Need Zama Carb Expertise - Main Nozzle Check Valve

[SteveSr]

Alright @backhoelover and other carburetor experts I am working on an intermittent issue that I have personally witnessed on two separate Stihl 1123 series (021, MS210) saws. One is an 021 with a Zama fixed high speed jet carb. The other is a MS210 with a C1Q-S77 limit capped high needle. I would appreciate it if you could lend some expertise.

When the saws misbehave they will idle fast like they are lean, fail to rev-up past part throttle and won't run at all at WOT. The 210 that I had the displeasure of trying to use this weekend would also occasionally stall out at idle.

The issue is that these symptoms come and go in a random fashion. The 210 that I tried to run Saturday morning worked fine when I got home cutting a couple of cookies out of a 14" oak log. A half hour later the saw wouldn't run and exhibited the above symptoms.

I am beginning to think that this may be the main nozzle check valve getting stuck open an causing these symptoms. Have you ever run into anything like this? Does this sound like a possibility?


Thanks,

Steve

 

[alexcagle]

Make sure your impulse hose is hooked up. Make sure you have the correct gasket between the metal plate, and that the impulse hole, and is matched to the carb impulse port.
That carb has an accellerator pump plunger in a bore actuated by the throttle shaft. If the plunger gets galled, or the o-ring gets a cut in it, it can cause a stall or hesitation.
If a HS check gets stuck open it will backfeed at idle and feed air through the idle circuit making it die. This happens on Stihl TS420 cutoff saws suffer the same problem in high hour saws.
There is an accelerator pump kit available from Stihl for TS420's, but the MS210-250 carbs are cheap enough to just replace.

 

[SteveSr]

That carb has an accelerator pump plunger in a bore actuated by the throttle shaft. If the plunger gets galled, or the o-ring gets a cut in it, it can cause a stall or hesitation.

This is more than a hesitation. It flat out won't rev up. It also doesn't explain how the saw runs great one day and not the next for no apparent reason. If t he accel pump is bad I would expect the symptom to be constant.

If a HS check gets stuck open it will backfeed at idle and feed air through the idle circuit making it die.

This is the only thing that I can think of that would cause these come and go symptoms. The check valve is gummed up and sometimes get stuck open or partially open.

 

[rd35]

I chased a similar problem on a Zama carb this past week. Then I looked up a replacement and learned that I could buy a brand new carb for $14.00. I ordered it, it came in a few day later, bolted it on, and it ran like a brand new machine.

 

[SteveSr]

I chased a similar problem on a Zama carb this past week. Then I looked up a replacement and learned that I could buy a brand new carb for $14.00. I ordered it, it came in a few day later, bolted it on, and it ran like a brand new machine.

Prices are falling! Where did you get a new OEM Zama for only $14?

 

[rd35]

The carb I bought was for an echo trimmer. An OEM for an MS210 runs around $18 on the bay! Aftermarket as low as $10.

 

[HarleyT]

Did you ever rule out the fuel line? Also, the impulse line can loose it's grip and not fit tightly on the nipple on the handle and cause trouble.

 

[alexcagle]

Spark screen is clear right?
Have you already checked the screens inside the carbs?
You didn't by chance use b-12 carb cleaner on the fuel filters or the carbs?
I've seen it cause fuel filters to become restricted after being cleaned with it.
It fuses the filter membrane somehow.
The fact that both saws are having the same problem lends credence to possibly poor fuel or water mixed in with the fuel. Cloudy fuel is a sign of water. shake it up.

 

[SteveSr]

Did you ever rule out the fuel line? Also, the impulse line can loose it's grip and not fit tightly on the nipple on the handle and cause trouble.

I cleaned up the area on top of the fuel line with brake cleaner and a flux brush. The rubber has lost some flexibility but I couldn't see any cracking. I guess it's time to pull the top cover but I'm still leaning toward the carb being the issue.

 

[SteveSr]

Spark screen is clear right?
Have you already checked the screens inside the carbs?
You didn't by chance use b-12 carb cleaner on the fuel filters or the carbs?
I've seen it cause fuel filters to become restricted after being cleaned with it.
It fuses the filter membrane somehow.
The fact that both saws are having the same problem lends credence to possibly poor fuel or water mixed in with the fuel. Cloudy fuel is a sign of water. shake it up.

These are all great ideas and have been checked. What I didn't mention is that there was about a year's time between incidents on the two saws so its not the fuel. Interesting note on the B-12. I know it will dissolve some plastics. It also may have dissolved and fused any varnish that was in the fuel filter and fused it together. I usually don't mess with fuel pickups. If it smells like varnish it gets replaced. If not it's probably good to go.

