Tuning Always Loaded 2-Strokes?

[trains]

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.

Regarding not being afraid of running a 4 mix lean, they are still lubricated by the fuel mix, it dosent matter if its a 2 stroke or 4 stroke, that doesn't matter, the oil mix lubes it.
if it was a 4 stroke, wet sump, like a honda motor for example then it wouldnt matter, as you just run regular fuel with no oil mixed in it, but to run a 4 mix lean is the same risk as any other 2 stroke.

 

[HansFranz]

Most of the trimmers I've owned seemed to be tuned real lean from the factory (maybe to meet EPA regs?) and it doesn't take too much gumming of the carb to lean them out to where they won't run under load except if set on half-choke...

 

[spuldup]

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 tune 4-mix lean at the limiter. I've not put a tach on mine but the limiter sounds low, so I think that really protects the engine. It needs adjusted seasonally to add some fuel (both H and L) in colder months. The valves also have a small lash spec and may need adjusted every year to maintain the tune. It is an easy adjustment. I run the S out of my KM90 as a 12" pole saw, blower, trimmer, and brushcutter. Heck, it even has an exhaust grind/polish and light muffler mod. And I do run a foot of line out. Make sure the heat always has fresh Stihl grease in it. Gets smoking hot but it's going on 10 years of what I would call hard homeowner use. I have ran a KM 91 and it was a much different animal. Seemed even lower revving, less power at peak but a smoother powerband overall.

 

[bwalker]

Regarding not being afraid of running a 4 mix lean, they are still lubricated by the fuel mix, it dosent matter if its a 2 stroke or 4 stroke, that doesn't matter, the oil mix lubes it.
if it was a 4 stroke, wet sump, like a honda motor for example then it wouldnt matter, as you just run regular fuel with no oil mixed in it, but to run a 4 mix lean is the same risk as any other 2 stroke.

The reason why running a twonstroke lean causes problems isn't about lubrication but rather the heat the piston runs at. Lean running two strokes seize not from lack of oil, but from lack of piston to bore clearance caused by heat swelling the piston.

 

[SteveSr]

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

How do you define rich and lean drop-off? Wouldn't this put the "L" tune at or near max RPM at idle? This may be too lean for a conventional carb but not necessarily for one with an accelerator pump. How do you tell acceleration on a blower? It sounds like a bog all the time.

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.

For the 25cc blower that started this question the max RPM and the end of 4-stroking appear to be at the same point. Going leaner doesn't raise the RPM noticeably. Seems to be a "flat" spot in the "H" tuning where noticeable screw adjustment doesn't cause a noticeable change in engine RPM at WOT.

 

[Vintage Engine Repairs]

How do you define rich and lean drop-off? Wouldn't this put the "L" tune at or near max RPM at idle? This may be too lean for a conventional carb but not necessarily for one with an accelerator pump. How do you tell acceleration on a blower? It sounds like a bog all the time.


For the 25cc blower that started this question the max RPM and the end of 4-stroking appear to be at the same point. Going leaner doesn't raise the RPM noticeably. Seems to be a "flat" spot in the "H" tuning where noticeable screw adjustment doesn't cause a noticeable change in engine RPM at WOT.

Lean drop off as stihl and states it is where the rpm’s start to drop once you’ve past peak rpm going clockwise. Rich drop off is the other way.
Finding the fastest rpm half way between the two is the next step. From there you richen the L until the rpm’s smooth out and become a nice rumble. Typically around 1/4 of a turn richer is a ball park place I have found. Then if you get a burble off idle, then lean it out 1/16 of a turn, if you get a dip, you need more fuel so richen it 1/16 of a turn.

Regarding WOT. That’s not surprising having the point at which is cleans up being close to max rpm. Each carburetor is different, the cheaper ones aren’t quite so refined either. On some Carburetors, not even turning the screw; but pressure applied onto them is enough to affect the fuel mixture. So when you’re tuning, work in tiny tiny increments and I bet you’ll find you can fine tune it better. Instead of working in 1/8 turn increments, work in 1/16 or 1/32. Tiny tiny tweaks!

 

[Lawless]

I have a KM91 I use mostly for trimming and it is tricky to get it tuned. I am still not sure it is at its best.

 

[SteveSr]

I have a KM91 I use mostly for trimming and it is tricky to get it tuned. I am still not sure it is at its best.

I believe that this is a 4-mix with a limited coil. When tuning these I usually put on a string head with line long enough to get the engine off the limiter. Then I tune for max power and back off a little on the rich side.

BTW, I don't think that I would recommend 32:1 Red Armor in a 4-mix engine. 4-mix are prone to carbon up with too much or the wrong type of oil.

 

[bwalker]

I believe that this is a 4-mix with a limited coil. When tuning these I usually put on a string head with line long enough to get the engine off the limiter. Then I tune for max power and back off a little on the rich side.

BTW, I don't think that I would recommend 32:1 Red Armor in a 4-mix engine. 4-mix are prone to carbon up with too much or the wrong type of oil.

I think the best bet for a 4mix is Stihl Ultra.

 

[Lawless]

I ain't getting involved with any discussions about oil here….

I believe that this is a 4-mix with a limited coil. When tuning these I usually put on a string head with line long enough to get the engine off the limiter. Then I tune for max power and back off a little on the rich side.

BTW, I don't think that I would recommend 32:1 Red Armor in a 4-mix engine. 4-mix are prone to carbon up with too much or the wrong type of oil.

I think the best bet for a 4mix is Stihl Ultra.

 

[Chevboy0167]

 

[bwalker]

I ain't getting involved with any discussions about oil here….

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Given the 4mix engines short comings I just think it's wise to use the oil that was designed for that particular motor.
While Ultra is garbage in a two cycle, it does help prevent valve issues with the 4mix four cycle.