carb repair for the ms 200T

[Marc-Antoine]

Frequently, after one year of daily use of the ms200T, this saw begins acting. No more straight acceleration, great loss of power, instable idle. We can get only some sort of useless WWAAAAABEEEUU.
The saw run very lean, no more enough gas to run properly.

It is too soon in the saw's life for the well known troubles with membranes and needle, usual air leaks, and the other gas starvations.
I already knew the cracking aluminum body on the small Zama (thanks Al).
But for the "1 year old" issue on the 200T, so common, it couldn't be those possibilities.
I understood that it could be some wear in the carb, but which one, how, what's going?
I didn't saw anything so quickly used in a so simple mechanic like a carb, able to give those symptoms.

So, I look after some informations on this f*** carb.

I found the answer on AS, where a guy put together a bunch of posts on this subject (explanations and solutions).
http://www.arboristsite.com/chainsaw/145024.htm


First consideration, in order to pass the new environmental laws, Stihl restrains his engines by the exhaust and by the carb, adjusted the most lean as possible, a story you all know very well.
Problem, the saw seems to become sleepy when man pulls the trigger, since it lacks on energy.

To wake it up a little, they add in the carb an accelerator pump (at a small scale) like the big carbs in the cars.
At each actuation of the trigger, the throttle's axle pushes a small piston, which send an additional drip of gas to the main jet. And go on, vroaaaar !

But the small brass piston (5 mm diam.) and his tiny bitty O-ring wear fast. The tightness is lost and some air comes trough the pump in the gas, before reaching the jets. Obviously, it doesn't run very well now, and instead of "vroaaar", it's more like waabeeeuuu !

Different solutions are possible, more or less costly, easy or long lasting (like putting a new carb or repairing the accelerator pump).
The one I choose costs a few bucks, is easy if you are a little "mechanic's lover" and lasts the saw's life. The onliest downside is a small loss of acceleration, (theoretically) but the max speed and torque don't change.


So, here's the way to repair and enhance the reliability of the ms200T's carb Zama C1Q-S61 (or S126 for mine).
The main goal is to neutralize the accelerator pump whose lifetime is too much short.
Given that this pump isn't really indispensable for the system, we can solve the problem definitely, and at a very low cost.
-- ---------

First step, we take the carb out of the engine. Even if the main operation could be done with the carb still in place, it's easier to work on it in hand.

The pics have some strange colors because my camera pull to the yellow with the kitchen light, and I have a hard time to give them again a "natural" aspect.

Here's the carb, seen from the engine side, with the throttle's valve. The brass cup (green arrow) closes the machining bore for the pump. This one is actually located deeper in the carb's body, between the throttle's axle and he choke's axle.

The carburettor, seen from the "air filter" side, with the choke's valve. The green arrow shows the small aluminum plate which closes an other machining bore. This one is there to drill the way between the pump and the usual internal canals (supply in gas for the jets).
We'll operate here.

With an sharp blade, pry out the small oblong alu plate. Take care of your fingers, the plate is both crimped and glued.
We find below two canal's entrances.

The left canal, very obliquely, reaches to the pump's bottom, located just behind the choke's axle.
The right canal goes down to the main jet (the bump on the top of the ventury tunnel). The needles help to visualize their directions.

The operation consists in blocking the way between the both canals. That's enough to completely isolate the pump and, more important, close the access to the gas supply and the jets.
One drip of grease remover and blow to dry the place.
Then, put a small quantity of epoxy glue to fill the both canal's entrances and the oblong machining cup. The good point with the epoxy-alu blend (like this Loctite or your JCB weld) is that it has enough viscosity to avoid deep penetration where it shouldn't go. A metal paper-clip has the right diameter to push a little bit of epoxy in the small holes. You need only one or an half rice seed of glue, but you will lost 10 times more just to get a good blend.

Be sure that the epoxy is well flattened and doesn't rise over the carb's back area. Otherwise, there will be a gap between the carb and the inox plate behind the air filter box.
Now, just wait one full day before the reassembly: the epoxy glue must be well settled to avoid trouble with the gas.

Then, adjust the settings for the idle (screw L and LA), eventually the H screw (if modified during the troubles), and the chainsaw is ready to go.

That's all.

The following is an option for the curious (like me).

