15 HP Mariner loses power

I have a 1997 15 HP Mariner tiller outboard. Bought it used this past winter. The motor's had little use and was very clean.

Previous repair was the switch box had been replaced with a used one. Compression was good and even. The motor had sat for awhile and the float needle was stuck. Bought a Seloc manual and rebuilt the carb. including primer and fuel pump.

Checked Hi and Lo speed timing. Both to spec. New fuel lines on Carb., new vacuum tubes and gas line and tank.

New spark plugs both evenly tanned and dry. Lots of spark, grounds tight, coils test OK on Ohm meter and no cracks or burns,

Reed valves are good, no cracks and all look even. Carb. gasket, good doesn't appear to be any air leaks.

Timing advance moves freely. Lots of cooling water spitting out.

The motor runs like a champ cold and doesn't miss a beat when hot, as long as I don't idle. When I reduce the throttle and troll the motor works great. When I open the throttle up again it accelerates very slowly and sometimes won't even get the boat up on plane.

The motor sounds fine, just no power. It's like I've barely cracked the throttle even though it's wide open.

I've looked at the throttle plate and it's wide open. Sqeezed the primer bulb and it's firm. Sprayed extra gas into the carb. and no change. I spray WD around the carb, gasket. No change.

Here's the weird part. If if put the motor in reverse and give it gas, it will rev. high like it's suppose to. Now, when I go forward it's good. It doesn't accelerate as fast as when it's cold but it will get the boat up on plane.

I'm stumped. I'm thinking that I might have too much float. When the motor warms up and I idle, it causes the motor to become overly rich and it boggs down. When I put it reverse there's less load on the prop and this allows the motor to rev. high and uses up the gas from the over rich condition. There is a little gas in the vacuum lines. Is this normal? Or am I mistaken and these lines are fuel over-flow lines??? One runs out of the carb. comes to a "T" with one line high and one line low on the crank case.

or

This motor has a theromstat. It appears to open and close fine. Could it be heating up the motor. I've read about an over heat slow down function but I don't know if this motor has it. I don't see anything in the manual. Could it be getting hot and when I idle I can't accelerate until it cools. The slow down funtion is only for forward so when I rev it in reverse it pushed extra water through the motor and cools it down enough for it to accelerate in forward.

or is it ignition. I hate to spend big money on ignition part without some clear indication that this is the problem.

I hope it's something simple, but as of now I'm out of ideas.

Your thoughts please.

Re: 15 HP Mariner loses power

Adjust your idle mixture screw 1/4 turn richer than smooth running. It needs that extra gas for acceleration. If you adjust for smoothest idle, it'll bog and die every time.

The tiny hoses are recirculation lines. They move puddled gas from one cylinder to another to be burned. Puddling occurs at low RPMs.

The motor will rev easier in Reverse because of the prop blades. In reverse, the blades are swept back. If forward, the blades are cupped.

Re: 15 HP Mariner loses power

SO, clockwise until it's ruff. Counter-cockwise til it's ruff. Count the turns, set the screw half way and then 1/4 turn counter-clockwise???????

Re: 15 HP Mariner loses power

I have exactly the same problem.
No power will barely plane.
What was your compression reading?
Mine started @90 psi and then next time it was only 60 psi.
Please let me know if you had a fix.
Thanks,
Rafiqi.

Re: 15 HP Mariner loses power

Did a compression test today. 110 psi on both cylinders.

Still haven't got my problem fixed.

WillyBWright. Thanks for the quick response.

I think your very close to being right.

I ran right out the next morning and adjusted the low speed mixture screw as you stated. The motor ran like a champ. Turned if off and started, trolled, accelerated all good. Went through almost a tank of gas. I really liked my motor. Right at the end I slowed down and tried to accelerate again and nothing. No amount of tinking would fix it. I felt like loosening the screws and dropping her off the transom.

Took the carb. off and took it apart again. I swear, I've done this so often, I can do it with my eyes closed. Everything looks good inside, very clean. I took the float off and let the float swing arm hang down. The float valve seemed to stick a little bit. I took it out and polished the hole with a Q-tip and carb. cleaner. More cleaner and air blown through all holes.

Very clean not sticking. Looking at the float. Not exacly at 1 inch. When I was rebuilding the carb. the diagram for setting the float had been omitted from the manual. I e-mailed Seloc and had a diagram within 24 hrs.

It shows the float set at exactly 1 inch. I assume this is critical. My float has a shoulder on it which is different from the diagram. When I say shoulder, I mean one side of the float is narrower that the other side like a raised rim.

Is the shoulder counted in the 1 inch? Is the measurement actually 1 inch? Should I replace the float with one that looks like the diagram??? I have an older WM carb. that looks the same as the WMC 49 that's on my motor. The float in this carb. looks the same as the diagram both in the manual and the parts list.

Put the carb. back on and it ran good for 15 minute then back to the same old. One thing that is curious. Everytime I take the carb. off, take it apart, put it back together and put it back on the motor the motor runs great. The performance goes down hill until it won't plane. The day I adjusted the screw as you instructed was the best it's ever ran.

Frustrated.

I checked everythink else today. Tons of spark. Thermostat opens an 120. all hoses tight. Throttle plate moving. All fuel line hose connections tight.

I'm sure it's fuel and I'm sure it's very close to being fixed.

Your thoughts,

Thanks in advance.

LesH