[st946tbf]

I know this is not a tractor question, but it's small engine type, so I hope I don't offend people by posting this.

The trimmer is Mac 2816, and when I take the carburetor out, I can't see the venturi hole anywhere. The problem is it runs in the full choke mode, but when I open the choke, the engine stops. The weird thing about this is that I have another trimmer (Robi), and it has an almost identical carburetor, with a few exception: 1) it does have a venturi hole 2) the choke is a sliding type, which is to open or close by sliding it, versus rotating the disk. The rest is almost identical with look and size. When I say identical, I actually open the diaphram, and also the valve, and they're the same. Would someone please shed some light on how the gas exits into the big hole to mix with air? I know in the full choke, it goes the other way.

Talk about full choke, I can't trace a path of how it goes from the tank to the engine. Specifically, it enters the carb, then through a 1st valve, go to a chamber, then through a 2nd valve, which it goes through a filter then to a hole where the needle blocks it up and the diapham would lift the needle up. But this path is for the non-full choke. For the full choke, gas appears to go from a closed chamber, out of a hole and into the engine. How is this closed chamber get gas?

 

[Tractor-Holic]

I've seen a few with a slider inside like a motorcycle carb has,others have a spiral rotating cylinder with a slot cut out that allows more ait to enter like a butterfly does on a regular carb..as for "choke" it may not even have on if it has the primer bulb,which injects raw fuel directly into the venturi..

 

[st946tbf]

I see. What about the missing venturi hole? The gas must goes somewhere from the diaphram. The only hole I saw is the screw hole coming from the other side (to tie down the valve cover). Unless the play creative with this hole, and have gas coming out of it.

For the full choke, I still have no idea how it works, and what the path from the close chamber to the engine (small separate hole) is for. Maybe that is not a gas path, but I don't really know.

Any guru mind to share?

 

[kaptain krunch]

I have a few Macs and some Weed Eaters in a box that got some problems, i'll go look at the carbs tomorrow, and let you know if i can find one like your talking about. Could you take some pics? make your missing something that one of us could see.

 

[Croc]

"I know this is not a tractor question, but it's small engine type, so I hope I don't offend people by posting this."

Not a problem at all, this is what the small engine and repair sub-forum is all about.:trink40:

Also, welcome to the forum!

 

[FergusonTO35]

I would get the numbers off the carb and look for one on Ebay. You can often find them for 10-20.00.

 

[st946tbf]

I found the hole, or holes actually. On the wall, there are two tiny holes. If the throttle disk is closed, then the two holes are on each side of the disk. The one near the engine is smaller, and that would supply idle gas.

So, here's how I see a diaphragm car would work:

1) The difference between open and close choke is only on the air flow speed, nothing else. Gas still goes in the same path.

2) Gas from tank would go through 2 valves. I can see the 1 valve can be used to prevent back flow. But the 2 valves before and after a reservoir probably is used for the assisting the pump. The pump is a piece of plastic divide the reservoir and a vacuum space. This vacuum space is connected to the engine's vacuum. So as the piston moves back and forth, this plastic piece pulse, and push the fuel forward. What I am not sure is the plastic appears to be rigid, so how would it be able to move.

3) Gas from the reservoir enters a filter, then through a small hole guarded by a needle.

4) The needle blocks the hole using a spring pressure.

5) The needle is connected to a chamber that has a diaphragm dividing it with outside air.

6) This chamber has 2 more ports beside the needle port. 1 port is for gas flowing back to the gas tank. Probably for release extra gas, and for circulation purposes. The 2nd port is for gas to go to the venturi holes (with interception in between by the adjustment knob).

7) As air is sucked in by the engine, venturi holes' air pressure is lower than ambient pressure. So, the air outside would push the diaphragm in.

8) This in turn pushes the needle up, open the gate for gas from the tank to enter this chamber.

9) As gas enters, the difference in pressure reduces. Add the spring pressure, it has an equilibrium point somewhere in the middle for the needle.

If it goes as described, then what if gas enters the releasing port (goes backward), and into the chamber? This is bad because this tube doesn't have a filter. It also defeat the purpose of the whole path previously described.