Carburetor Internal Parts Identification And How To Be Certain Which Ones To Use

[mikedennis0586]

long time reader first time poster here.

I've read through quite a few threads over the past few days in regards to carburetor rebuilds and carburetor kits. no where have i read or seen on youtube how to identify which parts for certain are to use. everything that I've read or watched so far you're instructed to use the same identical pieces out of the new kit in reference to what came out of the carburetor. what if you don't have those pieces? or what if 2 of the diaphragms look almost identical but they have slight dissimilarities? i am looking for information so i can learn how to be sure i am using what's needed. how to look at a carburetor and the internal passages to be able to identify what is needed. here's what i ran into.

a friend of mine asked me to work on a saw of his. it is a 011avt. the saw would run only if fuel was poured manually into the carburetor. so i took the saw apart and cleaned it up then moved on to the carb. after getting the carb apart i noticed that something was funny looking with the fuel pump diaphragm. it appeared that 1 of the 2 locator holes didn't line up properly and someone had put it together anyway. i took the carb to my stihl dealer and he chased down a kit for me. i showed him what i had found and was told that it was wrong. he pointed out the right diaphragm that i needed to use so i was on my way. when i got home i put the carb back together with what i thought was the right parts. the saw would not start. i could smell fuel real strong and it was leaking from the muffler as well. i took the carb apart to take a look inside just to double check and everything appeared to be in order so i put it back together and the same problems were happening. at this point i took the carb back off and apart again to think about it for a moment. i compared the new parts to old and found that the diaphragm i was told would be the right one was wrong. the locator tabs lined up perfect but the passage openings were different. the only diaphragm that matched from the new kit i was having the same problems with as what came out of the carburetor. the locator tab holes weren't lining up properly. i used a hole punch to oblong the locator hole and put the carb back together. everything seems to be ok now and the saw is running fine.

i would like to know how to be able to identify exactly which parts need to be used in a particular carburetor and why. why this diaphragm instead of that one. why one has a large opening on the fuel pump and the other doesn't. in what case does a person use the woven looking material diaphragm or not. i attached a few pictures of what i got. the first one is of the old fuel pump diaphragm with the damaged locator hole that came from the carb when i took it apart for the first time. as you can see it doesn't have the large opening on the rounded end. the second picture is of the 2 different types of diaphragms in the kit. the one with the large opening on the rounded end fit perfectly but when using it the saw would not run and flood excessively. the one next to it had to be altered but the saw ran great when installed. the last picture is of the woven type parts included in the kit. in what case are they used and why them over the other types of diaphragms.

thanks for any and all information that you are willing to share!

Mike

 

[buzz sawyer]

Good question.
Some kits have several versions of the diaphragms and gaskets. If you're the first one in there, you can carefully compare the orig. to the ones in the kit. If not, I would do what you did. I think the Walbro WT is one that has several possible combinations. Hopefully someone has a better answer.
In the second photo, I believe diaphragms like the top one would use a gasket that seals the open area. I had a similar problem with the the diaphragm on a Tilly HL.
As for rubberized fabric vs the woven ones in the last photo, I've used both with good results but I think the latter are more resistant to deterioration from fuel.

 

[Ifishsum]

The woven ones are better for ethanol fuel I think, but it doesn't seem like they are always an exact match to the rubber ones in the kit either so you still gotta match up the flaps with the fuel inlets in the carb body. So far I haven't done a saw carb that uses the more open design diaphragm, and if you got yours running well with one just stick with it. I won't use ethanol in my saws anyway. This page has series-specific breakdowns, including the WT that might help: http://wem.walbro.com/distributors/servicemanuals/

The full Walbro service manual can be helpful, and gives a bit more info on servicing each area: http://www.walbro.com/media/21936/SERVICEMANUAL.pdf

I'll admit that I usually don't do the welch plugs unless I see debris in the carb. Most of the time replacing the needle valve assembly and diaphragms/gaskets on both sides is sufficient but if the saw still won't idle right I go back and clean the circuits under the welch plugs.

 

[watsonr]

The kits are provided with several rubber and woven parts to fit a wide array of models. The K20-WAT kits fit about a 100 different models and instead of making a kit for each one, they lumped them together... good for us.

As stated above, match the rubber to your old one or you run the risk of a flap not in the right spot, that causes flooding or starvation. Not only the right part, but in the right order as well... the manual is your best friend. I always check the alignment before reassembly to see that the flaps are in the right spot, a passage isn't blocked or left open.

