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.
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.
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...
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.
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...
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? 