Before the reassembly, I wanted to see at what looks like this pump in real. The drawing on the parts list is very small and a little disappointing.
In order to see the pump (and get it), you have to remove the throttle's axle (the one on the engine side) :
- loosen the springs of the choke and the throttle
- unscrew the center screw on the throttle valve and take out this one
- take out the circlip on the end of the throttle's axle
- take off the carb side that hold the throttle stop (screw LA)
- rotate the throttle's axle 180° to push the small piston pump back in his room (in the normal position, the piston bears on a notch in the axle and prevent this one to get out, so do not force it as is).
- pull the axle out of the body, and here comes the piston, visible in the axle housing:

- with a thin rod and a little tap, push out the closing brass cup by the inside ( pass by the axle housing, very obliquely). Careful to not damage the aluminum body.
- work gently the piston and it goes out with its spring (small but strong for its size).

Sadly, my camera isn't good enough to show you the wear on the piston and on its O-ring.
At the bottom of the pump housing, there's a very small metal mesh filter, then the canal's entrance we closed at the other end.

Here is the arrangement of the different parts, like an exploded view :

Put again in place the axle, the circlip, the valve (position it very precisely) and its screw, the carb side, the brass cup (to be sure no debris could come in and jam the axle), then tighten the throttle's and choke's springs.
I trashed the piston and it's spring, so there's no more stress and wear on the axle from them.

End of the option.

Now, the chainsaw is reassembled, tunned and tested.
YES, it works!


You can see the pics from the first one, but actually, I repaired 3 Ms200T by this mean.
I took out the 3 pumps to avoid permanent pressure and wear on the axles.
All 3 chainsaws work very well. They get again a good beat in the wood, without any hole, speed loss, irregular idle, or dying.
And honestly,I'm not able to see the small acceleration loss. (but perhaps am I a little biased)

That's all good !

I hope that will be even useful for you as it was for me.

(and I hope too there aren't too much strange sentences by my translation and that remains understandable).

 

[MasterBlaster]

Preach on, brother man.

 

[Al Smith]

Real good info .I for one had no idea all that stuff was in there .Thanks .

 

[CurSedVoyce]

Well done sir! Well done! Worth the wait.

 

[Ax-Man]

Excellent thread. Well done

 

[Al Smith]

I just reviewed what info I have on these little Zama's The drawings I have are pertaining to the 16 a and 32 models of this carb but they don't show that accelerater pump as being a part of the carb .

Now I can't help but wonder if in fact these problems are related to just the model mentioned in this thread or just prone to all of these little Zamas .As I can't remember the exact models I have on my 200's I'll have to check to verify them .I can't remember either the one I sent to Eddie at Stihl for evaluation .That said it might be a good idea in my case to take notes except I'd most likely missplace them .

 

[top_notch]

Great thread Marc-Antoine!!! Gonna try this out tomorrow...hopefully, to see if it cures my 200's ailments...wish me luck!!!

 

[pantheraba]

Wow...great detail and explanations. You present a lot more detail than I am ready to deal with but I do recognize how thoroughly you researched this and how well you present the information. I have saved your info on my computer for future reference. Thanks much for taking the time to do such a detailed job.

 

[Al Smith]

After seeing this thing in pictures it appears that the pump is very similar to a piston oil pump used on old McCulloch saws with the o-ring seal .I wonder if just replacing the o-ring would be cure too ?

After thought : aha ,the o-ring is made of buna-n material no doubt .Real tollerant of gasoline but not so good with any type of alcohol .Ergo they fall apart with the damned ethanol in the gas and cause many to tear their hair out in frustration .Geeze after all this time you certainly would have thought Zama would wise up and use a material more robust .Then again I guess they make their money selling carbs so maybe therein lies the problem .

 

[Marc-Antoine]

Thanks for your kind words

Replacing the O-ring will cure the problem, but you'll get the trouble again some time later, probably quicker than the first time, because the piston wears too. It's abraded by the edge between the axle's housing and the pump's housing, as it's actually pushed sideway by the notch in the axle.
So the lateral play increases and the piston tilts in his housing, the wear becomes greater and greater due to those micro-movements (supposition only).

If you prefer to keep the carb in its original state, it's better to change the whole unit than just the O-ring. For that,Stihl sells a repair kit for this pump, 11291209700, "part" n°32, page 35 from the ms200T's parts list (piston, O-ring, spring, close cup and mesh filter).
But why messing up with this problem again and again, while you can get ride of it the first time ( and even better, preventively) ?

 

[Al Smith]

Yes I see what you mean .That thing is all wrong from a design stand point .With the thrust exerted by the flat on the butterfly shaft it can only put side stress on the pump piston .No easy remedy for that with the way it's built .

 

[Magnus]

I have to try this on the next that come in.

 

[Al Smith]

Well actually I might have to delve into that fix myself .It seems on my souped 200T the carb is giving me fits once again after working on the stupid thing 3 or 4 times .Leans out ,not bad but enough not to be able to get enough fuel to sustain a high speed run very long .

