Basic Repair: MS 360 carb for 034 project saw

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Stihl #1

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As I showed before in the other thread, the original Tilly for the 034, upper left in the picture below, had a split fuel inlet fitting which is what killed the saw.

DSCN0201.jpg


Since I have put on the MS 360 48mm piston kit, I will use a ZAMA carb off a MS 360. I will have to use a non-compensator metering cover, since I want to stay with the 034 shroud and filter cover, and all the MS 360 saws had a bigger filter and cover and used a compensator port. Fortunately I have one ZAMA with the cover I need, lower left above, so I can go through this pile of MS 360 carbs and get one going.
I see so many threads with folks struggling with carbs, so I will address some procedures that will allow you to verify if a carb has faults or not, so you can be sure it will work OK once installed. The way we used to work on carbs 10 years ago does not apply to modern carbs due to EPA changes as well as changes in materials and the lousy fuel we have to use today.
I guess before we get started let’s review a little theory. For me, if I understand why it is made the way it is and what is supposed to happen, it makes it easier to troubleshoot and repair. Maybe not for everyone but that is just the way my brain works. When I was just starting out in a mower shop in high school, I used to rebuild carbs all day by just swapping out parts and hoping it would run, and then finally an older gent showed me some things about the why and how of what was going on in there and that was a tipping point for me to start learning, and I am grateful to him for that. I would never have thought then that that interest in learning would lead me to become an educator myself, with 2 shingles to hang on the wall, and a dream job where I get to teach the best of the best all day. And I get to learn from them at the same time, Pretty cool! OK, I will quit preaching and start teaching.
A fellow named Bernoulli postulated that as the velocity of a fluid increases, its pressure will go down. This is a proven law of physics and it explains why an airplane wing creates lift, and why a carb mixes fuel with air.
You need to remember that carbs are dumb. No computer, no sensors, nothing to tell them anything about what is going on regarding temperature or altitude or anything else. All a carb can do is compare internal pressure with atmospheric pressure and react. The carb’s job is to mix fuel with air at an air fuel ratio of 14.7:1. That is 14.7 pounds of air with 1 pound of fuel. Or ounces or grams or whatever. At all engine speeds. Technically this is called the stoichiometric air-fuel ratio, also known as Lambda (l) = 1. Assuming complete combustion takes place, at this ratio all of the fuel and air is used, which should give maximum horsepower and minimum emissions. Of course, no engine is 100% efficient, so in reality, we have to deal with less power and some emissions.
When we say the engine is running lean it means there is less fuel being mixed with the air and if it is rich there is more fuel being mixed with the air. The engine will run lean or rich, but too lean and it will lose power and may overheat and fail. The fuel droplets mix with the air and atomize in the carb, but they are not completely vaporized until they come in contact with the hot metal in the engine. For a high-revving two-stroke such as a saw, the fuel is what is cooling the piston as it vaporizes right before the plug fires and combustion takes place. If you take away the fuel, the RPM goes up and the piston overheats; failure is eminent.
And too rich means the engine can’t make the proper amount of horsepower, will have high fuel consumption and high emissions, and eventually foul out the plug. Do not confuse a too rich fuel mixture with too much oil in the fuel. But that is for another lesson.
So the way a carb works is by having a venturi, which is a necked down area in a tube, so that when the air is rushing through the tube, as Bernoulli said, its velocity increases and the pressure in the restricted or necked down area goes down.

