What to do? - High Speed MS170/180 14.2-15K!

[SteveSr]

Hello,

I have both an MS170 and MS180 in the shop. Both have recently been completely rebuilt so there are NO fuel or air leaks. Pistons look pristine. Both needed new carbs for different reasons.

So I got the latest OEM Stihl/Zama C1Q-S57G carbs. Before I installed them I opened the metering cover and raised the metering lever to the Zama gauge correct height. Both units were about .030" low (lean).

So I put the MS180 back together first. I cut a couple of cookies to get the saw warmed up. I then put the tach on it at WOT and found the max speed at 14.1-14.2K. This seems to be pretty high but at least it is not 15K! I also noticed that the idle adjustment would lower the idle but not really raise it. Is this a characteristic of this particular saw/carb combo? I also noticed that the spec for the 018 is 400 RPM lower at 2800 than its 170 cousin.

Next came the MS170. Did the same procedure and it ran at 15.0K at WOT after being warmed up. The idle screw also seemed to have more effect I could up the idle speed such that the chain would start bumping forward.

So what should I do about these high max speeds? It would appear that the metering lever is not very sensitive in setting WOT max RPM. It appears that it would take a lot of height change to lower the max RPM very much and then it might over richen the low end.

I also did some cruising through some old IPLs and apparently the fixed jet in the HS circuit has gotten smaller over the years. The current "G" version carb comes with 0.44mm fixed jet. As late as 2012 it came with a 0.45mm and earlier but different carb model I found 0.46mm. Does anyone have access to the post 2012 Stihl tech notes that explains the reasons for revisions "B" -> "G"?

OPTIONS:
1. Trust Stihl knows what they are doing and leave everything as-is.
2. Raise the metering lever some more. How much?
3. Start swapping in larger fixed HS jets?

Your thoughts?

 

[TBS]

Imo raise the metering lever then switch to fully adjustable carbs

 

[tallfarmboy]

I would definitely look into getting an aftermarket carburetor that has H and L adjustment screws. Easy way to improve those saws.

Sent from my Moto G (5) Plus using Tapatalk

 

[Chainsaw Jim]

The S57 is a factory 14,000 carb.

 

[SteveSr]

I would definitely look into getting an aftermarket carburetor that has H and L adjustment screws. Easy way to improve those saws.

Unfortunately, these saws are for a trail club that I do volunteer work for so I will not be the primary user. They may also end up at the far end of the state where I will not be available to attend to any issues that might crop up. Therefore I cannot really change much (if any) from stock as I won't be the one working on them. It may be a generic saw shop or Stihl dealer and I wouldn't really want to challenge them with a frankensaw!

 

[SteveSr]

The S57 is a factory 14,000 carb.

So how many have you seen blown up that can be traced directly to this carb and the lean/high RPM?

 

[TBS]

The adjustable carb just bolts right up with no modification other than adding a hole to access the adjustment screws.

 

[Chainsaw Jim]

So how many have you seen blown up that can be traced directly to this carb and the lean/high RPM?

Lol... both models have another carb with specs of 15,500. Are you determining it being lean by the rpm level alone?

 

[Jasonrkba]

I used the farmtech adjustable carb for my ms170 and it works fantastic. Inexpensive also.

 

[SteveSr]

Lol... both models have another carb with specs of 15,500.

What is the part number for that carb?

Are you determining it being lean by the rpm level alone?

Unfortunately, yes since Stihl doesn't apparently publish max RPM specs for either saw. These will also be going out to possibly inexperienced sawyers who may abuse them by cutting with a dull chain and any other way you can think of! I keep finding new creative ways like improper sharpening making a super aggressive chain that burnt up a clutch and worm on an MS250.

I would just like to add a little "abuse margin" to the saw so that it is NOT on the edge of spontaneous disassembly!

Looking at the Stihl literature there appears to be a pattern of leaner and leaner on this series of saws. I am wondering if Stihl has determined that the saw can take it or t hey are being pressured by the EPA.

 

[atpchas]

See first page. It may help with your carb/rpm questions.

 

[Cliff R]

I had an MS170 in here recently and put a new carb on it after rebuilding the original didn't help. It did pretty much the same thing, revved to the moon and was too lean for my liking, with little response from the idle speed screw as well.

If they make a decent adjustable carb for them I'd go that route. I turned this one back to the owner as I already had too much time in it, and he very seldom uses it and just casual cutting anyhow, maybe a small tree limb down in his yard once or twice a year, etc.......Cliff

 

[SteveSr]

See first page. It may help with your carb/rpm questions.

Nice document! Wish I could find a current version as this one doesn't list the "G" version of the S57 carb. This list also doesn't include some USA carburetors so this list might be specific to the EU where I don't believe they have EPA-like emissions regulations to confuse things.

Based on this info it looks like Stihl intends for these saws to run this fast with the unknown of how it might affect longevity.

 

[morbius18]

I'd get a chinese adjustable carb and tune it a little rich for the inexperienced cutters. Newbie = Clogged filter + dull chain = junked saw.

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/310818212401

 

[a. palmer jr.]

I just ordered an adjustable carb for the 180 I'm working on, probably the cause of the scored piston on the saw was a dirty non-adjustable one...

 

[SteveSr]

I just ordered an adjustable carb for the 180 I'm working on, probably the cause of the scored piston on the saw was a dirty non-adjustable one...

Just out of curiosity... What is the model of the old carb? And what are the numbers stamped on the brass fixed HS jet in the metering chamber? Was the saw blown up while being used in cold winter weather (runs leaner)? Any other signs of abuse such as a terminally dull chain or burned clutch?

 

[a. palmer jr.]

Just out of curiosity... What is the model of the old carb? And what are the numbers stamped on the brass fixed HS jet in the metering chamber? Was the saw blown up while being used in cold winter weather (runs leaner)?

 

[a. palmer jr.]

Just out of curiosity... What is the model of the old carb? And what are the numbers stamped on the brass fixed HS jet in the metering chamber? Was the saw blown up while being used in cold winter weather (runs leaner)? Any other signs of abuse such as a terminally dull chain or burned clutch?

I'm not in the shop today but I know the old carb was a Zama non-adjustable and I ordered a Walbro look alike adjustable one. I'm not sure that's the cause of the scarred piston but I'm replacing the carb anyway. This isn't my saw so I'm not sure when it blew up or much about it, just know it didn't have much compression and the piston is bad..

 

[SteveSr]

I'm not in the shop today but I know the old carb was a Zama non-adjustable and I ordered a Walbro look alike adjustable one. I'm not sure that's the cause of the scarred piston but I'm replacing the carb anyway. This isn't my saw so I'm not sure when it blew up or much about it, just know it didn't have much compression and the piston is bad..

I think about all of the non-adjustable carbs are various Zama models. From the specs posted earlier in this thread they appeared to be set pretty lean. Which Walbro clone fits? The WT-215?

 

[a. palmer jr.]

I think the WT215 clone is what I bought. I've tried them before and unless you get a factory defect they seem to work just fine..