Stihl MS201CM Question for Stihl Techs

[ZeroJunk]

So, this is not a top handle saw. It has a primer bulb at about one o'clock under the starter cover.
Gentleman brings it in and says it is cutting off on him and he thinks it is because the primer bulb is not filling.
So, I replace the primer bulb and in the meantime check the old one by just sticking a piece of hose in a glass of mix and it will squirt fuel a couple of feet.
Now, the carb has a black solenoid which I replace with a green solenoid after taking the carb apart and making sure there is no debris in it. There is not.
And, it seems to run perfectly.
But, the primer bulb is completely dry.
If you take the input off the carb fuel spews everywhere, so there is no issue with the input line.
And, the saw runs fine.

So, what is the deal with the primer or purge pump with an Mtronic carb ?

Just an afterthought so they can use the same plastic molding ?

I can run this saw extensively and there is no fuel, at all, in the bulb.

Help me understand this.

 

[SteveSr]

The bulb is only a purge system and circulates fuel from the tank to the carb and back. Fuel to the engine/metering chamber does NOT have to go through the bulb. I have seen several engines with broken/disintegrated primer bulbs that run fine, however, you may have to pull them a lot to get the fuel up to the carb.

 

[ZeroJunk]

The bulb is only a purge system and circulates fuel from the tank to the carb and back. Fuel to the engine/metering chamber does NOT have to go through the bulb. I have seen several engines with broken/disintegrated primer bulbs that run fine, however, you may have to pull them a lot to get the fuel up to the carb.

I understand. But, I know of no other Mtronic that has a primer bulb.

The check valve is in the bulb itself. The bulb works correctly when checked with just a piece of fuel line in a glass of fuel.
There is plenty of fuel at the input of the carb.

I guess I was wondering considering the 201TC has no primer and doesn't need one if the primer on the rear handle is there just to have something to fill the hole on Mtronic versions and useful on the non Mtronic.

Or, is something in the carb stopped up.

Don't want to just replace the carb if they are all this way.

 

[SteveSr]

Or, is something in the carb stopped up.

As far as I know there is no path to push fuel into the primer bulb without actually pushing the bulb itself.

Don't want to just replace the carb if they are all this way.

An M-tronic carb is not magic. It is just a regular carb with a metering solenoid. Nothing more.

 

[ZeroJunk]

As far as I know there is no path to push fuel into the primer bulb without actually pushing the bulb itself.


An M-tronic carb is not magic. It is just a regular carb with a metering solenoid. Nothing more.

This is an odd duck. As opposed to a MS201TCM carb which has no primer if you look at the metering side of this carb for the rear handle you have the compensating opening side which goes in to the air filter and straight opposite of it you have a needle that goes in to the input side of the purge pump and the return straight back to the tank through said purge pump. This is in the top casting of the cover over the metering diaphragm.
So, it pulls fuel up through some pathway in the carb, through the metering needle , and back to the tank.

Apparently from the replies nobody on here has seen one or knows what I am talking about.
Probably because a rear handle MS201 Mtronic is rare.

Anyhow, it seems to run OK with that pathway blocked somehow, which makes sense since the top handles never had a primer to start with.

But, if he has more complaints I will try to figure out how to clear that passageway.
Could be there is another check valve in the carb itself in addition to the check valve in the stand alone purge pump. Don't know.

Hate to just replace the carb, but may come to that.

But, it pumps no fuel through the primer, and there is nothing wrong with the primer.

So, the pathway is stopped up in the carb, or there is a problem in the line. Unlikley since the saw is practically new.

 

[SteveSr]

But, it pumps no fuel through the primer, and there is nothing wrong with the primer.

Put a MityVac in vac mode to the line that goes to the carb. Use the catch bottle to keep any gas out of the mityvac. This should pull fuel from the tank and into the mityvac bottle. If it doesn't you have an air leak or stoppage somewhere or possibly a blocked tank vent. The carb line should go pretty directly to the metering chamber so should be easy to trace.

So, the pathway is stopped up in the carb, or there is a problem in the line. Unlikley since the saw is practically new.

The purge system has two check valves and the ones that I have seen are located in the purge valve assembly when they are separate from the carb. There may or may not be one in your carb. If there is one here it should allow flow out of the carb but not back in.

 

[ZeroJunk]

Well, no fuel makes it to the primer bulb at all. The primer bulb works properly with just a line stuck in a glass of fuel. But, even after making several cuts the primer bulb is empty. Saw seems to run fine.
I'm thinking if there was a hole in the line between the carb and pump that it would leak air and not run correctly.
I'll see if it will pull fuel through with a vacuum. May suck a bugger out of it, who knows.

I'll report back to you in case you are mildly curious about a primer bulb on a Mtronic.

 

[stihltech]

Yes, 201 top handle uses the primer bulb. I had one I needed to replace the molded lines to correct this. Also, do not be afraid to try a new carburetor. Pretty common for the internal check valves to fail as stated above. Do not let the "IT HAS A COMPUTER!" blind you. Just a saw with a carburetor that has an outside control.

 

[ZeroJunk]

Yes, 201 top handle uses the primer bulb. I had one I needed to replace the molded lines to correct this. Also, do not be afraid to try a new carburetor. Pretty common for the internal check valves to fail as stated above. Do not let the "IT HAS A COMPUTER!" blind you. Just a saw with a carburetor that has an outside control.

It had the black solenoid. I replaced it with a green. Started in three pulls and seemed to run fine.

I can't see how the carb on this saw is any further away from the tank than on a top handle. My conjecture was so they could use the same molding as the non Mtronic was the only reason to use the purge pump.

You are right. It has to be a bad molded line or a blocked passageway in the carb.
No problem with the input because it squirts fuel everywhere until you loosen the cap.

Yes, 201 top handle uses the primer bulb.

I have never seen one.

 

[alexcagle]

Try tilting the saw on it's side when you prime it, or try pumping it when it's running at partial throttle.

 

[ZeroJunk]

View attachment 1123889

Try tilting the saw on it's side when you prime it, or try pumping it when it's running at partial throttle.

I hate to say it. Normally I don't work on Mondays, but I needed to go to town and this saw was bothering me.

I was afraid the guy may have picked it up and had more problems that the green solenoid wasn't part of.

Well, now it primes fine. I'm like what the hell. I know I must have pressed it thirty times.

One of those things that make you scratch your head.

We don't sell top handle MS201's with primers. Have no idea why. Regional thing maybe.

Or, maybe it's something new. Don't know.

 

[SteveSr]

Well, now it primes fine. I'm like what the hell. I know I must have pressed it thirty times.

Well you may have a wonky input check valve in the purge assembly. Sometimes these things don't work if the check valves aren't primed in fuel/oil. If it does it again just replace the purge bulb assy.