MS250 - 14-14.5K Max RPM?

[SteveSr]

Hello,

Carb was recently rebuilt with an OEM kit. At that time I was careful to adjust the metering lever to the correct specification. As I recall I had to raise it up (richen it) to bring it into spec.

It is now relatively cold here in soggy NC, about 40F. Took the saw out to test run a new chain and brought the tach along. I was rather surprised that the saw would hit 14-14.5K at WOT out of the cut. I also checked the plug after a WOT run and it was pretty pristine ceramic white.

Checking the spec in the manual it is supposed to be 14K max. but don't specify at what temperature (assume 70f?). So considering that it probably won't get much use in temperatures much colder (leaner) than this I am tempted to just leave the carb alone and not raise the metering lever any more.

Your thoughts?

 

[Old2stroke]

Tuning a saw with a tach is a very inaccurate way of arriving at the "sweet spot" where it is slightly too rich at full throttle out of the cut and runs clean at full load in the cut. Tach specs given by manufacturers are selected to comply with mandated emission standards and are not necessarily good for the longevity of your engine. Many engines set this way from the dealer are just too lean and the pristine quality of your plug might indicate yours is too. I am not big fan of running any engine at full throttle with no load on it for the length of time it takes to get a stable tach reading.

 

[Sam R]

I can't test any saws I tune but I am in the habit of tuning them a hair richer than OEM calls for.

 

[HarleyT]

You will quickly know when you find that the rpms are too high...

 

[SteveSr]

You will quickly know when you find that the rpms are too high...

Care to elaborate? Seen many of these burnt up?

 

[SteveSr]

Tuning a saw with a tach is a very inaccurate way of arriving at the "sweet spot" where it is slightly too rich at full throttle out of the cut and runs clean at full load in the cut. Tach specs given by manufacturers are selected to comply with mandated emission standards and are not necessarily good for the longevity of your engine. Many engines set this way from the dealer are just too lean and the pristine quality of your plug might indicate yours is too. I am not big fan of running any engine at full throttle with no load on it for the length of time it takes to get a stable tach reading.

Good point about the EPA. I took a look at the specs for the 025, the MS250 predecessor. It is spec'd for 13K max. Although they did have a bunch of fixed "H" jet carbs in the herd. They weren't all adjustable line the popular WT-215.

 

[Natster]

L adjusts fuel air mix at idle, and runs at any time saw is running.
H adjusts mix at high rpm.
So, L affects high, just a little, but h only adjusts high.
If I get a bit of sludge, clogging L, it goes lean, at both low and high.
Often, open low jet, 1/2 to 3/4 turn, while running at moderate to high rpm, will let the junk by. Then, put L back where it was.
Just food for thought.
N

 

[Jed1124]

Is this an adjustable carb? If it is richen up the high side to 13500-14000 and all good.
If it’s a set carb it is what it is. 14500 shouldnt kill that saw with the fix jet. Get the adjustable carb if you want to bring it down some.

 

[SteveSr]

L adjusts fuel air mix at idle, and runs at any time saw is running.
H adjusts mix at high rpm.
So, L affects high, just a little, but h only adjusts high.
If I get a bit of sludge, clogging L, it goes lean, at both low and high.
Often, open low jet, 1/2 to 3/4 turn, while running at moderate to high rpm, will let the junk by. Then, put L back where it was.
Just food for thought.
N

Flushing out the Low circuit is easy just pull the needle and shoot carb cleaner in from the metering chamber side. I think I did that when I rebuilt the carb. The issue is the "H" side where it costs you a limiter cap to pull the needle. Because the saw runs close to (or actually at) spec I don't think it is clogged.

 

[SteveSr]

Is this an adjustable carb? If it is richen up the high side to 13500-14000 and all good.
If it’s a set carb it is what it is. 14500 shouldnt kill that saw with the fix jet. Get the adjustable carb if you want to bring it down some.

This is a limited range adjustment carb so theoretically you should be able to adjust the preset on the limiter to bring the max RPM down... Or just wait for the ambient temperature to go up 10-20 degrees! In this neck of the woods saws see temperatures from the 30's to the 90's so it makes it hard to come up with a happy medium fixed tune.

I remember setting up an 026 with a WT-194/394 carb at about 45-50 degrees for one of our trail volunteers. He then complained that it didn't run worth sh!t at 85 degrees! So I re-tuned it to the lower side of the RPM range at that higher temperature knowing that it would go up as ambient temperatures decreased.

BTW, Since I won't be the primary operator of this saw the limiter cap has to stay.

 

[HarleyT]

Care to elaborate? Seen many of these burnt up?

I have seen many that have been burned up.
I cannot say if they were going 14,500 or 14,600 though..... I wasn't there holding a tach on them when they went.....

 

[Jed1124]

This is a limited range adjustment carb so theoretically you should be able to adjust the preset on the limiter to bring the max RPM down... Or just wait for the ambient temperature to go up 10-20 degrees! In this neck of the woods saws see temperatures from the 30's to the 90's so it makes it hard to come up with a happy medium fixed tune.

I remember setting up an 026 with a WT-194/394 carb at about 45-50 degrees for one of our trail volunteers. He then complained that it didn't run worth sh!t at 85 degrees! So I re-tuned it to the lower side of the RPM range at that higher temperature knowing that it would go up as ambient temperatures decreased.

BTW, Since I won't be the primary operator of this saw the limiter cap has to stay.

You can pull the limiter, adjust and re-install after the adjustment. Sometimes they just don't allow you to give enough fuel.

 

[SteveSr]

I have seen many that have been burned up.
I cannot say if they were going 14,500 or 14,600 though..... I wasn't there holding a tach on them when they went.....

Thanks for the data point!

 

[SteveSr]

You can pull the limiter, adjust and re-install after the adjustment. Sometimes they just don't allow you to give enough fuel.

The limiters that I have seen are normally designed for one time use. pulling them off will break them requiring a new one to be installed. I'll have to look at an old carb and limiter and see if I can figure out the retaining mechanism and a way around it.

 

[HarleyT]

It would be interesting to see at what rpms/temps that will melt a piston. I always richen them up to max out at the factory recommended settings.

 

[SteveSr]

It would be interesting to see at what rpms/temps that will melt a piston. I always richen them up to max out at the factory recommended settings.

But again at what ambient temperature? Saw leans out as the temperatures drop and the air gets more dense. So which max do you use on the 025/250? 13K for the 025 or 14K for the 250?

 

[SteveSr]

The limiters that I have seen are normally designed for one time use. pulling them off will break them requiring a new one to be installed. I'll have to look at an old carb and limiter and see if I can figure out the retaining mechanism and a way around it.

It looks like these limiters on the Zama S76 carb can be reused. Must have been an early EPA oversight. So with the metering lever set to specs I'll just set the max RPM to probably 13-13.5K at an ambient of 35-40F. That should keep it in a safe area.

 

[ericm979]

The limiters on my Echo 352 bought less than two years ago could be reused.