fuel fluctation problem

I have a Canadian made 1996 mariner 9.9 Ser # 0G119661 on a 20 foot pontoon boat remote controlled. I seam to be having a fuel fluctuation issue. The motor has been ran a good bit over the summer and over the last few weeks I have noticed a change in performance it wont hold a constant rpm. I normally run it at 4000 and it will slowly drop off to 3600 to 3800 and if I add a bit more throttle it jumps to 4200 t0 4400 and then starts dropping off again unless i go wot, and even that has dropped off from 5400 to 5000,. And today while I was out it didn't want to idle, normal idle is 900 to 1000 in gear and today it dropped off to 700 or died. The only change was I did have to get 2 new fuel tanks due to the old ones being stolen so right now I am suspecting the fuel fittings on the fuel line, their less than 2 years old, got a new line last year. I use only pure gas from the marina. I ran into this same sort of a problem with a Nissan last year and it turned out to be a bad O ring in a fitting. My plan is tomorrow is to get 2 new fittings and give that a try.

Re: fuel fluctation problem

Well had a good day so headed out to the boat armed with 2 new fuel fittings and a can of sea foam. put in the sea foams first and took it for a 6 mile run and that seams to settled down the top end rpm fluctuation. Rand that tank dry and thought I would change the fittings. And ran into this. It appears the silver EPA approved fuel line has a thin plastic liner. while that may not be a issue I sure don't like it and will be replacing it. So I didn't really do much, I hate being forces to spend all day out riding around especially when it was a nice 70 plus day the leaves are starting to turn.


thought a pic of the line would interest a few, its a one year old attwood set up

Re: fuel fluctation problem

That liner is probly for the ethenol gas of today. Looks like a aftermarket way of cutting cost of a heavier liner inorder to keep the cost down. Replacing the fuel line with new line would be the best for your trouble. JMO
Oldman570

Re: fuel fluctation problem

It took a few hours and stops at several places but I was able to find some marine grade fuel line that wasn't the silver stuff. according to the weather it will be 2 days or so before the next trip to the slip and test.

Re: fuel fluctation problem

I'm no expert, but even though you are confident you have good gas, I would start with emptying the tank and putting in new gas. You may save a lot of time chasing other gremlins. Especially if you don't have fuel injection. I chased down a similar issue for weeks and didn't suspect the gas was the problem because it was new. Must have been a bad batch. I used up the gas, sprayed a lot of quicksilver cleaner in and added seafoam as usual. All the problems went away.

Re: fuel fluctation problem

Well I made it out to the slip today, wasn't expecting a good day bit had one. changed out the fitting and the line with real gas line and took it out for a long ride, and yes on a fresh tank of gas and sea foam and their was some improvement. I got my wot rpm`s back and the 3600 to 4000 fluctuation is gone to but the idle is still down, but I went ahead and set it up.

The idle may be just from the season air temp change. but replacing the silver line with the thin liner sure has helped. The motor dose have a fuel screen in it so I hope any chunks were caught but I wont know till I pull the boat in awhile. still have a good bit of fishing time left.

I am planning on a fuel pump and a carb rebuild over the winter just because its a 1996 motor. I suspect at least one issue with the carb any way. The man I bought it from told me it was changed to a 15 hp and if its like ever thing else I have found I would bet they drilled out the main jet and didn't replace with one for a 15 hp, and from what I can see from comparing part numbers the main jet is the only difference between a 9.9 and 15. The carb, intake, reed valves, pistons, and exhaust body are the same part numbers.

Re: fuel fluctation problem

I had a thought over night. I know that no idle or a motor that dies when dropped into gear is often a sign os a bad needle valve and or float, so would a motor that has a sudden change in its idle rpm be a sign of the needle valve wearing.