Tuning Always Loaded 2-Strokes?

[SteveSr]

Hello,

Here is question to ponder on your Sunday evening... What is the proper way to tune 2-stroke engines that are always loaded? For these engines I am thinking mostly blowers or line trimmers. Due to the loading you can't get to the traditional 4-stroking at WOT and can't really judge acceleration snap / response off idle due to the load. I am assuming that you don't have any spec for WOT RPMs so a tach is kind of useless.

Your thoughts?

Thanks,
Steve

 

[ammoaddict]

I have wondered the thing

 

[piglosi]

Hello,

Here is question to ponder on your Sunday evening... What is the proper way to tune 2-stroke engines that are always loaded? For these engines I am thinking mostly blowers or line trimmers. Due to the loading you can't get to the traditional 4-stroking at WOT and can't really judge acceleration snap / response off idle due to the load. I am assuming that you don't have any spec for WOT RPMs so a tach is kind of useless.

Your thoughts?

Thanks,
Steve

My 580 Husky blower has a max RPM of like 7200. The carb has an idle only if memory serves. I checked it with my tach, and it’s darn close. Few years back it seemed to be sluggish and I lost like 800 rpm on WOT. Rebuilt carb and it was back to normal.

 

[ammoaddict]

My 580 Husky blower has a max RPM of like 7200. The carb has an idle only if memory serves. I checked it with my tach, and it’s darn close. Few years back it seemed to be sluggish and I lost like 800 rpm on WOT. Rebuilt carb and it was back to normal.

I have the 570. Only has idle adjustment as well.

 

[cookies]

warm up then hold it wide open throttle, turn H needle out until it starts fuel bogging then turn in until it cleans up. I seem to need to adjust mine once at the end of spring and once at the end of fall for the temps. I like mine to 4 stroke when cold but run perfect once fully warmed up.

 

[huskihl]

Max rpm under any given load is said to be safe. I always richen slightly from max rpm

 

[MacAttack]

warm up then hold it wide open throttle, turn H needle out until it starts fuel bogging then turn in until it cleans up. I seem to need to adjust mine once at the end of spring and once at the end of fall for the temps. I like mine to 4 stroke when cold but run perfect once fully warmed up.

Yep... I was going to say this exact thing. Piece of cake.

 

[HansFranz]

For a blower, couldn't you cover the air outlet to create a no-load condition? If it's not moving any air, it's under no load. That's why shop vac motors speed up when you cover the suction hose.

For a line trimmer, I would consider it to be under no load if there were no string installed and/or if it was not actually cutting grass. If it's not cutting/moving any grass, it's under no load.

 

[trains]

if the L is set correctly, a way to fine tune from the above already mentioned ways, is to see how it behaves off wot.
if it drops down low, then slowly recovers to normal rpm, its rich, lean it off till you get minimal drop off wot, and your pretty close, thats how I set them.
I prefer to err on rich than too lean.

On motors like pole saws, trimmers, both hedge, and strimmers, if you go too lean at wot, the motor will drop in rpm and stumble, I find where it drops off like that, and then richen till its running strong at wot, and then fine tune as above.

hope that helps.

 

[SteveSr]

if the L is set correctly, a way to fine tune from the above already mentioned ways, is to see how it behaves off wot.

What is the best way to verify/set "L" correctly? Conventional wisdom is to find the max idle RPM and then richen it up to a certain RPM drop at idle. What should the RPM drop be? Does this same scenario/method work for carbs with accelerator pumps?

if it drops down low, then slowly recovers to normal rpm, its rich, lean it off till you get minimal drop off wot, and your pretty close, thats how I set them.
I prefer to err on rich than too lean.

This is where I'm at now with a 25cc blower. "H" is currently set to the rich side of max RPM at WOT. and the RPM does drop and slowly recover after a run at WOT. It sounds like I am still a bit too rich. The falling fall temps might fix this for me soon.

On motors like pole saws, trimmers, both hedge, and strimmers, if you go too lean at wot, the motor will drop in rpm and stumble, I find where it drops off like that, and then richen till its running strong at wot, and then fine tune as above.

hope that helps.

True. I tried this and too lean at WOT causes an RPM drop. I didn't leave it at this point for more than a second or two as I didn't want to burn the engine up from being too lean.

 

[ZeroJunk]

The only piece of equipment that has a constant load is a blower. I tune those for max RPM and then make it richer until you just notice it slowing down a few RPM. On a string trimmer I run the string out as far as it will go. But, a lot or most guys take the guard off so you can run it out a foot if you want. OK if you don't mind replacing shafts and gear heads. But, you can tune those the same.
Four Mix engines are just confusing. They hit a rev limiter. So, I lean them until they clean up and then turn them rich until you can hear the rev limiter skipping.
I tune a saw with a tach about 500 RPM rich from max WOT. If, they even have an adjustment.

I don't claim any of this is perfect. So, I'm open to any better ideas.

 

[bwalker]

My 580 Husky blower has a max RPM of like 7200. The carb has an idle only if memory serves. I checked it with my tach, and it’s darn close. Few years back it seemed to be sluggish and I lost like 800 rpm on WOT. Rebuilt carb and it was back to normal.