 

[alexcagle]

It's hard to guage sometimes if a filter is overly restrictive. If I suspect one is clogged I'll pull it off, stick a short hose on it and blow through it. I've seen some before that were almost impossible to blow through. Most filters I have ever tested have the same resistance whether blowing through it or sucking through it (after they are dried out on the latter of course). I would just replace them if I were you. They are cheap. That's part of regular maintenance anyway. Far as I know, I've never heard of a sniff test.

 

[Old2stroke]

I have seen the same problem, and it can definitely be caused by the nozzle check valve but only in saws running fuel with alcohol content. The alcohol causes the valve flapper to swell and stick open at times, this will allow air to bleed back into the idle circuit and lean it out to the extent that it won't supply enough fuel to transfer from the idle system to the main jet. When it happens, you can sometimes flick the choke on and off, this will get it rich enough to rev up and pull fuel from the main jet. Those nasty little beggars can be hard to find replacements for.

 

[Chainsaw Jim]

I haven't read anything to this point about rebuilding the carbs with new kits. That's usually a good idea to start with when troubleshooting carb problems.

 

[SteveSr]

I have seen the same problem, and it can definitely be caused by the nozzle check valve but only in saws running fuel with alcohol content. The alcohol causes the valve flapper to swell and stick open at times, this will allow air to bleed back into the idle circuit and lean it out to the extent that it won't supply enough fuel to transfer from the idle system to the main jet. When it happens, you can sometimes flick the choke on and off, this will get it rich enough to rev up and pull fuel from the main jet. Those nasty little beggars can be hard to find replacements for.

Thanks for the confirmation! I don't run ethanol fuel but I have no control over the end user. I just looked at both saws again today. Turns out that the 021 has a Walbro WT-503 on it. So it would appear that this issue is not unique to Zama.

 

[SteveSr]

I haven't read anything to this point about rebuilding the carbs with new kits. That's usually a good idea to start with when troubleshooting carb problems.

A kit won't fix an issue where the saw runs great one minute but not the next hour (or day). Diaphragms go bad over time and don't miraculously heal themselves overnight.

 

[alexcagle]

A kit won't fix an issue where the saw runs great one minute but not the next hour (or day). Diaphragms go bad over time and don't miraculously heal themselves overnight.

Ok.
Buy a new carb and be done with it. Pesky gremlins can be isolated by trying a known good part sometimes. Sometimes carbs can't be fixed. I've only had a couple like that that wouldn't fix, and it's agrivating.

 

[SteveSr]

Ok.
Buy a new carb and be done with it. Pesky gremlins can be isolated by trying a known good part sometimes. Sometimes carbs can't be fixed. I've only had a couple like that that wouldn't fix, and it's agrivating.

Yea, I am thinking that replacement carbs are probably the way to go. However I currently have 2 saws with 2 more on the way... 4-new carbs. This gets expensive for a volunteer organization. I just like to make sure of the diagnosis before spending hard earned donation dollars.

 

[bikemike]

My suggestion is to not use carb cleaner. I use brake cleaner on pump carbs. Seems a bit more rubber and plastic friendly. I have sprayed decent running carbs and they didn't run after that. Diaphragms went hard instantly

 

[Chris-PA]

I've had leaky check valves on Zama C1Qs long before there was ethanol in our fuel - it's kind of what I expect from them. I've had it happen on Walbros too, but less often, although that might just be my experience. Storing fuel in inappropriate plastic containers can leach out stuff that ruins them too.

Usually leaky check valves make them hard to start, although it could cause running problems at low rpm I suppose. However, a leaky check valve won't hurt high rpm at all, as the check valve is open then. So given the symptoms I'm guessing that the check valve is a red herring.

 

[Chainsaw Jim]

A kit won't fix an issue where the saw runs great one minute but not the next hour (or day). Diaphragms go bad over time and don't miraculously heal themselves overnight.

I see this on a regular basis...at least three times a week, and I've serviced hundreds of carbs. A little bad fuel starts the process but things seem to run just fine, then you add some fresh good quality mix and then weird things start to happen. The saw will start up and tune like normal, you make a half dozen cuts to fine tune it and all seems ok. You shut it down and go to use it the next day and get half way through a cut and it just dies. Sometimes the choke will help pull some fuel to get it going again and if it runs it will act like it has all sorts of problems which I've noticed can lead to unnecessary troubleshooting and in some cases, wasted money.
Carb cleaners and other solvents will seep into the vent hole on the metering diaphragm cover and that will stiffen the diaphragm and ruin it.

And by the way...check valves going bad will still operate at wot. They just won't idle worth a crap.