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Saw goes lean as it heats up

Pioneer

Pioneer

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Had a saw exhibit some weird behavior today, A Husqvarna 50 I have would go dangerously lean as it heated up, so much you would have to re-tune it drastically in both high and low speed. It starts easily when cold, the screws set exactly where they should be, and you can move it in all positions without an idle speed change. Within about 5-10 minutes of use it goes completely out of tune. Venting problems? fuel starvation related to heat somehow? Never had a saw do this before. +30C conditions today, so not crazy hot out. Carb mounted good and tight. My normal mix using premium, no E-10. No problems with another saw today using the same mix.
 
MaddBomber

MaddBomber

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I've never had a vaporization issue, but it definitely happens.
Definitely check compression when it gets up to operating temp (hot).
Also, after it acts up, slowly open the fuel cap. Listen for suction. Could be the fuel tank vent is clogged.
I've also had carb adjustment screws (needles) move on me. Eventually replacing the tension springs cured it.
Perhaps a thorough carb cleaning and throwing a rebuild kit in it is worth the effort.
If it runs fine cold, then loses power and gets wonky once warm, I suspect compression. I'd start there. Maybe pull the muffler and visually inspect your piston rings.
Keep us posted.
 
Pioneer

Pioneer

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New ring in it, compression is 160 cold, will check when hot. So far I suspect the vent, and perhaps the air filter is too close to the air horn and is slowly retracting as it warms up. The 50 uses kind of a poor air filter setup that doubles as the choke and can cause a richer mixture if it is partially collapsed.
Always starts second or third pull cold, no problem with it pumping fuel, at least at that point.
Will eliminate one possibility at a time, the usual thing. Start with the simplest and dig deeper if needed.
 
MaddBomber

MaddBomber

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Will eliminate one possibility at a time, the usual thing. Start with the simplest and dig deeper if needed.
Yup, fully agree.
I almost split a case once to fix seals.... I replaced almost everything looking for an airleak. Turns out the spark plug was loose.
Pitiful! Lol.
 

Okie

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I would go through the fuel system first, after looking at the piston for damage.
Could just be a fuel line with some cracks.
Right:
It's probably just the opposite of what you are thinking. You are thinking that it's goes lean as the engine warms up.
It's probably always running lean which causes the block to overheat as the engine is loaded. Fuel hitting the piston is what keeps a chainsaw block Jug/piston from overheating. Little too much air and not enough fuel hitting the piston causes overheating sometimes slow and sometimes fast heat build-up according to how lean and how much the engine is loaded into a cut. Do a pressure vac/test to see if the seals are ok. Take a gander at the piston through the exhaust port so as to see if the piston skirt shows signs of overheating.
Read this thread: post #7 also
Make sure the exhaust is not restricted and the cooling fins are clean.

 
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outdoortype

outdoortype

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Do you notice a weird smell when it starts to act up? I say this because I had a 51 that would go lean after heating up. It wouldn't four stroke when it got hot. I think the case was leaking/expanding and I was smelling burning bar oil. FWIW, I replaced the tank vent, impulse gasket , intake donut, etc. to no avail.
 
hector

hector

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I am not a saw expert or mechanic but I fix my lawn care equipment. a few weeks ago I repaired a friend a blower that had the same problem. started from the first. but when it warmed up the problems started. the problem was the intake manifold, I rectified it because it was a little bent and did not seal well. and I put threebond. sand paper 180, water and flat surface. it is sanded in a circular way, and the piece must be rotated every 5 to 10 passes. everything in a circular way so that it lowers evenly if you do it by hand
 
Pioneer

Pioneer

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Tank vent. The porous insert was plugged, I could barely force air through it. Had some duckbills, one fit perfect in the vent housing. A long fish hook remover is ideal for grabbing the vent inside the tank and pushing it out, makes a difficult job fairly easy.
I'm pretty picky about saw tuning and noticed it going lean right away. Stopped it and retired it for the day when tuning failed to correct the leaning out tendency. This saved it from burning up, no damage, compression still good.
If I have had this problem before, I would have known just to crack the gas cap to verify it. The giveaway is the amount of time it took. Previously I had only done short cutting cycles to break the new ring in, that's why the problem never manifested itself. It took a while to create a vacuum in the tank as the fuel burned off.
 

Okie

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A hint (about not venting properly)I keep a ear open for such is if the tank is completely full they will vac lock quicker vs a tank that is low on fuel.
 
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