Carb study

[Joe Kuhn]

I'm a hobby repairman and failed with chain saws until I gave up guessing and went to the internet to study the topic of carburetors. This drawing helped me the most with the saws I fix:



Note the sequence from :

Fuel filter inside tank to:
Carburetor inlet to:
Carburetor outlet to:
Primer bulb inlet, to:
Primer bulb outlet to:
Tank return.

If the above is not right, forget about starting your saw.

You can play with the primer bulb to figure out which connector is the inlet by pushing & releasing the bulb w one thumb while your other thumb is over one hole, then the other. When you release the bulb, take your thumb off the hole you're testing and you'll hear a slight sucking sound on the inlet but not the outlet.

Primer bulbs suck fuel through the carb, they do not push fuel through. That's the important rule to remember (I don't know of any exceptions yet). I guess it's a design feature to keep users from over pressuring carbs w fuel. The suck from the release of a bulb is consistent and cannot be overdone. Since it's a suck that moves the fuel, the sequence is always fuel filter to carburetor inlet...

How to id the inlet on a carburetor versus outlet? Look it up on the internet. Or if the fixtures are different diameters, it's the bigger one. (I'm sure there's more to this per the brand)

When the hoses are hooked up correctly, you should see the fuel and bubbles moving quickly through the hoses. If hoses are black and you cannot see the fuel, you can hear it moving when you push (squish) and there's a nice whistle when you release. If the hoses are hooked up wrong, you'll see a few bubbles move a little in one direction and maybe the other. When hooked up correctly, the fuel really moves!

Anyway, once you get the hoses right, you should be able to start it and tune the high and low screws on the carb. And that's another topic for study...

 

[Joe Kuhn]

Here's another tip someone told me about for threading new hoses through the tank. The procedure is a bit different from what you see in the picture.

Trim the end of the hose on an angle, feed the folded over wire down through the hole in the top of the tank and out the fill hole with the gas cap removed. Then wrap the wire around the end of the hose as shown above and pull the wire up through the tank. The wire may come off the hose, but you can probably grab the pointed end of the hose with a needle nose pliers and pull it the rest of the way through. Remember to install a new filter before you pull too far. Then proceed with the rest of your new hoses as shown in first post.

 

[Okie]

If you will thread the tag wire through the end of the tygon tube tapered tip (pierce the tip of the angled tubing with a small sharp awl or equal) push the wire through and then pull enough of the angled tip through to reach inside with long tip locking forceps and pull the tygon on through. You will be surprised how tough the tubing is with the tag wire through the pierced tip.
A little bit of lube on the tubing also makes it go easier. Sometimes the tygon may have to be split back and angled for 2 or 3 inches so as to reduce it's diameter for a start and to get a grip with the forceps inside the tank.

 

[Joe Kuhn]

... reach inside with long tip locking forceps and pull the tygon on through....

Thanks for your note. I need to get one of those tip locking forceps. Very good idea.

 

[Ax-man]

Not being picky but that is a hemostat not forceps. Better yet if you go to buy one ask for a good old 70's roach clip . Odds are they will know what that is instead of a hemostat or forceps .

 

[Joe Kuhn]

Forceps, got it.

What did I miss in my first post? I know it needs help with identifying inlets and outlets on these small carbs and that's a key part of getting the hoses hooked up correctly. I won't tell you how many times I've pulled on a starter only to have my son come out and say - hoses are set up wrong. Without even looking he knows because I've only gotten one little sputter at the very beginning. Then I'll step back and look and bang, hoses are usually all wrong. Only exception has been a plugged gas cap. I take some of the blame for the hoses because I've tried to short cut it and hook them up "the way they were". But I guess that's how I learn - by making mistakes.

 

[Okie]

Yes about the primer bulb connected wrong. You can also browse online for tips about testing connections, etc.
The Hemo/Forceps come in several different size lengths and some have curved tips. You have to watch the prices also, some are hospital grade and expensive but most of mine came from elctronic parts vendors I think and are stainless steel and not very costly. The long curved tip are the most user friendly ones for long reach into places where you cannot see what you are reaching for and their is also some long reach long handle needle nose pliers (not very expensive) that are handy sometimes when Hemo's might be little to weak for a good grip.
If you piddle around with carbs very much removing brass parts such as jets that are tight fit a set of Hollow ground gun smith screwdrivers are good to have handy such as Wheeler engineering type from midwayusa.
Hollow ground screwdrivers are straight sided and do not buldge out the brass slot and cause a internal jet to actually seize and wedge when force is applied.

