Pressure/Vacuum test on Crankcase w/scored piston

[ClemXP]

Hi Everybody,

So I've been reading as many posts as I can about Vac and Pressure testing, and I still have a few questions. But first, here is why I am testing this saw. My 346xp (1998 model, orange cover) was killed by a coworker...it had good compression when it left the shop, and the carb was tuned using a tach. They say that after a few minutes of running full bore with no load, shaving a spring pole (I know, I know) that it over heated, stalled, and wouldnt restart. He brought it back and the piston was fried on the muffler side, no compression to speak of.

Now I covered the intake, between manifold and cylinder, and exhaust between the metal muffler ?gasket? and cylinder, with rubber bike tube material, and bolted everything back together. Replaced spark plug, too. With my mityvac hooked to the impulse hose, I couldnt get any reading on vac...could this be due to a bad piston? I though that wouldnt matter. Any other ideas? Is it just a major leak I never knew of?

I'm considering rebuilding it, I got it for free, so it's still worth it to me, even at the price of a 346 p/c, just not if I can't find the original problem. Is it likely that a bad seal in the crankcase caused problems that were temporarily covered when I tuned the carb, which then got worse when they ran the saw later? Can a crankcase be tested without a cylinder bolted to it?

Thanks for all the great info on this site!!!

 

[Rookie1]

The scored piston would have nothing to do with the test. You are pressurizing the entire engine not just the bottom end.Pop the little vent thing off your mighty vac and pressure test the saw. Ill bet a seal is out of it. You may have to use some soapy water to see the leak if you cant hear it.

 

[Steve K]

Like Rookie said the bad piston will have no effecto a pressure or vacuum tests. I have had to drop the pressure on my air compressor down to 10psi and hook that to an impulse line to find a bad leak before. Hard to keep pumping the Mityvac enough to find bad leaks.

 

[Fish]

With everything blocked off, put the saw in a bucket of fluid and put some air to it quick, you will see the leaks quickly.

 

[Fish]

With everything blocked off, put the saw in a bucket of fluid and put some air to it quick, you will see the leaks quickly.

Whoa!!!!! Did I say that??????


Here come the posts...............

 

[ClemXP]

That's what I thought (about the piston having nothing to do with it)

I tried both vac and pressure with the mityvac, Im familiar with the top pressure nozzle, but I couldnt get any readings either way. How many pumps with that little guy should I expect before I get to 9mm/Hg, or whatever vacuum I am shooting for?

I may have to find an air compressor I can use. Thanks

 

[Rookie1]

I dont remember how many pumps it took. I do know the gauge doesnt register pressure. The Fish Way may work if the leak is that hard to find. Just dont vac it in the "fluid" or youll suck it into the crankcase.

 

[ClemXP]

Sounds good, i'll try again today. Exactly what type of "fluid" would you recommend?

 

[FATGUY]

With everything blocked off, put the saw in a bucket of fluid and put some air to it quick, you will see the leaks quickly.

+1 I keep an old fish tank in the shop for that reason. (I usually take the ignition off as well though. Just make sure you're suing pressure and NOT VACUUM with the saw submerged.

 

[stihl only]

I haven't had one that I've had to put in the dunk tank yet. I have used a hose and just used my lungs for pressure to eliminate all the noise and distraction of a hand pressure pump. I also use some home brewed block of plate in place of the putting the muffler back on thing, because the muffler in place blocks the cylinder base gasket area.

It sounds like you have the manifold blocked at the cylinder? If so, keep in mind you aren't testing the manifold if you do it this way. Not familiar with the saw, but if it's a rubber boot style manifold, it should be checked too!

A regulator and gauge is in the works, but not on the top of my to do list because I haven't really needed it yet!

 

[ClemXP]

It sounds like you have the manifold blocked at the cylinder? If so, keep in mind you aren't testing the manifold if you do it this way. Not familiar with the saw, but if it's a rubber boot style manifold, it should be checked too!

I did have the rubber manifold boot excluded, I'll have to back up the blocker to between it and the carb.

Thanks for the suggestions!

 

[blsnelling]

No need to submerge the saw. I spray windex on the suspected areas. It makes nice bubbles. You'll need to remove the flywheel and clutch first.

 

[ClemXP]

So what do people like to use to block off intake and exhaust? Rubber hose as a gasket and a fabricated metal plate? Husky or Stihl specific "tool"? What other suggestions are out there?

Also, if you are bolting something to the exhaust port, without the muffler, would you use different bolts? Place a spacer where the muffler should be? The same question goes for the carb side, too.

 

[blsnelling]

So what do people like to use to block off intake and exhaust? Rubber hose as a gasket and a fabricated metal plate?

Also, if you are bolting something to the exhaust port, without the muffler, would you use different bolts?

Rubber hose and plate on exhaust with same bolts. Rubber hose behind the carb.

 

[ClemXP]

If you use the muffler bolts, isnt there a couple of inches of space between the head of the bolt and the plate, where the muffler used to be? How do you keep the plate from popping off?

EDIT: Okay, I think Ive got it. Still not getting any pressure or vacuum, but I feel more confidant about the top end being sealed off. Now to the local Aquarium...

 

[blsnelling]

If you use the muffler bolts, isnt there a couple of inches of space between the head of the bolt and the plate, where the muffler used to be? How do you keep the plate from popping off?

By "behind the carb" do you mean between the carb and manifold?

Sorry, I didnt quite get what you said.

Depends on the saw you're testing. Stihl saws have short muffler bolts. And the carb mounts directly against the carb box. Sorry, I didn't notice you was working on a 346. I would still think you could put rubber between the carb and monifold. I would put it after the carb and not before. The throttle shaft will likely not be air tight.