2000 75 hp Mercury surge or misfires? Help[!]

[sumyunguyz]

I'm at my wits end on this problem I'm having with my 2000 75 hp Merc (S/N OT160329) on an 18ft. pontoon boat. I've search the forums and can't seem to find anything that's similar to this problem.

Engine starts and idles fine. Seems to start missing on one cylinder around 3500 rpm. As I gradually increase throttle between 4000-4400 rpm the cylinder seems to start trying to fire as the rpm continually increases then decreases. Prior to this problem I was running around 5100 rpm wot. Now it's running around 4600 wot.
I've pulled and cleaned all the carbs, (found some trash in top carb hoping that was it) OEM fuel pump was replaced back in spring. No restrictions or leaks in fuel lines or fuel tank. Have external Racor filter separator installed that has fresh filter in it. Only running non-ethanol fuel in it. New spark plugs also.

It was running fine up until a couple of weeks ago. I keep thinking it's a fuel problem causing the surging but can't seem to find anything.

Any feedback or ideas is appreciated!

 

[Texasmark]

Assuming you get rapid changes in surges you may have a CDI pulse pack going out. Problem could be a thermally sensitive component in the pack that has the capacitor dump switch (transistor or SCR) right at the firing point rather than well above it to ensure firing. Knowing a couple of things could help isolate it. First is when does the problem occur? Immediately on the outing after minimal warm up, or does it occur after operating for some time. Is it continuous once it starts? Does the time of day matter?

A bit risky under way since you need to be at considerable rpms and on the trailer would be too high to be practical, but if you pulled the cowl and got a battery powered timing light with someone to drive and someone to look, check the firing of the light on all 3 when the problem occurs. The clip on sensors are easiest to use. If it's ignition you'll know right quick. Actually, on those ignition systems the sensor just needs to get near the plug wire to trigger the light; don't have to actually connect it to the wire.

Otherwise you are looking at fuel but fuel is usually not rapid changes, sorta goes with the flow. I had a dropped cylinder on my 90 triple once that was intermittent and I pulled the carbs. All looked good, no varnish or anything until I hit the bowl (where the high speed jet is located) with compressed air and wala this little piece of hose came out of the hight speed jet and proved to be the problem. Other fuel problem could possibly be air in a fuel line. Since it seems to be isolated to one cylinder then it would have to be after the fuel line splits to go to the 3 carbs. A good place to have a problem would be right where the hose slips over the hose barb at the carb inlet.

On a 2000 engine, your problem may be fuel and it may be fuel supply problems caused by the fuel line deteriorating due to fuel and age. I had to replace the OEM gray line on my 2002 engine last year due to tan flakes coming from the inside of the hose and clogging up the fuel supply. If the bulb is sucked in when you have this problem then the fuel pump is starving for fuel to pump and is a clue. If so, the tan flakes will be all through the fuel supply line and it all would have to be changed and flushed out. I did the hose from the tank, bulb, pump overhaul and fuel filter in the engine. Flushing with the new components amounted to pointing the output of the filter into a suitable container and pumping the new bulb and all till you had a nice clean stream of fuel.

HTH,
Mark

 

[Texasmark]

I tried to edit the above but the software wouldn't let me. Change is in the last sentence. The disconnect for flushing needs to be at the INPUT to the new filter in the engine not output. You want to flush what's left of crud out before it gets into the new filter.

Mark

 

[sumyunguyz]

There doesn't seem to be any specific time of day or engine temperature when it happens. It's a constant problem. Seems like an easy method to check for firing problems using a timing light if my wife doesn't throw me out of the boat. I'll give it a try.

The new fuel bulb never needs to be pumped. The bulb always stays hard (unlike other things) even underway. I haven't tried changing all the fuel lines after the filter but it sounds like a good idea.

Is the high speed jet the same thing as the main jet located on the fuel bowl housing?

 

[sumyunguyz]

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