Missing at half throttle

[Uinthas]

I have a 87 85hp Force outboard. If I'm at full throttle the engine runs fine, but if I drop it down around half throttle the motor starts missing & is slugish. Here's the kicker if I'm at half throttle & I pump it up to full throtle, it bogs down more & sometimes almost dies and sometimes after a minute or so it will go up to full throtle again. But if I'm at half throttle jerk it down to almost N & then jerk it right back to full throttle it's fine again. I have always thought this was just a fuel delivery problem, so replaced the fule line from the tank to the bulb & replaced the bulb, but it still does it. Now I'm thinking I have either a bad coil or sparks are jumping from the plug wire. I have also changed the spark plugs with no luck, any help would be appreciated

 

[rbruce63]

Re: Missing at half throttle

It sounds like a carburetor is do to be rebuilt unless the fuel is stale and the oil ratio is other than 50:1.

 

[Uinthas]

Re: Missing at half throttle

Carb maybe?? I read another thread that said if you ignition is week then replacing the flat plugs with normal plugs could solve the problem because it doesn't take as much power to run the plug, is this true? if so what plug# would I use? <br /><br />Any other ideas??

 

[eurolarva]

Re: Missing at half throttle

Most problems are fuel related. Cheapest way to start is replace the fuel lines, rebuild fuelpump and carbs. These can be done in an afternoon and will cost about 15 bucks per carb and 9 bucks for fuel pump. If the diaphram in your fuel pump has pin holes or a tear you could be experiencing the troubles you are having. You should stay with the plugs the manufacturer recomends.

 

[Oldsaltydog]

Re: Missing at half throttle

You may want to test your whole fuel system FIRST. Squize the primer bulb until it is hard. If it loses its hardness, you have a leak beyond it someplace. Next take the hose off BEFORE the fuel pump, place it into a jar, and pump the bulb. If there is a good flow then reconnect. Weak flow may indicate a plugged gas tank air intake or a plugged fuel intake screen in the gas tank. Next disconnect the the fuel line AFTER the fuel pump, place a jar under it, and turn over the motor, with the ignition kill switch off. If you have a weak flow, it's your fuel pump. Rebuilding it is not really difficult and the cost of a kit is reasonable. Also test (blow through) or replace your fuel filter. If everything is fine up to here, your carb comes next, AND LAST -- adjusting, cleaning, rebuilding, etc.

 

[Uinthas]

Re: Missing at half throttle

When you refer to fuel pump, I assume your refering to the bulb that I pump to prime the gas correct?? If so I have replaced that, and the gas line from the tank to the bulb, I dont think I have a fuel pump, maybe I'm wrong. Also where would I find the gas tank vent?? I appreciate everyone's help, this should get me going in the right direction

 

[Oldsaltydog]

Re: Missing at half throttle

The fuel pump is usually located right on the motor, between the primer bulb and the carburator(s). It is sometimes round, sometimes square and flat. You can find the air vent by looking at the built-in gas tank. There should be attached a thick rubber tube -- the filler hose. And there should be a thinner hose as well -- at the end of it should by your vent opening. (In my case, the two lines meet at one location, because I have a combined filler opening and air vent.)

 

[Uinthas]

Re: Missing at half throttle

okay I'm going to run the test on the fuel pump.. Where might I find one of these rebuild kits for the pump should I need one??

 

[eurolarva]

Re: Missing at half throttle

Looks like you have a standard two stage pump. Napa sells the kit for 9 bucks. NAPA part number 18-7806. Do not mess with the press fit fuel pump valves. If you arent careful it is easy to break one of them trying to get it out. Just replace the diaphram, main gasket, fuel strainer gasket and the gasket for the fuel pump valve that is held in by two screws. The whole operation takes about 1 hour. Even though the diaphram may not have holes or tears in it it may be stretched to a point where it no longer works correctly.

 

[Uinthas]

Re: Missing at half throttle

I opened up & looked at the fuel pump.. I dont see any damage but I have no idea what to look for or how it works, any insight on that?? Anyway I ordered up the napa re-build kit, does it inculde all 3 fuel pump valves?? Took the boat out again today with some new plugs & it did the same thing, sure sounds & feels like is just runs out of fuel, I also wiggled around the plug wires while is was bogging down & it didn't do anything different or shock me .. I also pumped fuel through the fuel line after the bulb & seamed to have a good flow, so now I'm hoping it just needs the rebuild kit for the fuel pump.. Should I also replace the fuel lines after the fuel pump? Thanks for the help

 

[eurolarva]

Re: Missing at half throttle

yes replace the fuel lines. The rebuild kit does not include the valves. You can replace the gasket on the middle valve but I would not mess with the press fit ones. My experience is the gaskets in the rebuild kit do not fit on the seats anyway. I had to order them from a merc dealer and also a new valve because I busted one trying to get it out. The one held in by two screws is easy. The diaphram can be distorted and stretched out. For nine bucks you cant go wrong replacing it. <br /><br />When you rebuilt the carb did it have the needle with the rubber tip? When holding the carb upside down the float measurement should be 1/2 inch instead of 13/32 inch. Also when motor stutters try loosening the gas cap on gas can and see if that helps. Your gas tank may not be venting well and make sure you have the vent screw open.

 

[Uinthas]

Re: Missing at half throttle

I have not rebuilt the carb yet, hoping it wont come to that.. eurolarva did you have the same sort of problem before you rebuilt the fuel pump?? I will also try opening the gas cap next time I'm out on the water..

 

[eurolarva]

Re: Missing at half throttle

It was my very first post. Had trouble getting it to idle. Sometimes it would idle fine then kill going into forward gear, or driving along and motor would loose power and die. Recommendation to me was to rebuild carb, fuel pump and fuel lines and get rid of old gas. I did these things and it was amazing. Just like having a brand new motor. Then I had to keep it that way by using stabil in my gas, making sure I shut off vent screw during periods that I was not using boat, and draining gas in winter and fogging with fogging oil. These things are a pain but I finally trust my motor and the cost to do these things are minimal

 

[Uinthas]

Re: Missing at half throttle

Turns out my problem apears to be the #2 coil. I changed all the fuel lines & re-built fuel pump & the problem persist. I checked the spark on each cylinder & #2 was not getting any spark. So I swaped #1 & #2 coils & stil no spark from the #2 coil so guess it's time to replace it.. <br /><br />I appreciate all your replys