2 stroke fuel system

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  #1  
Old 01-08-04, 04:51 PM
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2 stroke fuel system

One thing that I never really could fully understand was the fuel system on a 2-stroke. Does the fuel tank have to be pressurized during opperation? How does it get pressurized? I know the primer bulb on a 4-stroke pushes a shot of fuel into the throat of the carburetor, but I think the way a primer bulb on a 2-stroke works is that it pushes fuel through the carburetor and back to the tank to fill the carb and fuel lines with fuel, is this correct? Also, from what I have heard, to set the carburetor on a 2-stroke, you adjust the low speed screw until it idles smoothly, then you run the engine wide open and adjust the high speed screw until the engine runs very smooth and fast, but then you have to back it out until the engine starts to run rough to ensure that it isn't running to lean, is this correct? What if the carb only has one fuel adjusting needle? And, is the high speed screw on both two and four stroke always the one closest to the air filter, and the low speed screw always the one closest to the engine? I know this is a lot, but if someone can explain it, I would really appreciate it. Thanks.
 
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  #2  
Old 01-08-04, 10:19 PM
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Hi Mower17!

The fuel tank doesn't always have to be pressurized on 2 strokes. Some are designed that way, some are not. Chainsaws usually are. I think you have the primer operation a little backwards. Usually, the primer pulls fuel through the carb and squirts it back into the tank. The suction side of the primer is usually supposed to be connected to the carb, not the tank line. Most carbs have a vent to allow the primer to pull some air as it pulls fuel through the carb. This air gets pushed into the tank by the primer to pressurize the tank as you prime. It is very little pressure.

You have the carb adjustments correct, and their placement is correct...high speed always closest to the air inlet end of the carb. If you only have one adjustment screw, you have to adjust it to get the optimum high-speed operation, and use the idle speed screw to set the idle speed.

Hope that helps!
 
  #3  
Old 01-08-04, 10:52 PM
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Remember that a two-banger pulls the fuel into the crankcase and not directly into the cylinder. The bottom of the rising piston makes a void in the crank case that is filled by the air rushing through the carb, where it mixes with the vaporizing fuel and is admitted into the crankcase through a reed valve. When the piston next decends the bottom of the piston pushes the fuel air mixture into the cylinder via the intake ports cut into the side of the cylinder wall. The mixture isn't blown back out through the carb because the reed valve has pressure on the crankcase side now and slams shut.

Two cycle engines employ the bottom of the piston as an air pump. This causes the crankcase pressure to oscillate with the rising and falling of the piston. Inside the carb is a small diaphram. One side of the diaphram is hooked via an air passage to the oscillating crankcase pressure. This causes the diaphram to bow in and out. The other side of this diapham is hooked to the fuel system and acts like a fuel pump that takes suction on the fuel tank.

The fact that the fuel in taken into the engine via the crankcase makes it difficult for an oil bath to exist to lubricate the crank and piston. That's why you mix the oil with the gas. The crankshaft, rod bearings & piston are lubricated by the oil in the fuel stream. Two cycle engines often are operated at extreme angles that would make it difficult to have a regular splash type lubrication so often seen on four cycle engines.

The carb on small two cycle engines can have a lot of variations so I don't know if there are any hard & fast rules. Adjusting the engine towards lean until it runs ruff then enriching it some is usually the correct proceedure for adjusting the carb.

As you can see the fuel tank doesn't need to be pressurize because there is a small diaphram fuel pump built into the carburator and fuel is pumped just like any other engine.

PS
The diaphram often goes bad or gets distorted and is the cause of many weed wackers getting pitched in the trash. I've saved a few with a cheap carb kit that included a small rubberized diaphram.
 
  #4  
Old 01-10-04, 06:26 AM
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Thank you both. Ya'll cleared up many things!!!!!!! Just two last questions pertaining to 2-strokes. Do all chainsaws, trimmers, and other small 2-stroke engines have reed valves? From what I caught so far, they open when the piston moves up and creates negative pressure in the crankcase and they close when the piston goes down and creates positive pressure. Is that correct? Also, how can you tell if/when they are bad/malfuctioning? Would compression pop out the carburetor throat like on a 4-stroke when the intake valve clearance is too tight? How often do reed valves require replacement? I have heard some people refer to reed valves as reed boxes. And the other question is, I have heard of people checking compression on chainsaws by holding onto the rope and letting the saw go. The rule of thumb is, if the saw falls on it's own, then there isn't enough compression, but if the saw stays up, then there is sufficient compression. Is this a good rule to go by? Would the same work for a trimmer? These are all questions that I have been meaning to ask and would really appreciate anyones help in getting me to be more familiar with these 2 cycles. Thanks a bunch!!!!!!!!!!!
 
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Old 01-10-04, 11:42 AM
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I can't comment too much on chain saws. Mine hasn't given me any problems so I haven't torn into it.

This morning I took a few pictures of a Sears weed wacker engine that I've learned to hate. You can see why by looking at the picture of the black plastic box that surrounds the carb when it's mounted onto the engine. That makes it very difficult to work on. It was pulled from a garbage can by my wife a couple of years ago. I fixed it and used it two summers. This year it started running poorly so I took it apart. The good news is that my wife got another weed wacker from the trash so I won't really have to fight this one too much anymore. You can easily spend all day working on these crazy little engines. If you had to pay for the repairs yourself you could easily spend the price of a new one to fix them. I guess that's why you find them in the trash.