 

[mikedennis0586]

buzz - in reference to you saying that using the top style diaphragm with a gasket that seals the open area on the rounded end that would be a great option but my kit didn't have that available. that's why i used the bottom style and had to oblong the locator hole to make it fit properly. also you can see that their's a difference in the passage openings on the bottom left of the diaphragms. which is the fuel needle inlet passage.

ifishsum - thanks for the links and tips. on the service manual page link i can't find 3 of my 4 carbs listed in the chainsaw model group. i believe the HDA section applies to my 440 carb but i am not certain to that. i have carburetors as follows.
ms390 - HD21B 150
ms440 - HD17A 425
ms660 - WJ69 304
066 - WJ69 022

randy - i understand these kits are made to be universal and fit many many different models. i am very aware of that. but in the case of my 066 i don't have anything to reference to as i got the saw in a box of parts. i don't have all the parts for the saw yet nor do i have the kit for the carb so i haven't crossed that bridge yet. i'm looking for information or manuals or whatever i need to be able to look at a carburetor and identify exactly which diaphragm or gasket i need. not so much the gaskets as the diaphragms because the gaskets seem to be pretty cut and dry.

update on the 011avt carb kit whith a few more pictures. i took the carb back apart more out of interest on what's inside and how the passages work and so on. i was digging through my spare parts and found a woven type fuel pump diaphragm that seemed to fit perfectly as the locator holes matched and the flaps matched better then the one i had currently installed. i put the carb back on and the saw runs great. i am personally happier with the way the woven diaphragm fits and lines up better than the rubber type. it didn't have to be altered at all and the flaps and passages seem to line up better.

the first picture is of the diaphragm installed that would flood the saw excessively and it wouldn't run at all. actually all i got with this one was a ball of flames through the muffler. the second picture is of the diaphragm that i had to alter to fit because of the locator holes not matching but the saw ran fine with this one. also you'll see that the passages and flaps didn't seem to match up quite the way that they should. the third picture is of the woven material type diaphragm installed and is on the saw now. it seems to be running great. now on to my burning question. why did the first one induce excessive fuel? as you can see the only thing different between it and the woven type is that the opening on the rounded end is open vs. not. after looking at the cover and the gasket i don't see how these two different types of diaphragms do not work the same. their isn't any passages being left open of shut. the gasket seals the large opening on the rounded end and that opening only has 2 passages in it so i don't see why the first one does not work.

 

[fin460]

Sub

 

[mikedennis0586]

sub? what does this mean?

 

[watsonr]

I'm surprised the first one didn't run right. It looks to be the right one, maybe a slight movement could have caused it, it doesn't have two guide pins to keep it aligned and could have moved a little. Maybe it was a little to stiff to allow the flapper to work correctly? The second one I'm surprised ran at all, but if the main purpose was to make the flappers work and that plug didn't matter, sure. I think you got lucky on #2.

I like the 3rd, you can see the holes through the material and see that its the right one.

Did you do anything but replace the diaphragm when you took it apart for these different shots... say move the needle settings, spray cleaner thru it again or make an adjustment? Had them in the wrong order the first time?

 

[mikedennis0586]

randy - actually yes all of them have 2 guide pin holes to line them up correctly so that shouldn't be the issue. this carb had a complete kit w/needle and linkage put in at the beginning. the linkage was adjusted properly. since the first 'complete' rebuild the only thing that i have been changing out and replacing was different fuel pump diaphragms. i thought that the first one would have worked properly too but id didn't. that's what i'm trying to understand.

 

[watsonr]

gasket was in the correct order... you never said

 

[mikedennis0586]

yes everything was in the correct order

 

[fin460]

sub? what does this mean?

I just wanted to post something to "subscribe" to the thread so I could get alerts and I could find the thread easy. I was on my phone and read the first couple paragraphs and wanted to read more but it was hurting my eyes on the phone.

 

[mikedennis0586]

gotcha...i'm really new to this online posting stuff so i'm still learing. i've read a bunch but this is my first actual thread

 

[watsonr]

yes everything was in the correct order

commenting on the pictures...
same gasket I'll assume. First pic, was the rubber a new one or cleaned by you, it has a slight bulge to it. Second picture looks well used, third new. Did you look at the gasket for comparison as well?

