Carburetor Question

[Douglas Firbanks]

Hello everyone! I had a quick question regarding carburetor function. I tuned my new carb on my CS-590 to an idle speed of 2800 RPM with a tachometer, after I blipped the throttle a few times and let it settle back down the idle speed was about 3100 RPM. Is this normal or a cause for concern?

 

[Old2stroke]

Get rid of the tach and do a search on how to tune a chainsaw

 

[buzz sawyer]

How long had it been it running when you set it at 2800 - compared to how long when it was at 3100? IF tuned correctly, it should be just a tad rich at idle. When you blip the throttle, it is clearing out the excess fuel so it may idle a little faster for a brief period.

Also, how old is the saw? Have you done a vac/pressure test recently?

 

[Douglas Firbanks]

How long had it been it running when you set it at 2800 - compared to how long when it was at 3100? IF tuned correctly, it should be just a tad rich at idle. When you blip the throttle, it is clearing out the excess fuel so it may idle a little faster for a brief period.

Also, how old is the saw? Have you done a vac/pressure test recently?

I have vac/pressure tested it a day ago and it passed. I had let the saw run for a few minutes at 2800 I only blipped the throttle and then checked again. Your statement about the saw clearing out excess fuel makes sense as it runs fine otherwise.

 

[WoodmanSr]

Get rid of the tach and do a search on how to tune a chainsaw

This. Adding remarks as might help others. Many sources to learn tuning by ear.

Firbanks - so you may still have misadjusted Idle "L" screw. A proper "L" setting should repeat +/- 50 rpm. You said "set idle speed by Tachometer" which sounds like using LA (low adjust speed) screw versus proper low mixture tuning.

A few things of mention;

▪︎Setting Idle is done more by "L" screw than "LA" screw. Learn to understand that principal.

▪︎ Clean air filter & get saw to normal warm operating temperature before tuning.

▪︎ Echo & other models with an independent choke lever: a cold saw might idle different for a few minutes. If 'cold starting' idles too fast, then try pulling out choke rod a bit... but not so far that re-enages the fast-start cam. Often feathering the choke in that manner will "richen" mixture & settled down a 'racing' cold idle speed. Push choke off as soon as idle settles down.

▪︎An excess idle speed = lean "L" setting (or LA threaded in too far), granted idle might load-up with excess fuel pooling, then blipping throttle clears it up. But, if idles too fast after releasing throttle, then it's too lean of "L" setting. The "L" should be rich enough to drop to 'set' idle speed very quickly. If hunting around at a too fast speed then it's a bit lean "L" screw or an air leak.

▪︎I agree that reliance on a Tachometer is often a mistake. Train your ear... then fine to verify how you did with a Tachometer.

Others may offer better explanation. Read more topics on "How to Tune a Chainsaw / How to Tune a 2-Stroke".

Maintenance Manuals will have 3 basic tuning steps for Idle Mixture setting;
>> note; must perform with Chain Brake Disengaged / Exercise required caution as chain moves.

1. Set "L" screw to Highest Indicated RPM. By ear, or Tachometer might help here (& teach hearing rpm).

2. Set "LA" throttle stop screw to position that causes chain to just slowly crawl over bar (so engaged clutch). Then, back out "LA" screw until chain stops or only occasionally bumps. Leave "LA" at this setting (disengaged clutch)

3. Richen "L" (CCW) until idle slows to desired idle speed. Can use Tachometer to target 2750-2800 rpm. Nothing wrong with 2600 rpm if smooth idle.

Fine tuning "L" demands more experience or iterations while learning.
-- Check that Throttles Up smooth [if stumbles on throttle snap, usually suggests that to Rich, set "L" more lean (CW)]
-- Check that settles down quickly after a moment WOT (if 'races' or slow return to idle, set "L" more richer (CCW)... ensure that rich enough to consistently drop to idle speed)
-- repeat swinging "L" CW/CCW to achieve best peak RPM near target 2800.
-- as mentioned, often "L" wants to be a slight bit Rich (too settle down RPM when lifting throttle, tradeoff is that prolonged idling may load-up, causing idle very slow or rich stall. Either clear fuel before stalling or lean "L" for improved long idling. But, maybe sacrifice the preferred "Rich L" pulling rpm down quickly)

Typically a slightly "Rich L" helps saw starting behavior but won't idle a long time without loading up.

If discover Idle Speed isn't ideal, only then tweak LA screw & repeat accel/decel checks.

 

[Vintage Engine Repairs]

Hello everyone! I had a quick question regarding carburetor function. I tuned my new carb on my CS-590 to an idle speed of 2800 RPM with a tachometer, after I blipped the throttle a few times and let it settle back down the idle speed was about 3100 RPM. Is this normal or a cause for concern?

It’s not surprising, just reset it back down. When you’re in the ball park like you are, do a few decent cuts to get the saw hot and clear the crank of fuel, then re-tune. Do another few cuts and a final tweak.

Don’t get obsessed with perfection, there is a reason manuals specify +/- 200 rpms.

A tach is a great tool to have in the tool box and use it to get use to what different rpms sounds like. In time you’ll start to know where to set it by ear and feel coming back when unsure or to confirm certain machines with it.

Tuning well, across a range of machines, is a skill that will take time. You’ll get there.