Fuel delivery issue

paul ages · Sep 16, 2010

1968 Mercruiser 120, seems to be in stock form. Since I bought it it's had fuel starvation issues, and I finally tracked it down to a vapor lock in the fuel line. I thought it was happening at the big fuel filter/ water seperator, so I bypassed it altogether. It's better, but still happening. After about 5 minutes at WOT, the engine starts sputtering. If I disconnect the line at the barb junction I used to bypass the filter, you can hear the hiss of the vacuum in the line as air rushes in.

The tank has a large (1 1/4" or so) vent on it, at the overflow. Thoughts on why this could be happening?

Bondo · Sep 16, 2010

Re: Fuel line vapor lock

Re: Fuel line vapor lock

Ayuh,... There's a restriction, probably in the in-tank plumbing...

Btw,... It's Not vapor-lock,.... It's Restricted...

paul ages · Sep 16, 2010

Re: Fuel line vapor lock

Re: Fuel line vapor lock

Bond-o said:
Ayuh,... There's a restriction, probably in the in-tank plumbing...

Btw,... It's Not vapor-lock,.... It's Restricted...

Okay, i'll check for restrictions. I don't see how venting the vacuum from the line in the middle would affect a restriction in the tank or line before it though..

Bondo · Sep 16, 2010

Re: Fuel line vapor lock

Re: Fuel line vapor lock

Ayuh,... If the fuelpump is suckin' against a restriction, it's gonna Hiss when released...
It's pullin' a vacuum that can't be satisfied due to the restriction......
Simple physics..

paul ages · Sep 16, 2010

Re: Fuel line vapor lock

Re: Fuel line vapor lock

Bond-o said:
Ayuh,... If the fuelpump is suckin' against a restriction, it's gonna Hiss when released...
It's pullin' a vacuum that can't be satisfied due to the restriction......
Simple physics..

Yeah, that makes sense to me... just not why the fuel flows again after I release the vacuum.

I'm a motorcycle mechanic. It's very common that fuel tank vents become clogged, especially after people line the tanks, thinking they are protecting them. After a bit of running a vacuum is created in the tank from lack of venting, and the fuel can't flow. Crack the cap, fuel flows, problem solved. This seems exactly the same, but in the middle of the line.

Fishermark · Sep 16, 2010

Re: Fuel line vapor lock

Re: Fuel line vapor lock

Run it without the fuel cap on and see if it still does it.

paul ages · Sep 16, 2010

Re: Fuel line vapor lock

Re: Fuel line vapor lock

Fishermark said:
Run it without the fuel cap on and see if it still does it.

Will do. I tried to do that last time it happened, but we had already vented it at the line.. then we ran out of gas and had to use the outboard before it starved again.

paul ages · Sep 17, 2010

Re: Fuel delivery issue

Ok, a little new info. The line is clear from the pump to the tank. The pickup tube in the tank is clear, as is the vent on the tank.

I took it out on the river last eve, and it ran fine until I kept it at WOT for more than 5 minutes. i opened the line, kept it slower, and then fished for a bit. On the return trip I was at a trolling speed almost the whole way, and it was fine.

It seems like demanding a lot of gas brings the problem. Any new ideas? fuel pump diaphragm? BTW, what is the line that loops around the fuel pump? it goes from the "in" junction on the pump directly to the "out" junction as if it were a bypass, but it seems like "bypassing" the pump wouldn't really make sense...

Fishermark · Sep 17, 2010

Re: Fuel delivery issue

You're supposed to have a clear line from the pump to the carb. (And by "clear" - I mean a plasitc / vinyl / tyvek whatever line that you can see thorugh). It serves as a safety in case the diaphram ruptures. Instead of spilling fuel into the bilge, the excess fuel goes to the carb. By your description, it sounds like someone has been doing some "modifying."

Fishermark · Sep 17, 2010

Re: Fuel delivery issue

Here's a typical pump - the fitting on the left is inlet from the tank. The lower right fitting is the "safety" line, while the top right is the outlet to the carb.

paul ages · Sep 17, 2010

Re: Fuel delivery issue

Here's mine:

the left is from the tank, right to the carb. loop?

