Help me fix my grandfather's poulan pro 260

[kevin711]

I'm missing something and i'm hoping the community here can give me a hand in finding out what it is so I can get my grandfather's pp260 running again. It's more for sentimental value than anything else and it would mean a lot!

I will post pics to try and better explain what I mean but here it goes.
Originally the saw stopped working because it had no spark. I replaced the ignition module with an OEM one and that took care of that problem. However, the saw became very hard to start and impossible to tune the high/low jets. I decided it was time for a rebuild so I ordered the kit for the walbro WT-391 carb, dipped it in berryman chem dip, then replaced the welsh(welch?) plugs and all of the gaskets/diaphragms. While I was at it I decided to replace these as well:
-new fuel filter
-New fuel lines
-New purge bulb
-new air filter
-New gas cap
-New oil cap

The saw has good spark and if I put a little fuel into the cylinder it fires up within half a pull for a split second.

The problem I am having is that when I push the purge bulb it sucks a lot of air with the fuel and now I have found out it is pressurizing the pump side of the carb. If I take the top plate off the pump side and let the pressure out I can see the fuel and air bubbles running through the fuel lines to the tank until all the pressure is out of the system. I can also release the pressure by pushing down on the (needle valve?) which results in the same thing... lots of air and fuel moving through the lines. The only way I can get the system to not suck air is to use my finger to close the hole that looks like it has a brass surround. It's the hole right below the word "side" in my picture. I didn't put an arrow to it.

Any ideas?

I also included a picture of the serial number in case that helps.

Thanks
-Kevin

 

[Andyshine77]

Sounds like you have the suction line hooked up to the dump side of the primer. The line from the primer to the carb should have vacuum not pressure.

 

[kevin711]

Sounds like you have the suction line hooked up to the dump side of the primer. The line from the primer to the carb should have vacuum not pressure.

The pressure side of the purge bulb is pushing fuel back into the tank. At the end of that there is a little one way valve which is mounted at the top of the gas tank. Purge bulb sucks from the left side of the carb. The right side of the carb is a thinner fuel line which goes into the tank and is connected to the fuel filter. I tried to get the orientation of the fuel lines and purge bulb in the picture but it's hard to see.

 

[Andyshine77]

OK. Did you lightly seat the H and L needles and turn them out 1.5 turns? This will get the carb settings close enough to at least fire up. If none of this helps you may need a new carb. Check the lines to make sure they don't have a pinhole.

You also mentioned the needles had little effect on tuning, likely a carb issue, but this can also be a system of an air leak. If resetting the carb and or replacing it doesn't help, a vac/pressure test is in order.

 

[kevin711]

I started out with both jets turned out 1.5 turns when I first tried to start it.

I put pressure on both fuel lines and they held pressure with no leaks. I was fooling around with them during this whole process so that's something I can check again.

I could not find another wt-391 carb so I bought a chinese replacement. It looks of decent quality but the armature for the throttle and choke are at slightly different angles than the original so it doesn't operate smoothly on this saw with the choke lever and air filter housing installed. I ended up putting the original one back on that I had rebuilt.

I did try the same process with the new chinese replica and I was still getting air. I only tried once so I'm not sure if the chinese carb was building pressure inside of it as well.

 

[Chris-PA]

If the bulb is pulling in air it will eventually pressurize the tank and therefore the line to the carb. Get a piece of tubing (or one of those vacuum splice adapters) and bypass the carb in the fuel system entirely. Pump the purge bulb and see if it draws any air bubbles. If not the fuel lines and purge bulb are good.

If there is no air without the carb then you have a typical leaking check valve. There are usually two, one in the H/Main fuel outlet, and the other from the fuel well to the purge bulb outlet. If the check valve from the fuel well to the purge bulb leaks the saw may still run ok, as the bulb itself has a valve and the lines may not let air in. If the Main fuel outlet check valve leaks it will not start and run, unless maybe you get it to fire holding it wide open throttle.

I've fixed check valves but it's a PITA, especially if you don't have a suitable press to push out the main nozzle. See if you can swap the shafts and such from the WT-391 to the Chinese clone - or perhaps it's the wrong carb?

 

[Andyshine77]

Yeah if the lines are good and you still see air, I call that a bad carb as Chris explained.

 

[kevin711]

If the bulb is pulling in air it will eventually pressurize the tank and therefore the line to the carb. Get a piece of tubing (or one of those vacuum splice adapters) and bypass the carb in the fuel system entirely. Pump the purge bulb and see if it draws any air bubbles. If not the fuel lines and purge bulb are good.

