Lean 372 help please

[gwat]

One of my 372 Xtorq saws has been acting up.
It seems to run lean, once its warm it hangs for a long time (20-30+ seconds) at high idle with chain spinning.

It is not the tank vent, cracking the gas cap does not fix it.
Thought it just needed a carb rebuild, but that had no effect.
Replaced the air filter, spark plug, fuel filter and fuel line, no change.
I swapped a carb from a good saw, no change.

At this point figured I must have an air leak but was surprised to see that the saw holds pressure and vacuum!
It does leak from the decompression valve threads, but I've heard this is not an issue?

What am I missing here? Any ideas why this thing is acting up?

 

[1hp]

metering valve adjustment?

 

[Czed]

One of my 372 Xtorq saws has been acting up.
It seems to run lean, once its warm it hangs for a long time (20-30+ seconds) at high idle with chain spinning.

It is not the tank vent, cracking the gas cap does not fix it.
Thought it just needed a carb rebuild, but that had no effect.
Replaced the air filter, spark plug, fuel filter and fuel line, no change.
I swapped a carb from a good saw, no change.

At this point figured I must have an air leak but was surprised to see that the saw holds pressure and vacuum!
It does leak from the decompression valve threads, but I've heard this is not an issue?

What am I missing here? Any ideas why this thing is acting up?

Check the rubber intake boot and impulse and fuel line's for cracks.

 

[Maintenance supervisor]

Transfer covers?

 

[mbrick]

What kind of pressure and vacuum does it hold? No change at all after 10 minutes?

 

[gwat]

metering valve adjustment?

I don’t have a gauge (yet) but I’ll check that. I did swap a carb on from another saw that was running fine..

What kind of pressure and vacuum does it hold? No change at all after 10 minutes?

I went to 7psi both ways.
The decompression valve threads leaked, I don’t have a proper plug so was holding my finger over the port and only tested for about 2 minutes each test. As far as I could tell the pressures held.
Is 2 minutes too short of a test to rule out a leak?

 

[mbrick]

I would think the issue you are describing should be a fairly gross leak that will fail in 2 min. But a small leak could have been masked by fiddling with your finger, etc.

The decomp leak is right into the cylinder though so it ould be the culprit?

I would replace the decomp valve with a plug or pull the valve and add a little threadlocker or PTFE pipe dope on the threads and re-vac test and make sure it will hold for 10 min with no change.

 

[John Stryker]

I would inspect the fuel line and filter. Sometimes the line can get kinked under the airbox floor also.

 

[gwat]

I would think the issue you are describing should be a fairly gross leak that will fail in 2 min. But a small leak could have been masked by fiddling with your finger, etc.

The decomp leak is right into the cylinder though so it ould be the culprit?

I would replace the decomp valve with a plug or pull the valve and add a little threadlocker or PTFE pipe dope on the threads and re-vac test and make sure it will hold for 10 min with no change.

Ya I was expecting to find a major leak, as you said. I’ll track down a plug and do a proper test.

 

[Backstage]

vacuum grease on the intake/carb boot can help. Sometimes it leaks there and depending on your method, vac test wont show it. .

 

[BuckthornBonnie]

Grab that crank and wiggle.

Visually inspect the throttle cable and linkage to make sure it’s engaging and releasing properly. Plug the decomp and redo your test. The plugs are available online and are handy to have around.

 

[huskihl]

A leaking decomp won’t cause issues like that. It doesn’t really cause any issues, other than a little black residue running down the side of the engine

 

[Lightning Performance]

Had a seal last spring that drove me nuts on an MS361. It tested good but ran terribly down low. When they get work hardened rubber it doesn't always show up during testing. Most times it's the PTO side but this one was shot on the flywheel side. Guess I missed it when the PTO side was done a few years back. All the carb settings were whacked on a carb swap to a good runner. The muffler mod made it worse imho.

Factory carb settings came right back vs [email protected] and the idol went down a full turn. The H was wondering a bit when it got to the break-in runs a Dewaynes this past spring. It was a simple port job and piston update with a second muffler exit. Hot harden rubber doesn't seal very well. It still past both vacuum and pressure testing before I ripped it out to be replaced.

 

[mbrick]

Good tip Lightning. If the seals are old (and maybe crappy) it may be worth replacing the rubber bits. Amazing how a new fuel line vs a 15 year old one feels for example.

 

[gwat]

Hot harden rubber doesn't seal very well. It still past both vacuum and pressure testing before I ripped it out to be replaced.

Dang that can make things frustrating.
Got my hands on a decomp plug and the saw holds pressure/vacuum well.
I’ll run it with the plug and see if it improves, but like you say huskhil I don’t think a leaking decomp should be a problem.

Now onto specimen #2 (three of my 372s are on the bench ). Major crank seal leak on the clutch side, it will not build pressure at all. No obvious wiggle in the crank….would I be crazy to simply replace the seal without cracking the saw open and doing the bearings?
How common it it be for seals to be bad but bearings to still be good?

 

[mbrick]

Replacing seals without splitting the case is common, for example Stihl makes a deal puller tool. I've done this on a few saws.

If the crank bearings feel good I would not hesitate to replace the seal only.
Be very careful to not scratch the case and bore when removing the old seal.

 

[gwat]

Thanks for the confidence. I’ll start with replacing just the seals. If they go again I’ll do a full tear down over the winter.

Seals are cheap…seal pullers not so much. Husky makes the tools - MSRP ~$50 each side .
Can anyone recommend/justify spending the $$ on the specialized pullers rather than rigging up a homemade puller (I’ve seen screwdrivers ground down to pull the seals).
I have four 372s in my fleet, all 4-9 years old so I will likely have more seal jobs in my future..

 

[mbrick]

I made a homemade puller to slip past the lip and slowly go around and work it out. It went well, no damage.
I'd say it depends on your handiness and ability to resist getting out a big hammer and causing damage.

 

[JoeMay]

Lisle makes seal puller that is small enough to work on chainsaws. I’ve used it successfully several times. It’s available at better auto parts stores. It works on either side of the case and it’s definitely less than 50 bucks. Then there is the old school method of carefully drilling a small hole into the metal perimeter of the seal, spinning a small sheet metal screw into the hole enough for one to grab the screw’s head with stout pliers and then pull the seal out.

 

[mbrick]

I've used the sheet metal screw method very successfully on car front and rear main seals... but I have not yet tried it on a saw. I think the seal was very narrow on the last one I replaced.
Good tips.