Merc 150 V-6 idle problems

[<Ricky_P_44>]

Just bought an 83 Nitro 17'er with an 85 XR-2 150 HP Merc. The boat sat in the owner's garage for three years without running (fuel drained and winterised). Compression is good (125-135 on all six) and on the muffs, it starts right up and idles smoothly. On the water, it starts but misfires at idle (pop/chuff out the exhaust) and will die frequently, especially after pulling back to idle after a run across the lake. If I use 1400 rpm as a no-wake speed it runs great; pull it back to 1200 and it will die. Pull it back into neutral and it will sometimes run fine at 1000 rpm and sometimes die. It will always restart, and the bulb stays firm at all times. Checked all six plugs and they are firing, none looks fouled, and as I say, it runs great above 1400 rpm. I also pulled all six slow speed jets and checked to see that they were unobstructed. I do have both a Clymers and a Seloc manual. I'm thinking maybe carb synchronisation or fuel pump diaphragm (too rich at idle?) Big question is, why does it run perfectly on the hose? Less back pressure? Really hard to fix when it seems fine in the driveway and only misbehaves on the lake (one hour away). Any help, suggestions, etc, are greatly appreciated.

[<SamtheMan>]

My guess is that you will find that one or more of your cylinders is not firing at idle. It may have spark but isn't firing. First start pulling plug wires (best to ground the loose wire) 1 at a time and see if you can find one cylinder that isn't pulling its share. Then concentrate on the mixture for that cylinder, too much or too little. If it happens to be the cylinder that your fuel pump is attached to, check the fuel pump diaphragms before you go any further.

[<Ricky_P_44>]

Thank you for the advice. I pulled the boat out on the street, hooked up the muffs, and fired it up. While running at idle (1100 rpm) I pulled each plug wire in turn. Very little drop in actual rpm (less than 100) but I did notice the exhaust note change as each plug wire went off then back on. All cylinders are firing. Now, when it's in the water there is a bit more back pressure, right? As I said before, I have already verified that all six low speed jets are clear--is there an associated air passage I should be checking? How about the small vacuum hoses--I can see the ones on the port side but on the starboard side they're hidden behind the ignition components. Might I have a vacuum leak that under load is causing an idle misfire? Here's the part that puzzles me--I've alway thought that if an engine spits back through the carbs, it's lean, and if is pops or fires in the exhaust, it's rich--at least that theory seems to work with four strokes...am I overlooking something obvious here? I thought I read in another post where someone said that if your fuel pump was leaking internally, you'd be rich at idle, maybe enough to cause symptoms, yet at larger throttle openings the excess fuel is negligable. Once again, your suggestions are welcome and appreciated!

[<SamtheMan>]

Leaking fuel pump diaphragms can cause the engine to run rich or lean depending on where the leak is. I have a Suzuki DT85 that was having a rough idle and ran rough until about 3500 rpm where it would run fine up to WOT. I initially thought it was the carbs, but ultimately the problem turned out to be a hole in the inner fuel pump diaphragm that was squirting fuel directly into the intake port of the bottom cylinder (the one the fuel pump draws vacuum and pressure off of) effectively killing that cylinder (too rich) until the demand at higher rpm could keep up with it.

Sometimes a leaking fuel pump just sucks air and can cause a lean condition due to inadequate fuel supply. Similarly bad fuel line connections can suck air.

Your carb float valves may be stuck open or improperly set causing a rich idle.

You can't evaluate and set the idle properly on muffs without the backpressure and drag of the prop in gear. Maybe if the lake is that far away you can set up a tank to submerge it in that is big enough to get it in gear.