If there is no air without the carb then you have a typical leaking check valve. There are usually two, one in the H/Main fuel outlet, and the other from the fuel well to the purge bulb outlet. If the check valve from the fuel well to the purge bulb leaks the saw may still run ok, as the bulb itself has a valve and the lines may not let air in. If the Main fuel outlet check valve leaks it will not start and run, unless maybe you get it to fire holding it wide open throttle.

I've fixed check valves but it's a PITA, especially if you don't have a suitable press to push out the main nozzle. See if you can swap the shafts and such from the WT-391 to the Chinese clone - or perhaps it's the wrong carb?

I thought all the valving in the carb was done by the diaphragm, but now I see that hole that I thought had a brass surround is actually a check valve. That would make sense as to why when I put my finger over that valve as I let the purge bulb come back out it sucks gas and not air... i'll have to see if I can find another wt-391 carb and i'll report back.

 

[MacAttack]

i had this happen on a Husqvarna leaf blower, no matter what i did i could not get the carb to prime with gas. All i could get was a tiny bit of gas and mostly bubbles in the primer bulb and fuel lines. I finally gave up and replaced the carb, and the new one primed and worked perfectly. It seemed like some passage in the carb was irreparably plugged.

 

[kevin711]

I just went out and bypassed the carb. the purge bulb moved fuel with no air in the lines. Going to have to search for a genuine wt-391 carb. The Chinese copy is the same carb but the linkage for the throttle and choke are different enough that I can't use it on my saw. Learn something new every day. Check valve pressed into a carb... who would have thought. Thanks so much for all your help!

To be continued.....

 

[kevin711]

If the bulb is pulling in air it will eventually pressurize the tank and therefore the line to the carb. Get a piece of tubing (or one of those vacuum splice adapters) and bypass the carb in the fuel system entirely. Pump the purge bulb and see if it draws any air bubbles. If not the fuel lines and purge bulb are good.

If there is no air without the carb then you have a typical leaking check valve. There are usually two, one in the H/Main fuel outlet, and the other from the fuel well to the purge bulb outlet. If the check valve from the fuel well to the purge bulb leaks the saw may still run ok, as the bulb itself has a valve and the lines may not let air in. If the Main fuel outlet check valve leaks it will not start and run, unless maybe you get it to fire holding it wide open throttle.

I've fixed check valves but it's a PITA, especially if you don't have a suitable press to push out the main nozzle. See if you can swap the shafts and such from the WT-391 to the Chinese clone - or perhaps it's the wrong carb?

I'm thinking about taking the check valves out of the chinese carb and replacing the oem ones. One of them is the the brass donut looking thing in the last picture I posted. Where would the other one be?

 

[vizette]

I had one carb I never could get to work right, I think on a string trimmer, maybe a hedge trimmer... anyway, it had a check valve of some sort that was in the body of the carb and not serviceable. If I remember right, I had a similar issue as you're describing. Ended up having to replace it - kinda frustrating after you've gone to the trouble to clean it and rebuild it.

I have used Qazakay and Aisen brand knock-off carbs before (Amazon) that worked fairly well. Definitely not oem quality, but they worked and I was able to tune as expected, however I was going to sell that gear off anyway so I wasn't all that uptight about it. For the price though, the knock-off carbs cost about as much as a rebuild kit for the original, so it was worth a shot, and I knew Amazon would refund me if it didn't work. Also have used a lot of HIPA stuff for rebuilds on my personal gear that seems to be decent.

 

[LonestarStihl]

I know it says out of stock but contact the guys there and a lot of times they’ll have one available just not updated on the site. Chainsawr . Com

 

[Chris-PA]

I'm thinking about taking the check valves out of the chinese carb and replacing the oem ones. One of them is the the brass donut looking thing in the last picture I posted. Where would the other one be?

I've never had any success removing those valves intact - I've gotten them out but there was nothing usable left. Now that I think about it I believe the WT-391 does not have a separate brass main nozzle that can be pressed out, just an outlet hole on the edge of the venturii, and the main check valve is internal (don't remember what it looks like).

I've used several of the no-name Chinese carbs with no issues and the shafts & linkages were fine. Given the price I'd try again - there are a ton on eBay.

