spark, gas, compression, but still won't start, what next??

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cityboy

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Got the cylinder plug for my 357xp to replace the auto decomp valve, and the compression seems good. I know, I need to do a compression test to be sure, but it feels like the compression is good and I can suspend the saw (with bar attached) by the starter rope and it holds. I don't have a compression gauge, so that's why I don't have a reading, but I'll get it measured as soon as I can. Assuming that checks out OK, what else should I try next? I have spark, and after repeated cranking the plug is moist with fuel, so I think I have spark, fuel, and compression, but it won't even sputter, nothing at all.

I thought about pulling the carb and doing a rebuild, since that's fairly easy to do but I'm at a loss as to why this saw ran fine one day, then two weeks later wouldn't even sputter.
 
I would drain out all the fuel, and pull until the cylinder dries out, then do not refuel, but prime a little to see if it hits then dies, then
you will know it is just flooded or the gas was bad.
If still nothing, look at the flywheel key.
 
I had a saw like that recently; the problem was the gas cap gasket. Years of oxygenated gasoline had rotten the rubber just enough that the tank wouldn't pressurize. I put a new O-ring on it and the saw started right up and runs better than it has in years.

I know it's a long shot, especially since you said it's getting gas. But if your saw has a vented gas cap make sure the vent is open, and if it has a pressure cap make sure that it seals.
 
Got the cylinder plug for my 357xp to replace the auto decomp valve, and the compression seems good. I know, I need to do a compression test to be sure, but it feels like the compression is good and I can suspend the saw (with bar attached) by the starter rope and it holds. I don't have a compression gauge, so that's why I don't have a reading, but I'll get it measured as soon as I can. Assuming that checks out OK, what else should I try next? I have spark, and after repeated cranking the plug is moist with fuel, so I think I have spark, fuel, and compression, but it won't even sputter, nothing at all.

I thought about pulling the carb and doing a rebuild, since that's fairly easy to do but I'm at a loss as to why this saw ran fine one day, then two weeks later wouldn't even sputter.

Have you tried putting a little mix right in the spark plug hole? The fact that your plug is a little moist suggests some mix is getting through, but its possible its not enough. If you squirt a little fuel into the plug hole & then get a sputter when you try to start it, then you know its a fuel delivery problem & you can isolate the issue to either fuel getting to, or from, the carburetor.
 
Change the plug first!
Then change it again, they are cheap!

If you have fuel, compression and ignition.......at the right time......it has to run, it has no option!
As Fish said check the flywheel key and the air gap between the coil and magnet if the above doesn't solve the problem.
I have seen both of those produce a good spark when the plug wasn't installed and NOTHING when you put the plug back in.


Mike
 
I just got my craigslsit $75 346XP running by pulling the plug and letting the flooded chamber dry out and regapping the plug. I got spark on my plug too, but the gas was set to double the factory spec...tightened it up, some fresh fuel and it fired on the third pull.

Seems overly obvious I know, but if I can get this lucky, anyone can!
 
Got the cylinder plug for my 357xp to replace the auto decomp valve, and the compression seems good. I know, I need to do a compression test to be sure, but it feels like the compression is good and I can suspend the saw (with bar attached) by the starter rope and it holds. I don't have a compression gauge, so that's why I don't have a reading, but I'll get it measured as soon as I can. Assuming that checks out OK, what else should I try next? I have spark, and after repeated cranking the plug is moist with fuel, so I think I have spark, fuel, and compression, but it won't even sputter, nothing at all.

I thought about pulling the carb and doing a rebuild, since that's fairly easy to do but I'm at a loss as to why this saw ran fine one day, then two weeks later wouldn't even sputter.

You say you have spark, a fuel moist plug then you say you think you have spark. I'd suggest you try a known good spark plug set to the correct gap.
 
Remove the muffler and look at the piston. I had the same thing happen to one of my saws. It felt like it had compression with the plug in it but it didn't have enough. The exhaust side of the piston was scored.
 
Change the plug first!
Then change it again, they are cheap!

If you have fuel, compression and ignition.......at the right time......it has to run, it has no option!
As Fish said check the flywheel key and the air gap between the coil and magnet if the above doesn't solve the problem.
I have seen both of those produce a good spark when the plug wasn't installed and NOTHING when you put the plug back in.


Mike


I haven't checked the ignition timing, and if its off that could explain what I'm seeing. I'm totally new to chainsaw repair, so I need a little coaching. Is the flywheel key a fixed woodruff key type of arrangement, where it would have had to physically break or fall out to change the timing, or is there something else that's adjustable that I need to check for proper alignment?

I'm pretty sure the air gap is correct, but I need to get a non-magnetic 0.3mm gauge to make sure. I used a piece of cardboard and micrometer, and it seemed to be just about right. Or, can I just put a regular metal feeler gauge in there?

Regarding some of the other suggestions:

Fuel tank pressurization:

I'm not sure exactly how to check this, but have done this so far: Removed gas cap, put it back in. Removed fuel supply line from carb, no gas shot out. Blew into supply line, waited a second, fuel then flowed out of line. I think this showed two things, one that the fuel filter was not plugged, two that the tank held pressure, but I don't know if this tells me anything or not. Any other tests I should do?

