18hp FastTwin weird intermittent problem

[870 Expressmag]

ok heres a little back story on this motor, i bought it with a 15ft meyers aluminum boat in june 2012. the motor didnt run right at the time, we cleaned and rebuilt the carb, replaced upper crank seal, points and cond., fuel pump and lines. Had it running but couldn;t get the lean sneeze adjusted out, took it to a marina and they adjusted the timing and sealed up a welch plug and the motor has run like a dream ever since, including trolling on a no wake lake we go to. now this year it has started acting a little funny. the motor always starts right up for me. we can go out fishing and start the motor up and move 3-4 times, but about half way through the fishing trip, at least once trying to start the motor it will start, kick a little and pop, if i get it into gear fast enough and give it some throttle it will pick up and we can plane out and the problem goes away, usually for the rest of the trip. sometimes it takes a couple times starting it to get it to run enough to get it into gear but usually once or twice is enough. but it doesnt happen all the time, i have no problem most of the time trolling this motor around at 2.5-3mph per my lowrance. last weekend dad and i were back out on the no wake lake, and we kinda figured something out. as we were trolling around i could hear the motor rpm start to climb, like an air leak on a chainsaw or other 2stroke, etc. then start popping, if i rev it up (which your not supposed to do on this lake) i can get it to clear up, the other thing that clears it up, is my dad would pump the fuel line primer bulb and the motor would settle back down. so we are thinking the fuel pump isnt pumping enough fuel at low rpm? but i still dont understand why it acts lean. however if that were the case it shouldnt do it to me randomely on the other lakes where i dont troll at low rpm like that, and in the bigger lakes it always does it to me on startup, once i plane out and clear it out i'm good to go. dad thinks there is an airleak somewhere in the fuel line from tank to motor and the fuel pump starts sucking air once in a while. i dont see any evidence of fuel leakage inside the cabinet of the motor, and no oily sheen on the water so i dont believe i have a fuel leak. we are kinda on the right track to finding problem but dont know exactly and i dont wanna start throwing parts at it until i get some advice. thanks for any help

 

[64osby]

Have you replaced the o-rings on the fuel line connectors?

Use a different hose, use a different tank and see if the problem still exists.

 

[870 Expressmag]

thats kinda what dad was thinking, i dont have any other tanks/hoses to try, are the o-rings available and easy to replace? tank says johnson on the side and connectors are 2 pin evinrude/johnson

 

[Tim Frank]

single fuel line or two line pressure tank?

 

[870 Expressmag]

Single fuel line

 

[HighTrim]

vent clear and working?

 

[Tim Frank]

Check compression.

 

[64osby]

Check what the other guys have said.

O-rings are available and pretty easy to trade out. A dental pic makes it easier

 

[870 Expressmag]

compression is 125psi on top cylinder, 123psi on lower cylinder

 

[870 Expressmag]

how do i check the vent? i am assuming you mean the vent on the tank? if so wouldnt that give me problems more often than described? also one more note, it doesnt matter if tank is full, 3/4, 1/2, 1/4 or almost empty, happens at all stages of tank fullness

 

[Tim Frank]

Try running with tank cap open.

 

[twocyclemania]

Just a couple of things to try. It sounds like the low speed is not getting enough fuel. Could be a piece of debris in the bowl/pickup tube and it could be something that just floats around inside and on occasion plugs up the intake of fuel. Drop the bowl, clean out any debris and also run a small wire in the uptakes and blow everything out with a shot of starting fluid. This may not be it but it's the best I can come up with from my limited experience.

 

[steelespike]

If the vent is closed or plugged the primer bulb would collapse.
Disconnect then fuel line at the carb, Use something to catch the fuel and crank it fuel should be in strong steady pulses.

