1985 170 HP Mercruser 3.7-Carb stays at higher idle after throttling down.

Hi Boaters,

I'll try to keep this simple.

At the end of last season I had spun out my engine coupler at the end of the season. While the motor was removed, I had my mechanic also replace a leaky oil pan gasket. I never put the boat back in the water in 2012, but it started fine in order to winterized it. This spring, everthing started fine, but I noticed as I throttled down to idle, the rpms would remain noticably higher. In the water, the rpms were high enough as to not allow me to put the boat in neutral. Looking at the throttle linkage there was a small gap between the fast idle cam (FIC) and throttle adjusting screw(TAS). If I put a small amount of pressure on the TAS to further seat to the FIC and the RPM"S would lower and I could then put it in neutral.

My resolution to the issue was to rotate down or clockwise 3 rotations, the throttle cable swivel linkage so the TAS would snug up to the FIC. And this worked...I think. I don't know if there exists a reason why I should not have done this. Why? Today, as we cruised the river at no wake or low speed, the engine rpm's seemed too low at times and the exaust had a rich smell. It seemed like the motor would miss or studder occasionallyand then return to normal. It seemed to get progressively noticable as the day went on. Almost like it wanted to stall, but wouldn't, until I was pulling her out for the day. The boat ran fine at higher speeds with no issues.

When I pulled the boat out, I thought that perhaps the idle was lower because the 3 rotations may have snugged the FIS too tight so I took her home and backed off the swivel linkage 1 rotation. I know it is out of the water, but I muffed it and it started fine and while idling down it returned to normal idle.

Anyone of you great mechanics (sincerely) believe what I did was wrong? If so, would you be so kind as to tell me what I should have done instead?

Thank you all for your insite.

Kurt

Re: 1985 170 HP Mercruser 3.7-Carb stays at higher idle after throttling down.

IMHO sounds like what you did was just fine, after all they make it adjustable for a reason. Sounds like you are running a little rich at low RPM. Might want to do two things: On muffs, start the engine and remove the flame arrestor to check that the choke has opened all the way, if it has not adjust it. Try adjusting the idle mixture screw: Engine off, note the present position, turn it all the way in until it bottoms on the seat (DO NOT FORCE IT). Turn it back out 1 1/4 turns. Start the engine. SLOWLY turn the screw in until the engine begins to stumble (too lean), then turn it out until it starts to stumble (too rich) then turn it back to half way between those two points. That will get you close. Set the RPM to 650-700 with the TSA screw and you should be OK. If you like you can make further adjustments once in the water. Good luck.

Re: 1985 170 HP Mercruser 3.7-Carb stays at higher idle after throttling down.

Ty, Terry for the great insight. . I will post back on what will no doubt be a success.

Re: 1985 170 HP Mercruser 3.7-Carb stays at higher idle after throttling down.

stonyloam said:

Might want to do two things: On muffs, start the engine and remove the flame arrestor to check that the choke has opened all the way, if it has not adjust it. Try adjusting the idle mixture screw: Engine off, note the present position, turn it all the way in until it bottoms on the seat (DO NOT FORCE IT). Turn it back out 1 1/4 turns. Start the engine. SLOWLY turn the screw in until the engine begins to stumble (too lean), then turn it out until it starts to stumble (too rich) then turn it back to half way between those two points.

Click to expand...

I made the adjustments and took her out yesterday. No real issues for about 4 hours, but when we returned to port she seemed to stammer or run rough coming out of high throttle into a no wake zone. It seemed to be running rich again like before. We limped back to the dock by pumping the throttle and she stalled coming in. I could not restart it at the dock, but when we pulled her out and got her home (an hour later)I put her on the muffs and after a couple times got her started. She would idle ok, but I can tell she's still running a little rich and rough.

Does it sound like a stuck or sticky float and would the heat of a days boating exacerbate the issue versus it running cold? Would bad points or condensor cause the same symptom. I rebuilt the carb about 5 years ago.

My gratitude for any insight from the forum. If you need additional info I will galdly provide.

Kurt