I've rebuilt hundred's of carbs and sell thousands of kits. The first picture looks to be a good rubber as far as flappers go. You think maybe something in the metering needle may have been stuck keeping the lever open and it passed through eventually? A flooding carb is normally associated to the metering needle, stuck open, not adjusted correctly or a H/L needle having been damaged. It could be caused by the wrong metering disk, being to tall in the button that activates the lever.

Does the metal disk have a different height center section to activate the lever. I realize your saying that the pictured rubber part is what you've changed.... at this point, the only viable explanation is the that the rubber wasn't responsive enough to open and close the valve when commanded. That little flapper is just acting as a one way check valve, the rubber was just floating open and never really moved to open and close it correctly. Once the pump is in place it doesn't really move much... maybe it was to restricted by the rubber material.


This thread is killing me as I'm a "need to know it all"!

 

[mikedennis0586]

randy - yes you are missing something according to walbro. the diaphragm with the disc on it is the metering diaphragm. the diaphragm with the flappers on it is the fuel pump diaphragm. take a look at this link and cross reference the part numbers to the description below the breakdown. http://www.ereplacementparts.com/walbro-hd21b-carburetor-parts-c-139716_142749_139832.html . all parts that i used were new. the second one was altered by me due to the locator holes being off. look at the gasket for camparison? i'm not getting what you're asking here.

yes i thought the first pictured diaphragm would work great as well but it didn't. i don't believe anything internally was causing the issue due to me taking everything apart and cleaning it before i put it back together. as soon as i took the first pictured diaphragm out and replaced it with either the second or the third the saw would run great. better yet with the third. no the metal disc didn't have a different height or design than what was removed initially.

 

[watsonr]

the three pictures don't show the gasket, just the pump. The gasket that goes over the top of the pump and under the metal cover... did that match up as well?

Brain fart... I revised by post. Were talking the same thing.

 

[mikedennis0586]

yes randy the fuel pump gasket matched up perfectly. the only difference in the first picture to the third is the type of material and the large opening on the rounded end. i just don't see why that opening makes a difference. i looked at it for a long time and couldn't come up with a thing

 

[watsonr]

I've re-read your original post.

The first picture is with a pump that has the large opening in it. One side of that is impulse from the cylinder, it moves the pump. The other side is fuel, pumping action sends the fuel into the correct passage, but they shouldn't mix. When you used the first pump, all that happened was the impulse just pulling fuel and filled it completely full flooding the saw. Eventually the case would have filled up with fuel as well.

Number two looks like a used one and stretched out. There needs to be tension. As the impulse pulls the rubber up, its response is to spring back, that one doesn't look like it would very well and the flappers are wrong to begin with. You fixed the flappers, the stretched rubber would eventually effect the saw. You cut with it or just ran it? I believe if you cut a few blocks it would have stumbled and adjusting the H/L wouldn't have helped.

The fuel pump is subject to about 10psi, if it can't take that pressure, stretching will fail the pump, not stretching enough will fail the pump as well. The first gasket never separated the fuel and 10psi, so it flooded. Second pump was the wrong flappers, you fixed that or it would have been flooding as well. The stretched rubber is questionable.

Number three. Yep.

 

[mikedennis0586]

randy - lets just forget the second picture because it was wrong from the beginning. now the first one though i still don't understand why it didn't work. i am very familiar with the impulse style carburetor and how it works. weather or not the style of diaphragm that i used had the large opening or not it seems to me that it should have worked. after the diaphragm is installed, which ever one is used, the gasket and cover are installed. the gasket seals off that large cavity and the cover seals to the gasket. all in all in the end it seems to me that the large cavity area is sealed it's just a little bigger with the open ended style diaphragm. where am i going wrong here and what am i missing?

 

[watsonr]

on one side is fuel with direct access to the metering needle. The needle moves up and down to provide just the right amount needed. It takes about 10psi to pop the valve. The other side is air, impulse air. It sucks the pump up drawing fuel into the carb from the inlet and pushes it into the metering needle that lets it escape above 10psi. feeding the saw.

If they were combined, no pump installed, the impulse doesn't have anything to push against to send the fuel into the metering needle. It would also be exposed to more than 10psi and the fuel would just flow freely, flooding the saw. The separation needs to be there so that doesn't happen, it acts to prime the saw, but once flowing it really limits the fuel because its almost free flowing to fuel the higher RPM. That's where the high and low needles come into play, they also limit the amount of fuel into the saw.

Where in eastern warshington are you?