How To Add A High Speed Circuit To A Walbro WG Carb

[srcarr52]

How modified must a saw be before you start drilling the idle feed hole and can you show how you plug the feed passage from the main nozzle when you do?

Also, do you change or make the main jet smaller? The only modified 3120 carb I've had my hands on was almost too rich with the stock jet so you'd have to have the added adjustment at 1/16 turn or less.

 

[Mastermind]

How modified must a saw be before you start drilling the idle feed hole and can you show how you plug the feed passage from the main nozzle when you do?

Also, do you change or make the main jet smaller? The only modified 3120 carb I've had my hands on was almost too rich with the stock jet so you'd have to have the added adjustment at 1/16 turn or less.

The way I port them, the carb works well as I posted. But......anything less than huge finger ports and big compression and it's not going to need the extra fuel. Most of the time, I plug the main jet completely.

Next time I do one of these, I'll post pics of blocking the connecting passage.

 

[srcarr52]

The way I port them, the carb works well as I posted. But......anything less than huge finger ports and big compression and it's not going to need the extra fuel. Most of the time, I plug the main jet completely.

Next time I do one of these, I'll post pics of blocking the connecting passage.

I had wondered but I didn't mod the carb so I didn't know what had been done to it. I wonder if a person could find those brass jets in different sizes?

 

[Mastermind]

I had wondered but I didn't mod the carb so I didn't know what had been done to it. I wonder if a person could find those brass jets in different sizes?

To make the jet size smaller, I just fill them with solder, and redrill them.

 

[Pintail120]

The way I port them, the carb works well as I posted. But......anything less than huge finger ports and big compression and it's not going to need the extra fuel. Most of the time, I plug the main jet completely.

Next time I do one of these, I'll post pics of blocking the connecting passage.

Hi Randy,
I am going to carry out this mod, could you please post the pictures mentioned ?
Is this mod still a valid mod and do you still carry the mods out on the carbs ?
Could you also please confirm the following:
1) The dimensions of the sleeve ?
2) size of the drill bit to go in the sleeve ?
thank you & kind regards
Gavin

 

[Mastermind]

Hi Randy,
I am going to carry out this mod, could you please post the pictures mentioned ?
Is this mod still a valid mod and do you still carry the mods out on the carbs ?
Could you also please confirm the following:
1) The dimensions of the sleeve ?
2) size of the drill bit to go in the sleeve ?
thank you & kind regards
Gavin

Pretty sure everything you need to know is in this picture. Instead of blocking the passage from the main nozzle to the low side, just leave it alone, and don't drill the feed into the low side well.

 

[kneedeepinsaws]

What would be really killer would be to use the same needle as the L jet for your high, and 3d print a new better air horn for the filter to accommodate it.

 

[Moparmyway]

What would be really killer would be to use the same needle as the L jet for your high, and 3d print a new better air horn for the filter to accommodate it.

Not so sure I’d agree on that .......

1. The hex head is brilliant, and because of its location, cures a huge problem you would have if you ever successfully did what you suggested to the carb.

2. Husky offers a foam element for the air filter for the 3120 saw, it is hands down, the absolute best filtration any OEM ever put out.

3. The stock air horn is perfect for the mod Randy does, and it works great with the foam element to eliminate all fines whilst milling.

4. If you’re thinking of racing 3120’s, DD is THE man !! But at that point, are you even running the stock carb anymore ?

 

[Moparmyway]

Hi Randy,
I am going to carry out this mod, could you please post the pictures mentioned ?
Is this mod still a valid mod and do you still carry the mods out on the carbs ?
Could you also please confirm the following:
1) The dimensions of the sleeve ?
2) size of the drill bit to go in the sleeve ?
thank you & kind regards
Gavin

Randy has modded a few for me, and I’ll be asking him to do more soon too.

Peace of mind is priceless

 

[kneedeepinsaws]

Not so sure I’d agree on that .......

1. The hex head is brilliant, and because of its location, cures a huge problem you would have if you ever successfully did what you suggested to the carb.

2. Husky offers a foam element for the air filter for the 3120 saw, it is hands down, the absolute best filtration any OEM ever put out.

3. The stock air horn is perfect for the mod Randy does, and it works great with the foam element to eliminate all fines whilst milling.

4. If you’re thinking of racing 3120’s, DD is THE man !! But at that point, are you even running the stock carb anymore ?

I personally found trying to find the high jet by sticking in an allen key was always a pain with mine is why I brought it up.