venturi.gif


If you put a tube in the necked down area and let it rest in a bowl full of fuel, and add a throttle plate to control the air, and vent the top of the fuel bowl to atmosphere you have a carb. So the carb is always reacting to pressure differential by comparing the pressure in the venturi to atmospheric pressure outside the carb. This next statement may be a myth buster for you: If the pressure in the venturi is less than atmospheric, then the atmospheric pressure comes in through the bowl vent and pushes down on the top of the fuel, causing it to flow up the metering tube and into the low pressure area in the venturi. Technically it is not sucked into the venturi, but pushed by the differential in pressure between the venturi and outside the carb. Plug the vent and the carb won’t work. Close the throttle and there is not enough pressure change to cause fuel to flow. This very simplified explanation of a float bowl carb shows you the physics that cause a carb to work. Bear in mind that the kind of pressure change we are talking about here is very small, not pounds per square inch, but maybe a small fraction of one pound per square inch. A carb is very sensitive to very tiny changes in pressure, and will react immediately and always so long as everything is working as designed. Old stale fuel, dirt or trash, overheating of the carb or fuel tank, the wrong kind of fuel or worn out or aged components are the kinds of things that cause trouble.
This simple carb we have made would work fine for an engine that ran all day at one speed and load, as long as the hole in the tube was metered to allow just the right amount of fuel into the venturi to maintain the correct air fuel ratio. But that is never the case. We need to be able to start the engine cold, run Ok when it is hot, and have the engine idle and accelerate and decelerate properly, all the while adjusting for load and maintaining Lambda of 1. So this is why carbs look so complex and have so many little holes and passageways and a choke or primer or both, and mixture screws and jets and accelerator pumps and a fuel pump and also why they drive us crazy trying to fix them.
One more thing to consider is that a float bowl carb doesn’t work very well when it is tipped over beyond a certain point, and we need all position operation for hand held machines.
So we use a diaphragm carb to allow all position operation, but it works the same way as a float bowl carb does to get the job done.
The next post continues...
 
Here is a schematic of the metering side of a diaphragm carb, and shows the fuel circuits as they are in the ZAMA I am going to use on my 034.

partloadjet.jpg


Pretend you are a drop of fuel coming through the carb. The fuel pump side of the carb has pushed you up against the tip of the inlet needle, the pointy thing on the left. The flexible diaphragm, stretched across the bottom, will be pushed up by atmospheric pressure coming through the vent hole in the cover, when the pressure in the venturi is less than the pressure outside the carb. This fills the metering chamber with you and your buddies from the tank. Think of the diaphragm as having a wet side, nearest the venturi, and a dry side, next to the cover. The low pressure in the venturi is felt in the metering chamber area because of the little passageways that connect the venturi with the metering chamber.
So the diaphragm flexes up and pushes against the fulcrum and the spring and opens the needle and you flow in. Depending on what speed the engine is running at will determine the path you take to get to the venturi. At idle the piston is pulling against the nearly closed throttle plate, so there is fuel flow through the low side of the carb, but not the high side. At idle, there is very little air velocity in the venturi, so the pressure in the venturi is about the same as atmospheric. The higher pressure holds the main nozzle check valve down and keeps it closed; otherwise air would leak into the metering chamber and mess up the idle mixture. You will pass through the H & L screw fuel feed hole and travel over to the L side mixture needle. As you pass the needle you will then go through one of the idle discharge ports, depending on where the throttle butterfly is, and mix with air and go into the engine. There is actually more going on here at idle, but that we will save for the advanced carb lesson!
Now let’s say the throttle is opening up from idle, so more air is drawn in by the piston, and the velocity in the venturi goes up. So as the pressure goes down the part load jet check valve opens and you may get drawn into the venturi through that passage. It is there to help the engine accelerate smoothly as the throttle is opened, so you don’t have a dead spot or hesitation as you try to rev up the engine. Older designs will not have a part load jet. As the throttle is opened further the low pressure sweeps back through the venturi and now the high speed nozzle check valve opens and you might go through it into the venturi. At wide open throttle (WOT) the engine needs maximum fuel flow, so fuel will come out through the low side, the part load jet, past the H mixture screw, and through the main bypass jet feed hole. It is the carb engineer’s job to get all of these jets and openings calibrated with the size of the venturi so that at all engine speeds the air fuel ratio is maintained. It doesn’t matter which passageway you travel, or how many are feeding fuel at once, the engine will always be in tune.
I have an old early production 066 I restored in my collection, and I have a Walbro, a Bing, and a Tillotson carb for that saw, as all were used at one time or another. It runs fine with any one of them, yet each is totally different from the other, yet all do what they do by obeying the laws of physics and mixing fuel with air based on pressure differential. And I can open any one of them up and figure out the fuel flow just by looking since I understand how the carb has to work.

