Carburetor Main Nozzle Replacement

[SteveSr]

These instructions apply to both Walbro and Zama carburetors and probably others as well. The main nozzle in the carburetor has the job of providing fuel during full power operation of the engine. Because of this the passage is large and will likely never clog up.

However, at low speed this nozzle would act like a rather large air leak if it was somehow left unsealed. Because of this main nozzles are equipped with a check valve disc to cover the nozzle during low speed operation.

When this check valve malfunctions is where the running issues begin. If the valve gets stuck and doesn't close all the way the saw will tend to bog when the throttle is opened up. If the leak is really bad you may never get the saw to transfer to the high speed circuit. Sometimes the valve will stick intermittently and the saw runs fine one minute and not the next.

These symptoms can be differentiated from a crankcase air leak in that a crankcase leak will cause an erratic idle or die at idle while with a nozzle check valve issue the saw will usually idle just fine.

Replacement main nozzles are NOT supplied in carburetor rebuild kits. However, they are normally available for a dollar or two from the carburetor manufacturer. Special thanks to @backhoelover for supplying the replacement nozzle for this instruction.


MAIN NOZZLE REMOVAL

CAUTION: Remove the high speed adjustment screw before beginning as the tip of the screw may extend into the nozzle bore in the carburetor body!

Removing the old nozzle is the hardest part. It is a brass cylinder that is press fit into t he diecast carburetor body. The original Walbro carburetor manual suggested just pressing the old nozzle through the body and into the throat of the carburetor. Recent conversations with Luis at Walbro recommended the safest way to remove it is to drill it and pull it from the metering side.

Tools Required:


1. 3/32" Drill bit
2. Fine pitch drywall screw with the tip filed flat.
3. 3/16" round drift.

Drilling setup:

A hole must be drilled in the metering side of the main nozzle to attach the drywall screw to be used to pull the nozzle out out of the carb body.



The nozzle is in the carb at an angle but must be drilled straight through. Here a Dremel collet is used as a balance to determine when the back side of the nozzle is level and ready to be drilled.

Next the back side of the nozzle is drilled... You don't have to drill all the way through the nozzle just far enough to get through the bottom of the nozzle. You'll know it when the drill bit breaks through.



Once the hole is drilled use the drift and a hammer to break the nozzle free by tapping it lightly into the bore

Next install the puling drywall screw...



Now it was time to get creative I needed to pull on the screw to remove the old nozzle. In hindsight warming the carburetor body might have helped some. Instead I got creative...


Rube & Goldberg would have been proud! Clamp the vice down to a Workmate and use an oversize ball joint fork and a hammer to extract the old nozzle. The recommended method is to use a small slide hammer which I believe comes in the Walbro special tool kit.

MAIN NOZZLE INSTALLATION
Use the drift and a small hammer to tap the new nozzle into place to the same depth as the original. Reinstall the high speed screw.

 

[Chris-PA]

I have removed and replaced a few nozzles on WTs by pushing them through - these were not because the check valve failed, but because I was boring the venturi. I sealed them with nail polish. One of these I removed and replaced the same nozzle twice with no issues.

On some carbs you can repair the check valve with the nozzle in place: http://www.arboristsite.com/community/threads/repairing-walbro-check-valves.269276/

Some of them use harder plastic disks which look to be more difficult to fix.

 

[SteveSr]

Chris, thanks for the input on check valves. All of the ones that I have seen lately seem to be of a woven fiberglass type which would be impossible to replace even if the valve was removed. I think that the carb manufacturers switched to this material for ethanol resistance. The valves are cheap so it is better to just replace the whole thing and know it is correct.

BTW, both Zama and Walbro have STOPPED using any type of sealant (i.e. nail polish) on main jets or welch plugs due to the fact that ethanol will soften and cause them to come loose and migrate to blocking various orifices.

 

[backhoelover]

i have done dozen from the walbro wa series which is the easiest. and the older hd series which takes a small special pull slide type tool to remove the nozzle assembly which is in the walbro tool kit. you also have to remember that on some wa and wt carb they dont use a nozzle but a accumulator set up with the clear gasket and a snap ring, whick cannot be test by suck and blow method all you can do is remove and replace. they are also set ups that use circuit gasket and plate as a main nozzle check valve. i have included the walbro service manual for anyone that would like to read it and gain more knowledge.

 

[El Moobs]

I just drive them into the bore. Sure is quicker.

 

[alexcagle]

These symptoms can be differentiated from a crankcase air leak in that a crankcase leak will cause an erratic idle or die at idle while with a nozzle check valve issue the saw will usually idle just fine.

*
I thought if your high speed nozzle check was leaking, it would allow air to be sucked into the idle circuit and cause idle problems.
Irregardless of whether the carb had isolated circuits or not.

 

[Chris-PA]

There must be a check valve in the H nozzle. When the engine is running the main fuel well is held at a pressure that is a fixed amount below atmospheric (this is what the metering lever & spring do), and this cannot work if you can draw air in through some hole connected to the outside. This is also why they had to eliminate the air bleeds used on other carbs to control the mixture, as they are essentially holes to the outside.

At start up the fuel well is still at atmospheric pressure, but for the L to draw fuel it pulls on the metering chamber and you can't just have it pull air in through the H nozzle or it wouldn't pull any fuel. If the check valve leaks you might be able to start it depending on how the choke plate is set up - if it really closes off the inlet then the cylinder will suck enough fuel directly from the fuel line. Some choke plate have pretty big bypass holes in them though. Still, it won't be able to idle that way.

