MS 170

[crappiemiser]

Ok guys, I've seen multiple posts about the ms170 having the same bogging issue that mine has but I never see a fix. Starts fine, idles fine, but as soon as you hit the throttle it bogs.
I've tried so far
1. Carb rebuild
2. New carb
3. Fuel line/filter
4. Clean tank vent
last night I thought I figured out that the bar sprocket was sticking b/c when I ran the saw without the bar and chain there was no bog. Tonight I tried again and the bog is back. I know these are inexpensive saws but at this point I'd like to get some closure with this pos. Any help would be appreciated.

 

[Brushwacker]

Is your carb adjustable ? Sounds like it needs richened. Tons of saws have the issue as they are set lean to pass EPA.
I've had 170's don't remember 1 that I wasn't able to set to run right. If you can't richen your carb, may be try opti 2cycle mix or Amsoil. I am 1 that likes and recommends more oil to gas in most cases but in yours I think you might get what you need, which I think is more fuel to air by a thinner fuel mix. I think opti is like 70 to 1 (1 tube oil to 1 gallon of gas) . I never used Amsoil but may be an option to do the same. Before you do that or much more, if you haven't, check your piston through the exhaust for signs of a light seizure.
I recently bought a used 180 and the previous owner gave me an adjustable ms250 carb to put on it. I guess he used it that way prior, I think he said the throttle linkage had to be bent a little to make it work. I haven't tried it yet, may be tomorrow.

 

[walterg]

Is the new carb, fuel line, and filter AM or OEM.?
If the carb is AM that might be your problem, sometimes you get a bad one.
If the fuel line is AM make sure it isn't kinked.

Make sure the carb linkage isn't binding.
Also did you check the muffler screen to make sure it isn't clogged up.?



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[crappiemiser]

@Brushwacker - No, the OEM carb wasn't adjustable and neither is the one I bought to replace it.

@walterg- It is a cheap knockoff carb, so it could have something to do with it. I Have a friend with a good running 170, so maybe I'll swap carbs with his and see if that's the issue. Carb linkage isn't binding, and the fuel line isn't kinked. And I cleaned the spark arrestor. I haven't changed the spark plug, but I don't think that would be it.

 

[bang]

I had the same problem and after putting in a carb kit and ultrasonic cleaning only to have the same issue, I got a new oem carb and solved the problem. Have you cleaned/replaced the air filter?

 

[crappiemiser]

@bang - no I haven't changed the air filter but I can. I'm interested in seeing if switching the carb from a working saw will make this saw run correctly. I'll keep you posted.

 

[Wood Doctor]

I ran into this last summer with a friend's non-adjustable carb on a 170. Same problem. His eventually stopped bogging down after running awhile, but it seemed forever, and it always stalled out until it warmed up. I hate non-adjustable carbs. They leave you helpless.

 

[Brushwacker]

Do the easy, cheap stuff first if your time and money mean much. I have had chainsaws and at least 1 motorcycle that all that was wrong in similar situations was the spark plug or the gap in the plug. Had a Makita brought to me that wouldn't run right, checked the plug and I could see it was gapped more like .030 to .040. so I gapped it .020 and it was fixed. He thought it needed carb work . Gap is fairly sensitive on some engines, not the first time I run into that. A bad 2 or $3 plug will often go unnoticed so might just try another known good 1 before switching carbs. Its to easy to be skipping. If your fuel line is chinese I'd check it inside the tank for texture and make sure the filter didn't fall off. The ones I tried only 1 of 3 or 4 made it through a tank of gas ( and then it was done for) before the end softened and the filter fell off. Did you check the piston through the exhaust ? 15 minutes you can eliminate a lot of possibilities before you swap carbs for the 2nd time.

 

[Cody]

From the factory mine was cold blooded as well but I put on a wt-215 and opened the muffler, had tuning issues at first because I'm no pro but last spring I got it dialed in and it starts and runs like a saw should.

