe, if may restrict output, I ran into this once, restricted high RPM operation......

Also agree with the need for A-1 hose, if it's not A-1 already. I would double clamp all fuel hose connections, clamps should be clocked 180* apart.

 

[shizny]

Thanks for all the replies... I'm still messing with this and with no luck.

I have a mechanic that looked at the boat and he was thinking that maybe my problem was the size of the nipples connected to the A-1 fuel hose coming off the pressure side of the fuel pump. So, he's replacing that A-1 stuff and also installing that line to the port that I capped up by the flame arrestor. I have a question on that though... I put my finger over the port and it was definitely "sucking". That's the correct place to hook the clear fuel line from the bottom of the fuel pump ?

The mechanic also told me that I should get a tune-up to see if that fixes the issue. So I did that, he said the timing was so far off that he didn't understand how it was even running before. So, I took it out after he was done and it drove exactly the same. Maybe a little worse.

Went back to the mechanic and he told me that my carb seemed to be adjusting oddly when he did that tune-up and maybe the carb is bonk (he rebuilt it two years ago). So, he told me to spray starter fluid in it when I'm trying to accelerate (and when the boat bogs down) and if it drives better then I've got a fuel side issue. He also said the ignition side was "spot on".

I went out and did what he said and when I sprayed the starter fluid in the carb while going at my top speed (10 miles per hour) and the engine cut out. Seemed like it had too much fuel or something. That's also the feeling I get from the boat when I gun it normally.

One more question. Why am I getting advice to not worry about hooking up my vacuum gauge to the port on top of the engine (where it was hooked). It seems like knowing the vacuum in the engine is good info to have in diagnosing engine issues .

 

[Bt Doctur]

remove the cap on the bottom of the fuel pump, that needs to be open for the diaphram to work correctly, run a secton of Ty-Gon tubing from there to the carb fitting.

 

[shizny]

ok, thank you... I'm not seeing the carb fitting though. Do you know what it should look like? I've got a brass colored bolt (it's in the first pic I posted on page one of this thread) on the side of the carb that may be covering the fitting you're talking about ?

 

[saltydogjeff]

The sucking part is a pressure thing... when velocity flows through your carburetor, it creates sucking at other places... this place is what you are talking about your mechanic hooking up on your carburetor. Incorrect.

The sucking helps pull full from the fuel pump if the diaphragm ruptures. There is no actual force there to push your gas, so the sucking pulls it from the diaphragm somewhere. Into the carb where it can burn instead of leak-by into your bilge.

So yeah, the vacuum is an "important" gauge, but it only tells you how much you are sucking. If your engine is running, you are obviously sucking because you have velocity going through the carburetor... therefore... it is not that important.

What does become important is making sure your bilge doesn't blow up.

The fitting outside of the carburetor is a 90 degree elbow (threaded on one end and tight fit on the other to tygon tubing). The fuel pump quote "I think has the same thing".... but in reality, it should be run to the carburetor to relieve the back pressure due to the vacuum in the carburetor to allow maximum pressure differential across the diaphragm to properly pump your gasoline to the carb.

The carburetor is more or less level controlled, not vacuum controlled. Level gets low, allow flow, level gets high, stop flow.

 

[shizny]

Thanks Jeffyo. I'm not seeing any 90 degree elbow coming off the carb. Could the place where is should be be where that brass screw is in pic 1 ?

 

[Alumarine]

That brass "screw or bolt" at the top of the carb is a plug and that's where the fuel pump rupture tube should go. You'll need a fitting for the tube.

 

[shizny]

thanks MarcC... Jeffyo, I told my mechanic what you said about the sight tube not only being a safety device but also contributing to how the fuel is delivered because of the pressure differential across the diaphragm in the fuel pump. He said that that wasn't how it worked, but I've read what you said other places as well and then somebody always chimes in and says what my mechanic says. I checked the mercruiser service manual and didn't get any info there. Are you for sure that the fuel filter needs that hookup for the diaphram to work correct.... can you somehow point me to some info on that? Just for clarity, the sight line does go into the side of the carb body so I would think there is a vacuum there and therefore what you said makes sense to me too. Also, would it make sense that the boat seems to bog down under load and heavy acceleration and can't get up past 2200 rpm because of this disconnection? It seems to be running rich. Thanks in advance.

 

[shizny]

Anyone got any thoughts on this last question I posted ?

 

[Rick Stephens]

I have seen those sight tubes hooked up by design to a port too high on the carburetor to get much if any vacuum - does it matter whatsoever whether it aids in diaphragm efficiency - I'd say who cares. It has to be hooked up for your boat to be safe to operate.

If you want to know if your fuel pump is up to the task of supplying fuel at higher RPMs, stick a fuel pressure gauge on the line and check it.

 

[shizny]

Thanks Rick. I'll get it hooked up and do the fuel pressure test

 

[Alumarine]

My sight tube goes in about 3/4" below the carb opening.
Above the butterflies but below the choke.
Not much vacuum there.

 

[thumpar]

I don't think it is about vacuum. The sight tube is for safety first but also it is a vent for the diaphragm. It is moving up and down. It needs a place to breath.

 

[77GlastronMT]

This is somewhat related, what kind of tubing do we use for that "sight tube"? I went to the auto parts store and they didn't have any translucent fuel rated hose.

 

[thumpar]

You need tygon tubing.

 

[77GlastronMT]

You need tygon tubing.

Thanks!!

 

[Alumarine]

Personally if I couldn't find tygon, as long as it was clear hose I would use it. It might yellow over time.
It should only have fuel in it once, and then you will repair or replace the fuel pump.