Cold Flooding vs. Choke Plate Bypass Hole Size

[SteveSr]

Hello,

I just received back from service an MS250 that exhibits the often complained about cold start flooding issue. It also liked to "load up" at idle. I assumed that it would need a carb kit and pulled it apart. The diaphragms were actually in good shape and the "L" screw was only about 3/4 turn open and not even the recommended 1 turn. So this wasn't the cause of the idle issue. I also checked the welch plug for leaks and none were found. So loading up at idle is still unresolved.

As far as cold start goes the only way I knew it "popped" was the starter handle jerking in my hand. Not hearing a "pop" seems to a common issue with this saw which leads to flooding.

While disassembling the carb I noticed that the bypass hole in the choke plate seemed awfully small. I then dug around my "dead carb" collection and found another Zama C1Q that was made to go on a 35cc FS120 brushcutter. As you can see that bypass hole is significantly larger! The brushcutter will actually run a few strokes before dying with that large bypass hole.

So I am wondering why Stihl used significantly different choke plates in different machines of similar displacement. The next question is would the saw actual make an audible "pop" if the hole in the choke plate was larger?

Anybody ever try this? How about it @backhoelover You're our resident carb expert!

 

[HarleyT]

Sounds like a leaky needle,
did you test it?

 

[HarleyT]

Or better yet....


1123 120 0603 CARBURETOR (MS250) 21.99


1123 120 0604 CARBURETOR (MS210) 21.99

1123 120 0605 CARBURETOR 28.49

 

[Chainsaw Jim]

There's a few things that will cause this problem. One is if the metering lever is adjusted too high which will let too much fuel through. Another is if the pop off is too weak. The remedy there is to slightly stretch the metering lever spring to give it just a little more strength.

 

[SteveSr]

Sounds like a leaky needle,
did you test it?

No I did not since I am going to rebuild it anyway. I'll check the rebuild before it goes back on the saw.

 

[SteveSr]

There's a few things that will cause this problem. One is if the metering lever is adjusted too high which will let too much fuel through. Another is if the pop off is too weak. The remedy there is to slightly stretch the metering lever spring to give it just a little more strength.

The metering lever is actually a bit low (lean). This might be why it was a little inconsistent at WOT. I was beginning to think that there might be welch plug sealant floating around the "H" needle chamber. Unfortunately, you have to remove the limiter cap to pull the needle and flush it out.

 

[Deleted member 150358]

Agree with HarleyT WT215 is worth doing. Even if your times your own it just seems like the better carb for those. Might even feel a bit stronger.

 

[SteveSr]

Agree with HarleyT WT215 is worth doing. Even if your times your own it just seems like the better carb for those. Might even feel a bit stronger.

If it were my saw I wouldn't hesitate to do this. However, this saw will be for a trail club that I volunteer for and I will not be the operator. So to put this saw in the 025 wayback machine with a WT-215 carb a bunch of other things have to be changed, Carb adjustment boot has to be changed, tank vent has to be relocated, muffler opening enlarged, and now without limiters the printed screw settings on the A/F cover are incorrect. Then the model badge is no longer correct so anyone else working on this saw thinks they are working on a MS250 but really it has been converted to an 025. I think you get the picture.

 

[SteveSr]

Another is if the pop off is too weak. The remedy there is to slightly stretch the metering lever spring to give it just a little more strength.

I'll put the pump cover back on tomorrow and check for leak and pop-off weakness. May find a piece of that welch plug sealant!

 

[SteveSr]

I think I may be on the right track with the choke bypass hole... I had a spare Walbro WT-215 and looked at the bypass hole in it. The hole is considerably larger! Remember that area is proportional to diameter squared.

I also looked at the larger tooling (I think) hole in the Zama and tried to rotate it around, but alas, the choke plate is oval and not round. Looks like the plate would have to be completely removed and drilled.


Walbro WT-215 Choke Plate

 

[SteveSr]

I'll put the pump cover back on tomorrow and check for leak and pop-off weakness. May find a piece of that welch plug sealant!

No needle / seat leak below 20 PSI (as high as I went) so should be good. Only seepage was between carb and pump diaphragm.

 

[SteveSr]

Well, I took a carb and enlarged/added holes to match the Walbro WT-215. I enlarged the existing .050" hole to .062" (1/16") then I added the an additional .078" (5/64") hole above it. I removed the choke plate to do this but this effectively undoes the factory Loctite and screw staking. If I have to do this again I'll see if I can do it without removing the plate with the carb in a drill press vise.

The results were encouraging. At 40F the cold motor actually fired and ran for a couple of seconds before choking out. There was no mistaking whether it fired or not. Previously it would sometimes pop once or just tug on your arm as the only indication that it fired. I have also looked at several IPLs for different carbs for this series. It turns out that ALL EZstart versions had different choke plates with larger holes in them. Looks like this "design change" could be a realistic "fix" to keep from flooding these saws.


Zama C1Q-S76 Modified Choke Plate

 

[Colt Marlington]

Most of my 2-stroke equipment can be set at partial choke.

 

[SteveSr]

Most of my 2-stroke equipment can be set at partial choke.

Are you perhaps confusing part throttle as part choke? If not, your equipment must be all pre-EPA. EPA wouldn't allow part choke as it would allow an engine to run super rich spewing unburned hydrocarbons into the air. I suspect that the fewer times it will fire on full choke the happier the EPA would be. Somehow Stihl got around this on their EZ-start saws and Pro brushcutters.

 

[boltonranger]

This is interesting...!
Steve would you consider posting a video of the saws cold start?
I’d like to see it’s improved behavior.
I agree—The soft “pop” is sometimes hard to hear.

 

[ericm979]

The 025 was what made me change my starting procedure from full choke until it pops to just turning the choke off after three pulls.

 

[boltonranger]

Yes Eric, that’s something I’ve had to do on my 026 as well.

Another thing I do is slowly pull it over several times before I even set the master control, if it’s been sitting for a while.

 

[Colt Marlington]

Nope.

Are you perhaps confusing part throttle as part choke? If not, your equipment must be all pre-EPA. EPA wouldn't allow part choke as it would allow an engine to run super rich spewing unburned hydrocarbons into the air. I suspect that the fewer times it will fire on full choke the happier the EPA would be. Somehow Stihl got around this on their EZ-start saws and Pro brushcutters.

Nope. No confusion on my end.
Almost all of my equipment will allow the choke to be set at full or partial setting.

The Dolmar 421 may be different as it has a master control like a newer Stihl. But I honestly have never had to mess with the choke on it much because it always starts very reliably, and I don't run it all that much.

And I have some brand new equipment that the choke can be set part or full closed.

You should get out more.

 

[SteveSr]

You should get out more.

In my neck of the woods Stihl has pretty much a monopoly on all forms O.P.E All the stihl's that I have seen the choke lever is either fully on or off.

 

[Colt Marlington]

My 009L can be set at any level.