Installed a Bing 48 carb on my 036

[Terry Syd]

Yep, that is a plugged opening for a drilled circuit. It appears to have been drilled vertically from the outside, so it should be easy to see where it extends. Just lay a straight edge over the wetside that runs from the plug.

 

[Backtroller]

With the stud lined up this is where it appears to run.

 

[Terry Syd]

Holy Sh!t, I think it may get its fuel from the main discharge nozzle tunnel (like on some Walbros). This configuration keeps the low speed circuit LEAN as the carb comes on the high speed circuit - it is another way to meet EPA guidelines. It tends to decrease the torque down low (and therefore less emissions) and makes the operator have to run the saw at a higher RPM where there will be less mixture blown out the exhaust port.

You will have to push out the main discharge nozzle to see if that circuit goes all the way to the main discharge nozzle tunnel.

If it does, we can mod this carb to work a lot better.

Stihlbro, if you can confirm that the brass jet does not have a discharge port underneath it then your carb will be an easier mod. All you have to do is drill out the limiter jet on the low speed circuit.

If it does have a discharge port into the main body, then it is an auxiliary jet to help with throttle response and the carb should have the same low speed circuit as Backtroller's.

 

[Backtroller]

You will have to push out the main discharge nozzle to see if that circuit goes all the way to the main discharge nozzle tunnel.

Ok, so the main discharge nozzle is the brass piece with the offset hole that I can also see from inside of the venturi? Just wanting to make sure. How do I "push" this out? I won't be able to try this tonight though.

I'll sacrifice this carb in the name of progress if need be.

 

[Terry Syd]

Hopefully you won't have to sacrifice anything but your time. Yes, it is the brass piece with the offset hole (jet) that you can also see inside the venturi.

The nozzle gets tapped out by using a flat punch on the back of the nozzle. It gets tapped back in the same way. Take note of the position of the nozzle and how far it sits in the carb body - you'll want to put it back that way. Since you have a couple of camera shots of it, it shouldn't be too hard to 'remember'.

 

[Backtroller]

Yes, I see a hole that lines up with the plug on the outside. It's the bright dot on the last picture. The drill is the approximate angle View attachment 37834View attachment 37835

 

[Terry Syd]

I think that is the hole from the High Speed needle. Remove the high speed needle and shine a light through there and you'll probably be able to confirm it. However, if the hole has been drilled at an angle, then you are probably right.

If the low speed circuit drew its fuel from the nozzle circuit, I would expect there to be TWO holes in the discharge nozzle tunnel. One from the high speed needle and one from the circuit that has been blocked.

If there aren't any more indications of blocked off circuits (look for more plugs on the carb), then it appears that both high speed needle hole and angled circuit have come together in one hole. In which case, the mod is going to be more difficult to do. If there were two holes, you would block the hole to the low speed circuit with a bit of JB Weld (I do that on the Walbros)

What we want to do is isolate both circuits from each other. If they both come together in the nozzle tunnel, then you will have to drill further back from that hole and into the feed to the low speed circuit, perhaps 5 mm will be enough. However, you don't want to drill too far back along the circuit or you may hit where the low speed needle comes into the carb.

After you have drilled down and found the circuit, you can probably open up the hole you drilled with a bit bigger drill to make the next move more easily. You need to fill up the hole with JB Weld so that the circuit is blocked and no longer connected to the nozzle tunnel.

Once the circuit is blocked, we will drill another hole over the low speed needle to provide fuel to the low speed circuit. - Then you can start tuning a whole different carb!

 

[Stihlbro]

Something else I'll mention is that with the 038 boot and Bing carb I'm having issues with the idle. It's extremely touchy to how it's tuned on the L side.
After making a cut (hot saw) the saw will return to idle as it should but if it's allowed to idle for approximately 10 seconds or so the idle will begin to drop off. Sometimes the saw will stall. I figure that what's happening is that its loading up with fuel just like if the L side was tuned to rich. I think that the larger 038 boot is lowering the intake velocity. Maybe the 036 intake boot would help this? Or maybe like Al said the intake port needs to be a bit higher. Idk for sure. Just thinking out loud here

I should of asked more of the condition of the Bing. New? Used? Fresh kit? Check condition of the needle, fulcrum arm, and spring. Check pop off pressure. When does it pop off? When does it seal back? Metering lever height? Metering diaphragm condition? Disregard my previous post till this is done.

 

[drf256]

Great stuff guys. I'm doing my best to follow along. I will try to pull one apart this weekend to see what you are all doing here. Seems one in my hand would make it more comprehensible.

So, if I'm following correctly here....

The H side nozzle gets its fuel 2 ways, from the drilled offset hole in the bottom of the brass plug (where the needle ends-adjustable) as well as the fixed hole closer to the needle screw externally? So the H has a fixed amount it can Ben adjusted.

The L circuit fills from a passage into the H circuit. It fills the Welch plug area.

If I have any of this correct, my question is:

Can we drill a small hole close to the Welch plug along the path of the L needle to fill the area at the tip of the needle and call it quits? Wouldn't that allow the L and H circuit to pull fuel from both places?

That way, the L wouldn't get leaned out when the H is pulling fuel and the H will still be partially adjustable. We would just reach a new equilibrium with both needles.

Make any sense, or am I way off here?

 

[drf256]

Unless the adjustable portion of the H circuit is the hole in the metering face and the fixed supply is the hole in the brass jet.

