Need help with carb flooding out saw

[Joe Bopp]

My dad has an old Craftsman/Poulan saw that really isn't used except as a backup to the 290 Farm Boss that does 99% of firewood duties. I went to get it going after sitting for a number of years over Thanksgiving. Carb diaphragms were completely shot so I bought a carb kit and rebuilt the carb. However, I am getting it where the saw will start, run for a few seconds and then flood out. I checked the carb multiple times and the rebuild looks good. I grabbed a piece of spare fuel line and tried to do a pressure test on the carb but it would not hold pressure and has a massive leak to the ports that go to the pulse hole coming from the crankcase. The carb is a Walbro WT391. The needle and seat appear to be holding as I can build pressure if I block the pulse port with my finger. I am guessing it is just pouring fuel into the crankcase through the pulse port. Is there a check valve in the carb that is blown? Is it something other than a typical carb rebuild problem? I have rebuilt 10+ carbs on other equipment so I think I did the rebuild correctly. Just bothers me that I couldn't get the saw going. I won't be able to get back to this for some time when I get to visit my parents again but I wanted to get a game plan for the next time I visit. Do I just need to replace the carb with a cheap knock off from Amazon and move on?

 

[3000 FPS]

Sounds like there is a leak in a diaphragm. The impulse has to push on the metering diaphragm and on the pump diaphragm.

 

[Old2stroke]

The impulse line from the crankcase only operates the diaphragm in the fuel pump section and should hold both vacuum and pressure. Sounds like the pump diaphragm is faulty, the wrong one, or the gasket is on the wrong side.

 

[3000 FPS]

The impulse line from the crankcase only operates the diaphragm in the fuel pump section and should hold both vacuum and pressure. Sounds like the pump diaphragm is faulty, the wrong one, or the gasket is on the wrong side.

So what operates the metering diaphragm.

 

[Old2stroke]

So what operates the metering diaphragm.

The main diaphragm responds to the engines' demand for fuel. As fuel is sucked out of the fuel chamber, the diaphragm is pulled down until it presses on the control lever and opens the inlet valve which allows fuel under pressure from the pump to enter the fuel chamber and lift the diaphragm back off the control lever. Small forces involved and little range of motion so it's important that the diaphragm is flexible and the control lever set to the right height. The pressure from the fuel pump or from pressure build-up in the tank should not be able to force fuel through the inlet valve. "pop off" pressure is the pressure required to force the valve open and should be high enough to prevent this.

 

[3000 FPS]

Ok so it looks like the metering diaphragm is operated from fuel pressure from the fuel pump diaghragm.
I stand corrected. Thanks

 

[thenne1713]

Few years back, bought a China carb for similar for less than a kit locally; turned out 1-inch long line carb to primer line was 1/8" too long, causing crimp; would prime, start 1st pull and die after 15-20-seconds and trimming line 1/8" cured, but both carbs fine.

 

[J D]

Ok so it looks like the metering diaphragm is operated from fuel pressure from the fuel pump diaghragm.
I stand corrected. Thanks

The metering diaphragm (& in turn valve) is controlled by the fuel volume in the chamber. When it drops to a certain level (due to the vacuum in the throat of the carb drawing fuel through the jets) it pulls the diaphragm in, opening the metering valve & allowing the pressurized fuel from the pump into the chamber.
Think of it like a ball cock valve, it is opened & closed by the water level in the tank & the valve has to be able to shut off whatever water pressure is piped into it

 

[3000 FPS]

Got it. Good explaination there JD.

 

[pioneerguy600]

Incorrect parts in the chicom carb kits are nothing new. When rebuilding a carb with those kits special attention must be given to each replacement part being used. Many pump side diaphragms have been wrong , several were missing the pump chamber altogether resulting in the fuel running back down the impulse line.

 

[heimannm]

And just to be clear, atmospheric pressure is pushing on the metering diaphragm, as fuel is used from the "wet" side atmospheric pressure pushes on the diaphragm opening the metering needle, allowing the fuel pump to refill the wet side chamber. 14 shows the vent to atmosphere, often you will find some fines/dust accumulated on the vent side. Some Stihl models provide a special duct to allow only filtered air to the atmospheric vent.



And FWIW, the yellow in the diagram shows the impulse signal from the crankcase to operate the fuel pump diaphragm.

Mark

 

[Joe Bopp]

Thanks to everyone for the replies! I now know a little more about the theory and operation side of things from the conversation here. I used a brand new Stens kit (Parts pictured below) and then realized that it didn't have a traditional rubber diaphragm that matched the carb so then I moved on to a genuine Walbro kit. I am fairly certain I don't have the diaphragm and gasket backwards but never say never. I know the Chinese carbs are hit or miss. I bought a kit for a Mantis tiller I have because I needed fuel lines and everything else. Let's just say the carb was junk and I could not get it to run at all. Moved the rubber parts over to the old carb and the thing ran great. I am currently 0/1 on using those kits successfully. I just wanted to make sure that I didn't have a bigger carb issue or that I was missing something. I guess next time I make it home I will go equipped with a new carb kit and try again. I will probably buy a new Chinese carb and kit as backup (or maybe look into getting a new OEM carb but I just don't know if getting an old Craftsman saw is really worth putting that kind of money into it) and then return it if I can get it going by just rebuilding the carb.