About drives me nuts ,that thing will wind up to over 15 thou if it's running right .Now it won't run as well as a stocker because of the carb .So it's a shelf queen until I get the time and motivation to fix it .

 

[top_notch]

Did the repair tonight and made about ten cuts...enough to warm the saw up, and it ran SWEET!!! Hope this fix is permanent, will see in the next few days when I have some jobs to do down here in FL. Thanks again Marc!

 

[Al Smith]

I'm about to do the same myself .The hot rod super dooper moded 200T isn't getting it done .Sucks when I get outrun by a Poulan---long story but it will never happen again by George .

 

[Al Smith]

I took some time a little while ago and pulled the carb on the super bee only to find out it wasn't the model with the acel pump .

Evidently at some time I had swaped carbs with the stocker which at that time I found a crack in the fuel chamber of the super and repaired it with red loctite or so I thought .

Now I'll be damned if this other one doesn't appear to have a crack also .So more red Loctite and try it in the morning .Either that or the red Loctite from before got dissolved by the gasoline .

The more I delve into this I'm beginning to think that in addition to the fine work Marc did on the acel pumps there may be more than one problem with these things .

 

[woodworkingboy]

Al, do you have any idea how I might get a better closing fit on a high speed needle when screwing it all the way in? After putting a larger carb on a modded saw, it performs well, but the high speed needle has to be screwed all the way in to get the sweet spot, and it still isn't exactly there yet when fully closed. There is still fuel passage when the needle is completely screwed in, a bit much by increments. I was going to saw an old carb in half to see what goes on all the way down at the bottom of the needle hole, what actually closes it off down there, the small tip or what? Possibly if I could somehow add some volume to the tip? Thanks.

 

[Burnham]

Put a dab of blue loctite on the tip of the needle before you screw it in...if that doesn't work at closing it off, nothing will .

 

[woodworkingboy]

Taking that idea under advisement, it keeps getting spit out of the know it all machine without even an answer.

 

[Burnham]

 

[Al Smith]

I'd imagine possibley the high speed is not passing fuel .They will run on just the low speed but not as well as with both jets working .

If you richen up the high speed does it act like it's running rich ?If it's leaning out it seems somehow it's just leaking or sucking air rather then getting fuel for whatever reason .

On the 200t carb I'm fiddling with the high speed had little or no response no matter where I had it set .

Now I've got a carb that was made for a 123 cc Mac which is on a 70 cc model and it does amazing .However the high speed did need cranked down just a tad bit from normal .

 

[woodworkingboy]

Seems to be working as designed, Al, opening up the screw gives more rich and gurgling sound as you would expect. Closing it gets the revs up, but just not quite enough to where you want it, still a bit too much of the gurgling. I interpret that the high speed still passes a bit of fuel when closed, and with the larger carb, it's enough to be a bit rich. Like I mentioned, it's almost there, just a bit less fuel and it would be dialed in.

 

[Al Smith]

You kind of have a catch 22 Jay .It's obviously from you've said is getting plenty of fuel but can't move enough air to support the charge .

It sounds like the same scenario as hanging an 850 cfm carb on a stock small block Chevy 327 .With stock valves and a stock cam the thing would need jetted down to run correctly .Unless the componets were changed and the cylinder heads were reported for flow it never would run right .

So you have a deal like this ,either find a smaller carb or get to work on the innards of the saw engine .If for example you have a larger carb venturi area than it has intake area it's almost like trying to cram 10 pounds of chit in a 5 pound bag,won't fit .Then if the area were large enough and the transfers wouldn't support the flow you're right back to square one .

 

[Al Smith]

Now depending on the carb the actual jets sometimes can be changed but it would be a monumental pain in the butt .I've never done it on a saw carb but I have on a auto carb which screw out .The little saw carbs are pressed in place excepting some older models such as a Tilley HL .Where you might find them or how to install them I haven't a clue

 

[woodworkingboy]

Al, just to see, today I sawed an old carb in half through the high speed screw hole to see how the high speed needle seats. As per my guess, the small tip is supposed to block the jet passage when closed, but the fit is less than precision, so there is still fuel moving through. Obviously with most saws it would be insufficient for proper running, but I have a slightly rich thing happening. If I could figure a way to get better blockage, I think I would have it. I dipped the screw tip in some liquid gasket and made a more bulbous end. Tomorrow I'll screw it into the same model carb that is on the saw, a defunct one, and in theory the end should shape itself to give a better seal. I don't have a clue if it will work, or how the end will hold up even if it does the purpose initially. I have run the saw for a couple days now with the high speed completely closed. No problem, fuel is definitely moving through. Your idea about increasing air intake would really be interesting, but it seems problematical. The air filter is already a pretty open type, and the ports are already modded.