engines not running right

[LiquidWrench]

Got my boat in the water yesterday for the first time this season. Last year I had carb problems so they were both rebuilt. Both engines started fine and on the way out ran great, 3500-3800 and singing their little hearts out. The problem came when we pulled into another port. As soon as we slowed down the starboard engine started missing badly and died. I could get it to start back up but it would not idle. If I could keep it up to about 1500 it would stay running. Turned around to head back to the ramp on one engine. After clearing the pier I was able to get the starboard engine running again, in gear, and the RPM up before it died. Running about 3400 on both engines on the way back and now the port engine is acting up. Made it back to the ramp but as soon as I pulled the throttles back the starboard engine is gone again. Port engine smoothed out at lower speeds.

Both engines draw fuel from the same tank, I have all new fuel lines and a new fuel/water separator on each engine.

I plan on pulling both separators and emptying them and see if I get any water or other crud out of them. When the carbs were rebuilt they were completely disassembled, everything cleaned, float levels set to Mercruiser spec.

Any other ideas of things to look for?

 

[hottoddie]

Sure sounds like water/contamination. How old is the fuel? Is it E10? Is the gasket/seal on the gas cap there and in good condition?

 

[LiquidWrench]

half tank was from last season, seal on the fuel cap is good but I did have a leaking filler hose so I may have got some moisture in there over the winter. I did put 20 gal of fresh fuel in just before heading to the lake.

 

[Mike Blake]

Vacuum leak around the carb maybe? Did you change all the fuel filters when you did the carbs?

 

[LiquidWrench]

Hadn't thought about the possibility of a vacuum leak, I will check that out as well. The filter in the carb inlet and pump filters are all that it originally had and both were replaced. I added the fuel/water separators (can't believe that Mercruiser or Sea Ray didn't install them). We have a good storm due so I probably won't get things checked out today.

Keep the ideas coming.

 

[Mike Blake]

Fuel pumps, electric or manual?? Pressure good?

 

[LiquidWrench]

They are mechanical pumps, one replaced just before I got the boat, the other is good. 4 PSI out of both pumps.

 

[LiquidWrench]

Well I am officially stumped. Both filters came out clean put new ones on just to be safe. Timing is spot-on, no vacuum leaks. Point Gap is perfect. Any other ideas?

 

[Mike Blake]

Compression and spark is all you need. I assume you checked for gas in the carb. You didn't by chance loose a timing chain did you? Do a quick compression check maybe 2-3 cylinders.

 

[Bt Doctur]

remove the spark plugs , keep them in order and post pics or tell us the color of the tips

 

[In Two Deep]

I have had that problem when the batteries are weak or not charged to 12+. Engine will start but not remaining running. If increase the RPMs on the other engine, I can get the stalled engine started and remain running. Once batteries have a good charge on them, the problem goes away. Maybe unrelated, but worth a look.

 

[LiquidWrench]

I will bring my tools home tomorrow to pull the plugs and run a compression check. Charging system on both engines is working well so no problem there. I had both engines running last night on muffs after changing out the filters and they almost act like they are running too rich, starboard engine especially, plugs will show that problem. Did Merc put out any SB's regarding float height like they did about timing and I just never found it? Before someone asks, yes the exhaust manifolds and risers are good, engine temps were right in the middle on the gauges out running on the lake. Running on muffs temps stabilize right at 160 and don't fluctuate more than about 4 degrees, this was verified with an infrared thermometer. Timing advance curve seems to be right in line with what I have found online from the factory manuals.

 

[fc3]

Sounds like the low speed circuit in the carbs are not setup correctly. Probably ran okay at low speed when cold because the choke was partly applied, but after running a while and the engines warmed completely, the choke was't partly closed to enrich the mixture when switching back to the low speed circuit.

I'd go over the carb again. Easy test, try turning the idle mixture screws out about a half turn at a time and see if that helps. First count the turns to lightly seat the mixture screws. It should be roughly two full turns. Then turn both out the same number of turns, at least two, and try to start the boat. Keep going a half turn more until it runs okay or you reach a total of 4 to 6 full turns.

 

[In Two Deep]

When I had a similar problem, the mechanic ended up replacing the IAC (Idle Air Control) Motors and the problem was solved.

 

[LiquidWrench]

Finally was able to get around to checking things out further. All of the plugs are black and sooty which confirms what I thought about the engines running rich. Compression check showed all cylinders at 120 cold and dry after having set for a couple weeks. Pulled the carb covers again and found my float levels too high. I must have bumped them just right when I put the carbs back together. Ill find out if this fixed the problem when i get the shift cables replaced and drives back on.