Idle mixture setting on 87 Force 125

sidenberg · Jun 14, 2010

engines runs best with idle mixture screws turn out 3/4 from lightly seated. Any richer than this it runs to rough at lower rpms,(vibrates and intermitten skipping). I understand this setting may be considered too lean, however it runs smooth at this level, (although still tends to smoke somewhat within the lower rpm range) and, shows no sign of overheating. So it seems the proper amount of fuel is entering the cylinders. I have noticed the idle mixture screws are somewhat rounded at the tip instead of sharp. Could this be the reason of the 3/4 turn setting? If not, what other things may have an effect on an engine needing such a lean idle setting? Timing?

jaybirddog11 · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

How long since you have cleaned the carbs?

sidenberg · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

I cleaned out both carbs within the last month. I installed a new inlet needle and seat in the top carb only as the bottom one was still okay. I also installed a water/fuel filter at the same time. I suppose it is possible the carbs may have become dirty again but if this was true it would seem that it would not run good no matter how the idle mixture is set. true? I quess the question is 3/4 turn too lean? The clymer book says start at 1 1/4 turn out but I just looked Force owners manual for 1987 85 and 125 HP that says start at 1 turn out. Given that 3/4 turn may not be that bad?

jaybirddog11 · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

It really should be set at 1 to 1 1/4 turn out. I have heard numerous times ,on this site, that these engines do not like to idle. They are meant to be run at higher rmps. When set to 1 1/4 turns out, how does your engine run at WOT? You might consider a decarb. Not sure if it will help, but it wont hurt either. If you run it to lean at WOT, you could melt the pistons.

sidenberg · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

The engine runs great at WOT no matter where the idle mixture screws are set.
I understand however that the idle mixture does not affect the carb at WOT as the engine pulls from the main jet at these rpms. I am wondering why the motor runs best at lower rpms with a leaner mixture and if my setting is actually considered lean given the condition of the mixture screws. If not what other conditions might contribute to this "need to be lean"?

jaybirddog11 · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

How long since you rebuilt your fuel pump, (new diaphragm, gasket and check valves)?

sidenberg · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

I have not messed with the fuel pump as it seemed to be working fine. However I did purchase all the parts needed. Could a fuel pump problem cause a "need for lean" issue?

jaybirddog11 · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

If your check valves are sticking or your diaphragm is either old or torn, you could and probable will see an improvement with your idle mixture setting. Does your motor ever backfire? A torn diaphragm is a likely culprit if your motor does backfire. FYI, backfires also can rupture diaphragms. As you may have read on this forum, the ethonal that is added to gasoline is causing all sorts of problems for these older outboards. You might consider adding seafoam, or a similar product, to every gallon of gas you use to counteract the effects of ethanol.

sidenberg · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

I appreciate your assistance here Jaybirddog. This forum is great and I have learned quite a lot from all the great people here. Regarding the fuel pump, I would think if it was not functioning correctly it would also make the engine run poorly at all speeds. How or why would a plugged check valve or a torn diaphram cause a motor to run poorly only in the lower rpms?

foodfisher · Jun 14, 2010

Re: Idle mixture setting on 87 Force 125

750-800 RPMS at idle in gear (forward). Mixture screws 1-1 1/4 out from light seating. Fine tune at WOT. Clean plugs, they will foul with excessive idling. Sounds like you could use a trolling motor/kicker for your easy cruising. Let me reassert that the 125's don,t like to idle for extended periods.

jaybirddog11 · Jun 15, 2010

Re: Idle mixture setting on 87 Force 125

sidenberg said:
I appreciate your assistance here Jaybirddog. This forum is great and I have learned quite a lot from all the great people here. Regarding the fuel pump, I would think if it was not functioning correctly it would also make the engine run poorly at all speeds. How or why would a plugged check valve or a torn diaphram cause a motor to run better only in the lower rpms?

I may not be correct, but anything that messes with your fuel delivery system affects your performance. A torn diaphragm would be more obvious to diagnose than an old one or one that has pin holes but for $4 it could not hurt to replace. If your check valves are sticky, I suppose it could be possible that the increased pressure at WOT could free them, (just a guess). I gave the advice to replace because this seemed to help my issues and its cheap to replace and they should be replaced occasionally. After I rebuilt my fuel pump and cleaned the carbs, I was able to adjust the idle screw to a proper setting. Again, I could be wrong. I hope one of the pros here can chime in and give a more technical answer or just tell me I am wrong (lol).

roscoe · Jun 15, 2010

Re: Idle mixture setting on 87 Force 125

If your idle SPEED is too low, then setting the idle MIXTURE at 3/4 turn will lean it out and cause the engine speed to increase at idle, thus it seems to run better runs better. But it is too lean.

increase your idle SPEED , get it up to at least 800 in gear, then readjust your mixture.