 

[Backtroller]

I think that is the hole from the High Speed needle. Remove the high speed needle and shine a light through there and you'll probably be able to confirm it. However, if the hole has been drilled at an angle, then you are probably right.

Yes you are correct. Light from the H shows there. No other holes that I can see

 

[Canadian farm boy]

I should of asked more of the condition of the Bing. New? Used? Fresh kit? Check condition of the needle, fulcrum arm, and spring. Check pop off pressure. When does it pop off? When does it seal back? Metering lever height? Metering diaphragm condition? Disregard my previous post till this is done.

The Bing carb I'm using is a used carb that I cleaned in my usc. It has a fresh kit. The kit didn't come with a new needle, spring or fulcrum lever. I had to reuse these parts. They are in good condition. The fulcrum lever in the Bing is actually plastic. I haven't checked the pop off pressure but I can try and do it tonight.

 

[Stihlbro]

Great stuff guys. I'm doing my best to follow along. I will try to pull one apart this weekend to see what you are all doing here. Seems one in my hand would make it more comprehensible.

So, if I'm following correctly here....

The H side nozzle gets its fuel 2 ways, from the drilled offset hole in the bottom of the brass plug (where the needle ends-adjustable) as well as the fixed hole closer to the needle screw externally? So the H has a fixed amount it can Ben adjusted.

The L circuit fills from a passage into the H circuit. It fills the Welch plug area.

If I have any of this correct, my question is:

Can we drill a small hole close to the Welch plug along the path of the L needle to fill the area at the tip of the needle and call it quits? Wouldn't that allow the L and H circuit to pull fuel from both places?

That way, the L wouldn't get leaned out when the H is pulling fuel and the H will still be partially adjustable. We would just reach a new equilibrium with both needles.

Make any sense, or am I way off here?

There is a small divot inline with the low needle. I think drilling this would feed the Welch plug and make it independent. Block off the hole from main nozzle to Welch plug. I agree with your thinking. I can do the carb I have as I don't need it. Don't even have a saw to try it on. If someone wants to try it, speak up and it's yours.

The Bing carb I'm using is a used carb that I cleaned in my usc. It has a fresh kit. The kit didn't come with a new needle, spring or fulcrum lever. I had to reuse these parts. They are in good condition. The fulcrum lever in the Bing is actually plastic. I haven't checked the pop off pressure but I can try and do it tonight.

Let me know, the plastic fulcrum so can be finicky. We can work from there. I noticed in your video the idle was pretty high, which tells me you was trying to compensate too much fuel.

 

[Terry Syd]

After you get the high and low speed circuits independent of each other, you can then tune them to get the fuel curves you want.

When you increase the fuel flow in the low speed circuit the high speed circuit will have to be leaned out. You may run into a common problem with these kinds of mods, that is, the high speed circuit can't be leaned out enough. What you then have to do is block the jet on the back of the main discharge nozzle. Just put a dab of JB Weld on the back of the nozzle.

Another potential problem will be the brass jet that Stihlbro has on his carb. I THINK (it hasn't been confirmed) that that jet is to help with throttle response. You can find out by seeing if it has a port that goes into the carb body for the fuel to discharge from. If it does, then it is an auxiliary jet for throttle response (a common configuration on these EPA carbs). - What happens is that after making the high and low circuits independent, the low speed circuit will likely go rich. In which case, that brass jet may provide too much fuel for right off of idle. If it tends to bog off of idle then you will have to block the brass jet with some JB Weld.

 

[Stihlbro]

The fixed jet does feed a second hole in the bore. Like it has two transistion holes.

 

[Canadian farm boy]

There is a small divot inline with the low needle. I think drilling this would feed the Welch plug and make it independent. Block off the hole from main nozzle to Welch plug. I agree with your thinking. I can do the carb I have as I don't need it. Don't even have a saw to try it on. If someone wants to try it, speak up and it's yours.





Let me know, the plastic fulcrum so can be finicky. We can work from there. I noticed in your video the idle was pretty high, which tells me you was trying to compensate too much fuel.

I checked the pop off pressure on my Bing carb. It pops at 16-17 psi.

Just for comparison I also checked 2 Zama C3A carbs. The one Zama pops at 25 psi and the other is 30+psi( my gauge only goes to 30)
The 2 Zama carbs have different looking wet sides. I'll post a couple pictures in a minute

 

[Canadian farm boy]

Neither of these carbs are the carb I was running on my saw in the videos. I'll post a picture of the wet side of the carb that I've got on my 036 in a few minutes.

 

[Terry Syd]

That first carb with the fuel hole for the low speed circuit is what we are aiming at by modding the carb. Notice it does not have a jet drilled in the back of the main discharge nozzle. In other words, the low speed circuit is flowing enough that the extra jet in the back of the nozzle isn't needed.

I wonder what saw that carb came off of? European? Pre-EPA?

EDIT: Take a look and see if that carb has the plug for the extra circuit.

 

[Canadian farm boy]

The Zama carb that I was running on my saw in the videos look exactly the same as the carb in the second picture. Both the carb in the second picture and the carb that was on my saw have pop off pressures above 30 psi.

The carb in the first picture has a pop off pressure of 25 psi

 

[Canadian farm boy]

The carb in the first picture doesn't have a snorkel type cover nor does it have the brass plug in the side near the L screw.
My other 2 carbs have snorkels and they both have the brass plug