1989 force 125 air/fuel mix adjustment.

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
Got my first boat. 1989 Larson runabout with a force 125. Runs pretty good. I've linked and sinked the carbs. I read somewhere the air/fuel mix setting is 1-1 1/8 turn open from closed position. I have found mine to be running best at 3/4 turn open from closed. I am doing this by ear not vacuum since I am a musician and have an excellent ear. My fear is running lean. What would be the explanation for this difference/ or is this normal?
 

NYBo

Admiral
Joined
Oct 23, 2008
Messages
7,107
Welcome to iboats!:welcome:

You are right in being concerned about running too lean- it can cause major damage (ask me how I know this). The fact is that carbureted two-stroke motors will often running a little rough and smokey at idle when the mixture is correct. Back the scwews out to 1 turn at least, then reset the idle speed.
 

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
Thank you for the reply. The idle adjustment is all the way out. Tower is as advanced as it can be by the idle screw. Runs really rough if at all at 1 full turn from closed.
 

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
I guess this does make sense. The idle was adjusted all the way out to compensate for the lack of fuel I guess. Will try one turn and back the idle off as you suggested. Thanks.
 
Last edited:

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
Plugs are new and cleaned in between runs. I was using the ear for air fuel mixture not rpms. Problem isn't rpms , problem is running rough at 1 turn or more. It runs better below 1. I haven't confirmed proper timing.
in hind sight rpms led me to realize there was a problem..
 
Last edited:

NYBo

Admiral
Joined
Oct 23, 2008
Messages
7,107
Thank you for the reply. The idle adjustment is all the way out. Tower is as advanced as it can be by the idle screw. Runs really rough if at all at 1 full turn from closed.
That sounds like what the settings were on my motor when I got it. Because it was running smoothly I never checked the mixture. I am now the "proud" owner of a motor with broken piston rings in two cylinders after maybe 3 or 4 hours of running. :facepalm:
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
The problem is uneven fuel distribution because of manifold design. One cylinder of each set of carbs will usually run an little leaner. This is OK at idle and lower RPM but since these carbs are designed so that the low speed circuit will still deliver fuel at wide open throttle, and since the high speed jet is sized to compensate for this extra fuel, setting the carbs too lean at idle will most certainly be too lean on one piston at wide open throttle. This will lead very shortly to piston damage, usually one of each pair serviced by a single carb.

. On SOME 3 carb , 3 cylinder engines, 3/4 turn out at idle is acceptable and will usually do no harm. HOWEVER, on the 4 cylinder engines it is best to run no leaner than one turn out from lightly seated.

Unfortunately, these engines like to idle and give a beautiful solid idle with the low speed needles set lean.

AS I have reiterated numerous times on this forum: No matter what the idle quality, do not set the carbs too lean. It is way easier and cheaper to change fouled spark plugs due to a rich mix than it is to change pistons.
 

Jiggz

Captain
Joined
Oct 23, 2009
Messages
3,817
When you mentioned you had the idle screw all way in, it tells me something is not adjusted right hence compensating by increasing rpm with a semi-open throttle. The initial setting of 1 turn out from slightly seated is just that "initial" because it ensures proper lube and minimum amount of fuel to start the engine. But for the motor to continue to idle, the idle screw must be set at least to 900-1000 in neutral on muffs.

I have exactly the same engine and yes, it sure idles more smoothly at less than 1 turn out on the idle mixture screw. The reason is obvious, there is better ratio of fuel-oil mix and air thus better combustion. In fact, in this setting, there is also less smoke from the exhaust. HOWEVER, there is also less oil going into the cylinders those reduced lubrication which can seriously damaged an engine.

If you still believe your hearing is a better tool for tuning up your motor, obviously you can do so. Personally, I go with the service manual or from experience which I believe is referred to a Good Engineering Practices. One piston replacement will set you back at least $250 if you do the job yourself or more than $700 if done by a certified mechanic.
 

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
The problem is uneven fuel distribution because of manifold design. One cylinder of each set of carbs will usually run an little leaner. This is OK at idle and lower RPM but since these carbs are designed so that the low speed circuit will still deliver fuel at wide open throttle, and since the high speed jet is sized to compensate for this extra fuel, setting the carbs too lean at idle will most certainly be too lean on one piston at wide open throttle. This will lead very shortly to piston damage, usually one of each pair serviced by a single carb.

. On SOME 3 carb , 3 cylinder engines, 3/4 turn out at idle is acceptable and will usually do no harm. HOWEVER, on the 4 cylinder engines it is best to run no leaner than one turn out from lightly seated.

Unfortunately, these engines like to idle and give a beautiful solid idle with the low speed needles set lean.

AS I have reiterated numerous times on this forum: No matter what the idle quality, do not set the carbs too lean. It is way easier and cheaper to change fouled spark plugs due to a rich mix than it is to change pistons.

thanks frank! I knew there has to be an explanation for this. It's the first time I have ever adjusted a carb by ear or vacuum and been too lean. I figured something must been odd. Usually with carbs being used a slightly more open adjustment will run smoother, not the other way around! Ha. For the record I haven't touched the idle screw, it was adjusted all the way in when I got it. I had just gotten to the point of adjusting it when I saw it was already all the way in but the rpm gauge was only at about 750-800, so I couldn't even advance it. That's when I knew something was screwy. Correct me if I'm wrong... (I completely understand no less than 1 turn open) in order to truly dial in the carbs they need to be adjusted while at wot. (I'm just batting around ideas here so if I'm aggravating anyone here I apologize and don't feel obligated to comment.) Of course this would have to be done on the water on a calm day. If 1-1 1/8 open is considered the end all adjustment to air/fuel then I'm just totally over thinking it. Btw Forget the "excellent ear comment" I guess I never should have said it. Apparently some took this to mean I prefer hearing over tools and knowledge... I've done initial setups by ear hundreds of times. I now know I can't on this motor, except at wide open throttle... (IF my understanding is correct)
thank you again gentlemen
 

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
That sounds like what the settings were on my motor when I got it. Because it was running smoothly I never checked the mixture. I am now the "proud" owner of a motor with broken piston rings in two cylinders after maybe 3 or 4 hours of running. :facepalm:

i got this boat recently. I checked compression, linked and synced the carbs, fired it up, adjusted air/fuel to purr, noticed the rpm at 750-800, went to adjust the idle and realized it was all the way advanced already! That's when I came the house and posted my topic. I knew something wasn't right. Sorry about your motor.
 

icebergz12

Cadet
Joined
Jul 29, 2014
Messages
10
I know this is very late, but got it running like a top shortly after my previous post. It turned out to be the accentuated adjustment screw on the linkage want in the proper position. I guess i missed that adjustment during my link 'n sync process. I couldn't get it to run 1000rpm at idle without being lean on the mix. Thanks again guys.
 
Top