2 Stroke Tuning Experts... I need some advice.

[wahconah98]

I just picked up a brand new Stihl FS 250 brush cutter yesterday to tackle an out of control area of my property. When running at operating speed the thing is an animal and runs great. The issues I'm having are while idling and coming off idle. When I start the cutter on the ground it will idle properly initially. When I pick it up to clip it to my harness (not touching the throttle), it will bog down and stall. After running at high speed for a while, it will stall shortly after coming back to an idle. I've noticed that I can affect the speed of the idle by holding the cutter in different orientations.

My other problem is the engine bogs down and will stall if I'm not careful at metering the throttle while the engine is idling or running at low speed. None of my other 2 stroke tools have issues like this (MS361, large Husky backback blower, light duty echo weed wacker). They all run perfectly regardless of orientation and you can hammer the throttle at idle and they rev up with no bogging or lag.

My FS 250 has an adjustable carb with high, low, and idle screws. I played around with the idle adjustment but didn't notice a kind of change regardless of the direction I turned it. My dealer is closed on Sundays and my work schedule makes it hard to get there during the week so I'm trying to see if I can tune it without bringing it back to them. Any insight or instructions would be great!

Also, all my 2 stroke tools run off the same mix. Just in case I made a fresh batch of mix with premium gas but it made no difference.

 

[bobt]

Sounds like the low speed jet is set too lean. Try to adjust it a little richer by turning the jet to the left a small amount. This will give it more gas at idle, and the throttle response will be better. You may have to set the idle adjustment a little faster after adjusting the low speed jet.

Of course taking it back to the dealer is the proper thing when they are open.

Hope this helps!

Bob

 

[backdoor]

I don't know very much about tuning any motor, but I recently bought a Husky trimmer and had almost the same problem as you have before I used the first tank of fuel that the dealer put in. The dealer fueled the trimmer up, ran it a while, showed me how to start and run it and let me run it a few minutes, and all was good. As I used it at home, I got the same issues as you have. I didn't turn any screws because I was mad enough to take the trimmer back to the dealer for fixing. I looked in the gas tank, and the fuel line had a curl in it that would let the fuel filter crawl up the fill spout at certain RPM's or certain angles that I held the trimmer. After several tries, I got the fuel line uncurled to where the filter stayed on the bottom of the tank, where the gas was, and no more problems. Hope this helps.

 

[wahconah98]

Thanks Bob and backdoor. I checked the tank out and the fuel pickup is where it should be. I tried richening the low speed mixture by a small amount and the problems got worse. I put it back to where it was and just happened to notice that the exhaust had dark smoke in it coming off idle suggesting the low speed mix might actually be too rich. I verified that the low speed adjustment was set to the factory recommended 1 turn off seated. I leaned out the low speed mixture by splitting the difference between the point where the engine starts to race and where it starts to bog down. Throttle response has significantly improved. It's not perfect yet but getting there.

My new low speed screw setting is only about half of the factory suggested 1 turn off seated. Is it uncommon to have such a significant difference from the recommended value?

Thanks again for your insight.

 

[wahconah98]

Thanks Bob and backdoor. I checked the fuel pickup and all appears to be well with it. I tried richening the low speed mixture slightly and my idle problems got worse. Before I adjusted it I noticed there was a decent amount of dark smoke while idling suggesting it might actually be running rich. I put the low speed mixture back to where it was and verified that it was set to the factory suggested 1 turn off seated. I then leaned out the mixture by splitting the difference between the point where the engine starts to race and where it starts to bog down. Throttle response got noticeably better. It's not perfect yet but I'm getting there.

What concerns me now is my new low speed setting is about half of the factory recommended 1 turn off seated. Is it uncommon to have such a large difference between the factory setting and my adjusted value? How can I tell if I have it too lean at idle?

Thanks again for your help!

 

[wahconah98]

Just wanted to follow up.

No matter what I tried I couldn't make it run properly. I took it back to the dealer and they had it sorted out in a few hours. Turns out it was a bad gas cap. They replaced it and readjusted the carb and now all is well.