576xp won't start, bad coil?

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The diaphragm is responsible for fuel delivery , if your saying that your getting too much fuel then this is probably the culprit. If I were you I would order up a rebuild kit for your carb and start there. Something is missed but i can’t pin it, unless someone else has an idea.
 
If there is a lot of spray back from the carb, that is suspicious. Is there a chance the saw had a bad porting job done before getting rid of it? Maybe the wrong piston installed?
I agree it didn't seem correct, which is why I made a note incase that raised a red flag for anyone. It also 'popped' and kind of backfired through the intake and died a few times while I was adjusting the carb screws.

The cylinder is OEM and looks untouched and not ported to me. From the photos I see it is the correct cylinder and not the different 575xp for example.

Is it possible the wrong piston is in it? I was also wondering that yesterday.
I think there are different height but same bore pistons for a 575 / 576. I do not have experience with the two to know if they are visually identifiable but it looks correct from a few photos I could find. The arrow is pointing toward the exhaust.

Brent, I first started with a genuine Zama rebuild kit and then switched back to the original parts to rule that out as a factor.

Photos of the piston / cylinder when I first disassembled the dirty saw:
IMG_20190331_112809.jpg IMG_20190331_112835.jpg
 
That black on the piston tells me your leaking compression may need a new piston/rings? How’s the clyinder look above the exhaust port?
 
That black on the piston tells me your leaking compression may need a new piston/rings? How’s the clyinder look above the exhaust port?
I think you have worked on a lot of these, does it look like the correct 576 piston to you?

I considered new rings, OK I can order Caber rings and try that. Do you think leaking compression could be responsible for how it is running?
I can still see machining marks on the piston skirts which tells me it hasn't worn much.

I don't have a photo right now of the exhaust side but the cylinder looks the same everywhere, and it wasn't ported at the exh. I also gave it a quick pass with scotchbrite.

Check the spark arrestor in the muffler, a plugged screen mimics your symptoms.
Ok, I will try running it without the screen to verify. I did clean it and it didn't look clogged.
 
With the kind of compession you are getting the rings are probably good but did you check the end gaps? If rings and end gaps are good, the only explanation for excessive blow back through the carb is a loose fitting piston skirt (did you mic the diameter and compare it to the bore?) or an intake port that is being closed too late in the downstroke either because the intake port has been lowered, the piston skirt has been raised or a piston with the wrong skirt profile has been used. I'm thinking, with all the tests you have done and things you have tried, the damn thing should be running fine unless there is some basic issue that's lurking in there. This is getting interesting, nothing better then a saw that won't run right.
 
Went back to look at the pics of the carb. There are 2 check valves in that one, the brass one is probably to supply fuel to the idle circuit and the other one is pressed into the depression and supplies fuel to the main jet. These check valves operate on a little rubber disc that can stick either open or closed due to distortion caused by alcohol fuel or sometimes can go missing because the carb was cleaned by high pressur air. If these valves aren't operating right, it can have serious effects on whole running of the saw and unfortunately are never included in a carb kit. Replacing the carb is often the only way go.
 
Old2stroke, I actually just learned about those check valves this morning and think that may be the issue. I looked at the Zama exploded diagram a few times but they were not shown.
Pretty good suggestion, thanks.

I already have a new carb on the way and will provide an update once it arrives and I test it.

I'll take apart the old carb if that turns out to be the fix and post photos since I've never seen photos of them.
 
If you want to remove those check valves, just use a 2-56 machine screw tap to thread the hole in the brass one, then insert a screw and use the screw to pull it out. The one in the angled depression is the nozzle check valve and can just be punched right through into the carb throat. Pay attention to how deep it is in the cavity before removing in case you want to put it back in. The brass valve pretty much gets destroyed in the removal process. With a walbro carb the other valve has a rubber disc that can be made and replaced but I believe the zama carb uses a hard fiber disc and probably can't be replaced.
 
Went back to look at the pics of the carb. There are 2 check valves in that one, the brass one is probably to supply fuel to the idle circuit and the other one is pressed into the depression and supplies fuel to the main jet. These check valves operate on a little rubber disc that can stick either open or closed due to distortion caused by alcohol fuel or sometimes can go missing because the carb was cleaned by high pressur air. If these valves aren't operating right, it can have serious effects on whole running of the saw and unfortunately are never included in a carb kit. Replacing the carb is often the only way go.

I suggested this ages ago. The way you test those is to first wet it with some gas, then put a soft hose over it so it seals and pull vacuum to about -.5 and it should hold for at least 4-5 seconds. If not *that's* (one of) your problems. That's the hi speed circuit and if it leaks the thing won't idle right.
 
