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Thread: OT: Chainsaw problems
05-01-2009, 05:08 PM #1
OT: Chainsaw problems
I have been unsuccessful at finding a good chainsaw discussion forum so I thought Id post it here. If you know of a good chainsaw forum let me know.
I have a craftsman (poulan) 18" 40cc saw that Im trying to fix. After starting, it runs for a few seconds and dies. If you give it fuel while its running, it dies even quicker. It will do this about 3 times, and then it locks up. Hydrolocking with fuel. This I know because the starter gets hard to pull, and if you push in the primer bulb, it will not re-inflate unless you let it sit for an hour or so. Also, the carb and fuel system gets pressurized and I if I loosen the screw on top of the carb, fuel shoots out and the primer bulb will re-inflate. If I let it sit for a few hours the pressure will bleed off and I can start it again, but only for a few seconds as I described.
If I take everything apart and put it back together again, the pressure is released, and I can start it again and then it will do the same thing.
I have checked everything I can think of, even replaced the gaskets and diaphragms in the carb. Right after I replaced the diaphragms, it ran best, and I actually thought I fixed it...it ran for several minutes, but then it went right back to the hydrolocking symptoms. I have made sure the gas tank is venting properly, everything appears clean and tight, starts on the first pull.
Any ideas what would cause this hydrolocking condition?
05-01-2009, 05:15 PM #2
Most the time a run die situation is bad seals. If you can feed fuel with a squeeze bottle filled with gas/oil and keep it running it is probably the seals on the crank, both of them need to be replaced. How old is the saw.
05-01-2009, 05:37 PM #3
05-01-2009, 06:30 PM #4
the saw is about 10 years old.
05-01-2009, 07:23 PM #5
The primer fills the carburetor, not the cylinder, so the fact that it is hard does not indicate hydrolocking - instead it would suggest that the carburetor has pumped in more fuel than it's dispensing.
Two fairly common run-die problems on an old saw worth checking.
1: clogged intake sponge in the tank. To run sideways, upside-down, etc., the fuel pickup is taken through a sponge. It can clog or deteriorate and turn to nearly impermeable gunk, and cause intermittent starting and running that can be extremely frustrating to troubleshoot.
2: clogged muffler screen (spark arrestor).
05-01-2009, 07:53 PM #6
Thanks for the suggestions
Definately not a clogged muffler, checked that.
The fuel filter was the first thing I checked. to be certain, I tried running without it briefly after cleaning the tank and putting new fuel in. it made no difference.
As far as the primer bulb goes, I know it doesnt fill the cylinder directly...what Im trying to say is that the carb gets full of fuel and pressurized, such that if you press the primer bulb, it will not pop back out unless you somehow allow air into the fuel system (either by removing a hose on the carb, or loosening the top screw on the carb) and when I do this, lots of fuel shoots out under pressure.
When this pressure is built up like that, it is very difficult to pull the starter cord. When the fuel pressure is bled off, it becomes easy to pull start again.
The pressure build up occurs after a few of the run-die cycles, and then I can no longer pull start it without bleeding the pressure off.
I hope that makes better sense.
05-02-2009, 01:22 AM #7
Your high speed mixture screw might be too lean. that's usually why it dies when you try to give it more throttle. Try opening it about a quarter turn or so.
05-02-2009, 02:11 AM #8
I've fixed a number of saws with this problem.
Most of saws I've fixed have Weber or similar carbs. They have a needle valve, just like a car or motorcycle carb. However, the needle is actuated by a vacuum-operated diaphragm within the carb itself instead of a fload. Usually, running on prime then dying, after non-operation for some time, is an indication of a gummed up carb needle or vacuum passage. To rectify this, you need to rebuild the carburetor.
This is straightforward work. The local saw shop or lawnmower place should have a rebuild kit for your carb in-stock.
I mis-read your first post. You've already done this ^^^.
05-02-2009, 03:06 AM #9
I am going to go with Carl on this one. I had a Poulan with pretty similar symptoms, and it turned out to be crankshaft seals.
05-02-2009, 04:15 AM #10
the fact that the engine is hydrolocking makes me think this is something more sinister than the standard clogged/gummed walbro carb syndrome, it's somehow flooding with fuel. you also don't mention the choke or if you're using it.
things i can think of:
the fuel regulator diaphragm or jet may be stuck letting the engine suck too much fuel and resulting in flooding.
the engine may also be sucking fuel in through the pressure pulse line that operates the fuel pump. you changed out the diaphragm but maybe it isn't seated properly?
and regarding crank seals: i run a lot of these engines with no seals other than what a rubber shielded bearing provides. they do leak a bit but the reduction in friction results in more power in my application. i've also had an engine that ran just fine with a 1/4" hole in the case that i forgot to plug and only discovered after half a year.
05-02-2009, 12:26 PM #11
I dont get it. This thing is starting to drive me nuts. If I remove the spark plug, I can pull the start cord effortlessly over and over. Then I put it back in, and I pull start once and then I just about break my hand trying to pull start again. I can spin the flywheel easily with just my fingertip.
I pull the starter a few times with great resistance and sore knuckles, and then suddenly it frees up and I can pull start effortlessly for a few pulls, then it locks. No real detectable pattern to it, just intermittently. I dont get it.
05-02-2009, 12:53 PM #12
The fact that the primer bulb doesn't pop back up after pressing indicates a clogged restricted fuel line or check valve between the primer bulb and the tank. You have two check valves, one on either side of the primer bulb. When you press on it, the one to the tank closes and the one to the carb opens. When you release the bulb the opposite happens. Since the bulb doesn't inflate, that would indicate the valve to the carb is closing properly, but no fuel is being allowed to come into the bulb from the tank. Could be the tank filter or a kink in the line.
