Poulan Pro 260 - idles great but stumbles at high throttle

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B.C.

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Have a Poulan Pro 260 that stumbles at high throttle.

Swapped ignition coil - no change
Adjusted carb meter lever up & down slightly, does not resolve
Adjusting H & L screws does not resolve.
Impulse is clear
New carb gasket.
No leaks detected.

Has Walbro WT 625 carburetor. Tried a very similar WT 324 from a 2375 wild thing that runs good but couldn't get it to run much at all with that carb on it. Ordered a cheapo china carb to try, out of ideas otherwise unless anyone has some sage insights or advice to try.

Heres a video of it's odd stumbling at throttle:
 
If you know the 324 is good/adjusted and fuel lines good, doesn't sound like fuel delivery. 324, 89, 891, 624, 625 are all similar. But worth trying a new carb - they're cheap. Exhaust is clear? - a clogged screen will idle great but stumble on throttle up. Compression is good? Maybe light scoring on P&C?
 
The lines look fine, I thought about replacing the supply line anyway but it really doesn't seem necessary. I first thought so too that it was fuel but then the 324 ran like poop, kind of not at all. I didn't want to try to readjust it a lot because I already had it dialed in for the wild thing real good, and theoretically it should have just ran fine without any change since it looks like the exact same cylinder? That made no sense. None of it really does. Then I tried the ignition coil swap from the wild thing 2375 which seems like pretty much the exact same saw with a few minor differences. Yeah I've tried the exhaust from both on it, that was first thing I tried to check, seems like I can see a big screen in each muffler but can't find a way to actually get it apart though. Tried both anyway, no diff. Its funny because the piston looks great, the wild thing's piston has some light-medium scoring but runs just fine still, the 260 piston looks great but has this stumble issue. Its really got me baffled. Compression after about 5/6 pulls (granted my shoulder is quite effed right now) was around 100, two more real good pulls it went up to almost 120. That was with a good Mityvac compression tester that is barely used and near new.

Also the tank vent duckbill is fine. Kitted with new walbro kit, also kitted the wild thing which did the trick for that one to go from running but kinda crappy to running great after dialing it in.

Oh i also swapped plugs. One funny thing about the plugs on a side note is they call for an RCJ7Y, but the R type plugs don't spark for beans on these, the non R types seem to me to work so much better.

Verified that gas line and fuel filter not clogged as well.
 
B.C , I would say that your PP260 needs a new ring .

If you are not using the correct spark plug for your ignition module then don't be surprised if your ignition module fails .

Look at the PP260 ipl for your saw and it will give you the correct part number for the ignition module and spark plug .
 
I suppose it is possible the ring is simply too worn, so 120 psi compression on this saw is not high enough? You know what, I should check the compression on the wild thing that runs good and compare since they are the same cylinder it looks like.
 
Compression test on 2375 was about 140 vs around 120 on the 260 Pro. So it is about 20 psi higher. Not sure if 120 is too low or not and is causing issue or if it's something else still. Probably gonna bench it till I get the cheapo carb in a few days. If I do have to ring it that would be a shame, although it seems relatively easy on these saws. Looks like it must all bolt together the case, crank bottom & cylinder using just the 4 bottom screws. Unless there are four more hidden under the bottom. Just about out of time for this year, gotta start working on my backhoe and finish that.
 
Quick update for those interested. Chinese carb ran like poop, would not tune for beans. Adjusted meter lever which was a little high, bout half a 16th. Walbro tool was flush after that. Still ran like trash. Put original carb back on there and was about to give up, just for sh*ts I wanted to make sure it at least still ran like it did, and it did after a few pulls. Idled fine, stumbled high rev. I wondered if I had an actual rcj7y plug like it called for since I found some more plugs yesterday, didn't have one but I had an rcj6y. I'll be darned if it didn't run just a little better. I can tell it's still not at full power and if I don't tune the H just right it stumbles just the same but I can get it to about 11,500 and its still not 100% but at least a lot better. There is still a hint of that stumble in there at the very best I can tune it but almost gone. This makes me think that for some wierd reason even though I swapped the ignition off the 2375 that works great, it's still some kind of ignition or timing problem. I coil gap to magneto is business card thickness. Same as for the 2375, I use a real thin strip of plastic on all of them that is about same thickness as a card but doesn't get torn up easy so I can use it over and over. Went and got a rcj7y, runs same as the 6 but it's much better than it was with the non R type cj7y. I finally put the bar & chain on and it cuts. I think I'm done with it so the mystery may never be solved completely, if I had another actual 260 new coil I'd def pop it on there out of sheer curiosity at this point but I don't think I'm gonna get one for it just to see at this point. I've fiddled with it to no end already.
 
Install an inline spark tester on that saw, to keep from chasing your tail. Testers are cheap.

Don’t run it without the top cover for very long, or you’ll overheated the top end & only have 60# compression, lol.

Sounds like a definite ignition miss to me, and the spark tester will show it. It needs another OEM replacement coil, new & not Chinee aftermarket. You can also run it at night, mist it with water, & see if the plug wire is jumping to ground somewhere.

Is the plug gapped at 0.025”?
 
The lines look fine, I thought about replacing the supply line anyway but it really doesn't seem necessary. I first thought so too that it was fuel but then the 324 ran like poop, kind of not at all. I didn't want to try to readjust it a lot because I already had it dialed in for the wild thing real good, and theoretically it should have just ran fine without any change since it looks like the exact same cylinder? That made no sense. None of it really does. Then I tried the ignition coil swap from the wild thing 2375 which seems like pretty much the exact same saw with a few minor differences. Yeah I've tried the exhaust from both on it, that was first thing I tried to check, seems like I can see a big screen in each muffler but can't find a way to actually get it apart though. Tried both anyway, no diff. Its funny because the piston looks great, the wild thing's piston has some light-medium scoring but runs just fine still, the 260 piston looks great but has this stumble issue. Its really got me baffled. Compression after about 5/6 pulls (granted my shoulder is quite effed right now) was around 100, two more real good pulls it went up to almost 120. That was with a good Mityvac compression tester that is barely used and near new.

Also the tank vent duckbill is fine. Kitted with new walbro kit, also kitted the wild thing which did the trick for that one to go from running but kinda crappy to running great after dialing it in.

Oh i also swapped plugs. One funny thing about the plugs on a side note is they call for an RCJ7Y, but the R type plugs don't spark for beans on these, the non R types seem to me to work so much better.

Verified that gas line and fuel filter not clogged as well.

B.C. , my PP220 (same 42cc engine as your PP260) compression first pull is 62psi , second pull is 92psi , third pull 110psi , forth pull is 122psi , fifth pull is 130 psi , sixth pull is 132psi .
 
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