Fishermark · Sep 17, 2010

Re: Fuel delivery issue

Okay.... that I have never seen.

ziggy · Sep 17, 2010

Re: Fuel delivery issue

hope ya got service manual #1. i think it explains what's happening here maybe.

there is a section called installing a fuel line check valve. it appears to be for the 110-120-150 models. i believe that it's an ac fuel pump. the check valve is is "to eliminate excessive cranking afer fuel tank or carb. runs dry."

look at pp 4A-1 s/m #1.

looks like ya got a carter fuel pump to me. no idea if that makes a difference though.

paul ages · Sep 17, 2010

Re: Fuel delivery issue

ziggy said:
hope ya got service manual #1. i think it explains what's happening here maybe.

there is a section called installing a fuel line check valve. it appears to be for the 110-120-150 models. i believe that it's an ac fuel pump. the check valve is is "to eliminate excessive cranking afer fuel tank or carb. runs dry."

look at pp 4A-1 s/m #1.

looks like ya got a carter fuel pump to me. no idea if that makes a difference though.

I have the manual, but can't find anything about it. I'll look closer tonight.

dubs283 · Sep 17, 2010

Re: Fuel delivery issue

you need a safety wire on the top clamp bolt and i don't see a sight tube as described in an earlier post

the coil looks like its seen better days, along with the plug wires - time for a complete tune-up??

have you checked fuel pump pressures at various rpms??

paul ages · Sep 18, 2010

Re: Fuel delivery issue

dubs283 said:
you need a safety wire on the top clamp bolt and i don't see a sight tube as described in an earlier post

the coil looks like its seen better days, along with the plug wires - time for a complete tune-up??

have you checked fuel pump pressures at various rpms??

More on the safety wire? Sorry, I am a mechanic, but new to the marine world.

I did tune it up, just didn't get to a few things. Still need to adjust valve lash and replace the plug wires. I ultrasonic cleaned and rebuilt the carb, set the points gap and dwell, timing, replaced the plugs, and replaced both fuel filters. The coil ohmage tests fine.

Regarding the sight line: Is just to see if fuel is leaking past the diaphragm? The oil is not gassy at all.

Fishermark · Sep 18, 2010

Re: Fuel delivery issue

paul ages said:
Regarding the sight line: Is just to see if fuel is leaking past the diaphragm?

Seeing fuel in the line is a good diagnostic tool.... but the real reason is to keep fuel out of the bilge.

The older boats, (maybe like yours), didn't have that feature. It was added as a safety factor later. Any of the new pumps you buy will have it.

Fed · Sep 18, 2010

Re: Fuel delivery issue

what is the line that loops around the fuel pump?

I have no idea why it's there but to me it seems that you'd just be pumping fuel around in circles and it could cause your starvation problem.
What have you got?
Tank-->Filter-->Pump-->Carby
or
Tank-->Pump-->Filter-->Carby
and where did you crack it to get the hissing noise?

1 &1/4" is a huge vent are you sure you were looking at the right thing?

ziggy · Sep 18, 2010

Re: Fuel delivery issue

I have no idea why it's there but to me it seems that you'd just be pumping fuel around in circles and it could cause your starvation problem.

s/m #1 stated why it's there.

"to eliminate excessive cranking afer fuel tank or carb. runs dry."

that's a quote from the s/m. now, why the fuel tank or carb. runs dry, i don't know. but that's what it says.

to me, since it's giving your the blues, and it's obviously in place. do you really have a check valve in place? if not. perhaps that makes it work poorly..

Fed · Sep 18, 2010

Re: Fuel delivery issue

I can't see any check valve in that looping hose if that's what you're talking about Ziggy.
Does your 165 HP have a looping hose, I know mine didn't, mine was just set up like every GM car I've ever seen, fuel in from tank...fuel out to carby.
I don't have any SMs to check, running on memory here.

Fuel delivery issue

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