Stihl MS 361 Vacuum and Pressure testing - need some help!

[2-StrokeDude]

Hey guys, another batch of questions for y’all…

Almost done with my Stihl MS 361 rebuild, this is the first saw I’ve completely split the cases on and replaced the crankshaft, crank bearings, cranks seals, and piston / cylinder.

This is also the first saw I’ve ever pressure and vacuum tested.

I’ve got a mighty vac MV 8500 hooked up to the saw right now and I’ve got the exhaust port closed off with some bicycle innertube rubber strips and I’ve rigged a cut off rubber chemical glove finger to the cylinder intake port.

Spent a while just getting everything sealed off correctly, first thing the muffler was leaking, so I had to double up the rubber layers, and now that’s holding, then the intake was the biggest pain because some between the carb and the rubber boot was leaking. So I just pulled the carb and the boot off and everything and I’ve just got that rubber glove and a heavy duty rubber band around that to seal it off.

Then I had the decompression valve leaking, so I pulled that out and because I don’t have a plug for it I put an NGK C6HSA spark plug in there which has 10 mm threads and I got off my 1981 Honda C70 Passport. It’s got a weird spark plug design, but came in handy this time.

The manual for the MS 361 shows doing a pressure test first, then a vacuum test. I’ve heard some guys say to do a vacuum test first because it won’t push the seal lips out but I ended up doing the pressure test first because that’s what the manual had in order. Not sure if it really matter a whole lot.

Holds 10 psi of pressure just fine. Had trouble with getting it to hold a vacuum until I got the intake sealed off much better. Now it’s holding 10 psi of negative pressure so I think it’s good.

I’m skeptical in my work with the seals, I did everything I could to make sure they were installed properly. The clutch side seal is the one on more confident in since I was able to use a thin piece of plastic and some grease to get it slid down in there easily. The seal on the flywheel side however, I put in when I didn’t really know what I was doing on that one, but that one’s holding up. That one I installed dry so I hope its good.

So after that long rant, here’s my question to you guys… Do you think the saw is good to go? Like I said, I’m a newbie to doing it complete engine tear down, I’ve repaired plenty of small engines and stuff, but I rarely ever go this far in repairing them.

Also, do I need to take the clutch out and the flywheel out and visually inspect the seals/spray some water or put some oil on them to make sure they’re not leaking? Does everything have to be removed from there for me to properly check them in test them?

Before I did the pressure/VAC test I have the saw running for maybe 10 minutes max. It’s not even broken yet. I had it idling and I held it at different angles and upside down. It does not shut up off or run any different at an angle.

Think it’s OK to run now? I’ve got three sets of seals, so I’m fine if I have to replace them but I’d like to not if possible since it’s a bugger.

 

[J D]

Ok, so you've established the saw holds pressure/vac. Ideally you should rotate the crankshaft while doing the test, reading should flucuate but not change overall. You need to sort out the leaky intake as that will cause you issues. Your blocking rubber should be between the intake boot & the carb so you test the entire intake track. The boot should not leak! You should also sort your leaky decomp. Sometimes they just need cleaning, a bit of lapping compound & a few rotations can work wonders. If that doesn't work then for testing purposes a little bit of grease in it can help it seal but thats not a fix it just makes testing easier if the leak is tiny & stops when you pull on the decomp lightly (ie not significant enough to warrant replacing it)

 

[SteveSr]

Your blocking rubber should be between the intake & the carb so you test the entire intake track.

I don't think so! The carb will leak at the throttle/choke shafts. The blocking rubber should go behind the carb... between the carb and the intake boot.

 

[J D]

I don't think so! The carb will leak at the throttle/choke shafts. The blocking rubber should go behind the carb... between the carb and the intake boot.

Good catch, thanks!
I should've written "intake boot"...
Fixed now

 

[2-StrokeDude]

Ok, so you've established the saw holds pressure/vac. Ideally you should rotate the crankshaft while doing the test, reading should flucuate but not change overall. You need to sort out the leaky intake as that will cause you issues. Your blocking rubber should be between the intake boot & the carb so you test the entire intake track. The boot should not leak! You should also sort your leaky decomp. Sometimes they just need cleaning, a bit of lapping compound & a few rotations can work wonders. If that doesn't work then for testing purposes a little bit of grease in it can help it seal but thats not a fix it just makes testing easier if the leak is tiny & stops when you pull on the decomp lightly (ie not significant enough to warrant replacing it)

While doing the vac test I rotated the crank up and down a little and the needle fluctuated but went back to normal when I was done.

