My 1998 Merc 40hp ELPTO rough idle

[tmanis0182]

Okay, so I took my boat out on it's 2nd fishing jaunt since repowering my 85 Bass Tracker with a 98 Merc 3 cyl. 40hp elpto. The motor pushes the boat very well, it gets up on plane pretty quick, runs smooth once I get the rpms above 1000 and runs in the recommended rpm range at WOT (5250 rpms) with no bogging. At idle however, things are different and it runs a bit rough. Enought to feel the vibrations throughout the boat. If you watch the motor at idle it bucks a little from left to right . Replaced the plugs, it has decent compression (116, 118, 118 psi)and i have run half a tank of premium unleaded with marine stabil (double dose, first run of the season mixture) through it, but still idles rough. Any thoughts, or am I being overly anal

 

[Pontoonfoon]

Re: My 1998 Merc 40hp ELPTO rough idle

my 2007 90 ELPTO does the exact same thing, runs like a champ from 1500 and up but at idle bucks, shudders and sometimes dies.

 

[CharlieB]

Re: My 1998 Merc 40hp ELPTO rough idle

Idle mixture adjustment is not quite perfect.

Above 1500 the main jet is beginning to flow the majority of the fuel and performance improves.

Idle adjustment must be done with the correct fuel mixture, doing so with a 'double' mix of oil would require final adjustment once you return to a 50 : 1 mixture.

Idle mixture must be set running IN the water and IN Forward gear. The water column in the midsection is required for the proper exhaust back pressure, the prop load is required.

Adjust each carb idle mixture screw to achieve the BEST idle speed, then further enrichen the mixture (back out the screw) and additional 1/8 turn to provide the necessary enrichment for acceleration.

Test buy slamming the throttle from idle in gear to WOT.

Any hesitation, lean cough, bog, or back fire, further enrichen the mixture (turn out) the mixture screws just the width of the screwdriver slot and retest.

Repeat acceleration test until perfection.

 

[tmanis0182]

Re: My 1998 Merc 40hp ELPTO rough idle

Thank you CharlieB!! That is what I was waiting to hear. I plan on getting out there this weekend and see if I can adjust those carbs as you instructed. I'll let you know how it goes.

 

[Pontoonfoon]

Re: My 1998 Merc 40hp ELPTO rough idle

YAY!! cant wait to try this.

just gotta figure out where these screws are and Ill be ready.

 

[wingsaircharter]

Re: My 1998 Merc 40hp ELPTO rough idle

My 40hp 3 cylinder does the same thing but that didn't help. Above 2000 rpm runs great. Below, it intermittently coughs. Are those mixture screws on the port side setting fuel or air? Assuming fuel, I backed them out to richen up mixture but still runs poorly. It does seem too lean. Maybe carbs need to be cleaned. Does it have fixed main jets and adjustable idle jets? What about midrange? Is there a jet for midrange?