If you want to remove those check valves, just use a 2-56 machine screw tap to thread the hole in the brass one, then insert a screw and use the screw to pull it out. The one in the angled depression is the nozzle check valve and can just be punched right through into the carb throat. Pay attention to how deep it is in the cavity before removing in case you want to put it back in. The brass valve pretty much gets destroyed in the removal process. With a walbro carb the other valve has a rubber disc that can be made and replaced but I believe the zama carb uses a hard fiber disc and probably can't be replaced.

I suggested this ages ago. The way you test those is to first wet it with some gas, then put a soft hose over it so it seals and pull vacuum to about -.5 and it should hold for at least 4-5 seconds. If not *that's* (one of) your problems. That's the hi speed circuit and if it leaks the thing won't idle right.

Your posts are being read and helping more than the OP
I have been thinking my problem is very similar the main difference is my H 359 sat more than 10 years with a scored P/C had the plastic intake clamp.. I haven''t tested these valves yet but will test them before I try an ultra sonic cleaner. I was going to just buy a cheap carb to help isolate my problem but I will learn more by being stubborn. Thanks
 
Ok, received the new carburetor and it runs exactly the same. Crap. Barely idles (have the screw in all the way to get it to idle). Spits fuel back when opening the throttle. It also seems to get hot pretty quick compared to my other saws?

I visually compared the spark to my 550xp that runs very well, and it looks the same.

On the old carb I pressed out the hi speed nozzle and inspected and tested the check valve and it works correctly. I did more reading and looked at the parts diagram and photos of the welch caps removed and I do not think there is a low speed check valve on this specific carb. Regardless, the new carb behaves the same as the old one.

So, what should I do next? Remove the cylinder and check the skirt clearance? Either new rings or new piston + rings?

Video below with the new carb:
 
It doesn't seem inconceivable that your rings could be bad. Since the ring ends are on the intake side a high end gap might be pushing the gas from combustion back out the intake. I'd do like Old2Stroke tacitly suggested and mic the skirts against the bore and check your ring end gaps.
 
Ok, I tore it down. Piston arrow was facing the exhaust. Let me know if you think I need to measure with a mic and I can borrow one from work, my Mitutoyo mic is a 0-1". Used Mitutoyo calipers.

Top ring gap: .017" (same throughout the bore)
Bottom ring gap: .013"
Service manual says spec is 1.00mm (.040") but that seems like a football field.

Piston: Front to back: 50.66mm at the top, 50.79 below the ring land, 50.86 at the bottom of skirt.
Side to side: 50.82 in all locations
No specs in service manual.
Bottom of the piston has a lot of burnt on carbon.
I'm reading .002" piston clearance when new so .006" at the skirt might be a lot?

Bore: 51.00mm front to back and side to side
Porting looks all stock on exh and int. Can't feel any marks with my fingernail (lighting makes it look worse). No damage above the exh as someone asked earlier.

I don't see a meteor piston for the 576, only 575 and they are different. Funny enough this P&C says both Husqvarna and Meteor so I guess they are the mfg for OEM.

What do you think?
 

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You have been very thorough but sometimes a mechanical object can be a cruel mistress.

Early on you mentioned a bit of runout in the flywheel. Are you sure the key hasn’t been sheared? That or an air leak would explain the saw getting hot.
 
Wow we have literally danced around all areas of this machine!!
Does anyone have the actual correct resistance values for this coil?

As for ring end gap I’m sorry I just don’t have any experience in this area and cannot comment
 
You have been very thorough but sometimes a mechanical object can be a cruel mistress.

Early on you mentioned a bit of runout in the flywheel. Are you sure the key hasn’t been sheared? That or an air leak would explain the saw getting hot.
I can't remove it right now with the P&C removed but the key is cast in to the flywheel and was in-tact when I re-assembled. The trailing magnet is approx at the middle of the coil at TDC which is a rough check from what I read. I noticed the runout when setting the coil gap, I think it is roughly around 0.05-0.075mm between the magnet side and non magnet side but I can measure it more precisely if needed.
 
While i have not tore down a 576xp before, i find it odd that the piston is wider at the bottom (skirt) than at the top. It seems to me the top is too loose (too far from 51mm). The one side of the piston in the pic looks like the oiler rings have rubbed off so uneven wear from the other side. It seems to me like its too loose in the cylinder. Without even using a mic, if you insert the skirt side first into the cylinder, does it seem very loose? Does the crank bearing and wrist pin bearing look and feel fine?

As for the ignition, as long as you have a business card width between the magnet side and coil, its usually fine, doesn't matter what the gap is too the other side of the flywheel...the plug wire and ground wire are intact and insulation not cracked?
 
Its probably an illusion, but the flywheel looks like its spinning the wrong way in the video?
 
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