M18C2 idle adjustment

[cgd27]

I was able to reassemble the carb this evening after locating and replacing the main nozzle that I assume was broken by the previous owner. I also noticed that the timing was also off as well as the linkage adjustment. I can only guess that these changes were made to get the engine to run long enough to get me to buy it. I reset the linkage back to 28mm and set the timing at WOT to the 25 degree mark. I was able to start and run the engine with the idle adjustment at 1.5 turns out from seated, I ran the engine in a trash can so was not able to fine tune the mixture. I noticed now that the engine seems to idle rather fast so my question is, what is proper procedure for lowering the idle? How much is dependent on the idle stop screw and how much is the idle screw on the carb itself? I do not have a manual so I was hoping someone could give me a little help on what I need to adjust first. I looked through forums but can't find many answers to my particular question.

 

[pvanv]

The idle RPM is set solely via the idle stop screw.

 

[cgd27]

What is the use for the idle screw on the carb or at least I assume it's an idle screw

 

[pvanv]

Low speed mixture. Carbs 101. The final idle mixture (pilot screw setting) is done with a good ear, a shop tach, and in conjunction with the idle stop. Once the max idle speed is attained by optimizing mixture (within idle speed specs), the pilot needs to be richened about 1/4 turn for stability, and the the final idle speed is set with the stop screw.

 

[cgd27]

I understand what your saying but what is the purpose of the idle screw on the carb? It acts the same way as the idle stop screw if you turn in, rpm picks up. Turn it out rpm decreases. I have the idle mixture theory figured out just don't understand the need for two different screws that ultimately achieve the same thing.

 

[Sea Rider]

Does not achieves same thing. Mixture screw mixes air and fuel for idle speed, good, even ralenti performance at idle. Once spot on adjusted idle screw will adjust idle rpm for a nice even min rpm while throttle is set at idle. Adjust rpm screw to a point that when engine is geared forward you can hear a soft pinion claking while engine doesn't die, if dies, adjust screw CW some more till this cond is met. Combo must be on water and engine at load while pushing boat, adjusting on a barrel or muffs is not same.

Useless to buy an induction tach just to set idle rpm which has erratic idle rpm as opposed to needle tachs for this sole purpose, unless wanting one to check max wot rpm, play and maximize prop pitches for specific needs.

Happy Boating

 

[cgd27]

That's what I was wondering because I have three different screws. One being the fuel mixture screw and the other two being the idle stop screw and idle speed screw on the carb that I guess works the butterfly.

 

[Sea Rider]

Strange, can you post a pic of that 18 C2 carb ? Newer 18 E2 has only 2. Usually the butterfly opening is controlled via the idle screw, the one that is located at a vertical position.

Happy Boating

 

[cgd27]

It will not allow me to upload. I have the pictures on my phone and selected it from the album but the page just says that it's working but doesn't do anything. The idle speed screw does work the butterfly but it is horizontal facing the front of the motor. the idle stop screw is located on the left side of the block if looking at the motor from behind. What I attempted tonight was to back out the idle screw so that the butterfly was completely closed and adjusted the idle stop screw to its lowest reliable setting then on the mixture screw. I backed it out 1.5 turns then ran it in while adjusting the idle stop screw. From that point I listened for the motor to start missing then backed it out 1/4 turn. Then adjusted the idle speed screw to slightly raise rpm so it was slow and steady. Not sure if that is proper procedure but is a start

 

[Sea Rider]

Backing 1.5 turns is a reference from where to start, you should adjust mix screw to a point you can hear the engine reving smoothly and even when fully warmed up. Then adjust idle screw to be around 1 K revs at idle. Both 18C2 & 18 E2 differs from each other being the latter more simpler to adjust. If you have adjusted carb while OB was on muffs, barrel will probably need to re adjust idle screw once combo is on water and load, if not, engine will probably die when gearing forward. Adjust idle screw CW to a point for engine not to die.

Happy Boating

 

[cgd27]

I plan to try it out this weekend and will bring along some tools so I can make any necessary adjustments on the fly. Thanks for all your help

 

[cgd27]

Took the boat out today for a spin, started out with a slight lean miss. Backed out mixture screw about 1/8 to 1/4 turn and all is well. It's amazing how this engine is now that I replaced the nozzle. No problem out running the fish. Thanks to everyone for your help once again.

 

[Sea Rider]

With engine fully warmed up at idle, usually you have to back out the mixture screw a tad from factory recommended settings. Slight richer is preferable than leaner. Enjoy that nice 18 horse.

Happy Boating