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Author Topic:   air in fuel line
scottfarm posted 06-24-2002 09:08 PM ET (US)   Profile for scottfarm   Send Email to scottfarm  
On 1995 75hp 2 stroke merc--getting air in fuel line. Replaced line and bulb and female fuel connecter. Took out fuel pickup tube, inspected and cleaned--teflon tape on all fittings and stainless clamps on all connections. While trolling or idling for about 10 minutes motor starts running rough then quits. Pump fuel bulb up and it runs fine for another 10 minutes. Runs fine at 1/3 to wide open throttle. Will surge sometimes below 1/3 throttle--pump bulb then runs fine. What to look for next.
jameso posted 06-25-2002 12:12 AM ET (US)     Profile for jameso  Send Email to jameso     
This is a puzzler, have you had the fuel filter changed recently? I would suggest an old trick we used on use to find vacum leaks on hot rods, run the engine and spray some type of spray lubricant or carb cleaner on all the fittings and mating surfaces of fuel components. Just be careful and don't spray these on anything that might be a source of ignition.
Let us know,
Jim Armstrong
Tom W Clark posted 06-25-2002 12:17 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

You don't say what boat you have. What is the fuel tank situation?

This sounds like a fuel restriction issue, not an air leak.

If you have a boat like a Montauk with on-deck tanks, then make sure the vent is open. If your boat has a built in tank then I would investigate the check valve. Several threads in the past have discussed this.

scottfarm posted 06-25-2002 06:10 AM ET (US)     Profile for scottfarm  Send Email to scottfarm     
The boat is a 96 17' standard converted to a montauk. It has a 31 gallon aluminum fuel tank. Vent is open and unobstructed. It does have a ball type check valve that seems to be working properly. Blew it out with compressed air.
flwhaler posted 06-27-2002 10:06 PM ET (US)     Profile for flwhaler  Send Email to flwhaler     
What kind of primer bulb are you using? After market bulbs are junk. Get rid of it if it is. Use OMC or Merc.


DrAlan posted 06-28-2002 06:18 PM ET (US)     Profile for DrAlan  Send Email to DrAlan     
2 200HP Merc OB's.
Just went through the EXACT same problem. Found it to be the primer balls. Had installed new ones, ended up they were defective, the whole lot at the store also. They were allowing the fuel to run BACK under the slightest fuel line pressure. Sometimes the "ball" responsibile for blocking back flow would stick, so when I went to prime the fuel lines again, I would hear a small pop and feel the pressure release... Invest in better balls. I wenrt to Quicksilver. Could tell the diff in quality right away.
Clark Roberts posted 06-29-2002 08:59 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Scott, check the fuel lines from pump to carbs... also maybe a faulty pump (diaphram).. just some thoughts! Love your 17 set-up with the high seats! Happy Whalin'... Clark... Spruce Creek/Lake Seminole Navy
scottfarm posted 06-30-2002 09:38 PM ET (US)     Profile for scottfarm  Send Email to scottfarm     
Ok, I found the problem. There's a check valve in the aluminum fitting where fuel hose attaches to tank. It has a spring and ball bearing inside this fitting. I drilled it out enough to let the spring and ball fall out, reinstalled it and now it runs great. Thanks for all the input. Clark, got the stickers installed. My boat looks like the marine patrol now.
Steve Leone posted 07-01-2002 12:39 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
that thing you drilled out was the anti-siphon valve. it has a safety purpose. in case of a fuel leak or fuel hose rupture the gas is unable to just flow out of the tank. here in california you cannot recieve a coast guard sticker if it is removed from a stationary tank. i suggest you replace it with a new one. make sure you get the right diameter. steve out.
kamml posted 07-06-2002 08:01 PM ET (US)     Profile for kamml  Send Email to kamml     
I had similar problems this last week on vacation. My motor would lose RMs and the looking into the auxilary filter I could see air bubbles coming through and the level of fuel dropping. I pulled the filter apart and found it clogged with rusty looking crud that was non-metallic and floated in the gas. It wouldn't pass through the filter and was clogging it shut. Cleaned the filter, end of the problem, nope. Ten minutes or so later same problem, more rusty crud, cleaned the filter once more. End of the problem for that trip. Since I have a plastic Pate tank it was obviously coming from the gas station tank. Didn't use that station the rest of the week. Running it back yesterday for the last time it clogged again, more rusty looking crud, just not as much, I stopped before the engine stalled this time. Change your existing filter and add an auxilary filter ahead of the motor filter in a place where you can see what is happening while the motor is running. It will save you time and headaches, especially at sea. Ken
Clark Roberts posted 07-07-2002 07:42 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Dennis, just a thought... your fuel pump may be getting a bit weak and not able to overcome the spring pressure of the anti-siphon valve and may soon need rebuilding/replacing! If you experience more fuel starvation symptons this may be it! Not good for the engine to starve out as it is running lean and this can hole a piston in a heartbeat! Happy Whalin'... Clark... Spruce Creek Navy
scottfarm posted 07-14-2002 09:14 PM ET (US)     Profile for scottfarm  Send Email to scottfarm     
It was a pinhole leak in fuel line between male plug and fuel pump. When I squeezed bulb I could hear gas coming out of the pinhole leak.

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