Stihl Ms 170 Carb

[djg james]

Where would there be an air leak; between the boot and carb?
The new carb is a aftermarket but is the same design. If switching out the diaphrams on the old carb doesn't work, then I'll buy and OEM one.

 

[TheTone]

When I grew up it was African-American Rigged (not the actual wording). Closely related to this form of make-do modifications was Redneck Engineering, a term which I still use.

 

[Vintage Engine Repairs]

Where would there be an air leak; between the boot and carb?
The new carb is a aftermarket but is the same design. If switching out the diaphrams on the old carb doesn't work, then I'll buy and OEM one.

Possibly the boot, more likely the oil seals. That said, this is all speculation. This is why if you intend on doing any of your own chainsaw mechanics, the very first thing to grab is a pressure and vac tester. I think Mityvac are over rated personally. Grab a cheapie on eBay. I have both and the cheapie 30 usd does a stellar job.

 

[SteveSr]

Where would there be an air leak; between the boot and carb?
The new carb is a aftermarket but is the same design. If switching out the diaphrams on the old carb doesn't work, then I'll buy and OEM one.

This is how this series of saws dies with a burned up P&C. They develop air leaks at the crank seals and pan seal. Now a small leak could possibly be compensated for by a carb adjustment. But since this series has a non-adjustable carb that is tuned waaay too lean to begin with it is not hard to go over the line and burn one up.

Even after I have rebuilt one of these and verified no leaks they still rev higher (leaner) than I would like. I have one close by in the flatlands at about 250 feet ASL. So far it is doing fine. The rest I have managed to send to the mountains where the higher elevation will richen them up and provide some safety margin.

 

[djg james]

Got back to the 170 today. I installed both fiber gaskets to hopefully seal against the air leak, but no luck. The saw still revs higher than it should and the saw will not idle despite the screw being all the way in (CCW). This is a new CHINESE Zama Carb. I never had air leakage problems before with the old carb so I see no need for a vac tester? All the linkages are working properly. Could this carb just be a bad one?

I'm going to remove both diaphragms as suggested and replace on the old carb. See if that makes a difference.

Any other ideas before I buy a Stihl carb?

 

[djg james]

OMG!! Reluctantly, I flip flopped both diaphragms from the old/new carbs while waiting on a reply. The 170 came back to life! Good rev and no stalling when the throttle is pulled.
Who knew all that needed to be replaced was the diaphragms. Lesson learned. To my untrained eye, they looked good.
Thanks @Spoon Carving With Tom for suggesting that. I never would have thought of it.

Two down (038, 170) and one (011) to go.

And @HarleyT , I checked and there was no expander ring on my saw. Maybe lost by someone else on a preevious repair. I'll try to locate a part No. and purchase one to put on at a later date.


Looks like there is a recess for one.

Thanks everyone!

 

[SteveSr]

OMG!! Reluctantly, I flip flopped both diaphragms from the old/new carbs while waiting on a reply. The 170 came back to life! Good rev and no stalling when the throttle is pulled.
Who knew all that needed to be replaced was the diaphragms. Lesson learned. To my untrained eye, they looked good.
Thanks @Spoon Carving With Tom for suggesting that. I never would have thought of it.

Two down (038, 170) and one (011) to go.

And @HarleyT , I checked and there was no expander ring on my saw. Maybe lost by someone else on a preevious repair. I'll try to locate a part No. and purchase one to put on at a later date.

View attachment 830676
Looks like there is a recess for one.

Thanks everyone!

That "expander ring" is not just an "expander" for the main bore as in larger saws. It also supplies the impulse to run the fuel pump in the carb through that little hole in the manifold. DO NOT run the saw without it! The carb should go straight on with NO gaskets. I can't tell if the rubber manifold has been mal-formed by incorrect gaskets or missing expander. If so you may need to replace it as well.

As you learned Chinese carbs are junk, especially the non-tunable variety.

 

[djg james]

Thanks Steve, I don't know how long I've been running without one. I have to fix the oiler now while I'm waiting for the part. Anyone wouldn't have a part No.?

