Puzzling problem with Merc 160

[thunderroad]

SN 2497444. It runs great at high rpm but if I have to idle or putt around long it starts sputtering. A short run at WOT gets it squared back up for a while but as the day progresses, even that doesn't clear it up. Early on, it starts fine when hot, later in the day, not so well. Temperature stays in the green at all times. Also...when I pulled it out of the water yesterday, I noticed the rear water line was a lot more black with exhaust soot than normal. Now..I did more idling than normal yesterday . I took the plugs (new NGK BR9ES gapped at .035) out today and they really didn't look all that black. Points are new, dwell is within range. Timing is 6 BTDC. New plug wires. New fuel filter. Took the carb apart this morning and cleaned all the passages. There was a little gunk present, but not all that much. It sounds fine on the muffs right now, but it did yesterday morning before I went out too.
Anyone have any ideas about something I should check?

 

[DJ]

Re: Puzzling problem with Merc 160

Look at the clear tube that runs from the fuel pump to the carb.

Is there fuel in it?

There shouldn't be. If there is, your fuel pump diaphragm is leaking.

 

[Don S]

Re: Puzzling problem with Merc 160

Took the carb apart this morning and cleaned all the passages. There was a little gunk present, but not all that much.

There is a lot more to it than that. They usually need soaked in carb cleaner, then the passages blown out with compressed air. Then reassembled and adjustments made. Once reinstalled, the carbs idle mixture screwes have to be adjusted to the engine. This is not just putting them to the preliminary adjustments stated in the carb specs. They are just there to get it running and warmed up for final adjustment.
The engine also has to have good compression and a good properly adjusted ignition system. Point dwell, timing, idle rpm, idle mixture all have to work together. The choke also has to work, and as DJ points out, a defective fuel pump can't be dumping raw fuel into the carb during all this.

 

[thunderroad]

Re: Puzzling problem with Merc 160

thanks for the replies. The compression seems to be ok. I checked it when this first started. It was between 125 and 135 on each cylinder. As for this clear hose, I don't have one. I do have a short black hose that loops from the top side of the fuel pump and tees into the main supply line to the carb. It appears to be factory because it has the crimped connections. I replaced the fuel pump with an OEM a couple years ago and it all hooked right up.
I'll do some adjusting on the carb next.
I'm not too cheap to pay a tech to work on it, but the closest (good) one is over 2 hours away. Lots of places nearby that work on outboards, but they start shakin their heads when I say I/O.