B&S 3.5 Max Walbro Carburetor refurbishment

[JHZR2]

I’ve talked about having a few older Snapper Hi-Vac mowers. One of them has a 3.5Max engines. It starts perfectly, runs great, but it does clog somewhat easily.

I figured since it was new to me, I’d give it a full service. I know the one thing I noted when I pulled the air filter, it seemed to be dry but as if it had been soaked with gas. The replacement with maybe 10 hours is the white one. You can see then yellowish color.

I figured this meant something was slipping by the carburetor or who knows what else, and regardless, it would be prudent to service the carb.

I pulled it, it’s pretty darn clean.

It seems clean inside:

I got a refurb kit. I’m not sure I need all the parts.

The instructions talk about removing Welch plugs, but I’m not really clear if all the parts in the kit are necesssary, and if I need to do much with them.

I’m not even really certain why the Welch plugs need to be removed.

I’ve looked at a number of videos online, including with carburetors like this. I don’t get much about pulling the Welch plug, and what else to do.

Taking apart and refurbishing the carb on my JD Kawasaki was super easy. I can’t imagine this is terribly complex. Does anyone have recommendations on what I should do on this unit while I have it out and open?

Thanks!

 

[Nukeman7]

Here is a good video about removing the Welch plugs on a B&S carburetor.

 

[JHZR2]

Here is a good video about removing the Welch plugs on a B&S carburetor.

Thanks, yeah, I saw that one yesterday.

Thing is, I don’t know how to tell if I need to do it on mine, and if there is anything more than the small holes underneath on mine.

Plus there are a lot of other parts that I can’t really them if they exist, or I need them on mine…

 

[Nukeman7]

The Walbro rebuild kit will have extra parts that you won't need for your specific carburetor since some models are different. I'm pretty sure that the small port holes are the only thing under the Welch plugs. I don't pull them unless they are rusted or I suspect the holes are gummed up.

 

[The_Eric]

You might also do a leak down test (or even just fill the cylinder with compressed air) to see if combustion pressure is leaking past the intake valve and giving you a bunch of reversion.

 

[JHZR2]

You might also do a leak down test (or even just fill the cylinder with compressed air) to see if combustion pressure is leaking past the intake valve and giving you a bunch of reversion.

Good point. Thing is, the mower starts and runs beautifully, so it’s not like I’m going to scrap it even if the mower has low compression or whatnot. But it would be good to see the overall “health”.

For now I am just trying to service it as best as I can.

 

[AJB0009]

Brings back memories. I had a Walbro carburetor on a 1972 Rupp Rouge snowmobile. 15 H.P. single cylinder engine.

 

[jeepman3071]

My guess is that the needle/seat is not sealing 100% and it is letting some fuel into the filter as well. I usually leave welch plugs alone. If they are corroded enough to need replacing, in my experience the entire carb usually needs to be replaced.

Also make sure the engine is running at a high enough RPM, that can contribute to the clogging. I typically set mower engines to run at 3000-3200 RPM. The engines are rated for 3600, so it won't hurt them a bit and helps the mower not bog down in higher grass. Nice mower.

 

[wag123]

I’ve talked about having a few older Snapper Hi-Vac mowers. One of them has a 3.5Max engines. It starts perfectly, runs great, but it does clog somewhat easily.

View attachment 221492

View attachment 221493

I figured since it was new to me, I’d give it a full service. I know the one thing I noted when I pulled the air filter, it seemed to be dry but as if it had been soaked with gas. The replacement with maybe 10 hours is the white one. You can see then yellowish color.

View attachment 221495

I figured this meant something was slipping by the carburetor or who knows what else, and regardless, it would be prudent to service the carb.

I pulled it, it’s pretty darn clean.

View attachment 221496View attachment 221497View attachment 221498

It seems clean inside:


View attachment 221499View attachment 221501View attachment 221503

I got a refurb kit. I’m not sure I need all the parts.

View attachment 221504

The instructions talk about removing Welch plugs, but I’m not really clear if all the parts in the kit are necesssary, and if I need to do much with them.

I’m not even really certain why the Welch plugs need to be removed.

I’ve looked at a number of videos online, including with carburetors like this. I don’t get much about pulling the Welch plug, and what else to do.

Taking apart and refurbishing the carb on my JD Kawasaki was super easy. I can’t imagine this is terribly complex. Does anyone have recommendations on what I should do on this unit while I have it out and open?

Thanks!

Sorry to tell you this, but chute plugging is a common characteristic of Snapper Hi-Vac mowers and is completely normal. This was easily the number one complaint that I got from my Snapper customers. You are wasting your time messing with that carburetor, it looks perfectly clean to me.
The only way that this situation can be marginally improved is by installing a high lift blade with bolt-on air lift kickers (the blade that has 3 holes in the end).

