Basic Trouble shooting... right here... :lol: Althought these will be real basic instructions, you'll get the idea. I’ll run through some steps that should be able to help you.
The main cause of poor idle on a chainsaw is a plugged carb screen (fuel filter), a carb that is out of adjustment or an air leak.
The easy fix is if it is the carb adjustment. While adjusting a carb is almost an art and can quickly cause you to pull your hair out, it can be done if you take your time and go one step at a time. If you get flustered, step back for a few minutes, take a break and try again a little later
Here is a quicky on how to try adjusting it.
First, make sure the spark plug is good. Change it anyways; even good looking plugs can be bad. Also check the air filter and clean it. If REALLY bad, you may have to replace it.
Pull the adjustment stops on the carb. You can use needle nose pliers to pull them off... just grab a hold of one edge of them and give a yank.
Now carefully close the needles noting how many turns it took to close them. Don't tighten them too much, when you feel resistance, that's enough. But since you know full size engine carbs you know all about this.
Open each needle 1 1/2 turns
Prime the carb 10+ times. That's right, 10+ times. The primer is not really a primer, but a "purge" bulb. It doesn’t shoot fuel into the cylinder but circulates fuel through the carb. Priming the extra times will help flush the carb screen of any gunk that might be there.
Set full choke and full throttle if you don't have a throttle lock.
Try starting. If it "pops", set choke to 1/2 and keep trying. It should start if everything else is OK (compression, timing, etc.)
Once it starts, if it won't idle, turn up the idle speed till it will idle.
If the engine is in good shape, the idle mixture screw should be set 1 1/4 turns to 2 turns. In rare cases it may take up 2 1/2 turns. Just try opening the idle mixture 1/8 turn and slowing the idle speed. You just have to play with each setting until you can get it so it idles and accelerates properly. This isn't the final setting, but we want to first get the idle mixture close before we move to the high speed.
If you have gotten this far, it's time to run it at high speed. Rev it up. If it sounds like it is starved for fuel or sounds like it runs good, open up the high speed needle 1/4 turn and try again.
Keep opening the high end until it starts to run rough. You will be able to hear the difference when it starts to "4 cycle"... almost like it is missing or hitting on every other stroke. Once you hear this, close the high end screw 1/8 turn at a time until this just barely stops or only seldom does this.
Note: Setting the high end too lean will surely burn up the motor. It's better to err on the rich (fat) side then the lean side.
If it smokes a lot and runs rough (we call this 4 cycling), then close the needle 1/4 turn and try again. Use the above procedure to set the high end needle.
Now it's time to see if it still idles properly. If it doesn't reset the low needle and idle speed so it does. This is a balancing act where you adjust the needle a little and cut the idle speed down until the chain stops. Now quickly pull the trigger, if the engine hesitates or sounds like it is starved for fuel before it accelerates, the low end needle is too lean, open it 1/8 turn and try again. If the engine loads up at idle, the low end needle is too rich.
The biggest mistake here is opening the low speed idle too much and opening the idle speed to compensate for the rich mixture. A sign of this is a saw that won’t idle for very long as it dies from flooding.
Once you get the low speed/idle set, go back and recheck the high speed setting.
A couple things to remember.
For the most part, adjusting the high speed needle will not affect the low speed. Adjusting the low speed mixture WILL affect the high speed setting.
Do not compensate for a rich low speed setting by cracking the idle up. The saw should idle for at least one minute without dying… that is the bare minimum.
Do not run the high speed too lean or you’ll end up needing a rebuild.
If your high and low end adjustments get too far out of wack when adjusting, you can go back to the 1 1/2 turns out for each and start again.
I’ll get to checking for air leaks in my next message.