A leaky H nozzle check valve will not effect high speed operation at all as the check valve is open then anyway.

 

[backhoelover]

i have done dozen from the walbro wa series which is the easiest. and the older hd series which takes a small special pull slide type tool to remove the nozzle assembly which is in the walbro tool kit. you also have to remember that on some wa and wt carb they dont use a nozzle but a accumulator set up with the clear gasket and a snap ring, whick cannot be test by suck and blow method all you can do is remove and replace. they are also set ups that use circuit gasket and plate as a main nozzle check valve. i have included the walbro service manual for anyone that would like to read it and gain more knowledge.

ok I have a retype on the above sentence where i am taking about the style of main nozzle that you can not test using the suck and blow method. where i called it accumulator that is not correct. the proper name is capillary

 

[Chris-PA]

Also, not all the WTs have removable nozzles. Some just have a discharge hole at the edge of the venturi and the check valve is under a welch plug. This one has been bored but you get the idea:


This probably gives poorer atomization because the outlet is in the boundary air along the side, but it does reduce the obstructions in the venturi, which it an advantage when the carb is too small. They seem to work ok on these smaller carbs anyway.

 

[alexcagle]

There must be a check valve in the H nozzle. When the engine is running the main fuel well is held at a pressure that is a fixed amount below atmospheric (this is what the metering lever & spring do), and this cannot work if you can draw air in through some hole connected to the outside. This is also why they had to eliminate the air bleeds used on other carbs to control the mixture, as they are essentially holes to the outside.

What about this Tillotson HS10-A?
This carb baffled me a while back.)This is a capillary design High speed circuit. The carb is 1968 and from an 041 early design.

No check valve.

According to the Tillotson IPL, what I pulled out from under the Welch plug (pic) was all that there was supposed to be in there

Screen, retainer clip, Welch plug,

I never could not get this saw to idle correctly with this Tillotson HS10-A carb.

 

[Chris-PA]

What about this Tillotson HS10-A?
This carb baffled me a while back.)View attachment 528799This is a capillary design High speed circuit. The carb is 1968 and from an 041 early design.

No check valve.

According to the Tillotson IPL, what I pulled out from under the Welch plug (pic) was all that there was supposed to be in there

Screen, retainer clip, Welch plug,

I never could not get this saw to idle correctly with this Tillotson HS10-A carb.

Without being able to play around with it directly I really don't know. Maybe I can find a diagram of it somewhere. However they accomplish it they have to prevent pulling air into the metering chamber, or at least enough to equalize the pressure to the outside atmosphere.

Maybe the H jet area when adjusted is small enough that it doesn't change the metering chamber pressure enough to matter, but I don't see how this can be - the only thing pulling on the metering chamber at idle is the L outlet, feeding through the L needle. You'd think the H would have to be bigger than the L, and air is thinner than fuel - so when the L pulls on the metering chamber it would just pull air through the H rather than pulling the diaphragm in to let more fuel into the chamber.

I'll have to see what I can learn about these when I have a chance.

 

[old guy]

Yup, fingernail polish will come loose & get into orifices.

 

[backhoelover]

Without being able to play around with it directly I really don't know. Maybe I can find a diagram of it somewhere. However they accomplish it they have to prevent pulling air into the metering chamber, or at least enough to equalize the pressure to the outside atmosphere.

Maybe the H jet area when adjusted is small enough that it doesn't change the metering chamber pressure enough to matter, but I don't see how this can be - the only thing pulling on the metering chamber at idle is the L outlet, feeding through the L needle. You'd think the H would have to be bigger than the L, and air is thinner than fuel - so when the L pulls on the metering chamber it would just pull air through the H rather than pulling the diaphragm in to let more fuel into the chamber.

I'll have to see what I can learn about these when I have a chance.

could the low needle seat be mess up. i have ran into this before. people trying to work on their own stuff

 

[backhoelover]

does the carb have a low adjustment needle

 

[SteveSr]

I just drive them into the bore. Sure is quicker.

It sure is quicker. However, Luis who wrote the Walbro book over a decade ago that @backhoelover referred to has just recently recommended to use the slide hammer to remove the nozzles from the metering site. I think that it may have to do with the inability to use sealer around the new nozzle.

 

[SteveSr]

*
I thought if your high speed nozzle check was leaking, it would allow air to be sucked into the idle circuit and cause idle problems.
Irregardless of whether the carb had isolated circuits or not.

Thanks for the correction. Unfortunately, I can't go back and edit my original post. According to Walbro leaky main nozzle check vales can indeed cause idle issues. However, the symptoms that I have seen are a bog or lean failure to get the engine to WOT. Is is this cause by an air leak into the idle circuit or by the check valve being stuck closed? Can anyone care to comment? In the cases that I have seen the saw appears to idle properly but can't transfer to WOT.

 

[backhoelover]

It sure is quicker. However, Luis who wrote the Walbro book over a decade ago that @backhoelover referred to has just recently recommended to use the slide hammer to remove the nozzles from the metering site. I think that it may have to do with the inability to use sealer around the new nozzle.

the slide hammer is only used on the sdc etc nozzle. dont see much of them anymore. there different then nozzle you are working on. the one you are working on i would just knock it into the bore as mention by @El Moobs ( sometime you have to remove the throttle plate to get them out)




the slide hammer tool hooks under the inside lip of the bottom left brass insert

 

[backhoelover]

zama still uses welch plug sealant. bad thing is you can but if from them. wish they would start selling it

 

[backhoelover]

this is the one i ment to upload instead of the svc

 

[backhoelover]

dam it ill get right eventually lol this is the right one promise