Amazon.com

The problem with this blade is that you need at least 4hp, and even 4hp is not really strong enough, it really needs 5hp. Even then, the chute will still plug regularly. 3.5hp is not strong enough to support the use of this blade.
Additionally, when the bag gets old it loses it's ability to flow air freely and contributes to the problem.

 

[JHZR2]

My guess is that the needle/seat is not sealing 100% and it is letting some fuel into the filter as well. I usually leave welch plugs alone. If they are corroded enough to need replacing, in my experience the entire carb usually needs to be replaced.

Also make sure the engine is running at a high enough RPM, that can contribute to the clogging. I typically set mower engines to run at 3000-3200 RPM. The engines are rated for 3600, so it won't hurt them a bit and helps the mower not bog down in higher grass. Nice mower.

The needle seemed ok, as did the seat. But I replaced the seat seal.

 

[JHZR2]

Sorry to tell you this, but chute plugging is a common characteristic of Snapper Hi-Vac mowers and is completely normal. This was easily the number one complaint that I got from my Snapper customers. You are wasting your time messing with that carburetor, it looks perfectly clean to me.
The only way that this situation can be marginally improved is by installing a high lift blade with bolt-on air lift kickers (the blade that has 3 holes in the end).

Amazon.com

The problem with this blade is that you need at least 4hp, and even 4hp is not really strong enough, it really needs 5hp. Even then, the chute will still plug regularly. 3.5hp is not strong enough to support the use of this blade.
Additionally, when the bag gets old it loses it's ability to flow air freely and contributes to the problem.

Thanks. I kind of figured it was the nature of the hi-vac design. It’s only when I face deep leaves or excessive grass. Which is kind of to be expected.

I agree, I was pleasantly surprised that the carb was so clean. I didn’t know how it would look, thus I had to pull it. But I’m glad it is in good shape.

 

[Nukeman7]

Additionally, when the bag gets old it loses it's ability to flow air freely and contributes to the problem.

This can be remedied by washing the grass debris/fine dirt out of the inside of the bag. It has a tendency to build up on the abrasion deterrent fabric liner on the top panel the most. I used to do it every 3rd mowing when I had a Snapper Hi-Vac mower and it helped with airflow notably. The real solution is to lower the ground speed when mowing and keeping the blade speed between 3000 - 3200 RPM, especially in thicker grass. My Snapper had the commercial Kawasaki engine, so adequate power was never an issue, even when using the winged high lift blades.

 

[JHZR2]

Well, unfortunately whatever I did to the mower has resulted in an inability to run. I can’t get it to run for more than a few seconds when choked, and as soon as I put it to fast or anything else it stalls.

The linkage that connects to the governor spring was bent, I realized that when looking at a photo online, so I bent it back to what it should be. But that isn’t it. The mower ran fine before I started messing with it. I don’t even know where to start now. It has fuel, it has spark, and it has clean air. It just won’t run.


I did not mess with any settings. Yes I know on the photos that choke is engaged, and it works properly to engage and disengage.

 

[Nukeman7]

I don't believe in coincidences when it comes to things not working after I have just worked on it. It is always cause and effect in these situations. Since it will run briefly with the choke set, that confirms you have spark, compression and timing. It obvious that you have air, especially with the filter removed. So it comes down to a fueling issue...likely something you did inside the carburetor, the spring, or bad gas. I suggest sleeping on it before tearing into the carb again with an uncluttered mind. You will solve it eventually, but not if you are agitated or frustrated.

 

[jeepman3071]

If it isn't running at all, my guess would be the float/needle are sticking.

 

[JHZR2]

I don't believe in coincidences when it comes to things not working after I have just worked on it. It is always cause and effect in these situations. Since it will run briefly with the choke set, that confirms you have spark, compression and timing. It obvious that you have air, especially with the filter removed. So it comes down to a fueling issue...likely something you did inside the carburetor, the spring, or bad gas. I suggest sleeping on it before tearing into the carb again with an uncluttered mind. You will solve it eventually, but not if you are agitated or frustrated.

That is very good advice. Too often I keep at something for too long and get frustrated with the lack of progress and wasted time.

I did take one more step and got it working.

If it isn't running at all, my guess would be the float/needle are sticking.

I don’t know if it was that, but this is what I did:

I knew there were all three elements needed. But I wasn’t sure about if there was enough fuel going through. After all, I did mess with the carb, and replaced the fuel line. I knew there was some fuel, it would run (carb was empty and dry when installed), and I could smell it.