 

[Old2stroke]

The check valve you had your finger on cannot be removed without destroying it, but they are cheap (have to order them) and easy to replace. I don't believe that carb has a main jet check valve, it looks like both low and high speed circuits are fed through the same check valve. If you just remove the check valve and plug the line going to the purge bulb, the saw will run just fine with more reliability but without the convenience of easier starting without the bulb function. A quick test of these check valves without taking the carb apart, is to plug the fuel inlet port to the carb and apply a vacuum to the purge port, if it doesn't hold a vacuum the valve is not seating. However if the valve isn't opening fully and this is a common problem with the rubber disc inside it swelling due to ethanol, it will cause an fuel starvation situation. MANY carbs get replaced because of these annoying little beggars.

 

[kevin711]

The check valve you had your finger on cannot be removed without destroying it, but they are cheap (have to order them) and easy to replace. I don't believe that carb has a main jet check valve, it looks like both low and high speed circuits are fed through the same check valve. If you just remove the check valve and plug the line going to the purge bulb, the saw will run just fine with more reliability but without the convenience of easier starting without the bulb function. A quick test of these check valves without taking the carb apart, is to plug the fuel inlet port to the carb and apply a vacuum to the purge port, if it doesn't hold a vacuum the valve is not seating. However if the valve isn't opening fully and this is a common problem with the rubber disc inside it swelling due to ethanol, it will cause an fuel starvation situation. MANY carbs get replaced because of these annoying little beggars.

Thanks for the info. I will run the vacuum test and then try the saw with the purge port blocked if it fails. If the saw runs with the check valve removed and the purge port blocked I might just leave it that way.

Thanks for all the help!
-Kevin

 

[Chris-PA]

All these carbs must have a check valve on the main outlet that is separate from the L/Idle outlet. Without that then at idle when air going past the idle outlet ports caused a pressure drop, instead of pulling fuel from the well it would just pull air from the H outlet into the fuel well.

Usually the main check valve is in the brass main nozzle, but on these it is not.

 

[Old2stroke]

All these carbs must have a check valve on the main outlet that is separate from the L/Idle outlet. Without that then at idle when air going past the idle outlet ports caused a pressure drop, instead of pulling fuel from the well it would just pull air from the H outlet into the fuel well.

Usually the main check valve is in the brass main nozzle, but on these it is not.

Not always the case. On this carb the body check valve is placed between the two fuel adjust screws where is can supply fuel to both high and low speed circuits and there is enough separation between the two circuits so that they don't effect each other

 

[Chris-PA]

Not always the case. On this carb the body check valve is placed between the two fuel adjust screws where is can supply fuel to both high and low speed circuits and there is enough separation between the two circuits so that they don't effect each other

I went up to the barn and looked as it's been a while. This carb uses a fine mesh screen instead of a check valve to seal off the H/main outlet and prevent air from being pulled back in. It relies on surface tension of fuel in the screen to block it off. So there is a check valve to the combined H/L jets, and the mesh screen to seal off the H.

It also appears to rely on the check valve in the purge bulb, as the purge port is just an open tube.

 

[kevin711]

Update: this was a huge learning experience for me and i appreciate the time and effort everyone here put in to help me out. Tonight I tested the carb and after it failed I took the check valve out. While I was doing this I bolted up the chinese carb that I had on before but left the choke lever off of it so it would fit. I still could not get the saw to prime and I wasnt in the mood to investigate so I put some fuel down the carb throat and she fired up almost immediately. The saw ran at very high rpm and I moved the idle adjust screw all the way out till it wasnt touching a thing. No effect I had to kill it with the kill switch. After that I decided testing the saw with the bar and chain on was a bad for my health so I took them both off. I wanted to try the oem carb with the prime side blocked off and the check valve removed so I hooked all that back up and watched the fuel in the line to see if it would get aucked through the carb from the pressure pulses. It did but extremely slow.

This is where things take a turn....

I decided it would be a good idea to spin the engine a little faster but with the kill switch engaged so no spark. That way I could watch and see if it would prime the carb. Got a drill and spun the shaft where the flywheel bolts to. It worked great until I was done and I saw that my genius resulted in stripped threads. I decided to use a self threading nut instead but then found I had also damaged the ignition module. No spark either.

Originally I wanted to get this saw running 100% in memory of my grandfather. I spent countless hours on it and lost a lot of sleep but I learned a hell of a lot from this in the end. I decided to take the parts to a local small engine repair shop to see if any of the organs can be donated.

Before I do that, if anyone here wants any of the parts they will be free just pay whatever the cost of shipping is.

Thank you everyone!
-Kevin