So now let’s look at our ZAMA:

carbcircuitview.jpg


If you will look back and forth between the schematic above and this picture you can now see what all the various holes are for and visualize how they are connected.
First I will take the non-compensator cover off the old carb.

DSCN0204.jpg


Notice that this carb does not have a part load jet or bypass feed hole for the main nozzle.
I wish you could hear this diaphragm when I flex it back and forth.

DSCN0205.jpg


It clicks like a Halloween toy; it is so brittle and stiff.
All I wanted was the cover so it doesn’t matter.
Now let’s look at one of the MS 360 carbs. First thing is to always open the throttle a little and check to be sure the throttle shaft isn’t loose in the bore. If it is you are done, don’t waste any time testing the carb. This one is tight and both shafts operate smoothly, so I will pressure test it just to see if it holds.

DSCN0206.jpg


On late model carbs, never pump them up until they “pop-off” or you may damage the fuel pump diaphragm. They are made of alcohol resistant material and will not stretch like the old rubber ones would. Only go up to 10 PSI and it should hold steady. If it leaks dunk it in a can of water and see where the bubbles come out. If they come out the impulse hole the fuel pump diaphragm is bad, and if they come out of the venturi the inlet needle is leaking. I want to know what kinds of faults I am looking for before I open it up. This one is rock steady at 10 PSI, and it should hold forever if everything is right.
Here is the carb with the cover off and it looks very clean. I got lucky on the first carb I grabbed!

DSCN0207.jpg


more coming...
 
So the next thing is to rinse it and flush it with some mild spray cleaner. I use a brake cleaner for this. Do not use a carb and choke spray for rinsing the passages as it is too harsh and may damage the check valves and if the carb has an accelerator pump, it may damage the o-ring on the pump plunger. I want to rinse it to clean it and I need the check valves to be wet, like they would be if there was fuel present, so I can check them to see if they seal under vacuum. Never blow high pressure air through the carb venturi or any of the passageways or openings, as that may damage the check valves as well. Just flush it out with the mild brake cleaner spray, and gently blow from a distance.
Be sure to wear safety glasses when you do this because it doesn’t matter what hole you spray the cleaner through, it will always come back out and go straight into you eyes! I usually wrap the carb in a rag and just uncover the opening I want to spray through.
For all the above reasons I mentioned you should never soak one of these carbs in a “Gunk” type cleaner either. I guarantee you will ruin the check valves if you do. If a carb is really gummed up with varnish and old fuel, you should probably just replace it. I have tried the ultrasonic cleaners but I have little success with really messed up carbs.

DSCN0208.jpg


As you spray through the jets, look in the venturi and you will be able to verify that the openings are clear by seeing the spray come out.

I use a semi-soft adapter with a small hole in it on my vacuum pump to very gently pull a vacuum against the check valves to verify that they do not leak.

DSCN0209.jpg


To do this you must first gently seat the L and H mixture screws to close off the passageways from the venturi. Carefully locate the adapter over the main nozzle fuel feed hole.

DSCN0210.jpg


Gently pull a slight vacuum against the hole and it should hold, or it can leak off very slowly, say over 4 or 5 seconds. It will take a little finesse to get the adapter to seal and stay in place while doing this, so be patient and play with it a little. I have used a soft piece of fuel line that I cut off square and had good results. If it will not hold a vacuum then the main nozzle check valve is bad. If the carb has an accelerator pump it could also be the o-ring. This carb does not have one. With this kind of leak you will never get the saw to idle right. You will play with the screws and keep tweaking them back and forth and you think you have it and then you rev it up, let off and the saw dies or idles fast. Or you think you have it right, and then two days from now you start it up and it is all out of tune again. A leaking high speed nozzle check valve will not cause flooding; it creates an internal air leak that messes up your idle mixture. At WOT the check valve is normally open anyway. On most of these carbs the main nozzle can be replaced but it is tricky and requires some special punches to be sure to put the new one in to the right depth.
On some of these carbs the Welch plugs can be replaced, on some not, so don’t just knock them out. Be aware that ZAMA uses a sealant around the edges, and I have seen the alcohol, or something in the fuel, dissolve it. It then plugs up the jets and causes lean running. I have had good results by sealing them with a clear glue called “Seal-All”.