When set properly, you should NOT have instantaneous acceleration when you punch the throttle.
If you do, you are too lean on the mixture.

You should have a slight lag/stumble, then acceleration.

OldNBold51 · Jun 15, 2010

Re: Idle mixture setting on 87 Force 125

sidenberg said:
engines runs best with idle mixture screws turn out 3/4 from lightly seated. Any richer than this it runs to rough at lower rpms,(vibrates and intermitten skipping). I understand this setting may be considered too lean, however it runs smooth at this level, (although still tends to smoke somewhat within the lower rpm range) and, shows no sign of overheating. So it seems the proper amount of fuel is entering the cylinders. I have noticed the idle mixture screws are somewhat rounded at the tip instead of sharp. Could this be the reason of the 3/4 turn setting? If not, what other things may have an effect on an engine needing such a lean idle setting? Timing?

FWIW, in all my years of jetting 2 strokes, I go by the saying " it's not right jetting nor wrong jetting, it either works well or it doesn't". I jet till it works well.

sidenberg · Jun 15, 2010

Re: Idle mixture setting on 87 Force 125

I went ahead and replaced the fuel pump diaphram and two of the check valves. I could not get the 2nd stage check valve out but it looked okay anyway. In fact, the diaphram and the other two check valves looked good as well. I have yet to run the motor after the replacement but I hope to this weekend. We will see if there is any difference.

sidenberg · Jun 18, 2010

Re: Idle mixture setting on 87 Force 125

Installed new fuel pump diaphragm and two check valves,(could not get 2nd stage valve out but old one looked ok anyway). Result is no change. Engine still runs at its best at cruising speed (midrange rpms) with idle mixture screws turn out 3/4 from seated. So at this point I have carbs cleaned, seats and float all good, linked and sinc completed, cleaned and replaced all fuel recirculation components, new spark plugs with verified good spark, compression all cylinders at 145 PSI, decarbed twice with seafoam. The fact is the engine runs great at this setting at all speeds. As I said before, if the idle mixture screw is turned out any more it still idles good and runs good at WOT but runs rough, skips, seems to lose power at cruising speeds (1500 to 2500 rpms) Turn the screws in and it smoothes right out. Some have said this is normal as these engines are born to run WOT and dont cruise well. This does not sound right to me. The only thing I have not checked is the timing.

Would an incorrect timing, too advanced or too retarded have an effect on the midrange fuel mixture requirement?

foodfisher · Jun 18, 2010

Re: Idle mixture setting on 87 Force 125

Timing should be a no brainer. It has to be right. You seem to be bucking a trend here. I'd hate to see you burn/pit a piston because of it.

sidenberg · Jun 19, 2010

Re: Idle mixture setting on 87 Force 125

Foodfisher, are you saying yes my timing could be off and that could be affecting the idle mixture setting or are you saying the timing would have no affect on the idle mixture setting?

jaybirddog11 · Jun 19, 2010

Re: Idle mixture setting on 87 Force 125

Timing needs to be right and it could throw off your idle mixture setting.

foodfisher · Jun 19, 2010

Re: Idle mixture setting on 87 Force 125

Timing too advanced = burned piston/s. Fuel mixture too lean = burned piston/s. Set timing right, set mixture screws 1 1/4 out from light seating. Then adjust idle speed, 750-800 in gear. Then adjust the mixture screws. Screw in till faulty sputters. Back out till faulty sputters. Back in half way between. Will probably end up 1 to 1 1/4 out. Not 3/4. Hope I'm right and this helps.

sidenberg · Jun 23, 2010

Re: Idle mixture setting on 87 Force 125 - better

Re: Idle mixture setting on 87 Force 125 - better

Much improved. I reset timing from 30* static to 28* static. As a result I needed to re-adjust the idle and then re-sync the carbs. At this last step I notice there was a slight gap between the cam and the eccentric screw roller that probably had something to do with the carburetors not getting the proper mix of fuel to air. Regardless, the engine runs much better in the lower rpms with the mixture screws set at 1 turn out. Now to the next issue in my new post.

Idle mixture setting on 87 Force 125

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