Also here a link to show several small engine carbs. and some diaphragm carbs and other stuff that review at times.
https://outdoorpowerinfo.com/repairs/
Also lots of carb info at the Walbro and Tecumseh sites for chainsaw carbs.

 

[Joe Kuhn]

Yes about the primer bulb connected wrong. You can also browse online for tips about testing connections, etc.
The Hemo/Forceps come in several different size lengths and some have curved tips. You have to watch the prices also, some are hospital grade and expensive but most of mine came from elctronic parts vendors I think and are stainless steel and not very costly. The long curved tip are the most user friendly ones for long reach into places where you cannot see what you are reaching for and their is also some long reach long handle needle nose pliers (not very expensive) that are handy sometimes when Hemo's might be little to weak for a good grip.
If you piddle around with carbs very much removing brass parts such as jets that are tight fit a set of Hollow ground gun smith screwdrivers are good to have handy such as Wheeler engineering type from midwayusa.
Hollow ground screwdrivers are straight sided and do not buldge out the brass slot and cause a internal jet to actually seize and wedge when force is applied.

Also here a link to show several small engine carbs. and some diaphragm carbs and other stuff that review at times.
https://outdoorpowerinfo.com/repairs/
Also lots of carb info at the Walbro and Tecumseh sites for chainsaw carbs.

Good detail in your link on Zama. Thanks.

https://outdoorpowerinfo.com/repairs/zama_c1u_m29a_carb.asp
Got there from this intermediate link that includes a couple of Walbros: https://outdoorpowerinfo.com/repairs/#id_diaphragm

 

[jp233]

Yeah I just went through this on the Poulan in my signature. For the primer bulb line, I cut the tube at an angle and fished it through. Some parts manuals show a little fitting that go on there, I dunno if they are a one-way valve or what. It started and ran for a while with the way I had it, so it may be OK but I may order the little fitting anyways.

https://www.amazon.com/Husqvarna-Cr...rds=poulan+fuel+fitting&qid=1589821389&sr=8-2
I dunno if those fuel fittings just pop into the plastic tank hole, or what. Would make it easier than fishing that tube in which is deep inside the tank. But a pair of long needle nose pliers did fine (they were angled too)

The main fuel line was much smaller but easier to fish through since it was close to the fuel cap opening, and had to pull a lot more of it through to attach the fuel filter.

 

[MacAttack]

On this diagram the fuel lines connected to the carb look backwards to me. On the ones I have worked on the path goes:

- fuel line with filter from tank going into carb near the diaphragm
- fuel line comes out of side (top) of carb furthest from the diaphragm
- into primer bulb
- then the fuel line out of the bulb that just dumps into the tank

Maybe it doesn't make a difference?

 

[Joe Kuhn]

On this diagram the fuel lines connected to the carb look backwards to me. On the ones I have worked on the path goes:

- fuel line with filter from tank going into carb near the diaphragm
- fuel line comes out of side (top) of carb furthest from the diaphragm
- into primer bulb
- then the fuel line out of the bulb that just dumps into the tank

Maybe it doesn't make a difference?

I can confirm that my Homelite 23AV (UT10822) does indeed run when the hoses are hooked up according to the picture shown. Both the top and bottom plates on my carb have a diaphram, so I'm not sure that's a rule I can use for this Zama. The thicker plate, on top, is where the input to the carb is, and output is below that down by the thinner plate. I just went out and double checked my hose routing on the tank and it's as shown in the diagram. I suppose we could switch the hoses and see if it still runs. You first. (kidding)

 

[Okie]

Joe Kuhn:

You better keep a heads up and make yourself a diagram if you even get into those oiler tubing connections. (from experience)

If you put the new OIL lines on by the way they go on the easiest they will be wrong and she won't oil and finding a routing diagram for them is not easy find either.

 

[Joe Kuhn]

Joe Kuhn:

You better keep a heads up and make yourself a diagram if you even get into those oiler tubing connections. (from experience)

If you put the new OIL lines on by the way they go on the easiest they will be wrong and she won't oil and finding a routing diagram for them is not easy find either.

