Poulan question

[Bonzo]

I know the bad rap these saws get but I have a PP 4218 AVX that was given to me and I keep it at our ATV club for light cleanup .
My question lies in the fuel delivery of this saw .
It starts and idles great but after cutting some wood it refuses to start again .
I have narrowed it down to excessive pressure in the fuel tank .
I am a mechanically- inclined guy and I realize fuel tanks must be vented to the atmosphere to provide fuel correctly .
What I can't quite grasp is how this fuel cap provides ( or is supposed to ) venting and sealing simultaneously .

 

[scottr]

There is a duckbill valve in the gas cap that lets air in to relpace the fuel used .

 

[Bonzo]

OK , thank you , that does make sense .
Then How do I explain the excessive pressure that builds up in the tank after a few cuts ? I I open the fuel cap when the saw is at operating temperature , It's like opening a soda-can .

 

[scottr]

The running saw aggitates the fuel like shaking up that soda can .

 

[J D]

The saw should be fully functional with upto about 10psi in the tank. The valve is only there to allow air into the tank to prevent a vacuum building up as the fuel is used.
The pressure that develops from evaporation of the agitated fuel should assist the fuel pump. Problems arise when this pressure overwhelms seals/gaskets. The most common culprit is the seal on the metering valve which, if leaking, will allow excess fuel through the carb & cause flooding

 

[3000 FPS]

It could be the hard starting is from the carburetor not adjusted correctly. Maybe richen the carb a little and see if it helps. A little pressure in the tank will not cause the problem.

 

[Wood Doctor]

Try this. After running for awhile until hot (say half a tank), loosen the tank cap a lot, tighten it again, and see if it still will not start when warm. If that fails, then likely it is not the tank vent (duck bill valve) that is causing the problem. I have not run into this problem with my 4218:

I have been unable to kill this saw. It keeps on going. Is your tank cap about the same as this one? Sometimes strange caps wind up there instead.

 

[ChillyB]

Refuses to start by any means? Or you have to choke it to start it?

There is a plastic insulator between the carb and cylinder. I’ve had one that was slightly loose to the cylinder, and one that was cracked. The screws that attach the carb are not the same screws that attach the spacer to the cylinder.

 

[Bonzo]

The most common culprit is the seal on the metering valve which, if leaking, will allow excess fuel through the carb & cause flooding

Do you mean the metering diaphragm ?

It could be the hard starting is from the carburetor not adjusted correctly. Maybe richen the carb a little and see if it helps.

OK , thanks for the reply . Richen the low speed screw ?

Refuses to start by any means? Or you have to choke it to start it?

There is a plastic insulator between the carb and cylinder. I’ve had one that was slightly loose to the cylinder, and one that was cracked. The screws that attach the carb are not the same screws that attach the spacer to the cylinder.

It wont fire when hot given 1/2 choke or full . Thank you.
Going to get this on the bench a little later this morning .

 

[J D]

Do you mean the metering diaphragm ?

Nope, the diaphragm pushes on the lever which lifts the metering valve, breaking it's seal & allowing fuel to pass into the metering chamber to feed the jets. If for some reason the metering valve doesn't seal well enough then fuel pressure will overcome it causing excess fuel in the metering chamber which will exit via the jets leading to flooding.
If this were the case I would suspect the saw to be running richer at idle (ie idle rpms lower than usual & maybe smoke a little).
Are you sure the tank pressure you are describing isn't vacuum building up in the tank? As others have suggested that will cause the symptoms you describe.
If you have a vac/pressure pump or even a bike pump with a guage you can test the carb by putting pressure on the fuel inlet... it should hold about 12psi all day & they usually "pop off" around 15psi or just over

 

[Bonzo]

Nope, the diaphragm pushes on the lever which lifts the metering valve, breaking it's seal & allowing fuel to pass into the metering chamber to feed the jets. If for some reason the metering valve doesn't seal well enough then fuel pressure will overcome it causing excess fuel in the metering chamber which will exit via the jets leading to flooding.
If this were the case I would suspect the saw to be running richer at idle (ie idle rpms lower than usual & maybe smoke a little).
Are you sure the tank pressure you are describing isn't vacuum building up in the tank? As others have suggested that will cause the symptoms you describe.
If you have a vac/pressure pump or even a bike pump with a guage you can test the carb by putting pressure on the fuel inlet... it should hold about 12psi all day & they usually "pop off" around 15psi or just over

OK , I think we're using different terms as you are in New Zealand ?
What you're calling the metering valve , I usually refer to as the needle .

 

[J D]

OK , I think we're using different terms as you are in New Zealand ?
What you're calling the metering valve , I usually refer to as the needle .

https://outdoorpowerinfo.com/repairs/zama_c1u_m29a_carb.asp

 

[scottr]

Do you mean the metering diaphragm ?


OK , thanks for the reply . Richen the low speed screw ?


It wont fire when hot given 1/2 choke or full . Thank you.
Going to get this on the bench a little later this morning .

A few years ago I had a friends PP4218AVX and I remember the hot start procedure was like my PP220 . When it is hot you pull the choke to full choke then push it all the the back in to set the fast idle then pull the starter .

 

[Bonzo]

OK , thank you guys for the info . Going to tear the carb apart ....AGAIN this morning and make sure all is in order .

 

[Old2stroke]

A few years ago I had a friends PP4218AVX and I remember the hot start procedure was like my PP220 . When it is hot you pull the choke to full choke then push it all the the back in to set the fast idle then pull the starter .

YES!!!! The most common problem with hot start on these saws is the operator does not understand how to set the carb to fast idle or even why it is necessary to do it. If the saw won't start after a couple of pulls without touching the throttle, you have to go to fast idle or you will be pulling a LOT to get it to start.

 

[Bonzo]

Thank you all for your help , had to cut a few downed trees today on the trail . One was a Birch maybe 12inches around and i think the other was a Maple . Zipped through them like butter and re-started fine .

 

[Bonzo]

Alright fellas , now this thing's leaving a puddle of fuel in the bottom of the case . any suggestions before I pitch it in the trashcan and go buy an Echo ?

 

[Wood Doctor]

Alright fellas , now this thing's leaving a puddle of fuel in the bottom of the case . any suggestions before I pitch it in the trashcan and go buy an Echo ?

One of the long fuel lines is leaking. There are two of them, one to the carb and the other to the primer bulb. Most of the time I replace them both.

 

[Bonzo]

One of the long fuel lines is leaking. There are two of them, one to the carb and the other to the primer bulb. Most of the time I replace them both.

They have been replaced with new Tygon lines .

 

[Wood Doctor]

And, you are sure you connected the right one to each destination? It's easy to make a mistake. That repair job is an exercise in frustration. I've done it several times.

There has to be a connection leak somewhere. I doubt it's a crack somewhere in the tank, the source of many fuel leaks. I suppose It could be a carb gasket, either missing, split, or not tight.