Stihl Backpack Blower BR420 Magnum

bertsmobile1 · Dec 8, 2018

Go to the Walbro web page.
Hidden in the bottom right corner on the cube carb page is a link to their service manuals and even a couple of videos.
Nothing hard about them except they need to be clean and when you work on them you need to be clean and the area needs to be clean enough to conduct open heart surgery in.
I do it in the workshop bathroom.
Get a diaphragm kit for you carb , should be around $ 10 (US).

Despite all of the fertilizer you see on U-Tube always replace the diaphragm & gaskets, they are very cheap and telling the difference between a good one and a not quite so good one is a difficult job.
Your problem sounds like either a clogged exhaust or clogged high speed jet.
Inside the carb is a little screen which gets bocked.
When left unused for long times fuel can leave gum inside the tiny holes.

However before you start, loosen or remove the muffler and try again.
If it runs then your muffler is blocked.

On the fuel tank there are 2 fuel lines.
One is the outlet & the other the return.
Pull then off at the carb, after tanking photos or lableing them then blow gently down the filler cap.
Fuel should flow out of one of them .
Put the cap back on. Pinch off the one that did not flow fuel and leave the tank in the sun.
Fuel should start flowing out of the outlet tube as the tank gets hot if the filter is clean enough to be used.

If the muffler and the fuel tank check out OK then time to pull the carb down.
And don't forget the most important tool in the box, your camera use it every time you remove any part.

Darryl G · Dec 8, 2018

AlabamaDan said:
Part #28. I may not have it anymore. PO may have taken in out already.

It seems that those screens frequently get dropped and "accidentally" stepped on before getting tossed in the trash. :laughing:

And you can run without the filter with a clean tank and fuel for diagnostic purposes but it may want to float.

AlabamaDan · Dec 9, 2018

So, I disassembled the blower again. I found little puddles of liquid around on top of the gas tank and cover. This is a little disturbing for me

These muffler bolts were ridiculously tight... way beyond what seems appropriate to me.

It was very odd. The muffler had some liquid in it? There was about an ounce of liquid inside the muffler. AND the muffler weighs about 5 lbs. Is that normal?

I took a little of the carb apart and it looks pretty good to me. This is exactly how it looked. I didn't do any additional cleaning.

Boudreaux In Eunice La. · Dec 9, 2018

Hey Dan..... La. here again......... This is a buddy of mine in this video.... He is a Stihl dealer and Authorized service center for Stihl........ BTW your metering diaphragm looks hard to me, that's the one that looks like a wagon wheel..........It's not doing it's job and causing the flooding from the carb that you see...... This vid will put you at ease ............

Enjoy Mon Ami ,,,, https://youtu.be/7-VpdJEJRRw

AlabamaDan · Dec 9, 2018

Boudreaux In Eunice La. said:
Hey Dan..... La. here again......... This is a buddy of mine in this video.... He is a Stihl dealer and Authorized service center for Stihl........ BTW your metering diaphragm looks hard to me, that's the one that looks like a wagon wheel..........It's not doing it's job and causing the flooding from the carb that you see...... This vid will put you at ease ............

Enjoy Mon Ami ,,,, https://youtu.be/7-VpdJEJRRw

That wagon wheel thing is supposed to be flexible? I don't recall if it was or not. I'll double check it. and I'll watch the vid...

Boudreaux In Eunice La. · Dec 9, 2018

Yep soft as silk.... If you can play a tune with it like clicker it's junk........ All that time setting up makes em hard.......

AlabamaDan · Dec 9, 2018

That's a great video. Thanks. Not only was it informative, but any beer will do... :lol

bertsmobile1 · Dec 10, 2018

Dan,
what you have done is fallen for the newbie mistake of ripping everything apart before you took the time to actually work out what was wrong.
Thus you have the original problem to contend with plus any / all of the problems introduced by yourself in ripping things apart.
Back in my original response I suggested that you try running it without the muffler .
So your only problem could have been whatever is inside that 1 lb muffler that is making it weight 5 lbs.

Next you have fuel pooling on top of the tank,
So you now have a fuel leak that you did not have before you started to fix the blower.
The fuel tank & carb are a low pressure, pressure vessel and run at around 5 to 10 psi.
If a fuel line has a leak in it or the grommets in the fuel tank do not seal or the carb is not airtight or the vent is not working or the fuel cap is not working, then the blower will start but not run for long.

There is a logical proceedure for these thongs and that is to start from the beginning, the fuel tank.
If it holds pressure and can also suck in air then chances are it is ok.
The next thing you do is block off the return line dip the carb in a jar of water and pump the inlet line.
You can just blow into it.
If all is working OK there will be no bubbles.
If you have a pressure tester with a gauge, the carb must hold 10 psi.
You will read all sorts of trash about "pop off pressure , ignore it. Forcing the fuel needle to lift off it's seat by applying too much pressure to the fuel intake tells you nothing other than you have been led down the garden path by a shaved monkey with less idea about what he is doing than you had before you started.
Now that the carb is apart, clean it and DO NOT BLAST HIGH PRESSURE AIR THROUGH ANY PASSAGE. All that does is jamb debris so hard into tiny holes it can never been dislodged.