The first picture of this two cycle engine shows the black box housing the carb mounted onto the bottom of the crankcase opposite the piston.




You can see the top view of the black box with the carb removed. There are a couple of holes going into the crankcase along with some small air passages going into the carb that run the diaphram fuel pump.




Inside the carb is a rubberized cutout that has the reed valve. You can clearly see the tabs that cover or uncover matching holes in the carb.



The last picture shows (not very well I'm afraid) the cylinder and the intake and exhaust ports. The four channels cut into the side of the cylinder wall are the intake ports. The rectangular hole on the bottom of the cyclinder is the exhaust port.



There are quite a few different configurations possible with any engine especially a two cycle engine. I see this engine doesn't have a reed valve between the carb throat and the crankcase but it does use two small reed valves in the fuel pump mounted inside the carb. It still does use the bottom of the piston as an air pump that draws the fuel/air mixture into the crankcase where it is channeled up into the top of the cylinder thereby forcing the remaining spent exhaust gas out the exhaust port.

Don't get the idea that this simple design is devoid of any technology. There are many, many things that can be done to improve the performance of these engines that are really amazing. A two-banger has a better horsepower to weight ratio than a four cycle engine. Cars don't use them much because you can't get the emissions down and fuel efficiency is a problem. All the fancy computer controls on car engines are needed to meet EPA specs and given the way things are going you will start to see that soon in lawnmower engines. You'll see it first in California.
 

Last edited by jughead; 01-10-04 at 12:12 PM.
  #6  
Old 01-10-04, 04:19 PM
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WOW!!!!! Thanks Jughead!!!!!!!! I have seen those little flaps on 4-stroke carburetors, they have little springs under them. Exactly what do they do? Are they some kind on fuel pump? I haven't seen an explanation in the briggs manual we have at school. You are right about trimmers giving hard times. I have been agrivated by them plenty of times. They will start with one pull one time and never start the next, that's why I wanted to ask every question that I have ever been stumped on because I always ask myself the same questions every time I work on a trimmer or chainsaw. These answers can't be found in any book I have ever read on small engines so they can only be answered from experience which having you guys on this board is priceless to me and many other people. Thanks again!!!!!!!!!!!!!!!!!
 
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Old 01-10-04, 05:57 PM
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You can have a diaphram type fuel pump on a 4 cycle engine as well. There is one on the 16HP Tecumseh I've been working on. They work the same way. There is usually a small rubberized diaphram with a spring on one side and a line going to the crankcase where they get the oscillating air pressure to drive the pump. The flaps are just little check valves that allow either gas or air to flow in one direction only. There are probably more combinations that you can count so any explaination I could give on a specific carb probably wouldn't apply to everything. The Briggs or Tecumseh manuals don't tell you near everything you need to know. You just have to take an engine apart and carefully look at the parts to figure out how they work and expect lots of variations on a simular theme. It always helps to have good background in the basics so you can understand how something must work by looking at the parts. Briggs & Stratton has a hardcover book called "Small Engines" that does cover the operation of the diaphram fuel pump.
 
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Old 01-11-04, 01:05 AM
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Good info!

I'll add what hasn't been covered...
Checking compression by holding the rope can give you some idea, but it's not a rule by any means. You can tell by pulling the rope when you have enough compression, once you get used to these things and work on them enough.

Reed valves and reed boxes are 2 different things. Usually a reed box is found on 2 strokes with more than one cylinder...like a boat motor. It is a plate or box with several reed valves in it.

Most trimmers and chainsaws don't use reed valves, they use the piston position to block one passage and open another. (the piston acts as the valves as well).

Usually reed valves will last as long as the engine, provided they don't get rusty. They may need to be cleaned once in a while, but seldom require it.
 
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Old 01-11-04, 12:06 PM
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Cheese, good call on straightening out the reed valve vs reed block. The type of "reed valve" that is in a chainsaw or trimmer such as Jughead illustrated, is not really referred to as a reed valve. Some older chainsaws do use reed blocks for their intake. Chainsaws use what is called "piston porting". As Cheese said, the piston closes one or more ports, and opens others at the correct time in order to make the engine function. Playing with the ports is a tricky thing, and can lead to either getting more power, or the engine not running well at all.
 
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Old 01-11-04, 06:57 PM
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Thanks for the help fellas, I now understand 2-strokes a good bit more. Thanks again!!!!!
 
  #11  
Old 01-12-04, 04:26 PM
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Mr Jughead,
What type digital camera (how many megapixels) did you use to take those pictures. Very sharp quality.
 
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Old 01-12-04, 04:46 PM
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I use a Casio QV-4000 digital camera with 4.1 megapixal resolution. Taking pictures of a cylinder bore turned out to be a bigger challange than I thought. In order to get the depth of field you need you have to use an 8.0 or 5.6 lens setting, but then you have to use a strong light or keep the shutter open for a long time. It's tough to keep out shadows as well. The picture of the two cycle bore didn't turn out as well as I think it can be done, but I'm not a pro. I've been unable so far to put the pictures directly into the posts. No matter what I've tried all I get is just a link to the pictures.
 
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