 

[Mastermind]

I personally found trying to find the high jet by sticking in an allen key was always a pain with mine is why I brought it up.

Shut it off.....tweak it.....fire it up.....check tune.....repeat.

 

[Maintenance Chief]

Is a high speed check valve, a valid option instead of the press in valve? I know the flow limit is set buy the hole in the bottom of the well and metered with the needle/jet.
Kind of like what I've seen in sdc carbs.

 

[Mastermind]

Is a high speed check valve, a valid option instead of the press in valve? I know the flow limit is set buy the hole in the bottom of the well and metered with the needle/jet.
Kind of like what I've seen in sdc carbs. View attachment 275329

No sir. The nozzle is a check valve, and a orifice in a one piece deal. The hole into the venturi is the same size all the way thru.

 

[Moparmyway]

I personally found trying to find the high jet by sticking in an allen key was always a pain with mine is why I brought it up.

Try a 3mm ball end allen key

 

[Pintail120]

Pretty sure everything you need to know is in this picture. Instead of blocking the passage from the main nozzle to the low side, just leave it alone, and don't drill the feed into the low side well.

View attachment 274991

Hi Randy
thank you for the picture, may I please confirm in the picture I submitted that the hole to the low side well is crossed out as I have indicated in my picture ?
Any reason that you are no longer implementing that mod ?
thanks and kind regards
Gavin

 

[Pintail120]

Hi
A few more questions illustrated on the picture you provided as below, apologies about my ignorance, I am a metric guy and I think the imperial measurements are possibly tripping me up.
thank you kindly for your help.
Gavin

 

[Mastermind]

Hi Randy
thank you for the picture, may I please confirm in the picture I submitted that the hole to the low side well is crossed out as I have indicated in my picture ?
Any reason that you are no longer implementing that mod ?
thanks and kind regards
Gavin

Unless the engine is heavily modified, drilling this hole turned out to be unnecessary. There is a unseen passage from the main nozzle over to the low speed circuit that feeds it out of the main nozzle fuel supply. In other words, all fuel, both high and low is provided through the main jet.

So, drilling that hole has the unintended consequences of adding unmetered fuel back to the high side....and in most cases it's not needed. This means the carb will be too rich, and can't be leaned out enough.

Better not to drill this hole without blocking that hidden passage.

 

[Mastermind]

Hi
A few more questions illustrated on the picture you provided as below, apologies about my ignorance, I am a metric guy and I think the imperial measurements are possibly tripping me up.
thank you kindly for your help.
Gavin

View attachment 275713

Most of these numbers are measurements used to locate the position of the holes you need to drill. Rather that spend a bunch of time trying to explain....let me ask a few questions.

Does your carb have a pilot hole where the high side adjustment needle needs to go?

How far are you taking this saw?

Finger ports? Cut squish?

 

[Pintail120]

Hi Randy,
thank you again for your help, I will be using this on my paramotor, it has a small 80cc engine on it, previously I have been drilling out the high jet to achieve larger diameters, I also use your suggested technique of filling the jet with solder and drilling it out. Taking the carb apart to make these changes is a very time consuming way of trying to get that perfect mixture. Mostly I try to set the high by running my engine full throttle and checking the RPM on the engine, by carrying out experimentation I can identify the max rpm which would be the perfect mixture which is usually around 9600 rpm. I then richen up the high end so that the RPM drops by about 300rpm, adding a margin of saftey should the motor go lean for some unknown reason.
My squish is set to 0.8mm
Yes the carb has a pilot hole.
I do not want to run my engine at the very edge of performance, but if I am at the field and I know its running slightly lean, it would be fantastic to tweak he high needle(which I currently do not have)
I am not porting it as I would like to keep it standard, changing the motor to that level I think starts to enter the realms of R&D which would add elements of uncertaintity to reliablitiy and that is not what I want while flying. I have attached a diagram of my limited understanding of this carb below, please let me know if this is correct or if I have got it wrong ?
Thanks & kind regards
Gavin

Most of these numbers are measurements used to locate the position of the holes you need to drill. Rather that spend a bunch of time trying to explain....let me ask a few questions.
Does your carb have a pilot hole where the high side adjustment needle needs to go?
How far are you taking this saw?
Finger ports? Cut squish?

 

[Mastermind]

Your diagram is correct.

To add the high speed circuit is very simple. I'll be doing two more of these later this week. I'll do a revised thread then. I'll tag you in it.