Mixture Screw

[Sea Rider]

Paul,

Does this makes sense : I'm not a big fan counting mixture screw turns when removing them from carb. Preffer measuring the height between crew head to carb base at 6 o'clock position with a vernier tool. Usually comes factory adjusted to 10 mm, after 2 stroke engines are sufficiently warrmed up at idle/neutral, when trottle is given, sometimes engine experiments a throttle hesitation which has beed addressed on the forums, it's not a warrming issue, 5 minutes is more than sufficient as owner's manual recommends mim 3 minutes.

Have found that on engines factory delivered with 12 mm + heights this hesitation issue is more notorious, could this conclude that carb delivering a richer fuel/oil mixture will tend to foul plugs more than when at 10 mm while being at idle/neutral ? To overcome this hesitation, with engine at gear, open throttle to slight passing starting position and let engine run briefly untill hesitation dissapears, then you can gradually open throttle throughout tthe whole throtttle range.

Hesitation seems a product of plug fouling/cold motor temp related issue specially on first morning starts.

Happy Boating

 

[pvanv]

Correct mixture is affected by a lot of factors, including air density, humidity, temperature, velocity through the intake system, fuel/oil mix, and fuel octane.

The starting point for the low-speed mixture (either by counting turns or measuring screw height) is just that -- initial setting -- that allows the motor to start/run. Fine tuning the mixture takes patience, a good experienced ear, and a very good shop tach. Screw height is only valid if the needle is exactly identical from motor to motor, and the carb casting and needle seat is identical from motor to motor. Counting turns will typically be more accurate, since with x threads per mm, the same number of turns out always produces the same distance from the needle tip to the seat. Yes, a richer needle will foul plugs faster. As the mixture screw becomes less of an influence at higher speeds, the main jetting then becomes the predominant factor in determining the mixture.

On carbed systems that do not have an accelerator pump, slow speed mixture is always a compromise... needs to be rich enough to mask bogging when opening the carb quickly, yet must be lean enough to reduce plug fouling.

Colder conditions and slower velocities tend to cause fuel dropout (with a leaner air stream reaching the cylinders), which is compensated for with enrichment; that goes away as the intake and crankcase warms, and at higher mixture velocities. That's why carbed motors have chokes or automatic enrichment systems.

Those velocity principles are used on modern EPA-rated motors, by using carbs with longer, narrower passages -- that have higher velocities, which promote better emulsification of the air/fuel mix.

 

[Sea Rider]

Thanks Paul,

Have not experienced what's posted on the initial thread with brand new engines, begins developing those small issues after 150 worked hours. Have even torqued the whole engine which was bit out of specs, overall gaskets were lose due to heat compression with no avail. Not an issue living with it as long you can address the isssue correctly to minimize hesitation frustration.

With respect to screw turns, on new boxed 2 stroke 18-25-30-40 HP engines most of the times comes factory adjusted around 10 mm height, probably matches their specified turns for each HP screw model. The issue with adjustting traditionally with turns is that it's very hard to determine which is a opt lightly seated screw condition from where to start backing screw up.

Happy Boating

 

[pvanv]

I usually tighten until the screw bottoms, and no more. just a significant increase in resistance. If you overtighten, you will distort the screw tip and the seat, making future adjustments difficult or impossible. Just lightly seated.

 

[Sea Rider]

Paul,

Other tech issue. I'm currently using a 2 stroke 18 HP L engine, came factory adjusted with carb reaching it's max opened butterfly just reaching the high peak of the cam. As this engine will not move a heavier load boat as a sailboat, have addjusted carb to fully open butterrfly at cam's end, same as factory delivered S models. Liking running wot some times, found this condition gave an excesive over actuation on carb's roller and arm and plastic parts.

Would you say that advancing butterfly to fully open just reaching cam's front (not mounting over it) will give more punch to engine to move better a heavier load, timing plate settings are factory spot on, no need to move, carb apparently doesn't. Have reset arm to fully open at cam's end, same as in S models and with mixture screw reset at 10 mm, was delivered with 12 mm. Engine purrs as a kitten, backing 2 mm has miracously near stop the heistation issue at idle/neutral as opposed to before.

Happy Boating

 

[pvanv]

If it is working for you, stick with it...