Air mixture screw

[MH Hawker]

Being as its been slow lately I thought I would ask about this.

Over last winter I converted my 1996 9.9 hp 0G119661 to a 15 hp and it has worked very well and I have put a bit over 80 hours on the motor and the only issue is a WOT RPM thing, the motor runs between 5400 and 5600, and it will just up and down a bit, I tested the tach, and went over the fuel system, and pulled the plugs for a inspection and that may be my first clue. The plugs were in very good shape but it appears it is a touch lean they are a very light gray, the outer shell was black with carbon so I turned up the mixture screw a 1/4 turn and that seamed to fix the RPM fluctuation or at least improved it.

I was wondering how much effect the air mixture screw has on top end performance. I had thought it was mostly at low RPMs.

 

[Texasmark]

Re: Air mixture screw

Most carbureted 2 strokes of the last who knows how many years have fixed high speed jets....a plug in the carb with a certain size hole drilled in it. The low speed is adjustable with a spring loaded set screw. Usually this is set for best low speed performance and the ability of the engine to take throttle on the hole shot.

Mark

 

[MH Hawker]

Re: Air mixture screw

Thats what I had thought, I appreciate it Mark

 

[Dave1027]

Re: Air mixture screw

I was wondering how much effect the air mixture screw has on top end performance. I had thought it was mostly at low RPMs.

You are correct. Idle adjustments have hardly any effect on top end but the fact that you are seeing a small improvement could indicate a lean condition at wot. You should try squeezing the primer bulb at wot to see if that helps. If so then rebuild the fuel pump. If not then check float level or try the next size larger main jet.

 

[MH Hawker]

Re: Air mixture screw

The carb is more or less brand new and the fuel pump is built in. Its one of the very rare motors that can be converted from a 9.9 to a 15. The conversion is a new 15 hp carb and a few adjustments, link and synch and so on. Because its a conversion its not run of the mill problem and its run fine all summer I just back off of the throttle a bit. Now that the season is drawing to a close I thought I would dig a bit deeper into it.. I am sort of eliminating things. It may be just how it is.

 

[Faztbullet]

Re: Air mixture screw

Reset the float level as it sounds like it on verge of running bowl dry....

 

[MH Hawker]

Re: Air mixture screw

I hadn't thought of that and I bet your spot on. I has acted like a fuel problem, because if i back off of it a bit its fine.

Thanks faztbullet

 

[Texasmark]

Re: Air mixture screw

Something else to factor into your current work is the weather. With it cooling off and all engines run stronger with cooler more dense air so you could get some change in your performance (to the good) just from the weather. Depending on temp, it can make a 2-3 mph difference in my WOT and noted hole shot improvement.

Mark

 

[MH Hawker]

Re: Air mixture screw

I realize this is a old thread but i may of found the problem and i thought id toss this out as a follow up, while doing the per spring launch maintenance while looking over the link n sink i noticed that the trigger wasn't going to full travel, a bit of oil fixed that. I launched yesterday it ran spot on like it should. will know a bit more after a few trips out weather permitting.

 

[MH Hawker]

Re: Air mixture screw

Got out again today and ran it awhile another 2 hours and it was fine so i think i found it.

 

[MH Hawker]

Re: Air mixture screw

Got out again yesterday and it ran fine so it was the timing advance. got about 6 hours on the motor.