yamaha 90hp outboard rough running ? PLEASE HELP

[soggysod]

if anyone wants a real stumper here you go.....

i have a 07 yamaha 90hp 2stroke, few hundred hours nothing too hard.bought it new has always run fine, and never let a tank of gas set more than a few weeks, even in the winter so never old gas in it.
this past week i was planning a trip to the gulf so i decided to do a little due maintenaine. fogged the engine with mercury power tune stuff, new plugs ngk br8hes, new water pump, new fuel filter (not yamaha brand) after fogging the engine, ran it a while on the muffs to clear it out, and shut her down.
went down to florida, hard to start off the bat (first time EVER!!) then it would run great at w.o.t for about 5 mins them rpms would drop way off like it was running on 2 cyls. choke wouldnt seem to change the symptoms.
i then check the fuel prime ball, it is hard, real hard, pump it a bit, it goes soft then pumps back up. boat then fires runs great for another 5 mins.
soooo i change old (non yamaha) fuel filter out for brand new yamaha one. also put in new plugs since i had them. boat "seemed" to run right 20 mins or so but then symptoms came back.
so over the course of a week i wasnt going to abort fishing, i went ahead a ran it. ran about 90 gallons of gas thru it (35 gal tank) so i can safely rule out bad gas. the boat ran consistently good below 4k rpms, in fact if the problem showed up once below that mark i dont recall. but when heavily loaded or in rough seas when i had to crank it up, 5 mins then sputter, pump ball, back on plane ran great for 5 mins, then pump again.
i pulled the fuel pump and it looked ok no leaks when pumping ball etc, fuel line "might" have had a pin hole, replaced it too.. and i plan on putting a new diaphram on, but short of that i hate to start throwing parts at it if someone has seen this before.
the only thing i can think is sticking float needle on one carb, really dont think electrical problem since 100% of the time, atleast 30 different times pumping the ball past being hard fixed the problem, no other modulation of throttle, waiting, choke etc would fix it.
oh i also left the fuel cap off incase it was a vapor lock in the tank type situation, and had no change to the problem,
thanks soggysod

 

[wrechin2]

Were the new plugs installed before or after the fogging process??? If before, it is possible to have fouled a plug and it is coming and going. I have seen alot of similiar issue and after a simple plug change, all issues go away. Cheap test. Just a thought!

 

[buzz]

soggy...this may help. It's dumb on my part..but. I bought a new Yamaha fuel line for my 50hp. I put it on....like you see 99% of them.....the prime bulb in the splash-well. WRONG!!! After finding out that the line was backwards...(who looks at those little arrows) LOL. My boat would idle and run for a short distance...then shut off. The bulb is now close to the tanks.....and have to open a compartment to prime it. Just a thought.

 

[soggysod]

Wrenchin, i put in the new plugs after fogging, and then just to be sure new plugs after the problem showed up, so 2 new sets of new plugs.
buzz, i was actually leaning towards the fuel bulb check valves and will probably replace it, though the old one is still in, i will be sure and watch the direction to avoid that problem, thanks!
thanks for the thoughts, i just thought someone might have seen exactly the same problem before and might save me some trouble shooting. it seems to me unless im missing something obvious i've hit all the usual culprits and it's probably not going to be an easy one!

 

[wrechin2]

The problem is the scenario you have can range anywhere from poor fuel quality to ignition breakdown. I can take 3 engine giving the EXACT same symptoms and each will have a different repair.

A simple test is to have someone pump the ball while the problem shows up. If the engine picks up RPMs then 99% of the time the fuel pump is at fault. The other 1% can be a needle sticking closed in a carb and the fuel pump is not strong enough to push it off the seat. With the issue going away for a few moments after the bulb is primed past firm, it may be in the carbs or coincidence that it goes away.

Also pay attention to the bulb when the issue is going on. If when it drops RPM the bulb is sucked together, this indicates either the anti-siphon valve is sticking or there is trash in the tank. This will restrict the amount of fuel that it can get and drop RPM's.

As far as fuel quality, most tanks will hold more fuel than what the pick up can reach. I drain tanks all the time. Most of the time when no more fuel will come out of the fuel line going to the engine, I can remove the sending unit and get anywhere from 1 to 10 gallons of fuel more out of the tank depending on size and shape. This area of the tank may have bad fuel or water. Under certain conditions, it may be picking up bad fuel in this area and then making it into the carbs. Use of a remote tank with know good fuel can rule this out.

As you can see these 3 fuel conditions can create the same symptoms of loss of RPM and are different problems. So for someone to be able to tell you exactly what the problem is, it is almost impossible. A lot of times someone had a similar issue like the primer bulb and it turns out to be it, but there is always a percentage that it isn't. These are fairly simple to rule out the last 2 scenario and fairly simple to rule out 1/2 of the first one. Just my .02 worth. emoToast

 

[soggysod]

thanks Wrenchin i didnt think of a remote tank and will try that.
the rpms pick up instantly when the bulb is pumped i can fairly confindently say that isnt coincidence as it happened 30+ times over the course of a week and about 50 -75 miles of running. i guess if the remote tank dont fix it i will throw in a new pump as the diaphragm isnt but like 20 bucks.
odd thing is the bulb doesn't go flat when it starts acting up, but it gets too firm like harder than it gets when you are pumping it up to crank an engine, pump it past this hard spot and thats when rpms pick up that is why i was thinking sticking float needle or other kind of obstruction.
soggysod

 

[wrechin2]

I seen where you pumped the bulb but understood it as you pump it prior to taking off. Not while the issue was occurring. Sorry.

The bulb would only go flat if a restriction exist from the bulb back to the pickup. Like sucking on a straw in a milk shake with fruit and picking up a piece of fruit. You will flatten the straw. Same principle.

If you have someone with you, when it happens again, take and spray a LITTLE bit of carb cleaner in the throat of the carb. Which ever carb picks up rpms will be the problem. Some are single bowl carbs and have 1 needle/seat for both cylinders. Some are dual bowl and will have a seperate needle/seat for each cylinder. Which do you have?