tach question

[cmills]

I just bought a Tech Tach 20k and I was trying it out on my Trimmer which is a Stihl FS 250R, I looked in the manual and it says the engine idle speed should be 2,800, I checked mine and it is a little over 3,000 but the head does not spin so I figured it is ok. The manual says that the engine cutoff speed is 12,300, when I went to check full throttle the highest reading I could get was 11,523 and the reading started jumping all over the place, way lower though. It kind of sounded like it was barely starting to miss out just a tad bit, almost as though it was hitting a rev limiter. Could somebody explain what is happening?

I'm pretty sure there is nothing wrong, as the trimmer idles and revs just fine, I was just messing around with the tach and am curious as to why its doing what it is.


thank,
Cory

 

[nomak]

not a carb guy but from what ive read it may be the H mixture screw needs adjusting...just my opinion though

 

[Stihl088stock]

as though it was hitting a rev limiter.

That sounds like your answer... richen it up a tad until you get a steady reading.

 

[BobL]

That sounds like your answer... richen it up a tad until you get a steady reading.

Yep - when it jumps around like that it's your rev limiter kinking in. If you go over the limit it will mess with some tachos. It's the same as what happens with one of mine. It's funny that you can richen it up so the revs audibly drop but the tacho will read slightly higher but steadier.

 

[dieselram]

Are all the tachs the same ,or is one better than another? Like a Stihl tach or the one you can get from Bailey's.

 

[cmills]

Yep - when it jumps around like that it's your rev limiter kinking in. If you go over the limit it will mess with some tachos. It's the same as what happens with one of mine. It's funny that you can richen it up so the revs audibly drop but the tacho will read slightly higher but steadier.

If the rev limiter is kicking in shouldn't my reading for the highest RPM have been around 12,300, that's what the maual said the the engine cutoff speed is? The highest reading I got was 11,523RPM.


thanks,
Cory

 

[BobL]

Are all the tachs the same ,or is one better than another? Like a Stihl tach or the one you can get from Bailey's.

The only ones I have used are the Max Tach (Bailey's), Fast Tach and the Stihl Tacho and they are all the same when used with limiters. The one that seems to jump around less than the others is the Fast Tach but that maybe because of the slower refresh rate.

 

[nomak]

If the rev limiter is kicking in shouldn't my reading for the highest RPM have been around 12,300, that's what the maual said the the engine cutoff speed is? The highest reading I got was 11,523RPM.


thanks,
Cory

rev limiters are meant to drop the rpms so as not to over rev it ... so this would cause the erratic RPM readout

 

[cmills]

rev limiters are meant to drop the rpms so as not to over rev it ... so this would cause the erratic RPM readout

But the rev limiter wouldn't come into play until the engine reached 12,300 right, then it would drop the RPM's causing the erratic readout, or am I misunderstanding the concept. According to the tach, the highest RPM it ever reached was 11,523 well under the 12,300 that the manual says is the engine cut-off speed. Sorry, if my questions seem kind of dumb, I'm just trying to figure out what is happening and why.


thanks,
Cory

 

[andybcumming112]

The specs for the high speed are 11,300 to 13,000 i believe you are ok. Pm me if you want to know how stihl is doing the low side idle speed.Do you have the zama c1q-s83 carb on this.

 

[cmills]

That's good, since I'm in that range I guess I'll just leave it alone, I was just curious as to what was happening.

As far as the carb, I don't know what kind is on it, it doesn't say in the manual. Will I be able to tell by looking at it? Does having that carb make a difference?


Thanks,
Cory

 

[andybcumming112]

No but you need to set the low side to 2800 rpms to get the best throttle response and crisp acceleration.

 

[andybcumming112]

The H screw has limited effect on adjustments because of the rev limiter built into the ignition module.And the combination of an internal fixed high speed port,however you want to make sure you are close to spec on the high side, in your case it is 11,300 to 13,000 rpms.This is not important as it is for chainsaws because they do not have rev limiters,well most don't and you can fry a saw fast if you are not careful.Chainsaws have a slightly different procedure for the H setting.

 

[cmills]

When I turn down the idle will I have to adjust anything else or is that it? And I assume all I have to do is make sure it's warmed up when I do it, right?