2005 Mercury 50hp 2 cycle surging under power

[lszet55]

Thanks for reading!
Driving the pontoon at 3k rpm will drop to 2k then back to 3k then to 2.5k and so on. I pump the ball and the rpm will stabilize. I have replaced the tank primer ball and fuel pump. Also pulled the fuel pickup and it was spotless, Carburetors have been rebuilt twice by two different boat mechanics. The pontoon has never run right. It seems to be starving for gas or running lean. Any ideas on what could be causing this condition or what to check next.

 

[dwitting87]

Is it misfiring or just erratic speed? What about the airline to the fuel pump?

 

[lszet55]

Seam to be misfiring or just not running smooth. I replaced the line from top cylinder to fuel pump.

 

[flyingscott]

Try pumping the primer ball when that is happening.

 

[lszet55]

it will stop surging when I pump the ball. Like I’m forcing gas into the carb bowls?

 

[jimmbo]

You seem to have a Solution

I pump the ball and the rpm will stabilize.

Sounds like the there is an Issue with the Fuel Pump.

 

[lszet55]

goofed up sorry

 

[lszet55]

JimBo I’ve replaced 2 fuel pumps pulled the tank outlet tube replaced the primer ball checked the fuel line connector at the motor, replaced the fuel line to the pump. It seem to still be starving for fuel?

 

[dwitting87]

it will stop surging when I pump the ball. Like I’m forcing gas into the carb bowls?

I wonder if the fuel pump is not actually running, i.e is the air line to it actually moving the diagraphm. I have no idea what typically fails on these types of motors, but I would start with fuel obstructions (assuming you've already confirmed the rebuilt carbs are good) and then move on to fuel pressure related issues (i.e. is the pump actually be actuated).

 

[dwitting87]

I wonder if the fuel pump is not actually running, i.e is the air line to it actually moving the diagraphm. I have no idea what typically fails on these types of motors, but I would start with fuel obstructions (assuming you've already confirmed the rebuilt carbs are good) and then move on to fuel pressure related issues (i.e. is the pump actually be actuated).

maybe one option would be to unplug the air line and see if it's actually working with or without it. Keep removing suspect parts/functions in and out of the equation.

 

[lszet55]

I took fuel line at carb off and cranked it. It’s pumping fuel not a crazy amount but pumping fuel. I looked for a volume but could not find anything

 

[lszet55]

Dewittng 87 there are two inputs to the fuel pump one from the upper cylinder and one under the pump itself from 3rd cylinder not sure what you mean. Your saying remove the line from top cylinder?

 

[racerone]

There is compression in the cylinders.----What are the actual values?----There is compression in each crankcase.-----It is compression pressure in the crankcase that drives the fuel pump.-----There may be a lack of pressure pulses in the crankcase.

 

[lszet55]

Compression is 200psi in all 3 cylinders just check last week

 

[lszet55]

I was wondering if crank seals could be leaking messing with the mixture in the crank?

 

[QBhoy]

I know nothing about pontoon boats (don’t get them here)…other than overwhelmingly so…I seem to be constantly reading on here about their absolute requirement to have the engine set up extremely accurately…and then even at that…they have a habit of gulping air or not staying hooked up at the prop. You sure it’s not maybe doing this ?

 

[racerone]

200 PSI seems high for a 2 stroke.

 

[QBhoy]

200 PSI seems high for a 2 stroke.

For sure. Got to be an optimistic gauge, all be it, not sure I’ve known a gauge to be quite that margin of optimistic

 

[lszet55]

200 PSI seems high for a 2 stroke.

Spec is 170 to 200 psi

 

[lszet55]

I know nothing about pontoon boats (don’t get them here)…other than overwhelmingly so…I seem to be constantly reading on here about their absolute requirement to have the engine set up extremely accurately…and then even at that…they have a habit of gulping air or not staying hooked up at the prop. You sure it’s not maybe doing this ?

The prop is not in good shape. I question if the prop could be cavitating. But at steady speed rpm goes up and down, once it started to cavita I didn’t think it would stop?