DSCN0211.jpg


The part load jet can be checked in the same manner. However, if that is the only fault I find, a leaking part load jet, I may try the carb anyway. It is such a small opening that it probably won’t cause any idle problems. I just like to verify it so I know if I do have idle tuning trouble.
My luck is holding, as this carb is working great. If the metering side isn’t contaminated, then I verify the check valves before putting any more time into the carb. Recall that I inspected the shafts and did the 10 PSI pressure test, then opened the metering side and rinsed the check valves so I could test them. They were OK so next is the complete teardown and cleaning. I want to work fast and smart. This only took about 3 minutes to get this far. So next I will take out the mixture screws and the fuel pump side and continue my inspection and cleaning.

DSCN0212.jpg


The fuel pump side looks OK and the diaphragm is still pliable. This one is Mylar; the tan colored ones are Teflon coated fabric mesh. Just so they are not stiff or deformed.

a little bit more to come...
 
Last edited:
DSCN0214.jpg


The limiter cap cage comes off with the screws, and since this setup has the springs on it I will leave the caps off. Some carbs need the caps to be installed to keep the screws from vibrating out of adjustment.

Once everything is cleaned up and dried off, put it back together. I do the fuel pump side first, and then put in the inlet needle and its hardware so I can do a pressure test again and be sure the carb is still holding. On most carbs you set the inlet lever flush with the carb body, but these gauges are a must have on the ones that don’t. Just gently bend the lever until it s where it should be.

DSCN0215.jpg


DSCN0216.jpg



This carb is a C3A so I find the edge of the tool with that model listed and check the lever.

DSCN0218.jpg


As you can see, the inlet needle lever is a little low. So carefully bend it up and check it again.

DSCN0219.jpg


Now it is flush.
Always make sure that the little spring under the lever, or fulcrum as some call it, is the right one. If you lose it don’t just stick something in there. The spring is calibrated to the carb, and they are not normally interchangeable. I remember years ago Walbro had some universal carb kits, and there would be about 10 or more springs all painted different colors. There was a chart that told you by the carb model number which color spring to use.
I also remember a saw that was driving me crazy, and the third time I had the carb apart trying to see what I had missed, I noticed the spring had a funny copper color. So I put in the correct one by part number, and it was different, and the saw ran fine. You never know who has been before you.

DSCN0220.jpg


Just a few more bits and pieces and this baby will be pulling some chips!
 
First class post. Some of this could take half a lifetime to find out the hard way. Many thanks for taking the time and trouble to share your experience.
 
:chainsaw: Sounds like you have been around the block more than once on these things . So heres the question. If you dont continue further after checking for any leaks at 10 psi then how do you know where the pop off pressure is ? Another member here contacted walbro with the question as to where the pop off pressure should be . They would not give him the answer only some hints. Neither of us could find exact #s anywhere. When I approached my dealer who has been in the business over 15 years he said the same thing that there had been a bulletin about not checking the pop off pressure anymore for the same reason you have stated. Ridiculous. We checked it before but not any more. Why not just remove the fuel pump diaphram if theres the possiably of a problem with it streatching when peforming the test. I recently rebuilt a walbro for a 310 & when the pressure was checked it was the same as a 25 lb. chihuahua sitting on the needle. So what if any thing would you do if you found it like this? :chainsaw:
 
