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Rough Idle

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TerryL

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I have a 1988 Mercury 90 hp outboard. I rebuilt the carbs and replaced the fuel pump, timed the engine checked compression and replaced the water pump this past winter. I took it out this weekend and it still idles rough and stalls. Could this be caused by too much oil in the gas?
 
How did you check the timing and setup your carbs? What value did you use for idle timing and WOT? Where did you set your low speed needles for the initial run up? How did you setup your carb linkage? With the engine warmed up, in F gear at idle, in the water moving along, what is the rpm?

How much is too much oil? If you are still using the engine internal oiling system it supplies oil at an 80:1 fuel oil mix at idle and that increases to a 50:1 ratio at WOT...more oil needed at the higher rpms. Too much oil would cause obvious visible smoking. Even with regular dino oil that engine doesn't smoke much. I use syn blend and it hardly smokes at all, even on the hole shot and I'm running pre-mix 50:1.

Answer these questions and I'll move on.

Mark
 
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Texasmark said:
How did you check the timing and setup your carbs? What value did you use for idle timing and WOT? Where did you set your low speed needles for the initial run up? How did you setup your carb linkage? With the engine warmed up, in F gear at idle, in the water moving along, what is the rpm?

How much is too much oil? If you are still using the engine internal oiling system it supplies oil at an 80:1 fuel oil mix at idle and that increases to a 50:1 ratio at WOT...more oil needed at the higher rpms. Too much oil would cause obvious visible smoking. Even with regular dino oil that engine doesn't smoke much. I use syn blend and it hardly smokes at all, even on the hole shot and I'm running pre-mix 50:1.

Answer these questions and I'll move on.

Mark
Click to expand...


I have the assistance of a marine mechanic for timing and carbs. My engine is not oil injected it is a 50:1 mix.
 
Okaye. Then we will assume that the carb linkage is set up properly with the common rail opening all butterflies fully at full advance and the common connection linkage white lines lined up with the linkage relaxed. The low speed jets are somewhere around 1.5 turns open. Timing is around zero at idle and 20-22 at WOT. Rpm under conditions stated would be 675 +/- 25, adjusted by the idle knob which varies the timing around zero BTDC.

Get a battery operated timing light and take to your place of boating on a day when the launching ramp isn't busy. Pop the cowl and back the trailer in the water with the boat still secured. Ensure there is adequate water over the water pump and check the tell tale when operating.

Start the engine and let it warm up. While warming, connect the timing light to battery power (input terminal to starting solenoid and engine block works for me). Take the sensor and just move it in the viscinity of the spark plug wires. Just getting within 1 inch should light it off. Check all 3 wires. If all fire good. If not, check that plug and CDI. End of elect test.


Shift to F and goose it. If it won't take throttle open the low speed jets the width of a screw driver at a time until it will, if it will, till you get to about 1 3/4 turns open and it will take a goose without stuttering. Unless you have something else wrong, you should be there by now.

Mark
 
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