Motor Bogs down at very high rpms

I have a 1985 115 hp Mercury “Tower of Power”. The motor runs great for the most part. I was out this weekend and when I was on plane running around 4500 rpm everything was great. If I gave her a little more and got up near 5000 rpms the motor would bog down and start to sputter. When this happened I backed off the throttle and got it back up to 4500 rpms where it ran smooth again. The motor would not die, it would just bog down and lose the power.

I replaced my primer ball last fall and have inspected the fuel lines for any damage or loose fittings. Any ideas?

Thanks,

1. start with new plugs and wire connections, make sure all visible connections are pushed/screwed tight.

2. make sure the primer bulb is not collapsing. I have seen bad check valves (though you replaced it can fall).

3. check the amount of fuel that can be pulled throught the line, maybe a filter is partially plugged. might be a pick up filter in tank, in line or fuel pump filter?

These are just places to start before pulling carbs, from your description I would lean more towards fuel rather than electrical. Espicially if this was the maiden voyage of the year, did you drain the carbs last season, fuel addatives are not that good or maybe forgot about it? -PM for further info. -Mark

I did replace my plugs but not the wires. The primer bulb is not collapsnig, I did have my buddy check that while it was happening. I did not drain the system this winter. I was under the impression that the 2 stroke had enough oil in it that ii would prevent rust. I stored the boat with full tank of gas/oil and had a can of sea foam added. I will check my filters. Besides the fuel filter in the motor where do I go to check the tanks filter? Any other specific spots that have filters I should look?

Thanks,

Sometimes there is a filter on the pick up line in the tank. With a in hull tank you have to pull the access panel in floor. However, you said the bulb is not collapsing so I would not even worry about that.

I would put money on your carbs being gummed up. You want to drain your carbs prior to storage and not rely on addatives as they dont prevent this.

What happens is that the gas that is in the carb evaporates and leaves behind a film, and with oil mixed in it it can be even worse than strait gas. This film biulds on the walls of your carbs jets and orafhices very similar to blood vessle restriction in oud bodies from cholesterol and fats. Now the carb jets (that measure fuel) are restricting the amount of fuel that can pass. There are very small holes and passeges inside carbs that are very sensitive to gumming. I have not found an addative that will prevent this, at best they buy you more time before this happens.

Any carbed small-engine that sits for more than 2 months between uses should be drained.

As for you mentioning rusting, this only affects steel tanks. It can also be effective to use a fogging oil to spray into your intake when shutting down a motor for the season to protect internal parts from rust. This is debatable though and the amount of time you are going to be storing comes into play.

Get the carbs cleaned. -Mark

I agree on the carbs.

But here’s a strange thing…my Merc did the same thing 2 years ago. Turned out, the inner lining of the gas line–Mercs best–has a separate membrane. This membrane–very thin–separated inside the line. When I would cruise at 2/3 throttle or less, no problem. Give it the full throttle and she’d bog down because it was crushing the inner membrane. New hose for $40 and problem solved! Not saying is your issue, but it’s worth considering.

Where do I have to go to get the new plug wires and gas line? Is this something I have to get from a marine dealer?

We just changed out the bulb on my room mates boat doing the same thing..problem solved. May not be yours, but check the cheap things first.

I had the same problem last year Russ. I took it in this spring and it turned out to be the power pack was shot. It was only firing on 3 cylinders. $460 later and it runs like a dream. I thought it was a gas issue also but they did a compression check and found the problem. Hopefully its something cheaper than that but a good marine mechanic should be able to diagnose the problem for you. Dan

Did your engine actually completely shut down/die, or did it go to a low idle condition ??

Thanks

Would SeaFoam hurt an outboard?

jwellsy wrote:

Would SeaFoam hurt an outboard?

seafoam will not hurt an outboard,use the directions as stated.
in other words,dont dump a whole can into a six gallon tank for example.

85Lund,standing from the back of the motor looking forward,on the upper right side of the cylinder block closest to the rear you should see a little black or gray box with two yellow wires coming out of it.
that is the rectifier,look at it closely and see if it appears to have a bulge or burnt look to it where the wires come out of it.

the rectifier changes A/C current from the charging coils to D/C current.
if I remember right if it is burnt/blown out,it can play havoc with the ignition system at higher rpm’s.
if the motor is not charging your main battery,or is taking longer than normal to charge it,that is a good indicator that the rectifier is bad.

a few things can fry the rectifier,a battery jump/charger hooked up backwards for even a second will trash it.
the key left on for too long without the motor running will burn it out when the battery gets low.
and of course time and use will cause one to go bad as well.

check the balance tubes between the cylinders,they are the small hoses between pairs of cylinders on the starboard side of the motor.
you can remove the coil/power pack to gain easier access to a couple of them.
if run too long with them being cracked/broken,you will score a cylinder.

I agree with others,if its been a while the carbs are due for cleaning anyway.
please post what is found out so others with a similar issue may benefit.

to clarify the small balance tubes a bit,one will come from just under the flywheel as this is where it hooks to the upper main bearing cap.
it runs down along the motor behind the power pack.
another one will come up from the lower main bearing cap.
I believe there is one more between the center pair of cylinders.

they are hooked onto small brass fittings that have a check valve in them,make sure there is no carbon stuck in the fittings when you remove the hoses.
use dedicated fuel line to replace them,windshield wiper fluid hose will not work and will fail quickly.
most likely you will have to go to a merc dealer to get the right hose.