1987 115 2-stroke in-line six surging at higher RPM?

[Abubba]

My old faithful one owner 1987 Mercury 115HP inline six seems to be starving for fuel at higher rpm's. The motor starts right up and idles smoothly without any missings and no fuel leaks. Once the throttle is opened it will immediately jump on plane and run well at 5000 RPMs for a couple of minutes then the RPM falls off to about 3500, stays there for about 15 seconds, and then climbs back up to 5000. This surging continues on as long as the throttle is WOT. The motor runs smoothly even when it begins to lose RPMs and while it gains them back again. No cutting out, no backfires, just losing RPMs like you would when you pull the throttle back. Any suggestions on where to start looking for this problem. Fuel filter, power pack, fuel lines?

 

[Dave1027]

When it begins surging, have somebody pump the primer bulb. If that improves the surging then the likely cause is the fuel pump.

 

[Abubba]

When it begins surging, have somebody pump the primer bulb. If that improves the surging then the likely cause is the fuel pump.

Is there some other way to check the fuel pump besides somebody pumping the primer bulb while running? The location of the bulb will make it unsafe for someone to pump the bulb at speed, especially with the boat changing speeds/RPMs.

 

[Dave1027]

If you never rebuilt the fuel pump, you should go ahead and do it as a preventive maintenance any way. You should check all fuel lines and the primer bulb to rule out any restrictions. Also, make sure the fuel tanks vent is working.

 

[Abubba]

Thanks, the fuel pump has never been rebuilt so, I guess I should go ahead and do that. Would the pump beginning to go bad be the cause of the WOT RPMs to surge up and down at speed?

 

[Chris1956]

If the fuel pump cannot supply the required fuel flow for high speed, the speed will ocellate. It could be something as simple as a stuck one way valve in the fuel pump, or a clogged fuel screen.

 

[Abubba]

If you never rebuilt the fuel pump, you should go ahead and do it as a preventive maintenance any way. You should check all fuel lines and the primer bulb to rule out any restrictions. Also, make sure the fuel tanks vent is working.

Finally got my parts (all Quicksilver brand parts) in and successfully rebuilt the fuel pump. Changed out all the internal fuel lines under the engine cover and leak tested, no leaks. Hooked up a garden hose with good water pressure to the foot and started the motor. The motor started right up, the primer bulb held firm, and no leaks on the rebuilt fuel pump and new fuel hoses. The engine idled very smooth (like it always does) no cutting out or missing. Rev'ed the engine up to 4200 RPMs for a second and no hesitation or missing. Took the boat to the lake and ran it under load and it did the same thing. WOT to plane-out (no hesitation or missing) and when the engine gets to 4200 RPMs it starts to fall back to 3000 RPMs while WOT (still running smooth but you can hear the carbs are WOT). So it's not a fuel delivery problem, I guess. Any ideas??

 

[Abubba]

If the fuel pump cannot supply the required fuel flow for high speed, the speed will ocellate. It could be something as simple as a stuck one way valve in the fuel pump, or a clogged fuel screen.

Rebuilt pump, new fuel lines, new filter, and no change. Scratching my head and thinking but, nothing's happening? The engine never runs rough while at idle or WOT and doesn't stall out, just runs smooth. When RPMs exceed 4K it drops back to 3K but, still running smoothly with the carbs moaning at WOT. Would it be a power pack or trigger? I always thought they would run rough or not at all if one of those went out. What do you think?

 

[racerone]

Test run with another tank and hose.

 

[Abubba]

Test run with another tank and hose.

Just installed a new hose and bulb from the oil reservoir to the engine and a new hose from the tank to the oil reservoir. The 20-gallon tank is a factory-built aluminum below the deck floor and hull mid-ship. I don't have access to an external Mercury tank to try. The bulb pumps up hard and is not losing prime. I will try and access the fuel pick-up in the tank and check to see if there is any blockage. There is an access point on the floor. This boat is a Fisher Marine 18' V-hull bass boat and everything was built-in at the factory and can be very difficult to access.

 

[racerone]

Is this motor running with an " autoblend " mixing unit ?

 

[Abubba]

If the fuel pump cannot supply the required fuel flow for high speed, the speed will ocellate. It could be something as simple as a stuck one way valve in the fuel pump, or a clogged fuel screen.

Just rebuilt the fuel pump and replaced the primary fuel lines and fuel filter. No change.

 

[Abubba]

Is this motor running with an " autoblend " mixing unit ?

Yes, it is. The original "auto-blend" mixing unit. Could this unit cause fuel starvation?

 

[Abubba]

If the fuel pump cannot supply the required fuel flow for high speed, the speed will ocellate. It could be something as simple as a stuck one way valve in the fuel pump, or a clogged fuel screen.

Rebuilt fuel pump using Quicksilver parts. For being so old the fuel pump was clean and the check valves and diaphragms still appeared in good shape. Reinstalled with new fuel lines and tested under load, no change.

 

[Abubba]

My problem seems to be a fuel delivery issue but, could it be a power-pack failing or maybe the trigger? Can someone tell me how to test the power packs and trigger so I can verify that they are in spec? I don't think they are the problem since the engine starts and idles so well. Plus, no engine hesitation at WOT up to 4K+ RPMs before they fall back to 3K RPMs.

 

[racerone]

Most folks would say----" take that autoblend to the landfill. Heave it high up on the pile "