Confusing Carb problem??????

[Mike G]

Ok I once again must call upon the 2-cycle wisdom of the members of AS.

I was running and tuning (or attemting to ) a newly aquired stihl trimmer. I went to adjust the High screw to the rich side, and nothing happend............................. I was sorta stumped so i tried again, and as the screw neared its last threads, the trimmer continued to run very fast and very lean when accelerated. No hint what so ever of 4 cycling. H screw only set the mix too lean, and never over richened it.

My question is what causes this? is it a dirty carb or somthing worse?

So i installed a different carb. This time things went ok, all screws adjusted like they should. However (there always seems to be a downside when i work with 2 cycle engines ) when she gets good and hot from running, she will not idle. Spark is good when this occurs. But after a long run of cutting at mid throttle (mighty well i must say, runs and accelerates great when asked to work) she returns back to idle and right on past to good old 0 RPM. Try to restart, and she sputters, but will not untill she sits for a bit.

I am not haveing very good 2 cycle luck these days, Lakeside, thanks in advance for your help, :greenchainsaw:

 

[Lakeside53]

First.. you can't adjust a trimmer or a blower by listening for "4 cycling"... It doesn't happen as the device is under load. It would be like trying to adjust your saw in the cut... Use a tach or leave it at standard settings.

Many of the trimmers run even with the H screw all the way in or out as the adjust is only partial. Many trimmers have rev limiting so when you bump up against this the results are plain confusing.

Not idling when hot could mean you've got the L too rich, or worse, you have the machine so overheated from a bad H adjustment that it's lean seizing... I see this all the time when guys take of the cutter/guard and run extra long string.

Before we get too far into this, what model are you talking about?

 

[Kenskip1]

Fast Idle

Mike, from what you described it sounds like a possible air leak. From your limited description as to how old the implement is, I will assume that this is an older piece.If you had say, A bad crank seal and the engine is drawing air around this, then the carb adjustments will be futile. This is a worse case senerao.Possibly a bad or worn set of rings.Do a compression check and see what you come up with. Anaything over 100 PSi should be good to go. Here is something to remember, Air like electricity and liquids all take the path of least resistance.If you crank seals are worn, it is easier for the engine to draw the air from this source than the carb. This is possibly why your adjustments are having little effect. One other thing, How clean is the gas filter? Maybe a kinked gas line? Good luck, Ken

 

[Mike G]

It is an FS 65 AVE

ok, so if the cutting head was removed would one then be able to adjust as if it was in a no load setting? I had a polycut on her at the time.

Can a overly rich H setting cause overheating? i was again trying to use the chainsaw method, incorrectly as i gather now! I got worried and put the rebuilt carb from another trimmer, and she adjusted as a i would have thought it should have. I guess from reading all about chainsaw tuning and lean siezure, i thought one should adjust any 2 cycle the same?

She runs good when throttled up. and accelerates well too. Compression feels ok, (i have to get a new end for my tester)

Lakeside what method should be used for adj a trimmer if one does not have a tach?

 

[l2edneck]

I am also new to the adjusting thing........

Am i to think that if im runnin it with it smokin and it seems to be screaming that im not killin it?(i was taught better to smoke than not and yer tryin to find the happy medium)Pls by all means correct me ive been doin this work for a long time and am just now really liking the "working" on them side of it.But i also dont want to kill them so i try to be careful.I also have limited tools. Thanx for any input you can give.

 

[Lakeside53]

It is an FS 65 AVE

ok, so if the cutting head was removed would one then be able to adjust as if it was in a no load setting? I had a polycut on her at the time.

Can a overly rich H setting cause overheating? i was again trying to use the chainsaw method, incorrectly as i gather now! I got worried and put the rebuilt carb from another trimmer, and she adjusted as a i would have thought it should have. I guess from reading all about chainsaw tuning and lean siezure, i thought one should adjust any 2 cycle the same?