And I did not use the fiber gaskets this time.

Found this:
https://www.ebay.com/itm/New-Carbur...ihl-017-018-MS170-MS180-Chainsaw/163797787487
Not sure how the collar ring fits when I replace the boot also, but I see once I get the part.

 

[DND 9000]

If you mean that ring on the right of your picture, it`s part no. 1130 022 2000.

 

[djg james]

If you mean that ring on the right of your picture, it`s part no. 1130 022 2000.

Thanks, but I'm specifically talking about the ring pictured in the middle.

 

[DND 9000]

That ring is 1130 141 1800. It goes on top of the manifold and has a hole for the impulse.

 

[djg james]

Well again I thought I had it fixed.
To recap, I had used the diaphragms from the chinese carb that I had bought and the saw ran fine.
It was pointed out that I was missing the expander ring that sets on the intake boot. I ordered and installed a new one. It seemed to run fine for awhile, but it's acting up again.
The saw starts up fine and runs without over-revving, but only up to 1/2 - 2/3 throttle. When you give it full throttle, the saw starts to die until you let up.

Is this indicative of any single problem? Is it still a bad carb or could it be a air leak as previously stated?

 

[SteveSr]

Well again I thought I had it fixed.
To recap, I had used the diaphragms from the chinese carb that I had bought and the saw ran fine.
It was pointed out that I was missing the expander ring that sets on the intake boot. I ordered and installed a new one. It seemed to run fine for awhile, but it's acting up again.
The saw starts up fine and runs without over-revving, but only up to 1/2 - 2/3 throttle. When you give it full throttle, the saw starts to die until you let up.

Is this indicative of any single problem? Is it still a bad carb or could it be a air leak as previously stated?

Sounds like it is not getting fuel. If it was an air leak it wouldn't idle. Could possibly be a sticking main nozzle check valve in the carb. This failure can work fine one minute but not the next. I see a new OEM carb in your future...

 

[rupedoggy]

Start at square one and check everything that could slow fuel delivery. Kinked hose, partially clogged fuel filter, small rip or pin hole in fuel line or impulse passage, gasket or gasket cement partially over the passage way for pulse, metering lever to low, small screen in carb plugged, high speed adjustment screw not correct, fuel pump diaphragm in carb stretched and more. You are learning, don't stop now.

 

[djg james]

Thanks again Steve. I didn't want to order another carb if it wasn't needed.

This time I'll get a new OEM carb.

 

[djg james]

Start at square one and check everything that could slow fuel delivery. Kinked hose, partially clogged fuel filter, small rip or pin hole in fuel line or impulse passage, gasket or gasket cement partially over the passage way for pulse, metering lever to low, small screen in carb plugged, high speed adjustment screw not correct, fuel pump diaphragm in carb stretched and more. You are learning, don't stop now.

Thanks,
It has a new fuel line and filter.
The impulse passage gasket (intake manifold expansion ring) is new.
This is a Zama carb not a Walbro so no High/Low adjustment screws.
Both diaphragms are new.

Can you think of anything else other than a plugged carb which I have completely disassembled and soaked at least twice?

 

[L34]

Had nothing but trouble with non adjustable carbs.
Both on saws and whipper snippers.

 

[SteveSr]

Can you think of anything else other than a plugged carb which I have completely disassembled and soaked at least twice?

You need to erase from you consciousness that ALL carb defects can be FIXED by soaking or changing diaphragms. It ain't so... Dealer's don't even mess with carbs anymore. They just replace them.

 

[djg james]

You need to erase from you consciousness that ALL carb defects can be FIXED by soaking or changing diaphragms. It ain't so... Dealer's don't even mess with carbs anymore. They just replace them.

Yes after playing with this one as much as I have, I believe you're right.

I just hate to be beaten by a little hunk of metal.

 

[Ando81]

I have an OEM Zama brand new for the 170/180 saws if you are interested.

Just out of interest, how much are you asking for the carb mate? Is it a tunable carb or the factory set type?