:chainsaw: Sounds like you have been around the block more than once on these things . So heres the question. If you dont continue further after checking for any leaks at 10 psi then how do you know where the pop off pressure is ? Another member here contacted walbro with the question as to where the pop off pressure should be . They would not give him the answer only some hints. Neither of us could find exact #s anywhere. When I approached my dealer who has been in the business over 15 years he said the same thing that there had been a bulletin about not checking the pop off pressure anymore for the same reason you have stated. Ridiculous. We checked it before but not any more. Why not just remove the fuel pump diaphram if theres the possiably of a problem with it streatching when peforming the test. I recently rebuilt a walbro for a 310 & when the pressure was checked it was the same as a 25 lb. chihuahua sitting on the needle. So what if any thing would you do if you found it like this? :chainsaw:

Well Oilhead,
I recall many years ago that bigger carbs like on the SeaDoo did have a spec and you would literally stretch the spring to increase the pop off pressure, but my understanding of these late model EPA carbs is that it is not recommended or necessary. I use this procedure for evaluation and have only had a couple of carbs that had internal faults or problems that kept them from working that didn't show up in the process of checking, and never has it had anything to do with finding out what pressure the needle opened up at. For me it does not tell me anything. A flexible diaphragm and the right spring, with the inlet lever set right, then no problems. ZAMA and Walbro both say it is not necessary.
Sorry but that is the best answer I have for you.
 
Excellent post!! I have a question for you. I rebuilt a WT on my 028S, a blue plastic pump diaphram was the only one in the kit that matched with the old rubber one. Is this the new pump designed for ethanol? I was concerned about it, as it was so stiff. All worked out well, it runs like a champ.

Thanks,on
 
Scootr,
Thanks!
If it was shiny and fairly stiff it is probably Mylar. I haven't looked at a STIHL kit for a 028 in a long time so I don't know about the color. Was it from STIHL or aftermarket, or from the carb manufacturer?
 
In order to function properly any membrane carburetor needs a well set pop-off pressu

Well Oilhead,
I recall many years ago that bigger carbs like on the SeaDoo did have a spec and you would literally stretch the spring to increase the pop off pressure, but my understanding of these late model EPA carbs is that it is not recommended or necessary. I use this procedure for evaluation and have only had a couple of carbs that had internal faults or problems that kept them from working that didn't show up in the process of checking, and never has it had anything to do with finding out what pressure the needle opened up at. For me it does not tell me anything. A flexible diaphragm and the right spring, with the inlet lever set right, then no problems. ZAMA and Walbro both say it is not necessary.
Sorry but that is the best answer I have for you.
Interesting but unless the muffler was clogging up think thats the wrong way to go. Heres the link : www.aerocorsair.com - Inventions Page - Pop Off Pressure By: Alex I have set all the saws between 17 & 19 psi sence reading this. They kick butt now.:deadhorse:
 
Just figured out the rep bullets, was wondering where they were. I should have asked, as this post deserves some rep. A silver bullet for you Stihl#1.

Question, how far does the pump move from the impulses? It can't be too much, the Mylar is so much stiffer than the rubber. I was concerned that it would not pump enough.
 
Just figured out the rep bullets, was wondering where they were. I should have asked, as this post deserves some rep. A silver bullet for you Stihl#1.

Question, how far does the pump move from the impulses? It can't be too much, the Mylar is so much stiffer than the rubber. I was concerned that it would not pump enough.

At 12000 RPM the spark plug is firing 200 times a second, so that is basically how many pulses pass through the impulse back and forth to drive the fuel pump. So I guess you could say it is vibrating and that is enough to keep fuel flowing. And we are not talking about high pressure here, just enough to maintain a constant flow of fuel and keep it flowing whenever the needle is open. The fuel pump cannot build enough pressure to overcome the spring on the inlet lever if it is the correct spring and adjusted right. Flooding is usually due to the needle or seat being damaged, trash under the needle tip, or corrosion sticking the needle.

And thanks for the rep!
 
Excellent information here!!:clap:

Got any good info on the strato-charge stuff on the 441? I understand how it works, just would like to see some of the Stihl stuff on it as far as transfer port timing and such.
 