Haven't gotten to oiler tubing yet. I did learn that the pressure from under the piston provides the pressure for the chain oil. Beats the old thumb pump.

 

[MacAttack]

I can confirm that my Homelite 23AV (UT10822) does indeed run when the hoses are hooked up according to the picture shown. Both the top and bottom plates on my carb have a diaphram, so I'm not sure that's a rule I can use for this Zama. The thicker plate, on top, is where the input to the carb is, and output is below that down by the thinner plate. I just went out and double checked my hose routing on the tank and it's as shown in the diagram. I suppose we could switch the hoses and see if it still runs. You first. (kidding)

I'd be the first to say I am not an expert on the theory and function of these little pumper carbs, so there may be some significant differences across brands and models. The ones I worked on that the inlet on the carbs near the bottom were some old Walbros and some of those Chinese Ruixing carbs on some string trimmers. A Zama might work very differently.

 

[Joe Kuhn]

I'd be the first to say I am not an expert on the theory and function of these little pumper carbs, so there may be some significant differences across brands and models. The ones I worked on that the inlet on the carbs near the bottom were some old Walbros and some of those Chinese Ruixing carbs on some string trimmers. A Zama might work very differently.

I agree. Expertise is relative. I worked with Walbro once and those guys are experts. I had an old Craftsman that was probably 30 years old and could not find a replacement carb. So the guys at Walbro set me up with a kit and the thing ran fine once I got the hose connected from under the piston to the the carb for pumping fuel. Looked all over the place and finally found the issue - that's a piece of hose at the red arrow.



Look at the documentation here. I really learned alot from those guys.

https://www.walbro.com/wp-content/uploads/2016/01/HDAseries.pdf
They have a support system and will email you back and forth.

 

[Okie]

I stumbled across this primer bulb you tube link from ereplacements link awhile back when I received a chainsaw that carb and primer bulb had been removed by someone else and was in pieces.
This link will give you tips on how to determine how to correctly determine the correct connections for a primer bulb type carb system, primer bulb on the carb and external primer bulb type. Note the external bulb type will prime fuel but not as good as when connected correctly.

 

[MacAttack]

I agree. Expertise is relative. I worked with Walbro once and those guys are experts. I had an old Craftsman that was probably 30 years old and could not find a replacement carb. So the guys at Walbro set me up with a kit and the thing ran fine once I got the hose connected from under the piston to the the carb for pumping fuel. Looked all over the place and finally found the issue - that's a piece of hose at the red arrow.

View attachment 828755

Look at the documentation here. I really learned alot from those guys.

https://www.walbro.com/wp-content/uploads/2016/01/HDAseries.pdf
They have a support system and will email you back and forth.

I really like Walbro carbs, I seem to have fewer issues with them than these Chinese carbs. In your picture it looks like you had to reconnect the pulse line to the crankcase, that will definitely keep it from pumping gas!

I've had at least one small Ruixing carb that I finally gave up on fixing, i think an internal passage was hopelessly plugged. I just replaced the damn thing with a new carb on a Husqvarna leaf blower.

I just ordered a rebuild kit for the Zama carb on my McCulloch SP40, I'm interested to see inside it.

 

[RedneckChainsawRepair]

Depends on the carb set up.

I marked this one for a guy once to use online. A B C on all ends to where.

A tank with fuel filter on end to carb
B primer to back in tank

 

[Joe Kuhn]

I stumbled across this primer bulb you tube link from ereplacements link awhile back when I received a chainsaw that carb and primer bulb had been removed by someone else and was in pieces.
This link will give you tips on how to determine how to correctly determine the correct connections for a primer bulb type carb system, primer bulb on the carb and external primer bulb type. Note the external bulb type will prime fuel but not as good as when connected correctly.

Best video yet! Summary (for self): If you don't know how the fuel flows through the bulb or the carb, test with some fuel. I did notice for the carb with the external primer bulb, the input line is nearest the plate with the screw in the center. That will be a rule I remember and check. In the end, a check with a little fuel does it!

 

[Joe Kuhn]

Depends on the carb set up.

I marked this one for a guy once to use online. A B C on all ends to where.

A tank with fuel filter on end to carb
B primer to back in tank


View attachment 828958

Now he has a permanent record for the future. Sounds good. Which line has the fuel filter on it, inside the tank?