I pulled the bowl, fuel valve closed, and it wasn’t particularly full. I then opened the tank fuel valve, and it flowed perfectly. I checked the float and the needle and it was just fine. I checked the tank vale again, I think it may be getting leaky, but it was ok.

After all that, I closed it up and it started as usual. I let it idle a while, and it didn’t stop.

So I guess it’s fixed… not sure why, could there have been an airlock in the fuel hose?

I don’t know what else it could have been. Or what I could have done.

i do think the mower runs a little slower than it should. What’s the best way to measure speed, and increase it, on these engines, if it’s too low?

Thanks!!

It also got the oil changed to Rotella SAE 30.

 

[Nukeman7]

So I guess it’s fixed… not sure why, could there have been an airlock in the fuel hose?

I don’t know what else it could have been. Or what I could have done.

i do think the mower runs a little slower than it should. What’s the best way to measure speed, and increase it, on these engines, if it’s too low?

The cheapest way to measure the RPM on a small 4 cycle or 2 stroke engine is with the tach/hour meter shown below.

I actually have the Oppama Pulse/Echo wireless tach for small engines, but it is pricey for infrequent use.

You should be able to increase the top end speed by bending the stamped metal anchoring point of the spring that you straightened out. More tension on the spring should increase the maximum speed.

 

[wag123]

Well, unfortunately whatever I did to the mower has resulted in an inability to run. I can’t get it to run for more than a few seconds when choked, and as soon as I put it to fast or anything else it stalls.

The linkage that connects to the governor spring was bent, I realized that when looking at a photo online, so I bent it back to what it should be. But that isn’t it. The mower ran fine before I started messing with it. I don’t even know where to start now. It has fuel, it has spark, and it has clean air. It just won’t run.


I did not mess with any settings. Yes I know on the photos that choke is engaged, and it works properly to engage and disengage.


View attachment 221581View attachment 221582

You have a couple of problems here. The wire governor throttle link looks like it is bent improperly (about 90 degrees off) and the governor spring is also bent/stretched and compromised. I would start by replacing both of those. The parts are inexpensive.
As far as setting the RPMS, I can personally tell within 100 RPMs by just listening to the engine when it is running, but that comes from experience. In your case, a cheap handheld tach will work, like this one...

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www.amazon.com

Set the no-load RPM to 3400. This will get the best cutting/bagging performance out of the mower.
The way that you adjust the RPMs is to bend the metal throttle control arm that the plastic slider moves back and forth and contacts the choke. I have a tool that B&S supplied to do this bending adjustment, but a long needle nose pliers will also work.
BTW, you might want to check the cooling system to see if it is clean. Airflow in the shroud controls the governor and needs to be clear to operate properly.

 

[JHZR2]

You have a couple of problems here. The wire governor throttle link looks like it is bent improperly (about 90 degrees off) and the governor spring is also bent/stretched and compromised. I would start by replacing both of those. The parts are inexpensive.

You mean this thing?

I was having a heck of a time getting it back on both ends. I looked up that part picture, and realized that mine was bent wrong - longer. I bent it to look kind of like above, and it works. I guess… maybe not enough.

I have a thread on the spring itself, and did just get a NOS.

Old Briggs 3.5 Max governor spring

Old but seems to work…. But look at the crease. Runs well. No concerns. Rebuilding carb just because. Though it’s clean it seems. So do I need a new spring? It’s NLA. Will any spring of the same length work? This engine has manual speed control and choke on the engine.

bobistheoilguy.com

The way that you adjust the RPMs is to bend the metal throttle control arm that the plastic slider moves back and forth and contacts the choke. I have a tool that B&S supplied to do this bending adjustment, but a long needle nose pliers will also work.
BTW, you might want to check the cooling system to see if it is clean. Airflow in the shroud controls the governor and needs to be clear to operate properly.

You mean item 916 curved arm, which connects to 131 via the link above, and the spring?

The curved arm on 916 is a cut piece of metal, I’m. It sure I could bend that..

You mean the curved piece top center? Does it get bent at the end with the hole, where the spring attaches, or more twisted at its base?

 

[Nukeman7]

As far as setting the RPMS, I can personally tell within 100 RPMs by just listening to the engine when it is running, but that comes from experience. In your case, a cheap handheld tach will work, like this one...

Loading…

www.amazon.com

The tachometer that you linked above is impractical to use on a lawn mower engine. Those laser based tachs require that you attach a reflective adhesive strip on the crankshaft and shine the laser onto the strip to measure the RPMs. For small OPE engines, the most practical inexpensive tach is the type where you wrap the inductive pickup wire around the spark plug wire.