05-02-2009, 02:36 PM #13
I have a Poulan Pro myself. It doesn't run either. I'm still puzzeled over it after many years. It belonged to my dad, so I will figure out what's wrong with it. To your issues: Has the machine been disassembled to the point where the crankcase halves were apart? Your description of the "hydrolocking" may be a binding of bearings inside the machine. They are built with minimal clearances inside, and if there is any misalignment of the components, it could cause the locking you describe. I have five other chain saws of various ages. The newer ones are the hardest to diagnose. I use the old ones for most of the hard work, and the newer ones, if they will start and stay running, I use for trimming. Now to the carb. There is nothing like a ten year old chain saw to keep one humble. Every mechanic should have one. They usually get rode hard and put away wet. Some of the symptoms you describe indicate a dirty carb as many of the guys mentioned. After hours of fiddling with these things, I will confess, a half-assed job of cleaning the carbueretor will not suffice. Pull the thing all the way out. Take as many parts off of it as you can. Take photos as you go. Expose as many lines, pinholes, filters as you can. Use some WD-40 to shoot out all the holes you can get at, then blow them out with compressed air. The Walbro has an internal super fine mesh filter just the other side of the diasphram that will clog easily with rubber fragments, dust or dried fuel residue. I have removed pieces of fuel debris that looked like celophane. After I totally cleaned the carbs, they all worked fine.... except for the Poulan. That's still a mystery. Whatever you do to the machine, I guarantee you will leartn something. WWQ
05-02-2009, 04:33 PM #14
Sounds like the timing is too far advanced. Is it possible to do that on that engine?
I have an almost NOS early Homelite XL-12 that drove me nuts for a few years. Periodically I'd work on it then I'd give up on it. Fooled some other good mechanics too. Several sets of diaphragms and gaskets, soaks in carb cleaner, tank cleaning, etc etc etc. It would start but not idle right, then would Idle OK but no power, then would shut off, then would refuse to start, then would start but not idle, would kick back, etc etc etc with occasional moments of being almost OK. There are 2 very fine screens in the carb that I never removed because I could not get replacements. I finally got a kit from this guy on ebay that had the screens - http://stores.shop.ebay.com/RANDYS-E...__W0QQ_armrsZ1 . Randy is very knowlegeable and helpful on the phone too. I pulled the screens and sure enough there was debris behind those screens that would move around and cause trouble. In hindsite it's now obvious, but EVERYONE who looked at it said "there can't be anything in there now". There are no mechanical devices that won't eventually give up their secrets, it's the relentless application of logic that prevails. Look again at what you are sure is OK.
05-02-2009, 05:25 PM #15
APD, it sounds like some way the crankcase is filling with fuel and when you clear the cyl and install the plug it pumps fuel back into the chamber and locks up. You will have to pull the carb and reed assy. and dump the fuel out of the crankcase.
I have had that happen but it's rare and in your case I don't know what is happening. I would have to see it to really know what to do. You could have the carb diaphram needle set to rich or assembled wrong. Did you get the little lever on the needle valve right? Did you adjust it correctly.
05-02-2009, 10:09 PM #16
It's not hydrolocking. The pull string may be hard to pull sometimes and I can't tell you what the problem is, but I can tell you that it's not from hydrolock.
Think about it... How much fuel would it take to hydrolock that engine? About half a tank. Is it sucking down half a tank after a minute of running? Not only that, but since it's a two cycle, you can't hydrolock it by taking in fuel through the carb. If you're chugging that much fuel into the crankcase, there's no way it would run. Ever. Not for a few minutes... Not ever.
Want one more.... So when the string is hard to pull and you take the plug out, do you get a cup of unburned fuel running out of the spark plug hole when you pull the string? No? Nope, not hydrolock.
I'm no expert, but I suspect that you've got two unrelated problems. First, you've got a fuel delivery problem where you're developing a vacuum on your fuel supply side. It will run fine for a few minutes until the vacuum becomes so great that the engine starves for fuel.
Second, you've got an unrelated problem that makes your pull string hard to pull sometimes. The overly advanced timing possibility is intriguing... I don't know if those chainsaws use a two pole magneto for timing, but the old 2-cycle Lawn Boys did. If it's firing on the high speed pole when it shouldn't be, then it would kick back when you pulled the rope. Maybe it only does this when it's hot?
05-03-2009, 12:36 AM #17
The more I fiddle with this, the more I agree with FTF Engineering. Ive taken it apart at least a dozen times now. Here is the latest:
The fuel pressure/primer bulb issue is fixed. The primer seems to be working as it should now.
However, I cant pull the starter cord at all now. Too much compression. Remove the spark plug and it spins freely. If I turn the saw upside down, I get a drop or two of fuel from the spark plug hole, not enough to lock up the cylinder.
When attempting to start it, I occasionally kicks back, as if trying to run backwards....so maybe it is a timing issue.
What I dont understand is how the timing can be altered without moving the flywheel in relation to the crankshaft. The coil is held on with two bolts, but doesnt adjust.
....It just occurred to me that maybe the flywheel has slipped and is no longer in the correct postion on the crank? Is that possible, I thought they were keyed onto the crankshaft? I'll investigate tomorrow.
05-03-2009, 01:40 AM #18
05-03-2009, 07:43 AM #19
Bingo! Maybe the flywheel key stripped, or in the case of a few 2 cycles, there is no key; just a tit which is part of the flywheel casting and is 'supposed' to align the flywheel to crank. Actually it is a ploy to make you buy a $75.00 flywheel instead of a 75 cent woodruff. (Read Stihl)
05-03-2009, 10:46 AM #20