Peacock,
see:
http://www.arboristsite.com/showthread.php?t=65949&page=5&highlight=strato

Page 5, post number 63 is where I explain stratified charging and why it is used and how it works. The carb may have a second butterfly for the scavenge air, but other than that it still works the same as the carb in this thread. The only exception is that many of the late model carbs have an accelerator pump, such as the MS 361, the MS 441 and others.
 
Peacock,
see:
http://www.arboristsite.com/showthread.php?t=65949&page=5&highlight=strato

Page 5, post number 63 is where I explain stratified charging and why it is used and how it works. The carb may have a second butterfly for the scavenge air, but other than that it still works the same as the carb in this thread. The only exception is that many of the late model carbs have an accelerator pump, such as the MS 361, the MS 441 and others.

Thanks!

I knew the carb worked the same as I've had mine apart. I was more curious about how the timing of the strato ports worked and effected the rest of the jug. Looks like that's the stuff I needed.

Jonathan
 
[UOTE="Stihl #1, post: 1068845, member: 6287"]
DSCN0214.jpg


The limiter cap cage comes off with the screws, and since this setup has the springs on it I will leave the caps off. Some carbs need the caps to be installed to keep the screws from vibrating out of adjustment.

Once everything is cleaned up and dried off, put it back together. I do the fuel pump side first, and then put in the inlet needle and its hardware so I can do a pressure test again and be sure the carb is still holding. On most carbs you set the inlet lever flush with the carb body, but these gauges are a must have on the ones that don’t. Just gently bend the lever until it s where it should be.

DSCN0215.jpg

DSCN0214.jpg


The limiter cap cage comes off with the screws, and since this setup has the springs on it I will leave the caps off. Some carbs need the caps to be installed to keep the screws from vibrating out of adjustment.

Once everything is cleaned up and dried off, put it back together. I do the fuel pump side first, and then put in the inlet needle and its hardware so I can do a pressure test again and be sure the carb is still holding. On most carbs you set the inlet lever flush with the carb body, but these gauges are a must have on the ones that don’t. Just gently bend the lever until it s where it should be.

DSCN0215.jpg


DSCN0216.jpg



This carb is a C3A so I find the edge of the tool with that model listed and check the lever.

DSCN0218.jpg


As you can see, the inlet needle lever is a little low. So carefully bend it up and check it again.

DSCN0219.jpg


Now it is flush.
Always make sure that the little spring under the lever, or fulcrum as some call it, is the right one. If you lose it don’t just stick something in there. The spring is calibrated to the carb, and they are not normally interchangeable. I remember years ago Walbro had some universal carb kits, and there would be about 10 or more springs all painted different colors. There was a chart that told you by the carb model number which color spring to use.
I also remember a saw that was driving me crazy, and the third time I had the carb apart trying to see what I had missed, I noticed the spring had a funny copper color. So I put in the correct one by part number, and it was different, and the saw ran fine. You never know who has been before you.

DSCN0220.jpg


Just a few more bits and pieces and this baby will be pulling some chips!


DSCN0216.jpg



This carb is a C3A so I find the edge of the tool with that model listed and check the lever.

DSCN0218.jpg


As you can see, the inlet needle lever is a little low. So carefully bend it up and check it again.

DSCN0219.jpg


Now it is flush.
Always make sure that the little spring under the lever, or fulcrum as some call it, is the right one. If you lose it don’t just stick something in there. The spring is calibrated to the carb, and they are not normally interchangeable. I remember years ago Walbro had some universal carb kits, and there would be about 10 or more springs all painted different colors. There was a chart that told you by the carb model number which color spring to use.
I also remember a saw that was driving me crazy, and the third time I had the carb apart trying to see what I had missed, I noticed the spring had a funny copper color. So I put in the correct one by part number, and it was different, and the saw ran fine. You never know who has been before you.

DSCN0220.jpg


Just a few more bits and pieces and this baby will be pulling some chips![/QUOTE]

Where did you find the non compensating end cover?I'm having problems finding one 1125 121 0803 for an 034,,dealer has ordered but has been on back order for a long time
 

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