She runs good when throttled up. and accelerates well too. Compression feels ok, (i have to get a new end for my tester)

Lakeside what method should be used for adj a trimmer if one does not have a tach?

The FS65AVE is a really old machine...

An overly rich H won't cause overheating.

The 4 stroking is actually a poor way to set smaller 2 strokes (like trimmers) and many of the newer smaller chainsaws- even under light load you simply won't hear it.

If you don't have a tach, make sure your line is the correct length and set the H screw to the default setting, like 1 turn out, and make small tweaks from there, but don't vary much from that setting.

If you do have a tach, either set it to the max allowable if the engine is in top shape, or, better - peak it to whatever it will do (assuming it's less than the max allowed), and then back it off about 250-400 rpm.



Go though your fuel system carefully - make sure the tank is venting and the fuel lines/filter are in good condition. Chuck the filter if it's more than a year old. Take off the pump end of the carb (one big screw) and check the inlet screen. It might be blocked.

 

[Lakeside53]

Mike, from what you described it sounds like a possible air leak. From your limited description as to how old the implement is, I will assume that this is an older piece.If you had say, A bad crank seal and the engine is drawing air around this, then the carb adjustments will be futile. This is a worse case senerao.Possibly a bad or worn set of rings.Do a compression check and see what you come up with. Anaything over 100 PSi should be good to go. Here is something to remember, Air like electricity and liquids all take the path of least resistance.If you crank seals are worn, it is easier for the engine to draw the air from this source than the carb. This is possibly why your adjustments are having little effect. One other thing, How clean is the gas filter? Maybe a kinked gas line? Good luck, Ken

It won't start at 100 psi... use the correct compression tester (valve at the plug end tip) and you'll get a true reading.

 

[RED-85-Z51]

What is this 4 cycling you speak of?

When I set a 2 stroke, I just get it to start, typically an overly rich mixture to start. THen warm it.

Then I get it to idle, I put the base Idle needle in about halfway, then start with the Idle circuit mixture screw. If the saw wont idle at all, I screw it in until it does.

Then I move to the High end. I hold the throttle Wide open, if it falls on its face, I open it up 1/2 turn. If it revs but stumbles around, I turn it in 1/2 turn.

Then I hold it wide open, and screw it in until it starts to peak..then 1/8 turn back.

Now I let it idle for a few seconds. Then I punch it wide open. If it falls on its face off Idle, that means the Idle is too lean. so I turn it out 1/4 turn, and compensate by turning the Base screw in 1/2 turn. Then Ifine tune the bottom end that way. I like to idle my 2 strokes pretty low. But I set my customers at the reccomended speeds.

Then I test it under a load. If I put a saw in a log, or a trimmer in some grass, and it sounds flat up top, I richen the H screw 1/8 turn. Once I get the L screw so I have good response off Idle, and a smooth Idle, I dont mess with it.

So far Ive never had a single solitary failure doing it this way, and Ive set huskys, Stihls, Macs, Homies, Ryobi Trimmers...you name it.

Its about comfort up top. I run it just lean enough to do the job.

 

[Lakeside53]

4 cycling is a term applied to the sound of a WOT LIGHTLY loaded engine that is running slightly rich of peak. The sound goes away (flattens out) when load is applied. It's been the traditional way of tuning a saw by ear, but it's less applicable with the newer lean-running EPA saws than before.

Almost all 2 stroke carbs are calibrated to guarantee a slightly rich start and run when adjusted to the manufacturers initial setting. It depends on the carb (some are 2 to 2.5 turns out), but it is mostly 1 turn out for both H and L from gently seated. Then you tune from that point, and if you need to move a lot to get a stable idle and acceleration, you have a problem either in the carb or the engine (leak etc). It's done this way to take the guess work out of the process.

Talking in terms of 1/2, 1/4, 1/8th turns etc will not get the desired result on all carbs - most newer carbs only allow partial adjust with the screws so the adjustments are exaggerated.

Adjusting to peak and then just backing off a small amount may leave you way above the max WOT. Sure it may run well, but for how long?