576xp air leak, carburetor??

[Lanedrew]

Have a 576xp that I rebuilt last year (non AT), new crank seals, case gasket, base gasket, you name it it’s been replaced. Reassembled saw with the gaskets and a little bit of yamabond, needless to say saw ran great the first time I ran it for about half a day. Sometime later I go to use it again, and runs awesomely from 1/4 to full throttle, doesn’t miss a beat in the cut or anything. Problem is sometimes (not all the time) I’ll left off the throttle to let it idle, and it will either idle perfectly, or idle for about 5 seconds then start to race like it’s running lean, blip the throttle a few times and all is well, makes no sense to me. Once that started I sprayed brake clean at the carb where it meets the intake while the saw was running, and it stalled. So took intake manifold out and found some slight cracking, figured I’d replace it and that would fix my issue. Replaced intake manifold and still same issue, only wants to lean out occasionally and still stalls or try’s to stall when I spray the carb, carb has been rebuilt, could the carb itself have a leak somewhere causing it to suck extra air at idle? Ive worked on a many of saws and haven’t had one with this issue yet, I’ve always been able to solve an air leak problem, thank you much!

 

[TLG]

How did you replace the PTO side crank seal. The seal is an integral part of the bearing??

 

[Lanedrew]

How did you replace the PTO side crank seal. The seal is an integral part of the bearing??

My bad should’ve worded that different. I changed both crank bearings as well, yes the pto side seal is part of the bearing, flywheel side is the separate bearing and seal

 

[TLG]

I find the 570,575, and 576 very challenging to pressure/vac test. Are you pulling through the impulse port?

 

[mbrick]

Did you pressure/vac test it after the rebuild?

I made a cover for the exhaust and two separate covers for the intake and tested through the impulse port. It was a touch hokey but helped me find a leaky stator side crank seal. You also need to block the oil pump holes.
Test setup worked fine and held without the needle moving once I changed the leaky crank seal. Lightly greased before installing these two small "plates" on the intake side.

Not the best photo but gives you an idea.

 

[Old2stroke]

Throttle plate shaft badly worn and sometimes comes to rest in a position that leaks a lot of air and blipping the throttle resets it to a better position?

 

[mbrick]

Interesting theory, I do have spare carbs for this saw. The OEM carb is $$$.

 

[Lanedrew]

Did you pressure/vac test it after the rebuild?

I made a cover for the exhaust and two separate covers for the intake and tested through the impulse port. It was a touch hokey but helped me find a leaky stator side crank seal. You also need to block the oil pump holes.
Test setup worked fine and held without the needle moving once I changed the leaky crank seal. Lightly greased before installing these two small "plates" on the intake side.

Not the best photo but gives you an idea.

Have not done any pressure testing yet, waiting for my new tester to come in. But to me if it were a major air leak a blip of the throttle wouldn’t settle it down, I’ll blip the throttle and it will idle for 10+ minutes no problem, won’t lean out and stall or anything, I’d just chance ordering a new carb if they weren’t $170

 

[Woodslasher]

Have a 576xp that I rebuilt last year (non AT), new crank seals, case gasket, base gasket, you name it it’s been replaced. Reassembled saw with the gaskets and a little bit of yamabond, needless to say saw ran great the first time I ran it for about half a day. Sometime later I go to use it again, and runs awesomely from 1/4 to full throttle, doesn’t miss a beat in the cut or anything. Problem is sometimes (not all the time) I’ll left off the throttle to let it idle, and it will either idle perfectly, or idle for about 5 seconds then start to race like it’s running lean, blip the throttle a few times and all is well, makes no sense to me. Once that started I sprayed brake clean at the carb where it meets the intake while the saw was running, and it stalled. So took intake manifold out and found some slight cracking, figured I’d replace it and that would fix my issue. Replaced intake manifold and still same issue, only wants to lean out occasionally and still stalls or try’s to stall when I spray the carb, carb has been rebuilt, could the carb itself have a leak somewhere causing it to suck extra air at idle? Ive worked on a many of saws and haven’t had one with this issue yet, I’ve always been able to solve an air leak problem, thank you much!

I'd see if you could tighten the carb cover plate screws a bit more, personally, and once the tester shows up check the carb. I've encountered a few saws that had leaks between the carb body and a paper gasket, swapping in a cover from a junker carb of the same model or lightly smearing sealant on the gasket would fix that for me.

 

[TLG]

Isolate the carburetor. Pressure test carb submerged in solvent through inlet port. Check for bubbles, leaks.

 

[Lanedrew]

Isolate the carburetor. Pressure test carb submerged in solvent through inlet port. Check for bubbles, leaks.
I believe that is what I will have to do

 

[TLG]

I'd see if you could tighten the carb cover plate screws a bit more, personally, and once the tester shows up check the carb. I've encountered a few saws that had leaks between the carb body and a paper gasket, swapping in a cover from a junker carb of the same model or lightly smearing sealant on the gasket would fix that for me.

This carb is oring sealed to the intake. I might suggest a light grease film. It will pinpoint a poor parallel surface.

 

[mbrick]

I'd see if you could tighten the carb cover plate screws a bit more, personally, and once the tester shows up check the carb. I've encountered a few saws that had leaks between the carb body and a paper gasket, swapping in a cover from a junker carb of the same model or lightly smearing sealant on the gasket would fix that for me.

The intake manifold on this saw does not have a paper gasket . It's a rubber gasket on the cylinder side and on the carb side.

I would not hesitate to try some proper grease like Molykote or Krytox to see if that helped, though.

 

[Woodslasher]

This carb is oring sealed to the intake. I might suggest a light grease film. It will pinpoint a poor parallel surface.

My post was misunderstood, I see. I meant on the carb itself I've found leaks from around the paper gaskets that go between the carb body, diaphragm material, and the carb cover plates. Either the cover plate or the carb body was warped a wee bit and the paper gasket wasn't thick enough to properly seal the gap, so air would intermittently leak and it drove me nuts. I was not referring to any other part of the intake system (ie the intake boot, filter horn, etc.)

 

[TLG]

My post was misunderstood, I see. I meant on the carb itself I've found leaks from around the paper gaskets that go between the carb body, diaphragm material, and the carb cover plates. Either the cover plate or the carb body was warped a wee bit and the paper gasket wasn't thick enough to properly seal the gap, so air would intermittently leak and it drove me nuts. I was not referring to any other part of the intake system (ie the intake boot, filter horn, etc.)

NP, @Woodslasher, Any input gives a different approach to diagnosing. Are you the sole proprietor of Jack Nut Saw Shop?? Love that gigs name sir. Are you practicing any climbing??

 

[Woodslasher]

NP, @Woodslasher, Any input gives a different approach to diagnosing. Are you the sole proprietor of Jack Nut Saw Shop?? Love that gigs name sir. Are you practicing any climbing??

Jack Nut Saw Shop is mostly fictional, I do small engine work for folks as a kinda side gig and some prick on O P E was trolling my threads calling me Jack Nut so I decided to name my fictional "business". I'd love to practice climbing and I have several people who I can nag about climbing but I've only used gear to climb a tree twice. That being said, I've climbed 60-90' up in trees freehand for fun but all the climbers I've talked to have advised against it as a career. I'm tempted to pick up the set of spikes Rarefish has listed and borrow a neighbor's harness and ropes so I can practice on my own, though.