Skip the beer.
Anyone who touches an engine after drinking either has a bad booze problem or is an idiot.
Sorry Bou, I know Steve is a friend of yours and I know he knows a lot by what he says, but beers are what you have when you have successfully fixed your equipment and kocking off a few tinnies while you are working will not help the outcome one tiny little bit.

Boudreaux In Eunice La. · Dec 10, 2018

Bert ........... You and I have always helped people on here and agreed on a lot of stuff........ First of all Dan said in post 4 he is scared to even take a carb apart , that's why he soaked the whole carb in Chen Dip for 2 hours, THE whole carb still assembled ........New to me on that one.....

Post 11 you did say say to run without the muffler... I agree all the way ... IL Engine told him to take the arrestor screen out... Dan said none in there and the muffler had some fluid and weighed 4 pounds ...... I doubt he weighed it though .... and it might have have some issues..... If he tried to run it without the muffler off we don't know and it hasen't ran in a couple years anyway.... So i don't think it would run anyway......

In my first post I told him that his diaphragm might be bad ..... Then he showed a pic of it and it sure looks that way to me ........ That is sure sign of flooding the carb and leaking on top of the tank......Plus soaking the whole carb in chem dip..... Every bit of trash is still in the carb..... Do you agree ??
Post 11 you said a high speed jet might be clogged and the screen inside clogged also..... You gotta tear a carb apart to get to that screen ...... So he didn't tear the carb apart prematurely When I saw that I opted to show him a good video to put him at ease to show him how easy it is do that, since he is not tech inclined like he said he was .....
Steve's Small Engine Saloon is just a name for his channel..... It's a name only and while he is running his business he don't drink........He announces a different beer in each video .....Also he has sponsors from companies and that's why he doesn't have ads / commercials on his channel..........He doesn't get drunk and act like a idiot on his videos.....

Now let's please see what he does tomorrow and help him out further along in his issue >>>>>>>

Plus Tard Mon Ami ~!~!

AlabamaDan · Dec 11, 2018

bertsmobile1 said:
Dan,
what you have done is fallen for the newbie mistake of ripping everything apart before you took the time to actually work out what was wrong.
Thus you have the original problem to contend with plus any / all of the problems introduced by yourself in ripping things apart.
Back in my original response I suggested that you try running it without the muffler .
So your only problem could have been whatever is inside that 1 lb muffler that is making it weight 5 lbs.

Next you have fuel pooling on top of the tank,
So you now have a fuel leak that you did not have before you started to fix the blower.
The fuel tank & carb are a low pressure, pressure vessel and run at around 5 to 10 psi.
If a fuel line has a leak in it or the grommets in the fuel tank do not seal or the carb is not airtight or the vent is not working or the fuel cap is not working, then the blower will start but not run for long.

There is a logical proceedure for these thongs and that is to start from the beginning, the fuel tank.
If it holds pressure and can also suck in air then chances are it is ok.
The next thing you do is block off the return line dip the carb in a jar of water and pump the inlet line.
You can just blow into it.
If all is working OK there will be no bubbles.
If you have a pressure tester with a gauge, the carb must hold 10 psi.
You will read all sorts of trash about "pop off pressure , ignore it. Forcing the fuel needle to lift off it's seat by applying too much pressure to the fuel intake tells you nothing other than you have been led down the garden path by a shaved monkey with less idea about what he is doing than you had before you started.
Now that the carb is apart, clean it and DO NOT BLAST HIGH PRESSURE AIR THROUGH ANY PASSAGE. All that does is jamb debris so hard into tiny holes it can never been dislodged.

Skip the beer.
Anyone who touches an engine after drinking either has a bad booze problem or is an idiot.
Sorry Bou, I know Steve is a friend of yours and I know he knows a lot by what he says, but beers are what you have when you have successfully fixed your equipment and kocking off a few tinnies while you are working will not help the outcome one tiny little bit.

It may be a newbie mistake, but diagnosing things is something that's a little difficult for me. Once I find the issue, I can fix just about anything I just have to find out what to fix.

I feel like I was taking it piece by piece. I started by removing the carb and soaking it whole. I've had a lot of success with this, for better or worse it has been a reliable go to on fixing my small engines the last year or so. Best results actually. Unfortunately that didn't work. So now I've dove a little deeper in the carb and while I was at it I took the muffler off.

My inspection of the fuel lines look good.

Judging by the nastiness of the motor and everything behind that cover as it was originally, I'd say it's been leaking or spewing or flooding or whatever for quite some time. I don't think that's a new thing.

I like the advice of running it without the muffler to see what happens. I'll reassemble, although it probably needs a new diaphragm, and try to run it without choking it. I'll also pull out the in tank filter and replace it.

I am apprehensive about buying some china made carb. I thought I'd try this walbro first.

I'm not 100% sure I understand your tests for the carb, but I'll read it again just to make sure.

No high pressure air - got it. Haven't done it yet so I should be good. Glad you told me.

As for the beer, that's just a joke.

Stihl Backpack Blower BR420 Magnum

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