 

[870 Expressmag]

i will check fuel pump output, but it was replaced less than 2 years ago, i know thats not saying it didnt go bad, as we own a lawn and garden dealership so we see that stuff all the time, but i would think a poorly pumping fuel pump would also give me issues all the time, not intermittently

 

[870 Expressmag]

I know this is an old thread but we are still trying to remedy this problem, had the boat out for the first time this year yesterday and today, after replacing fuel line ends and fuel line it still does the same thing. Yesterday motor started as normal, first time out for season, ran full throttle no problem, never skipped a beat, bring the motor down to trolling speed and runs good and smooth, as it trolls longer and longer, it will start to spit, if I throttle up most of the time it picks up and goes no problem, then bring it back down and it will troll again for a,little bit. When it starts to spit if I shift to neutral and twist grip to start position it sometimes fixes it, but sometimes it will just idle down and die even with grip in start position, but other times it will keep idling in start position and other times it will keep running but still spit. It's really driving me crazy as we are taking this boat and motor to Canada for a week this year and I need it to be running good. When I took it out today, when I first started I didn't think I was gonna get it going, started as normal, backed away from dock, shifted to fed and hit throttle, fell on its face, recovered, tried again, died, started right back up, tried again, fell on face, recovered it, feathered a little, finally got it to go, full throttle boat planed out fast, with just me in 15 ft boat and all our fishing gear I was doing 21mph per Lowrance gps on sonar, never skips a beat at full throttle I could go forever like that. My dad still thinks it's fuel pump, which was new 2 years ago, as a small engine mechanic it doesn't quite make sense to me but stranger things have happened, seems if it were a fuel pump it would be opposite, fine at idle, bad at WOT due to larger call for fuel, ????

 

[Tim Frank]

If you are a small engine mechanic, why would you guess?
Check/verify the fuel pump output with a gauge.

 

[870 Expressmag]

If you are a small engine mechanic, why would you guess?
Check/verify the fuel pump output with a gauge.

we dont use a gauge to check fuel pump output on small engines, we do it by sight, if i were to gauge it, what would flow specs be? and also how would i replicate problem without having engine running in the lake and trolling? since it runs so well at WOT i would think cranking engine over to check fuel pump output would produce enough fuel. I'm not simply "guessing", i'm trying to bounce my problems off of others for feedback as a direction, as we try to have a direction to go when working on motors, rather than tear in and hope to find a problem

 

[Tim Frank]

we dont use a gauge to check fuel pump output on small engines, we do it by sight, if i were to gauge it, what would flow specs be? and also how would i replicate problem without having engine running in the lake and trolling? since it runs so well at WOT i would think cranking engine over to check fuel pump output would produce enough fuel. I'm not simply "guessing", i'm trying to bounce my problems off of others for feedback as a direction, as we try to have a direction to go when working on motors, rather than tear in and hope to find a problem

Every O/B I have owned has a fuel pump functional test described and spec'd in the OEM manual.....the setup of the pressure gauge is pictured clearly.
(I had a fuel issue with an E/rude 6 HP last year....tested the fuel pump......which had a "reasonable" output against an open ended hose , so "looked" OK....it was out of spec....changed the pump and the problem was solved.
But you are a small engine mechanic, and so probably don't feel that you "need" the manual.

I AM giving you feedback to try and help you with this problem....which according to your thread is almost into year 2.
You suspect the fuel pump, but are testing it by "appearance". I was not trying to be insulting by referring to that as guessing, but that is what it amounts to.
Hopefully your verification of the spark and compression were done more systematically and not by just grounding the plugs and putting a thumb over the spark plug hole.

 

[870 Expressmag]

I get what your saying, didn't mean to sound like I had taken offense, just frustrated and right now we are so busy at work due to the season the last thing I want to do is work on this motor, I want to fish! So I apologize if my frustrations surfaced in my response. And I am guilty of looking for a short cut, the equipment I work on for a living I can hear it or look at it and tell what's wrong usually with minimal or no testing, just frustrating I can't do it with this when I "think" I should be able too. I know I should buy a manual, but for sake of speed, does anyone have that section they could post a pic of the pages for fuel pump testing so I can try it soon? My motor is an 18902b

 

[tomhath]

When you say it starts to spit, are you sure it's running lean? It might be the float needle is leaking and causing it to flood at low speed; at higher RPMs it can keep up with the fuel entering the bowl.