 

[Chevboy0167]

Funny thing Joe Bopp..... my shift manager at work has an old Mantis. Like... 1990's or something old. Nozzle check valve in the original Zama carb was damaged and was pouring fuel out when just sitting there not running.

So I buy another new OEM Zama carb in the box, wrapped in factory sealed plastic... it does the same dang thing.
Like you, I bought a fuel line kit that came with a China carb, but used only used the lines with the new Zama. Found the new Zama nozzle check valve leaking just like the original. So, what the hell... I slap on the China junker just to see the results.

The damn China carb works perfect! Tunes proper and don't leak. Got lucky on this 1 I guess.

 

[Chevboy0167]

Now the flip side..... got a Husky 55 Rancher on bench now. Somebody put an aftermarket piston and cylinder kit on with a China no name carb.

Saw no start, got good spark, good compression, pour mix down carb throat and she starts. Not getting fuel. Pull China carb off and apart, bone dry inside. Everything looks and feels correct. Still no go. Tested the new fuel line and filter and they pass.

Got a used OEM carb from ebay on the way with a Zama rebuild kit ready. Win some and loose some I guess...

 

[J D]

Funny thing Joe Bopp..... my shift manager at work has an old Mantis. Like... 1990's or something old. Nozzle check valve in the original Zama carb was damaged and was pouring fuel out when just sitting there not running.

So I buy another new OEM Zama carb in the box, wrapped in factory sealed plastic... it does the same dang thing.
Like you, I bought a fuel line kit that came with a China carb, but used only used the lines with the new Zama. Found the new Zama nozzle check valve leaking just like the original. So, what the hell... I slap on the China junker just to see the results.

The damn China carb works perfect! Tunes proper and don't leak. Got lucky on this 1 I guess.

The nozzle check valve won't stop fuel coming out, it's there to prevent flow back into the carb. If the check valve is "leaking" out you will probably find the metering valve is leaking or there's an issue with one of the gaskets or diaphragms

 

[J D]

Now the flip side..... got a Husky 55 Rancher on bench now. Somebody put an aftermarket piston and cylinder kit on with a China no name carb.

Saw no start, got good spark, good compression, pour mix down carb throat and she starts. Not getting fuel. Pull China carb off and apart, bone dry inside. Everything looks and feels correct. Still no go. Tested the new fuel line and filter and they pass.

Got a used OEM carb from ebay on the way with a Zama rebuild kit ready. Win some and loose some I guess...

Could be as simple as the gasket around the wrong way & blocking the impulse. I'd take a few minutes to check the obvious before writing it off, no fuel draw can only be a few different things

 

[Chevboy0167]

JD...
On the Mantis new Zama, I even put a complete OEM Zama kit in the new carb with metering lever set well below spec and she still poured out the front. Gaskets were in proper.

On the 55, I tried a new rebuild kit in the China carb also... no dice. Must not be a passage drilled correctly I guess.

Sorry to hijack the OP's thread. Just some things I have experienced.

 

[rmh3481]

The nozzle check valve is to keep barometric pressure out of the metering chamber. Check to see if you have your diaphragms and gaskets in the correct order, as they are different from pump side to metering side.

If you used an aftermarket kit, you may have received what is known as the 'high nub' metering diaphragm. The problem is the nub on the metering diaphragm is so high you cant set the lever without it flooding. Take a look at the pic as its easy to miss.

 

[Joe Bopp]

The nozzle check valve is to keep barometric pressure out of the metering chamber. Check to see if you have your diaphragms and gaskets in the correct order, as they are different from pump side to metering side.

If you used an aftermarket kit, you may have received what is known as the 'high nub' metering diaphragm. The problem is the nub on the metering diaphragm is so high you cant set the lever without it flooding. Take a look at the pic as its easy to miss.

That's very interesting!!!! I think I noticed the difference in the diaphragm design but didn't really think about it. It looked slightly larger but not nearly as large as what you have pictured. I will have to take a closer look at that for sure. If you install the metering lever from the kit will it resolve the issue? I guess how would that work at all then if the kit comes with parts that aren't compatible?

 

[J D]

Changing the metering lever won't make a difference as you would set the new lever to the correct height & then have the same issues as with the original.
If you were to measure the height difference between the two & either be file the high nub down or adjust the metering height appropriately (if you can) that should work