Echo chainsaw "bogging" when throttle applied

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The purpose of limiter caps is to prevent end users from turning the Hi screw out enough to make the saw run rich enough that it starts to "4-stroke" at peak rpm.

"4-stroke" is a colloquialism for what happens to a 2-stroke engine when its fuel pump begins to push so much fuel down it's throat that it gets so rich that it can't continue to increase rpms.

It's the sound you hear when a saw goes from a clean climbing howl to a ratty disorderly overfed braaap.
It's definitely not that grainy thing you hear when a lean saw goes from a clean climbing howl to a thin raging scream.
The former is life. The latter is death.

While buried in wood, if your hot saw howls cleanly up to peak rpms and pulls hard but instantly goes ratty braap if you lift the bar slightly while holding full throttle, that ratty sound is known as "4-stroking".
If it goes grainy screaming faster, you are lean and killing the motor.

The way to achieve 4-stroking is to richen the main jet array (turn the Hi screw counterclockwise) enough that a hot saw goes ratty when you lift the load off it at peak rpms. Basically, this is the engine revving to a point where the richness curve associated with the cyclic rate of the carb diaphragm halts the rpm progression ramp. It's an organic rev-limiter if you will.

This setting usually results in optimal power output, nearly always results in longest possible engine life, and farts out 7ppm more undesirable gases than a somewhat leaner engine.

EPA decided that the additional fartgas was way too much, and they instructed the saw manufacturers to put limiter caps on the screws so that end users could not set the saw slightly rich and get peak power and a ratty 4-stroking organic rev-limiter. Instead, the engine would run slightly lean and never see enough fuel to reach the richer peak crossover point where the carb pulls more fuel than the engine can handle.

The manufacturers were like, "Cool man, this will cut engine life in half, so we can sell twice as many saws!"

Real men circumvent the stupidity of EPA and the greed of manufacturers by modifying limiter caps. Just pull em off, cut off the limiter tab, and stick 'em back on. At least that's how it used to be. I have no idea how they are doing it now.

Usually, the idle and Lo screw adjustments are ordinary.
Idle - set it first. Set it so that the chain just stops spinning when you idle.
Lo - set it next. Set it so that the leap from low to high rpms is fat and angry, not starved, and not bleah boggy rich.
Hi - get the saw hot and bury the bar. Tweak your Hi screw until your saw instantly 4-strokes when you lift the bar out of the load but not while it's in the load.

Caution: When tuning the Hi screw, if you go too rich, the saw will just be ratty and disorderly at any high rpm, which is not harmful to the engine. If you go way too lean (too far clockwise), the saw will rage into screaming rpms that burn up the motor.
Better rich than lean. Be careful as you seek a good setting.

It is not unusual for experienced cutters to tweak the Hi screw daily or even several times a day as conditions change: very small changes, made when things don't feel and sound just right.
Not unusual in settings where 50° intraday temperature swings are completely normal. Like today in Baker County. 38 when I got up, 85 at 2PM.

OK then arak op man. Hope this helps. Good luck with your saw.

I made an account just to reply to this comment. This, and your earlier one towards top of thread, is by far the best most detailed explanation of how to diagnose a chainsaw issue as well as tune the carbs and actually understand what's happening and what to look for. I've searched for this info for a long while and got most of it over time, not all, and this just…man I wish this came printed out with every chainsaw. Thank you so much for making it all so clear and concise. I printed it out and put it on my wall in my shop.