small engine walbro carb questions... (long)

Old 06-24-05, 11:47 PM
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Arrow small engine walbro carb questions... (long)

Hi guys,
I've been messing around quite some time with 2 cycle trimmer engines trying to "repair" them. In between patience, frustration and sporadic moments of fake success im trying to understand the following:

1. The flow of the fuel thru the carb once the primer bulb is being squeezed, could someone over the top please explain that to me? (i.e. fuel comes from the filter to the carb, fills what holes, diaphragm does what(sp?) etc..) I just want to make sure I got it right (more less ).

2. (With proper safety precuations taken) When a carb is assembled with lines attached, but not screwed against the engine, and its primer bulb being squeezed normally, is fuel supposed to come out thru one of the small holes on the side of the carb? If yes, are all 25cc walbro trimmer carbs supposed to do this?

The other thing that Im trying to figure out are issues with primer bulbs:

3. What would cause a p.bulb to stay stuck and sloooooooowly come back up to normal a position?
And after sqeezed: I can see small bubbles coming out from the side of the "circular rubber thing (color orange or black)" thats on top of the carb and inside the p.bulb mounting area. I can't figure out why it is doing this, I have cleaned the carb more than 6 times and have also used the wire method to make sure nothing is blocking the passages but have had no luck.

4. After more than 5 times of assembling and disassembling another carb: Why isn't a correct primer bulb for this unit sucking any fuel at all?? Did I stick the wire thru something I shouldn't of?

5. What do the small bronze or copper colored things at the bottom of the carb do? regulate the gas?

6. I read a post on DIY about mixed oil/fuel coming out thru the air filter area, that happens to me after a few short seconds before a unit Im trying to fix dies out on me, does that mean that the carburator is worthless? any tips?

7. Are there specific books or any type of literature with these kind of topics?

I have tried continously to find resources for my questions on search engines and have come up with nothing specific.

Thank you for reading this post and will appreciate very much any info anyone can give me regarding the previous questions.Take care people.

BTW, Great site.
Old 06-25-05, 09:34 AM
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Hey Chaps,
Be aware that string trimmers and chainsaws of the economy variety are considered "Disposable" by most small engine techs, simpy because the low quality required to sell at the low price desired, does not usually effect a repair in a profitable way.
They can indeed be a valuable teaching aid however as you may spend several hours in cleaning assembling and dissasembling to no avail.

If such a unit comes to my shop and it will not at least offer to fire with a bit of fuel in the plug hole or carb throat, it is soon to become a project for a student at the local High School.

I am not sure what search engine you are using, but if you use google..."Engine theory operation maintenance" should yeild several results. To further narrow add "2 cycle" or "2 stroke" to the above description....etc.

As far as I am aware, none of the 2 cycle walbro carbs are designed to prime fuel into the throat of the carb. The purpose of the primer bulb is to remove the air from the fuel system. This includes the supply line, the carb and the return. The choke is used to enrichen the fuel/air mixture at starting where some 4 cycle engines with primers are actually in place of a choke and do squirt fuel directly into the throat of the carb to enrichen the mixture for starting.
If the primer bulb will not expand after being depressed, or is slow to return, it is most likely due to a restriction in the fuel system.
When the primer is pressed, it pushes air or fuel (which ever is in the return line) back to the tank via the return line from the primer. When the primer attempts to expand, it should suck/draw fuel from the filtered line in the tank to and through the carb through the primer and back to the tank.

Here is one link on theory of 2 cycle that looked kinda cool ...lots of others as well from only one google search. Also check Sears Service Center...they usually carry repair manuals covering most common makes and general repair as well.
Amazon will have almost ne book you could ever find on the subject. Your local Library is an invaluable source of info as well.
Old 06-25-05, 10:39 AM
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small engine walbro carb questions... (long)

I've had some identification problems with an old Walbro carb, and found the folks at Walbro to be responsive and helpful. Try

Last edited by JCCremer; 06-25-05 at 10:45 AM. Reason: E-mail address did not show up on my reply
Old 06-25-05, 11:30 PM
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Arrow thanks guys...

The information you have given is very useful, thank you.

Before posting the previous long thread, I did find stuff around the internet but usually subjects related to the engine per se, mostly on how things inside the combustion chamber work (2 cycle theory). Then started to get frustrated since I didn't have luck on finding the "science" of the carburetor, or its parts roles (diaphragm, copper colored parts, particular little holes etc..) in relation to the trimmer 2 stroke engine. By this I also mean something like a complete diagram of a 25cc (or other) trimmer with gas tank and lines, carb with primer bulb, engine, etc. with a little more accurate explanation on how it works from the first primer push forward . Secondly, the diagrams used by the different sites ( is one) were similar but not exactly the same as the engines I have on hand.
For instance, by taking the diagram from the site you have given me, one example is the reed valve, and the drawing they have regarding the passage or "tunnel" from where air/fuel mixture comes thru. Where would that reed valve and "tunnel" be on, lets say... a weed eater or craftsman 25cc 17" trimmer?
Don't get me wrong, I did understand your explanation in regards to the "carb-p.bulb", It's just that I like working on these small engines, especially on one thats giving me a challenge.

I will start more searches and certainly take your advice (library, amazon, sears) into consideration. Thanks again for responding and sharing some of your professional experience.

Thanks for the walbro address, I'll definitely be looking into that also.

Old 06-26-05, 12:30 PM
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Your looking for schematics I believe. Indeed if you find ne of small carbs such as your working on, share them here.
If you want to get into schematics and further physics of carbs, try looking at general aviation applications.
Old 06-26-05, 09:45 PM
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You're right sir.

Thanks Azis.

Looking into what you said I typed "how a carburetor works" and "small engine carburetor schematics" in google and just wanted to share with you guys that has some basics on how the 2 stroke carb works, even some video of it!
I plan on doing some extensive searching, but wanted to point that out beforehand just in case someone else is interested in the same subject.

Last edited by chaps; 06-26-05 at 10:33 PM. Reason: started to find some good carb info....
Old 07-21-05, 10:09 PM
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If I did this right, it should be a big help in what you are looking for as far as diagrams are concerned. Let me know if it gets you to the diagrams. I just recently used this site to reassmble some carbs look up parts for different engines I have.

Just copy and paste it.

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