Stihl MS170 will not rev high

[Sand-racer]

Hello and thank you for your help in advance, it is appreciated.

I have a MS 170 and it is not able to rev high. It starts and runs through the mid range well then bogs heavily at ~3/4 throttle on.

The saw had been running fine. It does the same thing nomatter if I gun it or try to bring it up slowly.

Initially i thought the chain may have been dragging and so i took off the chain and the bar and it does the exact same thing.

I checked the spark arrestor screen and it was clear as well.

To me it feels like once it hits a certain RPM the brake starts to drag and bogs it down. Is this possible?

There is no hi or lo speed carb adjustment on this saw.

Please help me. DOes anyone know what it could be or have had this issue before?

Thanks

Wes

 

[Somesawguy]

Have you checked the fuel filter and the air filter?

 

[Sand-racer]

Yes i have checked both. The fuel filter was clean and i even took the air filter out and ran it for a moment to make sure it was not that.

I even took apart the carb and everything seemed to be in order. Main jet was clear diaphrams seemed to be in good condition.

Thanks

 

[Scooterbum]

Process of elimination, pull the clutch cover off and check the brake would be the next move.

I've had this happen ( can't remember which saw though) loaded with saw chips and binding.

 

[Somesawguy]

There was a thread awhile back, where they had installed the wrong part, or installed it incorrectly. It caused the clutch to overheat.

http://www.arboristsite.com/chainsaw/200966.htm

 

[diggers_dad]

Sounds like a simple fuel issue. I have one on the bench with the same symptoms and it's getting a fresh carb kit as well as a thorough cleaning. The one I'm working on had a lot of trash in the screen inside the carb, restricting the amount of fuel it could pull through. If you've taken the carb apart, it would be a good idea to put in a new kit as well as a fuel filter if it is in question at all. The one I'm working on has only one adjustment screw as well and the carb MUST be clean for them to work properly. An ultrasonic parts cleaner might be in order.

Good luck!

 

[Unit44]

Did you actually take the screen out and look through it? At a angle (my 181) looks clear but will be so stopped up it will hardly run. Take out the screen and run it?

 

[Sand-racer]

I will get a carb kit and new fuel filter and advise.

Thanks

 

[mountainlake]

As Unit44 said first check the muff screen again, also pull the muff off and look. The exhaust port could be carboned up bad.. Steve

 

[Sand-racer]

I put in the carb kit and new fuel filter, now the revving problem is gone. I new problem has arose though. It will not idle or rev in mid range. It is full throttle or off. I know this is associated with the carb and I will take it back apart tomorrow and clean it but does anyone have an idea of what component it may be in the carb?

Thanks

Wes

 

[bennn*e]

Was it a complete repair kit or just diaphragms and gaskets?

 

[Sand-racer]

It was the complete repair kit.

 

[angelo c]

I put in the carb kit and new fuel filter, now the revving problem is gone. I new problem has arose though. It will not idle or rev in mid range. It is full throttle or off. I know this is associated with the carb and I will take it back apart tomorrow and clean it but does anyone have an idea of what component it may be in the carb?

Thanks

Wes

sounds like an air leak...either through an aging fuel line, failed seal or incorrectly set choke.

 

[Sand-racer]

I have now taken this carb apart what feels to be 100 times. I can make the saw either run well at high rev but it will not idle or run at low rev. THis is accomplished by having all diaphragms set correctly. If i swap the gasket and diaphragm on the pump side it will idle and run mid range but dies at high rev.

Does anyone have some thoughts, could it have a clog somewhere?. I do not see a leak in the fuel line.

Thanks

 

[angelo c]

I have now taken this carb apart what feels to be 100 times. I can make the saw either run well at high rev but it will not idle or run at low rev. THis is accomplished by having all diaphragms set correctly. If i swap the gasket and diaphragm on the pump side it will idle and run mid range but dies at high rev.

Does anyone have some thoughts, could it have a clog somewhere?. I do not see a leak in the fuel line.

Thanks

You won't see always see a "leak" in the fuel line. You need to vac test to see if it's weak and it's walls are collapsing under pressure due to aging rubber. Test all the lines and remove those from the equation . Case and seals too. Then go back and figure out what's wrong with fuel delivery...ie carb.

Have you looked at the clutch springs as well ?

 

[bennn*e]

If the needle lever was replaced as part of the kit it may not be set at the right high causing the metering diaphragm to open the needle to much or to little

 

[MS460WOODCHUCK]

I have now taken this carb apart what feels to be 100 times. I can make the saw either run well at high rev but it will not idle or run at low rev. THis is accomplished by having all diaphragms set correctly. If i swap the gasket and diaphragm on the pump side it will idle and run mid range but dies at high rev.

Does anyone have some thoughts, could it have a clog somewhere?. I do not see a leak in the fuel line.

Thanks

When I bought my boy his 170 my dealer told me he does'nt waste his time rebuilding the carbs on the 170 because they are only 22 bucks new so he just changes them. Just buy a new carb is what I would do.