Fuel delivery:

I don't know if the impulse line is working properly or not, but I'm not sure exactly how to check that. I think that works off of crankcase vacuum to suck fuel into the carb diaphragm, is that correct? If I'm seeing fuel on the plug can that problem be ruled out?


Manually priming fuel:

I would like to do this, but I'm not sure how much fuel to squirt into the cylinder. Too much would probably flood the plug, too little may not ignite. How do you know if you've put the right amount in?


Pulling muffler:

I know this is easy, but I have one question. What's the chance that 8-year old bolts will just snap off in the cylinder when I try to remove them? I really don't suspect compression since the saw ran fine last time, and I have no reason to believe the piston/cylinder was damaged during the last usage. I don't want to create a new problem for no reason, so I've avoided this step for the time being. If this is zero-risk, and I can reuse the old gasket, then I guess I have nothing to lose. If I need a new gasket then I have to go pick one up. I also have to go buy a long allen wrench socket since the one I have isn't long enough to reach the bolt heads.

New plug:

I'm going to do this because its so simple, but in 35 years of experience with cars and motorcycles I've never seen a bad plug yet, but I guess there's always a first time.
 
Have you ever had a air cooled engine with a spark plug that would not fire ?

No, but I haven't owned much in the way of OPE until the last few years, maybe they're different.

Prior to that the last air cooled engine would have been a late-'80s Yamaha motorcycle. Rode all my motorcycles regularly, year-round, and maintained them properly so I never had to troubleshoot them much.
 
If the flywheel key was snapped and timing was off, you would still get some backfiring, sometimes thru the carb. No POPs at all, check plug, plug wire, etc... while the plug is off, squirt about 1/2 teaspoon of fuel mix in there and see if that makes the saw POP a couple times. Do you have an in-line spark plug tester, so you can test the spark without pulling the plug?
 
If the flywheel key was snapped and timing was off, you would still get some backfiring, sometimes thru the carb. No POPs at all, check plug, plug wire, etc... while the plug is off, squirt about 1/2 teaspoon of fuel mix in there and see if that makes the saw POP a couple times. Do you have an in-line spark plug tester, so you can test the spark without pulling the plug?

Unfortunately I don't have anything in the way of test equipment for engine diagnostics. I have the inclination to fix this thing myself, but don't feel like spending much money on stuff that I'm not going to use more than once every few years, if that often. I finally found a local guy that has a good reputation for fixing small engines, and he told me for $20 he'd diagnose the problem and then its up to me to decide if I want to fix it myself or have him do it. I can't even buy a compression gauge for that price, so if a plug swap and fresh fuel doesn't fix the problem its going in to him on Monday AM.

I'm also going to look at the flywheel key just to rule that out, but I can't imagine how that could have sheared off without subjecting the saw to some very abusive treatment.

I may still have a weak coil, so I'm going to try one of those cheap in-line testers from Harbor Freight, but I'm not sure how good those will work for $4. If that doesn't detect any problems I'll just let him figure it out on Monday.
 
You do not need all the tools. checking spark is easy, first pull the plug from the cylinder then plug the spark plug back into the wire lean the base of the spark plug against the cylinder or one of the fins so u can see the diode, pull the cord and look for bright blue spark and listen for crackle. If this is impossible then grab the spark plug with bare hand and pull the cord, no crying because it only hurts for a second and u saved some money. If the spark is yellow or orange or u did not pee yourself then it is not getting enough spark and your problem will lie in the electrical side of the saw. on the gas side open up your needles about 2 turns pull choke and set throttle lock, if after a few pulls gas is not visibly leaking out of the muffler you have a fuel flow problem, start from the tank and work your way to the carb. This is not rocket science just seems that way until u get your hands dirty a few times.
 
If you have a lighter handly for a flooded cylinder, just take the plug out and light the cylinder ........... dries out real fast. Had my nephew running the 361 and at first he couldn't get the hang of pulling the cord fast enough to spin it and start, anyways, he flooded about 3 saws in short order. I happened to have BBQ grill lighter, or one of those long skinny ones, handy, and I just stuck it through the enlarged side port from modding the muffler and light the gas inside the cylinder, it didn't really do anything exciting except burn for a few seconds. I fired them up right after that without any problems.
Oh, and reset your L and H screws to 3/4 out or whatever settings is correct for your saw. If you just got the saw or something and not sure they were set correctly before, this causes issues, as well as, the aforementioned advice.
Sam
 
Remove the muffler and look at the piston. I had the same thing happen to one of my saws. It felt like it had compression with the plug in it but it didn't have enough. The exhaust side of the piston was scored.

I too had that happen to me on a Jred 670 Champ. Saw would hang by cord, pulled over like it had comp. Changed coil, plug, rebuilt carb., pulled hair, talked ugly, pulled muff no visible score on piston. Finally did comp. check 75 lb. Pulled cyl. was scored aroud the corner where I could'nt see it through the ex. port.
Shep
 
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