 

[TBS]

Another thing to look at is the carburetor metering lever, it should sit just above the surface. if it isn't set high enough diaphragm won't pull enough fuel to rev with a load. I had this happen on a 170 that the dealer said was junk after putting a new zama on it then sold my boss an ms180.

 

[Rockjock]

Might be worth a try...
Check the plastic band around the rubber intake boot. I replaced mine with a band type from and 024 and it solved the issue right away.

 

[crappiemiser]

So last night and this morning I started the saw without the bar and chain and it seemed to work just like it's supposed to. There is a small hesitation, but after a few rev's the saw takes off and then idles just like it should. I didn't change anything other than the parts listed above so I am unsure about the fix (really, don't think it is one yet) but I'll keep you'll posted. Shouldn't the carb either work right away or do they sometimes take a while to adjust? Thanks.

 

[TBS]

They are supposed to work right away.

 

[Okie]

So last night and this morning I started the saw without the bar and chain and it seemed to work just like it's supposed to. There is a small hesitation, but after a few rev's the saw takes off and then idles just like it should. I didn't change anything other than the parts listed above so I am unsure about the fix (really, don't think it is one yet) but I'll keep you'll posted. Shouldn't the carb either work right away or do they sometimes take a while to adjust? Thanks.

When you put the bar and chain back on the saw leave the chain real loose and check the idling, then tighten the chain to normal and gets worse at idling the crank seal is bad, it leaks when a strain in placed on the crank and the crankbearing can still be ok. Seal is just dried out and bad. You can confirm such by applying about 3 lbs of crankcase pressure and turning the crank while the case has pressure using liquid soap around the seal to detect bubbles.
AND yes about a cold bog on some stihls when they are first started with a cold engine, but it should start and fast cold idle good just let it run at fast idle for about 10-15 seconds and then when it takes throttle it's usually good to go.
If it's slow warming up to get past a bog try enriching going out or CCW with the idle screw 1/4 turn if it has a adjustable L screw and then you may have to go to the LA screw and increase the idle little bit.

Some of the 170 stihls don't even have any holes in the plastic so as to get too L and H jets for adjusting a carb, just a very small hole by the right side handle for the idle adjust. Bogging can be too lean due to air leaks into the crankcase also.

 

[crappiemiser]

@Okie - I'll try that with the chain attached and see if that is the issue with the crank seal. The carb is non adjustable other than idle so I can't adjust it.

 

[Okie]

@Okie - I'll try that with the chain attached and see if that is the issue with the crank seal. The carb is non adjustable other than idle so I can't adjust it.

AND
some aftermarket carbs have adjustable jets but you cannot access the adjustments on the adjustable generic carbs unless you mutilate the plastic by drilling access holes because not enough room to get a user friendly adjust using 90 degree adjustment tools on the jets.

Your best bet for a test is too install the carb off the good running saw that you mentioned. I had two of those saws in for repairs awhile back, one would not take throttle and the other one just needed fuel filter and fuel line and when I swapped the good running carb to the sick saw the problem was the carb. I usually try to kit sick adjustable carbs if they are above $35 and have a reasonably priced kit of not more than $12 and ultrasonic clean the carb before kitting but not in this case, not worth the effort and I heard rumors on this site that the little non-adjustable carbs do not like a kit quite often. I went back with a OEM Stihl carb instead of aftermarket due to being non-adjustable. I think the OEM Stihl carb was around $20. Could not bring myself to mess around and place trust and time in a non-adjustable clone carb on a good saw knowing it just needed a good carb.

 

[crappiemiser]

I put a new bar and chain on the saw and tried a few cuts with it. It cuts fine for short periods of time but it's still bogging just not as bad as before. I'm gonna swap carbs and see if that fixes anything, then I guess it may be time for a leak down test.

 

[Khntr85]

Swap carbs first, I have had AM carbs act weird myself..... some are good some junk.... I am glad I rebuild enough saws now to always have a "good" carb around to test with LOl!!!

 

[pafire]

another thing that you may want to check is the screen in the muffler. if you cant get air out, you cant get air in.

 

[crappiemiser]

I have removed the screen for now, and it was clean anyway. I wish it was that easy!