could my fuel tank be the problem?

Over the winter i bought a new 6 gallon fuel tank to replace my three 3 gallon tank. The label said the tank had a self venting cap, which it didn't. I could tell because the tank would swell up with pressure after awhile, so i bought a new cap for the tank at the fleet farm i bought it from. Well, my motor ran fine ('93 15hp merc 2 stroke), and now all of a sudden it seems like the RPM's have dropped. with two people i can do 18-19mph easily, with just me 23mph. now it seems to take longer to plane out. I keep the tank inside after a day of fishing so i know it can't be water. I opened the vent on the top of the cap all the way, and at times that does help. I've never had this problem with my three gallon tank. Anybody know what this could be?

What speed are u getting now with reduced rpm? Just wondering as my 2009 20hp is doing something similar. Fortunatley I have warranty left. My father inlaw said he had a 9.9 that did the same thing and it was the power pack.

I would check the following:

1. Is the gas fresh?
2. Is the fuel hose in good condition?
3. Are the spark plugs clean and gaped properly?
4. Is the fuel filter/screen clean?
5. Do you have the right fuel/oil mixture?
6. Are the spark plug wires clean and in good shape?

Once you do all those easy things, then its time to start thinking about cleaning/re-building the carburetor and or fuel pump.

The new tank could have had some small pieces of plastic, dirt, or whatever in it that got sucked into your carb and clogged it.

I'm inclined to think that it's an air leak at a tank fitting. Hook the old tank backup and see it you still have the issue. If not then the issue is with your new tank. You didn't say if you replaced any of the lines are fittings.

I had issues where mine 20 HP Merc would run fine one time them the next not or when you stopped to fish and started backup it would run like crap. As it turned out the quick disconnect at the motor was bad and leaking air. No signs of a leak and it would hold pressure when you pumped up the primer but it sucked air when running and when it did the idle was also erratic.

Hi
It is probably the ethanol in the fuel. I just replaced and cleaned the entire fuel system on 2 of my costumers boats. ONLY to find out that the NEW gray fuel line was the problem. The clear plastic liner inside the gray line was partiality dissolved and was separating from the outer grey line. How I found out was I tried to blow air through the line only to find out hardly any came through. I thought dirt dobers may had got into it before I had a chance to install it. NOTE:I live in Fla and these little bugs love holes in anything to make a nest.
Then I really looked into the line with my LED light only to see the liner was loose. SO I cut back some of the line only to find out it was all the way through.
MY remedy was go to my local Car Quest "old dude" and he put the new alcohol proof line in my hand. ABOUT $3.00 a foot
I installed it and NO MO PRO blem as in blemish. This is easy and simple to check. I think he called this line GREEN line. It isn't pretty as it is black and not UV stabilized like the gray but you Yanks don't have sun up there do ya?
Try this and let us know if it works for you. Hope this helps
I have several boats and NONE of them have this on them now.
Capt Rhan

lovedr, im getting maybe about 10-13mph out of it with two people in the boat. usually i can run 17-18mph with two in the boat. mrsimon, i everything on your check list i've done except for #2 and #4. it could be the fuel line, i bought it about 3 years ago, it looks to be in pretty good shape, but i think i'll change it out this friday when i go out. I also bought new spark plugs last month. I noticed the spark plugs have two shims on them. Should i take those off? Groghog, ill see if i can find some of that green line here. there is an auto zone and o reilly's about 5 mins away from my house. gramps50, i also have a feeling that it's the new tank also. I'll let you guys know this friday or saturday how it turns out, plan on getting some fishing in before i head up north next week for some more hot fishing action in the chippewa flowage

GrogHog said:

Hi
It is probably the ethanol in the fuel. I just replaced and cleaned the entire fuel system on 2 of my costumers boats. ONLY to find out that the NEW gray fuel line was the problem. The clear plastic liner inside the gray line was partiality dissolved and was separating from the outer grey line. How I found out was I tried to blow air through the line only to find out hardly any came through. I thought dirt dobers may had got into it before I had a chance to install it. NOTE:I live in Fla and these little bugs love holes in anything to make a nest.

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I've seen this problem with the gray or white fuel lines caused by ethanol.

The BLUE colored hose is rated for ethanol.

And as for the dirt daubers, yep, they will make a nest anywhere they can squeeze into. Yesterday, my girlfriend launched her Yamaha VX110 jet ski, then hauled it right back out because she said the indicator stream on the port side wasn't shooting any water out within the first minute of operation. (I've always told her that the indicator stream is a vital sign for an engine, if it ain't peeing in the first minute or so, or if it stops at any point while running, shut it down and find out what's wrong)

So, I hooked it up to the garden hose, cranked it up, sure enough, no water from the indicator fitting. WTH? I looked at the hole in the fitting, to see if there was a piece of marsh grass or something blocking it, and that's when I saw a dirt plug in there. Dug it out with a piece of aluminum wire, and water started flowing, still had a little dirt in it.

I've been riding jet skis for 25 years, and I've seen some strange stuff happen during winter storage, but the dirt dauber plugging up the indicator fitting was a new one for me.

Anytime a problem mysteriously appers, it's usually related to the last thing you did. Put the old tank back in and see if the problem goes away. That will eliminate a lot of tail-chasing.

I like chasing tail ..... but I guess that's a story for another thread.

Anyone know a good on-line place to buy these fancy ethanol rated fuel lines?

Anyone know a good on-line place to buy these fancy ethanol rated fuel lines?

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Defender has Fuel Hoses, and Fuel Line Assemblies

I think I found the problem. Was outside today and decided to check out the fuel tank again to see if I see anything out of the ordinary. I noticed the fuel line connector on the the fuel tank wasn't screwed in all the way. I think that was the problem. I took it off and cleaned it, and wiped off the fuel tank and then put it back on tight with some thread lock tape so it wouldn't back out so easily. I think this might be it. I thought about it, and when i first bought the tank and used it the engine ran fine. It didnt start with the low rpms until about 3 weeks ago. I did notice the connector wasn't screwed all the way in, but payed it no attention. well, fishing trip tomorrow. going to bring the old tank with me just in case.

Sorry for being so late, but I ran into a vent problem similar to yours. I bought a brand new three gallon tank and the vent screw on the cap didn't vent the tank even though it was back out as far as it would go. I ran the motor with the tank cap loose to let pressure in. I eventually got a brand new three gallon tank with my new Honda motor that cured that problem.

Found out what the problem is. I'm on the water right now. It was the fuel tank. Nomo, I had the exact same prob. Hooked up my old tank before I headed out. Motor runs strong again. Thanks for the input guys!