Actually the 580 has a LS and HS screw. They are very small and sort of hidden. They also have plastic plugs covering them.
Husky has a tuning sheet that you can find by doing a search. That will get you very close.
The HS screw doesn't matter a whole lot as they are usually very close. However the LS screw is very often way lean which makes them hard to start in cool weather. I fatten the LS up until it starts easy and then set the HS to the factory no load setting.

 

[bwalker]

The only piece of equipment that has a constant load is a blower. I tune those for max RPM and then make it richer until you just notice it slowing down a few RPM. On a string trimmer I run the string out as far as it will go. But, a lot or most guys take the guard off so you can run it out a foot if you want. OK if you don't mind replacing shafts and gear heads. But, you can tune those the same.
Four Mix engines are just confusing. They hit a rev limiter. So, I lean them until they clean up and then turn them rich until you can hear the rev limiter skipping.
I tune a saw with a tach about 500 RPM rich from max WOT. If, they even have an adjustment.

I don't claim any of this is perfect. So, I'm open to any better ideas.

I can't comment on the 4mix much as I refuse to buy one. The only thing I woukd say is being a four cycle motor I wouldn't be afraid of running them lean at all.
As for saws. The factory no load spec is in most cases very conservative. Setting it 500 richer just makes this worse. I wouldn't be afraid to set it at factory spec and send it out the door. Of course you do seem to deal with a high percentage of morons so running pig rich isnt the worst thing that will happen that equipment.

 

[ballisticdoughnut]

warm up then hold it wide open throttle, turn H needle out until it starts fuel bogging then turn in until it cleans up. I seem to need to adjust mine once at the end of spring and once at the end of fall for the temps. I like mine to 4 stroke when cold but run perfect once fully warmed up.

Spot on.

 

[JoeMay]

This thread is perfect for a project I’m into right now. A Ryobi 700c trimmer given to me because the previous owner gave up on it and bought a new Husky trimmer. Old fuel with ethanol had ruined all the plumbing, purge system and carb essentials. He had done away the limiter caps and collar earlier. The H and L screws were provided loose, meaning not inserted at all. It’s got a Zama carb. Once the carb is rebuilt, should the H and L screws be seated gently as usual before running them out 1 turn for a starting point? Remember that all the plastic limiting parts are gone and the screws are not in threaded in at all now. I’m wondering how the adjustment screws are seated and turned out at the factory to accommodate the height of limiting system. I’m concerned that 1 turn out from normally seated will be too lean because the height of the limiting system collar is not accounted for.

 

[82F100SWB]

When the screws are seated, it is the taper on the end of the screw seated against its seat inside the carburetor, and nothing to do with the head of the screw itself. The thickness of the limiting collar is not relevant.

 

[JoeMay]

Thank you. That’s the confirmation I was hoping for.

 

[huskihl]

This thread is perfect for a project I’m into right now. A Ryobi 700c trimmer given to me because the previous owner gave up on it and bought a new Husky trimmer. Old fuel with ethanol had ruined all the plumbing, purge system and carb essentials. He had done away the limiter caps and collar earlier. The H and L screws were provided loose, meaning not inserted at all. It’s got a Zama carb. Once the carb is rebuilt, should the H and L screws be seated gently as usual before running them out 1 turn for a starting point? Remember that all the plastic limiting parts are gone and the screws are not in threaded in at all now. I’m wondering how the adjustment screws are seated and turned out at the factory to accommodate the height of limiting system. I’m concerned that 1 turn out from normally seated will be too lean because the height of the limiting system collar is not accounted for.

Don’t be set on 1 and 1 as being oem screw settings. They could very well each need between 2 and 4 turns out each

 

[JoeMay]

Yes, I am aware of that. Zama technical support recommended 1 turn out from seated as a starting point and that’s what I typically start with. Zama also mentioned final settings will likely deviate from that starting point. I’ve got it starting, idling and accelerating pretty well now with good top end. I’m about 7/8 out on the high and 1 1/4 out on the low. I’ll soon try it with some string spooled on. I’ll likely finish slightly rich on the H as I usually do.

 

[Vintage Engine Repairs]

L - find lean drop off, find rich drop off, go half way, the richen for good off idle acceleration.

H - when they’re fully loaded like a leaf blower or even a hedge trimmer doesn’t get much more load, find max rpm, then find where it just cleans up from 4 stroking and go half way in between. Maybe a bit leaner for a hedge trimmer.

The problem with hedge trimmers is that they see very little extra load with very high reciprocating mass. This means ideally we’d want to run them quite sharp of a tune and therefore high rpm, but the high reciprocating mass doesn’t hold up too well when we do. It’s a balance but more often than not, they don’t tend to run too cleanly both high end and consumer grade, they’re either bouncing off the limiter causing spooge or set rich from factory to keep rpm down and spooging. Some times manufacturers limit throttle opening with a stop, this also causes spooge. It’s a lose lose anyway you look at it. In fact I just worked on one today, customer states it wouldn’t start…