How does the choke work on a Homelite Vac Attack UT-08542?

[trouts2]

Homelite Vac Attack model UT-08542
Carb part number 308028004

The issue is the choke does start the engine. If I block the carb throat with a rag the engine starts and runs fine.
If I pump the primer bulb 20 times and pull the start line the plug does not get wet.

The carb has a weird butterfly setup. When I look into the throat there is a half moon type shape in there that I think is the butterfly.
When I put the choke on, arm up vertical it catches on some of the throttle parts on the top of the carb to stay up. I don’t see a normal/usual butterfly close inside the carb.

How does this carb choke work to create an increased vacuum inside the throat? What is the action that happens when the choke lever is raised verticle? What parts on the top of the carb change what happens inside the throat?

 

[FowVay]

Those tiny carburetors use a drum/barrel choke. There is a plastic barrel that fits inside the throttle body and rotates when you apply the choke. The barrel has a hole going through the middle of it that allows air passage during normal operation but when you apply the choke the drum turns and the opening becomes restricted.

There is the standard butterfly that controls air flow with the throttle lever but that has nothing to do with the choke operation. If you're having problems of running only on choke or with air inlet restriction then you likely have a fuel line cracked or broken which is allowing unmetered air into the carburetor.

 

[trouts2]

Those tiny carburetors use a drum/barrel choke. There is a plastic barrel that fits inside the throttle body and rotates when you apply the choke. The barrel has a hole going through the middle of it that allows air passage during normal operation but when you apply the choke the drum turns and the opening becomes restricted.

There is the standard butterfly that controls air flow with the throttle lever but that has nothing to do with the choke operation. If you're having problems of running only on choke or with air inlet restriction then you likely have a fuel line cracked or broken which is allowing unmetered air into the carburetor.

View attachment 246857

 

[trouts2]

FowVay,

Great post and very helpful. I was on the couch with a flashlight and the old carb trying to figure out the operation. You have cleared up that it is a barrel with a hole in there versus what I thought was like a flat half circle and possibly in there to take the place of a usual flat circle butterfly. I have never seen this type of butterfly setup before.

The carb has the barrel as you mentioned and two holes in it. When I move the throttle arm to full throttle the holes open to both ends of the carb throat. That seems normal like a usual butterfly so ok.

When the choke is off and throttle released the air filter side of the barrel and throat is not completely covered. There is a small quarter moon of uncovered area on the left side of the throat viewed from the air filter side of the throat.

Looking into the throat from the engine side there is also a small quarter moon of uncovered area on the left of the throat.

With the choke on (up position with no throttle) the hole opening on the in the throat tsays the same i.e a small quarter moon. Area of the quarter moon opening is smaller than a quarter of the full hole in the barrel so not much. The widest part of the middle of the small quarter moon is about 1/8th of an inch. So that is 1/8th opening on the air filter side of the barrel and 1/8th on the engine side of the throat. I would think that is enough area to defeat any choke action in the throat i.e. won’t allow gas to be pulled in from where ever the primer stores in inside the carb, probably from a small hole in the throat. (this was on the original carb).

I just checked the new carb on the blower and it has the same half moon opening only slightly more maybe 3/16ths on both the air filter and engine sides of the throat.

Do you know if the half moon sections are normal? May be the barrel is not coming around completely on both carbs. ?? Again if I block the throat with a rag on the air filter side the engine starts right up. Without the rag it won’t start unless I put come gas into the throat.

If you can please send me a link to the source of the photo you posted either post it or email it to trouts2@gmail. What you have seems to be a tech manual of some sort and likely interesting with carb info or possibly Vac Attach service stuff.

I may have something wrong in the throttle wire connection or doing something wrong with the cruise control. I think I have both right but since I can’t get choking action something is wrong but I’m still puzzled about where. Tomorrow I putz with this some more.
Thank you for the info.

 

[FowVay]

I am guessing that the half moon plates are the actual choke restrictor plates. The barrel is the throttle valve and is what controls engine speed and the half moon plates should be attached to the little red or yellow lever on the side of the carburetor.

The carburetor is a common Walbro WYC series so maybe you can do a search for this and also watch some youtube videos. Keep us posted with how you progress with your project.

 

[trouts2]

I am guessing that the half moon plates are the actual choke restrictor plates. The barrel is the throttle valve and is what controls engine speed and the half moon plates should be attached to the little red or yellow lever on the side of the carburetor.

The carburetor is a common Walbro WYC series so maybe you can do a search for this and also watch some youtube videos. Keep us posted with how you progress with your project.

 

[trouts2]

There is only a barrel in the throat a nothing else. The barrel is dual purpose. It controls the air in or blocks air depending on how much the barrel is turned. For choke the barrel turns to close the holes. For throttle the barrel turns by the throttle button and opens the air flow by turning the barrel in the opposite direction than choke action. There are no plates inside the throat. The moons are caused by the barrel not closing fully. When not closed fully the two moon like openings happen which is just the barrel not turning enough to close fully and leaving the moons openings. I'm going to adjust the throttle wire to the carb which I think is not adjusted right. That and the throttle stop setting that came with the carb. Backing off the stop lets the barrel turn more and close the moons.

 

[trouts2]

Just to be clear, there are no plates involved in making the moons. The moons are only created by the barrel not closing fully. By not closing fully the moons are left. They are actually the area not closed by the barrel and that opening looks like a 1/8th moon. The only thing in the throat is the barrel with two holes. I am going to adjust the throttle line this morning which I think will allow the barrel to travel more and close off the holes that let air in which I have been calling moons as the holes look like 1/8th crescent moons. Sorry if that name was confusing. The moons are a concept versus a physical thing.

 

[trouts2]

It turns out this new to me type of choke and throttle arrangement is simple but requires some proper adjustment when installing a new carb.
The issue with not starting was the choke butterfly, if it can be called that, has to be reset when installing a new carb. For a new carb to come off the metal throttle cable has to be removed from the throttle above the carb. When the new carb goes on the throttle cable has to be adjusted so the choke butterfly inside the carb will work properly as the throttle cable influences how well the circular barrel with two holes inside the throat will work. If the tension on the throttle cable is not just right the closing of the throat will be slightly off. That is what caused the 1/8th moons (open areas) to happen on the filter and engine sides of the carb throat. By adjusting the throttle cable just right the choke lever will turn the throat barrel just enough to block off air into the carb throat so no open areas for air to get into the throat (no moons on either end of the throat).
I can now start the engine with the choke on which pulls gas into the throat and cylinder for starting.
The new design of using a barrel with two holes to take the place of the throttle and butterfly plates inside the carb throat is a simple and good idea but requires some fussy setting to work properly. It will probably be less of a pain should I ever have to adjust another one.
As an overview here is what happens inside the throat. There is only a vertical barrel in the throat. It has two big holes in for air to pass through. To choke the throat the barrel is turned so the holes roated about 90 degrees away from the open ends of the throat. The throat is then closed off. With the throat closed the vacuum can pull gas into the throat and cylinder for starting. Once the engine starts. pressing on the throttle button causes the throttle cable to pull on the throttle plate on the top of the carb. The throttle plate is connected to the barrel so the barrel turns. The barrel turns and the holes rotate to be at the carb throat front and back. That lets air pass to run the engine. The throttle button pull also releases the choke which has a return spring so allows the barrel to turn.
So the initial problem of not starting with the first carb was just an adjustment. I got a second carb thinking insides were plugged up but the new carb had the same issue. It also needed an adjustment so the barrel could closed off the throat. After realizing there wass a barrel in the throat with holes and understanding how it work the issue boiled down to simple adjustment.