Fuel flow problem

J

jlohse

Member
Joined
Nov 19, 2002
RO Number
10770
Messages
6
Greetings, I bought a sport craft 205 WA with a Mercury 150 OB. The boat runs great for about a half hour then stalls. It seems be starving for fuel. I have changed the filters and the fuel lines. I had the tank pumped out and the fuel filtered. There were no signs of corrosion in the tank. I removed and cleaned the dip tube and replaced the anti-syphon valve. The vent seems clear, but runs down to the bilge and back up to the vent. If I connect a portable fuel tank there are no problems. I have run out of ideas. If anyone knows what would cause this, please reply.
Thanks Joe
 
Was the vent line replaced as one of the “Fuel Lines?” Run the power plant with the tank fill cap loose/open/off and see if the 20+ minute problem reoccurs. The power plants fuel pump maybe weak and fails after 20 minutes when sucking fuel from the built in tank BUT has sufficient suction when running with a remote tank that is physically closer to the fuel pump.
 
Congratulations on a thorough investigation and report. Clearly the issue is from wherever you hooked up the small tank backwards unless, as above, the fuel pump is weak. A simple test is to run with the fuel fill open to rule out vent issues. Have you heard any air flow when you open the tank fill after it stalls?
 
Although it is not uncommon to see this either out of routing necessity or failure of hose mounting clamps or straps, the vent line should not dip down below the top of the tank.
If it does, the farther down it dips and the longer the hose length below that level, the more fuel is trapped and the more fuel pump suction is needed to pull vent air down through the liquid trap to get back up to the tank.
Even if some fuel still gets to the engine, as the tank level goes down ,increasing negative "head"(height of fuel pump above tank fuel level) pressure also requires greater pump suction to lift the fuel farther , so the combination of the trapped vent hose fuel and lowering tank fuel level may overwhelm the pump's ability to provide sufficient fuel to overcome a lean condition or complete starvation.
That combination may also increase the odds of air being sucked in from filter gaskets or hose connections,etc..

I would do as Michael suggest above - try running without the cap if conditions allow. If that helps you know the problem is somewhere in the venting system.
 
Thanks for the replies. I had run the boat on the portable tank for about 45 minutes, and had no issues. When running on the built in tank the primer ball gets sucked flat just prior to the motor stalling, so I think the fuel pump is working properly. I am going to reroute the vent hose, if possible. I did open the fuel fill, but it was after the motor shut down, not before, so I'll try that. Thanks again, and I let you know how it works out.

Joe
 
Don't run much with that condition. When you get that kind of fuel starvation you're leaning out the motor severely and two strokes don't like that. In fact they hate it. And there is a possibility (probability?) you could learn to hate it also. If the bulb is being sucked flat, you've definitely got a restriction, probably vent, in the line. If you can, replace the vent line completely and make sure there are NO loops or dips in it. Those Merc 150s are reliable as sunrise IF they are treated right. Don't let it lean out.
 
Flat bulb nails it. Clogged vent.try blowing it out . The design may not be perfect but it has worked .
 
If your vent has a little chrome "button" on the side of the hull, sometimes the screening (called 30/30 mesh) "grows" closed with corrosion. A clean or hose connected to it won't cure the problem.
 
Spider webs. Mud dauber wasps or just plain dirt can degrade the vent fitting thru hull very fast.
 
Sometimes a few shots of PBBlaster at the vent screen will help clear up it's nasal passages. That's part of preventative maintenance on my boat.
Flat bulb just means there is a substantial restriction anywhere upstream of the bulb, but you did say you had already replaced a lot of the Usual Suspects.
 
I just looked and it isn't possible to reroute the vent hose. The boat is a 1987 so it has worked the way it is. I removed the vent hose which didn't look bad, but I am going to replace it anyway, but first I am going to run the boat with the vent hose running through the service port in the deck and into the cockpit without it going below the tank. Is there enough suction through the vent that I could feel it at the hose end? The vent is not obstructed neither is the through hull. I had blown the vent out before I changed the fuel lines, it did not seem to be obstructed. Thank you all for your replies.

Joe
 
If the vent hose is original 1987, it is not likely USCG-approved SAE J1527 alcohol-resistant hose and may have delaminated or otherwise deteriorated where the fuel sat in the dip trap. If that had happened, it's possible flow could be restricted more in one direction than the other.

Especially if you are running E10 , all your vent , fill and distribution fuel hoses should be upgraded to compliant hose for safety if that has not yet been accomplished. E10 can make a real mess out of old type hoses.
 
You must log in or register to reply here.
Back
Top