runs great, then stumbles and stalls, then gets worse through the day

[joenapier]

I've been wrestling with a fuel issue for almost a year now.

She will run WOT for about 10 mintues, then stumbles and stalls. Cranks right up again and we are underway. Now it runs for 9 minutes and does it again. Cranks up again...by the end of the day, it will only creep along at 2000 RPM and even then it will finally starve out for gas.

Its a 1985 Merc 260. 4 bbl Rochester carb.

Temp is good at 150-170 with a new thermostat (engine has never overheated, I just change t-stat cause it looked like crud)

I have not found the problem despite doing the following:

Replaced the fuel pump - mechanical pump
Replaced entire fuel filter / water separator unit
Replaced fuel line from tank to fuel filter
Replaced vent line from tank to vent in side of hull
Rebuilt the carb

???

Any other ideas? I am pretty sure I had this problem before I rebuilt the carb, but not sure (i'm easily confused anyway). Could the float have something to do with this? I set it pretty carefully, maybe I should set it extra high and see what happens?

 

[Pete104]

Re: runs great, then stumbles and stalls, then gets worse through the day

Have you looked at pick-up in the tank? You probably have an aluminum fuel tank. Remove both the elbow & fuel line fitting (barbed). The barbed fitting should be an anti-siphon valve. They do fail but not usually the way you are describing. At the male part of the elbow, there should be a screen. If it's there, it's probably plugged up with gunk. I would take it out all together. Another thing that happens, debris of some sort, gets pulled to the pick-up & the suction holds it over the tube, blocking the fuel supply.
It's easiest just run the boat on a 6 gallon tank just to isolate a "tank" problem!

 

[joenapier]

Re: runs great, then stumbles and stalls, then gets worse through the day

Have you looked at pick-up in the tank? You probably have an aluminum fuel tank. Remove both the elbow & fuel line fitting (barbed). The barbed fitting should be an anti-siphon valve. They do fail but not usually the way you are describing. At the male part of the elbow, there should be a screen. If it's there, it's probably plugged up with gunk. I would take it out all together. Another thing that happens, debris of some sort, gets pulled to the pick-up & the suction holds it over the tube, blocking the fuel supply.
It's easiest just run the boat on a 6 gallon tank just to isolate a "tank" problem!

Thanks, Pete. Aluminum tank it is. I almost change the siphon valve, but the symptoms don't match, as you say. Trash in the tank is the next place to go. Would the junk make it to my fuel filter / water separator or be caught by some screen on the pickup tube?

A buddy with a small outboard is gonna be missing a plastic tank this afternoon for a few hours.

Thanks - I'll let you know how it goes.

 

[Don S]

Re: runs great, then stumbles and stalls, then gets worse through the day

Here are some tests to run to determine where your problem may be.
http://forums.iboats.com/showthread.php?t=167561

 

[joenapier]

Re: runs great, then stumbles and stalls, then gets worse through the day

OK - that was the problem. I ran it this afternoon until the stumbling happened. Then I switched over to a portable tank it it ran without a problem. When I went back to the tank, it started acting up again. it;s that old tank.

Thanks Pete and Don for your help. What a relief to find the issue - not I will see what I can do to rinse the crud out of that tank.

 

[joenapier]

Re: runs great, then stumbles and stalls, then gets worse through the day

Yep - sure enough I got the pick-up tube out (had to remove the entire gas tank) and there was some stuff that looked like a pinch of Skoal around that little screen. It must have been a little leaf or something that got into the tank.

I removed the screen altogether and put an in-line fuel filter near the fuel / water separator - much easier to monitor and change out if it gets clogged again.

 

[Fishermark]

Re: runs great, then stumbles and stalls, then gets worse through the day

I removed the screen altogether and put an in-line fuel filter near the fuel / water separator - much easier to monitor and change out if it gets clogged again.

Just so you know - the inline filter goes against the Coast Guard regs. It seems better to clean the tank and then you don't have to worry about it again in the near future. Just a thought. Especially since you had to remove the entire tank. Very easy to clean with the tank out of the boat. Just sayin'....