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MS170 Non-adjustable Carb Blues

TheTone

TheTone

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Just rebuilt a Stihl MS170. I did an ultrasonic cleaning on the carb block and rebuilt with a kit (metering lever flush). It was very hard to start, and I had to do some judicious bending of the choke rod so that the choke would close completely (thanks to an archived post). Although I couldn't find anything in the service manual that was relevant, somewhere I read that the initial adjustment on that single left hand thread blunt screw that does who knows what is 2.5 turns clockwise. Tried this and it started right up but idled too high, around 4500 rpm (no bar or chain). Fiddled with this and found that by turning counterclockwise (backwards), I could get the idle down around 3200. Idle was pretty steady considering the engine had new rings, but higher rpm's were a problem. I had to feather it a lot to get to around 9000 (bad flat spot in the acceleration) and as i gradually went to WOT it suddenly screamed to 12000 and I immediately backed off. Adjusting the screw did not solve this problem. The manifold does have the plastic ring and not a clamp, but I put Dirko in the outer groove of the cylinder intake port so the manifold would have a little help sealing. It sounds to me like it's running lean, not getting enough fuel. I'm suspicious of those closed carb passageways that can't be cleaned, and wonder, even with US, if they came clean. The plan is to replace the carb with an adjustable one and see if I can get a good idle and smooth transition to high rpm's. The question is, does it need more breaking in (wood cutting) before deciding what to do, or is this taking a chance on burning up the piston.
 
HarleyT

HarleyT

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Did you double check the little impulse nub in the intake boot? Make sure it isn't puckered shut and lined up properly.
 
Aggie 2012

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Same problem with mine that I rebuilt.

Always sounds like it's running lean on the high side.

I think Brad did a thread on converting this saw over to an adjustable carb. Looked like quite a bit of hassle so I post-poned my conversion.



Sent from my iPhone using Tapatalk
 
TheTone

TheTone

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Did you double check the little impulse nub in the intake boot? Make sure it isn't puckered shut and lined up properly.
Little thin plastic ring is properly placed in intake boot and carb is aligned correctly.

12,000 RPM at WOT? Spec is significantly higher than that.
True, but I was just doing a preliminary adjustment with brand new piston rings and no bar to check idle stability and transition to high rpm's. The problem was the flat spot on acceleration and the sudden scream from 9000 to 12000 that made me concerned about a lean condition. The question for me is do I keep things as they are, put a bar and chain on it, cut some wood and get it good and hot before making any changes, OR . . . swap carbs now (or do an extensive leak check) to avoid risking damage.
 
TheTone

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Blocked off intake manifold and exhaust port and took pressure and vacuum readings through spark plug hole - held 7 in. vac. and 7psi steady. That carb just "ain't right" and there's nothing more I can do for it. It will be replaced with an adjustable carb.
 
HarleyT

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Little thin plastic ring is properly placed in intake boot and carb is aligned correctly.



True, but I was just doing a preliminary adjustment with brand new piston rings and no bar to check idle stability and transition to high rpm's. The problem was the flat spot on acceleration and the sudden scream from 9000 to 12000 that made me concerned about a lean condition. The question for me is do I keep things as they are, put a bar and chain on it, cut some wood and get it good and hot before making any changes, OR . . . swap carbs now (or do an extensive leak check) to avoid risking damage.
I mean the nub under the boot.
 

Okie

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I was into couple of them MS170's sometimes back and only carb adjustment was the idle.

I bought a direct replacement non-adjustable from flea bay just as a test carb and it worked great on both saws just to get a test run. I had heard rumors that those non-adjustable little carbs sometimes don't like a kit even when been in a ultrasonic for a good cleaning so instead of running them in my ultrasonic and trying a kit I just bought the direct replacement Stihl. Don't remember the price from the Stihl man but was within reason and think maybe a kit costs almost as much as price of new carb from Stihl dealer. I just did not trust the $10 generic flea bay carb on a good Stihl saw going out the door for long term service and the flea bay carb had served it's purpose for me to save labor/time so as to make sure the saws had only carb issues.

I had considered getting a adjustable carb for a test or replacement but noticed when looking at pictures of the adjustable jet carbs that I could not easily access the adjustment jets on the 170's and might have to mod the plastics for access.

Anyway the cheap (probably China) flea bay test carb told me that a good carb would fix both saws.
 
TheTone

TheTone

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I had considered getting a adjustable carb for a test or replacement but noticed when looking at pictures of the adjustable jet carbs that I could not easily access the adjustment jets on the 170's and might have to mod the plastics for access.
Yes, I have heard that these carbs work well when they work, and replacing with a new stock carb is an option. When you place the adjustable carb on the black air filter box, the pre-drilled adjustment holes line up with the screws, so that part is easy. Three holes in the plastic sides of the case, which wouldn't show, and it could be adjusted with the box left on, top cover off. This saw was kind of a basket case with missing parts and I replaced the top cover with a cheap Chinese part, so I won't have to modify an original Stihl part. Finish with three more holes through the top cover and it's done. The hard part will be lining up these holes. My concern with the non-adjustable carbs is that the small passages in the high speed circuit could get narrowed with crud and lead to a lean mixture at high rpm - which I believe was happening when I ran the test.
 
likesaws

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I installed a adjustable carb from a MS 250 works great. Had to drill cover and plug one hole in carb. There is a old thread on this here somewhere. Open muffler it will run great
 
likesaws

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I installed a adjustable carb from a MS 250 works great. Had to drill cover and plug one hole in carb. There is a old thread on this here somewhere. Open muffler it will run great
Look for thread MS 180 carb replacement.
It is a Wt 215 carb.
I think this thread was started by
bssnelling. Don't know how to tag him on this.
 
TheTone

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Found the thread and happen to have a spare wt215. When you say "open up the muffler" how much do you mean and how exactly did you do it?
 
likesaws

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Found the thread and happen to have a spare wt215. When you say "open up the muffler" how much do you mean and how exactly did you do it?
I folded open the front cover and removed the baffles. Drilled out opening behind the outlet cover. It is a little loud now. May want to try leaving baffles and try just opening the outlet.
 
Snowchaser

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Yep, wt215 carb. $10 on ebay. Follow blsnelling s list of directions on it and its a piece of cake. Couple things to do, but nothing is permanent other than holes in the cover to adjust the h and low. "Ms180 adjustable carb" is the name of the thread.
 
Snowchaser

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here are Brads directions from the thread. I actually figured out how cut and paste on my phone. Woohoo!


Your doing good their, Brad

I have a 215 carb.......I'm just waiting on you to get all the bugs worked out before I proceed with mine :).......kinda of chicken sh!t, of me.

Gary
Just do it!:)

  1. Remove the brass plug from the impulse passage on the rear face of the WT215.
  2. Plug the original passage with JBWeld or something similiar.
  3. Install the carb. Do not use the Intellicarb cover from the original carb.
  4. Block the hole in the filter housing where the Intellicarb port inserted.
  5. Drill holes in the plasic for access to the screws.
  6. Install a small screw behind the throttle linkage, blocking the slot, so that linkage will have enough travel to go WOT. Do not bend the linkage or the carb may not fully return to idle.
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