Also, yeah it would be nice to have the carb and the boot hooked up and then block it off and do the test, but I was having too much of an issue with it sealing up for the vac test. Basically it would hold pressure but no vac, and I could hear air leaking in from where the carb was. Seems like it was just an issue with the piece of rubber inner tube I had wedged in between the carb and the intake boot.

The reason I disconnected the carb and boot and used the piece of glove to seal off the cylinder is to make sure that the saw was holding vacuum since I know it was holding pressure. My main concern is not that the saw holds vac/pressure at the carb / boot (which yes I know is important but is easily fixable) My concern is that I installed the crank seals and the case gasket and base gasket correctly because I have never done crank seals before and I don't really trust that I did it right the first time.

 

[hotshot]

Great job on the leak testing. Vacuum unit of measure you tested at was 10”Hg which is roughly 5 psi, but you’re fine if it held there.

Yes, the crank seals should be made visible when soap bubble testing. Since there’s no change in your gauge pressures, then no reason to tear it back down to soap test them.

Some saws, Lawn Boy mowers, & leaf blowers can be a real PITA to P/V test, most require making steel blind off plates & rubber gaskets.

 

[lone wolf]

I always spray it all over with soapy water to make sure it's perfect with zero micro leaks. Seals,plug,decomp,cyl base etc.

 

[Lightning Performance]

I just use rubber flaps cut on an angle to slide in tight. The carbs are a pita to pressure tests so usually don't.
Pto seals are bad on half the stuff out there that got cooked or worse. Goes with fried clutch springs from slipping the clutch and purple drums.

 

[2-StrokeDude]

Thanks for the advice guys! So overall do you think I'm fine to run it? Again I'm not sure if I should trust my work since this is the first time I've done it and had to seal the engine all off. If it didn't really leak during testing I'm thinking I put the seals in right so I think I got lucky with this being the first time I've done that on a chainsaw... hopefully I didn't jinxs myself there!

I will work on making sure the carb/boot seals up good, like I said I think it was just a matter of how I had the piece of rubber in there it might have been wrinkled a little causing a leak.

Also what about the decomp valve? Should I replace it? If I recall correctly when I pulled off the plastic shrouds and stuff there was a little bit of what appeared to be oil or extra fuel around where it threads into the cylinder so maybe it would be good to replace it?

One other thing, the impulse hose is old and hard, I'm worried it could leak at some point and cause a disaster or a hard starting issue, could I just replace that with a generic yellow fuel line like the one I had the tester hooked up too?

Thanks so much to all you guys who have helped me here, you have saved my butt and helped me learn a lot!!

 

[Lightning Performance]

Thanks for the advice guys! So overall do you think I'm fine to run it? Again I'm not sure if I should trust my work since this is the first time I've done it and had to seal the engine all off. If it didn't really leak during testing I'm thinking I put the seals in right so I think I got lucky with this being the first time I've done that on a chainsaw... hopefully I didn't jinxs myself there!

I will work on making sure the carb/boot seals up good, like I said I think it was just a matter of how I had the piece of rubber in there it might have been wrinkled a little causing a leak.

Also what about the decomp valve? Should I replace it? If I recall correctly when I pulled off the plastic shrouds and stuff there was a little bit of what appeared to be oil or extra fuel around where it threads into the cylinder so maybe it would be good to replace it?

One other thing, the impulse hose is old and hard, I'm worried it could leak at some point and cause a disaster or a hard starting issue, could I just replace that with a generic yellow fuel line like the one I had the tester hooked up too?

Thanks so much to all you guys who have helped me here, you have saved my butt and helped me learn a lot!!

Every decomp leaks vacuum. If it gets dirty replace it. Make sure it super tight first.

NO TYGON ON 361 IMPULSE LINES!

It will melt. Ask me how I know.... m***. OEM or Echo/Shindiawa OEM only!

Husky line sucks. It curls pita.

 

[stihltech]

You are on the right track. I like how you had issues and found a way to cure them.
When I was in automotive tech school, we had to rebuild a transmission that had been apart about 50 times for training. When I did it it, it did not work well.
I then proceeded to tell the teacher why the transmission did not wok. After listing the reasons for all the issues, he said " If it had worked the first time ,you did not learn anything. Now you learned a lot".
Diagnosis fixes problems.
Now break it in a bit and run it.