Blown 661...again

[angelo c]

Now just reverse it and you got it right. Vacuum to test seals first then pressure.

Thanks. Have always checked both but never focused on the sequence as being critical. I think we have confirmed the saw holds vacuum and pressure.

 

[angelo c]

You’re not listening, instead you’re replying with statements that are irrelevant to my question.
I want to help, but I’m not going to continue to repeat myself. Good luck with your repair.

Yes. I don't understand your point. And I also don't understand why so high of a vacuum is needed when a saw normally runs at 5-10 in.
The bottom line is the saw holds pressure and vacuum while static and rotating the crank.

 

[Vintage Engine Repairs]

Yes. I don't understand your point. And I also don't understand why so high of a vacuum is needed when a saw normally runs at 5-10 in.
The bottom line is the saw holds pressure and vacuum while static and rotating the crank.

You using a mityvac?

 

[angelo c]

You using a mityvac?

Yes. 8500

 

[Vintage Engine Repairs]

Yes. 8500

Right, then I recommend you do as I suggest when testing vacuum, because if you don’t you’re not testing it correctly.

If you’re pulling a vacuum up to 5 (which reads in inches of Mercury) on vacuum, you’re pulling just over 2psi… that’s way to low. There are roughly 2 inches of Mercury to 1 psi.

You should be pulling a minimum vacuum of around 16, ideally 20 inches of mercury which equates to 8-10 psi, then start rotating the crankshaft 5-10 times.

 

[angelo c]

Copy.
Never really looked that closely at the 8500 dial. Just looked for "10"
Thanks.

 

[lone wolf]

Pressure to seek leaks (soapy) and vacuum to confirm. It's not always the seals that leak. Had base gaskets fail, impulse batbs leak, cyl bases crack...ect

Vac will test a seal where it seals on the crank itself because pressure will push the seal closed on the inner lip ! I have seen them pass pressure, but vac failed. Vintage Engines offered good advice. Dmb said he would like to see the piston, so would I. Pics?

 

[angelo c]

 

[dmb2613]

when you pull the cylinder off I would like to see pics of the piston side and top

 

[angelo c]

when you pull the cylinder off I would like to see pics of the piston side and top

Intake correct ?

 

[dmb2613]

both sides and top

 

[mbrick]

The Stihl service manual outlines how to test seals, what order, and the vac/pressures to use. I suggest giving it a read. Vintage is correct so I also suggest following his suggestions and re testing for leaks.

Also, echoing several prior requests for photos of the piston with the cylinder removed.

Very knowledgeable folks here and you are being resistant to their feedback and light on details. For example it required several posts to figure out the year of the saw, which mtronic it has, and what you already tried.

"Help me... help you" as Jerry Maguire said.

 

[J D]

Is that just RTV sealing the cylinder or a proper fuel rated sealant?

 

[bwalker]

The smaller Mtronics have issues with more oil. 261 in particular. Never had issues with 661 and 40:1

That's not my expiereance.

 

[bwalker]

Is that just RTV sealing the cylinder or a proper fuel rated sealant?

I see that too. Looks like he is running a base gasket with silicone. No need to do that and gasoline disolves silicone.

 

[cscltd]

If it was silicone, could it have been dissolved by fuel thus allowing an air passage into cyl to cause piston issue as cyl would not be sealing against block?

 

[lone wolf]

If it was silicone, could it have been dissolved by fuel thus allowing an air passage into cyl to cause piston issue as cyl would not be sealing against block?

Isn't that a metal gasket in there too?

 

[lone wolf]

Intake correct ?

Don't pull it until you pressure and vac test it.

 

[huskihl]

You guys realize that Dirko is silicone, correct?

Never met Angelo, but I know him. Been in group convos with him for a couple years. He’s not inexperienced at this. It would be nice to see some help offered instead of going in raw like he’s a newb with no idea.

As far as the p/v test, I agree with the vac first. But I’ve learned on my own time that seal issues that affect running characteristics are instantly noticeable on the needle. If it holds 7 on the vac or pressure side without falling off while you rotate the crank, it’s not an air leak that’s causing the issue. There’s no way in hell that a hairline leak at + or - 10 psi, but no leak at + or -5 psi is going to score a cylinder. At 10k rpm there isn’t time for that kind of pressure differential to happen on every revolution anyway. You’re talking 2-3psi each way from 0 max

 

[bwalker]

You guys realize that Dirko is silicone, correct?

Never met Angelo, but I know him. Been in group convos with him for a couple years. He’s not inexperienced at this. It would be nice to see some help offered instead of going in raw like he’s a newb with no idea.

As far as the p/v test, I agree with the vac first. But I’ve learned on my own time that seal issues that affect running characteristics are instantly noticeable on the needle. If it holds 7 on the vac or pressure side without falling off while you rotate the crank, it’s not an air leak that’s causing the issue. There’s no way in hell that a hairline leak at + or - 10 psi, but no leak at + or -5 psi is going to score a cylinder. At 10k rpm there isn’t time for that kind of pressure differential to happen on every revolution anyway. You’re talking 2-3psi each way from 0 max

I am aware that Dirko is silicone and I wouldn't use it for that reason. I had a failure with a silicone based Threebond product years ago and vowed to never use silicone anything again.