MS170/180 New OEM Carb issues

[SteveSr]

Hello,

I have both a 170 & 180 that have been recently overhauled/rebuilt. Rubber has been replaced and bot passed leak test. Both are about 10 years old and needed carbs for different reasons. I bought 2 Stihl OEM carbs and installed them.

On the first test run @45F the 180 ran 14.2K and the 170 at 15K! I also noticed that the LA screw didn't have much effect on the idle speed except to lower it. I did some investigation and found that contrary to Stihl's 5/2018 IPL that 0.44 fixed jet had been installed instead of the specified 0.45.

So I took the 0.45 jets out of the old carbs, cleaned them, and installed them in the new carbs. I test ran both saws again today at the approximate same ambient temperature. The fixed jet change made a significant increase in idle speed which had to be adjusted down. However, much to my surprise, the WOT speeds remained the same!

@backhoelover or any other carb experts explain why changing the fixed jet had no discernible effect on the max speed of both of these saws? I would have expected it to richen it up enough to at least see a measurable decrease in the max RPM. Why did the jet change appear to effect the low end? I thought that the idle circuit was separate and fed from a fixed orifice under the metering lever arm.

I work on these (and other) saws as part of a state wide volunteer trail organization so I won't be the end user and the saw may end up at the other end of the state where I won't have access to it to monitor. I hate to send these out running so fast that they might destroy themselves but I also can't change to adjustable carbs and turn them into Frankensaws as I likely won't be the one doing any possible repair work on it.

Any suggestions on how to proceed?

Thanks,
Steve

 

[two4spooky]

Probably not what you want to hear after buying two new OEM carburetors but I used an AM adjustable carb on an 017 with success. The AM came from HL Supply and has both H and L adjustments, drilled another hole in the cover for access. Maybe a dumb question but are you checking WOT with the cover and air filter on? My little 017 will turn ~14k without the restriction of the top cover and air filter.

 

[SteveSr]

Probably not what you want to hear after buying two new OEM carburetors but I used an AM adjustable carb on an 017 with success. The AM came from HL Supply and has both H and L adjustments, drilled another hole in the cover for access.

As I originally stated, unfortunately, I cannot turn this into a frankensaw as I will not be the end user.

Maybe a dumb question but are you checking WOT with the cover and air filter on? My little 017 will turn ~14k without the restriction of the top cover and air filter.

Yes, I have been checking with AF and AF cover in place. I live about 200' above sea level and the temperatures have been fairly cold (about 45F) during my testing. These conditions result in "worst case" for leanness.

Had a long talk with Stihl tech support and they basically say it is a small or intermittent air leak or a potential issue with the ignition timing being too far advanced. Looks like my next step is to tear it all apart again and replace the seals and see what happens.

 

[1990Husky262]

The rpm's your are seeing are what i normally see when i work on these models. As long as they have passed both pressure and vacuum tests and sure the carbs don't have any issues (which obviously yours most likely are good since the are new oem). I remove the main jet and carefully ream out with a torch tip cleaner or better yet a micro drill bit, carefully though because a little goes a long way sometimes. Usually takes me multiple tries to get dialed in. I usually shoot for around 13500

 

[SteveSr]

13.5K is a good number for these saws. Stihl tech support says that they don't see 200 hr. durability failures at 14-14.2K but don't want you exceeding that.

I have attached a couple of references for available fixed jets for Zama carburetors. Stihl seems to think (from their testing setups) that a single size change is only good for a couple of hundred RPM change. With my 170 I didn't notice any changes when going from 0.44 to 0.45. Might take 3 or 4 more steps to get it down and you'll notice that the Stihl jets jump from 0.46 to 0.50.

The 170 is 14 years old and if the seals are getting "weak" I would rather change them now than have them fail and burn up the saw. My Mityvac 8500 shows that they will leak down but then hold about 5 PSI vacuum almost indefinitely which is within spec. However, this is at the bottom of the range on the MV8500 so it is unknown how accurate it is. I have seen new seals hold 8-10 PSI on the same gauge.

 

[1990Husky262]

Ya if the seals are getting weak then that should be addressed before compensating by adjusting jet size. I tend to find a lot of times the sealant on case half's leaking, usually sand cylinder and bottom half flat on piece of glass to flatten, i usually chute for zero leakage up to 7 to 10 psi both vacuum and pressure cause you got to figure if they leak a little like yours are they are on there way out. Plus they probably leak even more as the engine is running.

They might not see failures at 14000 to 14200 but i find they usually have better torq when richened up to around 13500. I usually find when reaming the jets it does seem to take a few different size trys and then you get to a point and then a little makes a big difference so that lines up with what you are seeing

 

[SteveSr]

I tend to find a lot of times the sealant on case half's leaking, usually sand cylinder and bottom half flat on piece of glass to flatten, i usually chute for zero leakage up to 7 to 10 psi both vacuum and pressure cause you got to figure if they leak a little like yours are they are on there way out. Plus they probably leak even more as the engine is running.

I have also seen clamshell case leaks, usually at the junction of the bearings and case halves. You want the bearings to be tight so by definition there is a gap in the case halves at that point. Some serial numbers are worse than others. This is why I don't use Motoseal as it is not gap filling and doesn't set or harden like Dirko.

They might not see failures at 14000 to 14200 but i find they usually have better torq when richened up to around 13500. I usually find when reaming the jets it does seem to take a few different size trys and then you get to a point and then a little makes a big difference so that lines up with what you are seeing

I have noticed t hat the fixed jet seems to have a much more noticeable effect on the low end with idle adjustment. With the furnished 0.44 jet I could lower but not raise the idle speed. With the old 0.45 jet I could do both. Seems like it enriched the low end quite a bit. In fact, I was surprised when I pulled it apart just how oily everything was inside. I don't recall letting it idle a lot before shutting it off the last time.

BTW, Zama is back to using welch plug sealant again. This time it is some sort of red, crystaline looking compound. Cyanacrolate maybe?

 

[SteveSr]

Well, as a followup I tore the saw apart again and replaced the seals. The new ones hold about 10-15 PSI pressure or vacuum.

I had to wait until today for winter to return to get comparable ambient temperatures with what I had tested before. Today it was about 45F and the saw maxed out at 14.4K with the 0.45 fixed jet. This is still high but it came down about 600+ RPM from before changing the seals. Based on this I am concluding that even though the existing seals held 5-6 PSI pressure and vacuum that they were "weak". It looks like it would take a 0.46 or 0.47 fixed jet to bring it down to Stihl's 14K "recommended" max range.

Next week I am going on a mission trip to help remove damaged trees from Hurricane Florence in New Bern, NC. Hopefully I will get some run time on this saw and get a chance to see how it runs in warmer temperatures which may lower the max RPM some more.