pressure testing 101

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Hey Guys, Can you give me the run down on pressure/vacuum testing a saw? I honestly do not know the first thing on pressure/vacuum testing a saw.:cry: I have rebuilt plenty of saws and carbs, and always got lucky I guess. When do you do it? How do you do it? What are you looking for? What tools/gauge's do you need. What are the most common symptoms to make you run a test. I tried to do a search and only got bits and pieces. Any help would be appreciated.
 
I do a vacuum test if I suspect that the saw has an air leak, that is to say, air is entering the crankcase from somewhere other than the carb. The most common area's are the crank seals. I only do a pressure test to find the source of the leak. Saws that have an impulse line are the easiest to do. Place a piece of rubber over the intake and exhaust ports on the cylinder. Plates behind the rubber work best, but you can just use the muffler and the intake manifold/carb. once you have the cylinder sealed off, attach your vacuum tester to the impulse line. I pump it up to 15" of mercury. Slowly rotate the crank over (by hand) 3 times and watch your gage. If your saw is holding vacuum, you have no leak, if it's droping so quickly that you can see the gage move, you have a pretty substantial leak. I don't like it to drop more than an inch or 2 over 20 minutes. On saws without an impulse line, you still must block off the ports but you have to find a way into the crankcase or cylinder. I made a fitting that screws into the spark plug hole with a quick-connect to my mighty-vac. If you do find a leak, switch over to pressure and pump the saw to 8 psi. spray soapy water around the crank seals, the sparkplug hole, the ports and even the base gasket. Wherever the saw bubbles is where the leak is. I had a leak so tiny once that I had to drop the saw in a fish tank and watch for bubbles. (The saw had an impulse type oiler and that's where the leak was.....
 
You are going to do a pressure/vac test to check for any kind of air leak in the engine. I try to be a Through as possible on every job i work on and have made it a standard practice on almost everything. Any air leak at all will cause the engine to run poorly. Depending on where the leak is most air leaks get missed by people richening the carb to compensate for the extra air. since the entire engine is at stake I feel that there is no need to have ANY leak. Most manufactures list a bleed down of 1lb over a min. But ive been able to fix that by making some warped flanges flat and using sealer on certain spots. It takes some extra time to do but is far better than having a pissed off customer because it came back because the problem got misdiagnosed. Even if the customer isnt mad they will always have a shred of doubt about your abilitys.

A pressure test is to pressurize (possitive pressure) the inside of the engine to find an air leak. ususally everything except the crank seals (unless they are shot)
A vacuum test is the same thing but your are removing all of the air in the crankcase to see if you can pull in outside air. If you cant create a vacuum and hold negative pressure then there is a leak. Usually it will be the crank seals or rubber boot intakes.

To do either one you will need to remove the carb,muffler,and plug your impulse hose. You will need some block off plates to seal the exhaust flange and intake. Manufactures sell these plates but I just make my own with plate and rubber gasket material. The best combination Press/vac pump ive found is from Power Probe. I like having both tools in one,...save space in the tool box.I purchased the Power Probe from Echo.
Once the intake and exhaust are sealed you will need a plug with a male nipple that will screw into the spark plug hole. Hook up your pressure/ vac pump and pump up to 10psi pressure and look for a leak. If it doesnt hold pressure look for a leak. Soapy water will help if its a small leak. If it holds pressure then you can do a vacuum test. I pump it to neg10 psi. New seals will hold this just fine but if they are old it may hold at around 5psi. When they are that low I replace the seals because once the engine is warm they usually leak alot more. If it wont hold pressure it definatly wont hold vacuum. So if you find a leak you need to retest after fixing it to find out if you got all of them. If its old equipment or just abused there is usually more then one.
good luck.

Steffan
Zombie Performance
 
I've studied the link JStone provided before. Nice to put some added explanation with it. Rep your way Zombie. Nik, must have gotten you recently because I'm out of bullets. Great post guys, one I'll be bookmarking.:clap:
 
Zombie Perf. said:
You are going to do a pressure/vac test to check for any kind of air leak in the engine. I try to be a Through as possible on every job i work on and have made it a standard practice on almost everything. Any air leak at all will cause the engine to run poorly. Depending on where the leak is most air leaks get missed by people richening the carb to compensate for the extra air. since the entire engine is at stake I feel that there is no need to have ANY leak. Most manufactures list a bleed down of 1lb over a min. But ive been able to fix that by making some warped flanges flat and using sealer on certain spots. It takes some extra time to do but is far better than having a pissed off customer because it came back because the problem got misdiagnosed. Even if the customer isnt mad they will always have a shred of doubt about your abilitys.

A pressure test is to pressurize (possitive pressure) the inside of the engine to find an air leak. ususally everything except the crank seals (unless they are shot)
A vacuum test is the same thing but your are removing all of the air in the crankcase to see if you can pull in outside air. If you cant create a vacuum and hold negative pressure then there is a leak. Usually it will be the crank seals or rubber boot intakes.

To do either one you will need to remove the carb,muffler,and plug your impulse hose. You will need some block off plates to seal the exhaust flange and intake. Manufactures sell these plates but I just make my own with plate and rubber gasket material. The best combination Press/vac pump ive found is from Power Probe. I like having both tools in one,...save space in the tool box.I purchased the Power Probe from Echo.
Once the intake and exhaust are sealed you will need a plug with a male nipple that will screw into the spark plug hole. Hook up your pressure/ vac pump and pump up to 10psi pressure and look for a leak. If it doesnt hold pressure look for a leak. Soapy water will help if its a small leak. If it holds pressure then you can do a vacuum test. I pump it to neg10 psi. New seals will hold this just fine but if they are old it may hold at around 5psi. When they are that low I replace the seals because once the engine is warm they usually leak alot more. If it wont hold pressure it definatly wont hold vacuum. So if you find a leak you need to retest after fixing it to find out if you got all of them. If its old equipment or just abused there is usually more then one.
good luck.

Steffan
Zombie Performance
Click to expand...


Sweet info. Rep to you, and welcome aboard.
 
Thanks Guys for taking the time to explain it. I know its a lot of typing, and a bunch of great info. Thanks Again, Repped who I could!
 
Last edited:
I use 1/4" x 1 1/2" aluminum flat bar for blocking plates, it's easy to come by in hardware stores and easy to drill. Just cut off a piece of flat bar long enough to catch both bolt holes on the carb and another for the muffler. Mark out the mounting holes from the carb and muffler and drill the screw holes a bit over size. Gasket material can be made from used inner tubes at a local tire shop. Just cut a strip to fit between the screws on the muffler and carb.

Mityvac makes a dual purpose tester model #8500 that will take care of vacuum and pressure testing (get a service manual for your saw and you will find info on the procedure) this tester comes with numerous fittings to hook up to many different tubing lines.

After blocking off the carb, muffler and spark plug I just hook up the Mityvac to the impulse line that I remove from the back of the intake area and do the test. Some saws don't have the impulse line and you can adapt a piece of that aluminum flat bar to cover the intake area completely (including the impulse hole to the carb) then drill a hole in the center of the flat bar so that you can attach the Mityvac. This hole is centered over the carb intake area. The Mityvac makes all of this really easy and can be had pretty cheap on Ebay.

Mityvac MV8500
 
Steve K said:
Some saws don't have the impulse line and you can adapt a piece of that aluminum flat bar to cover the intake area completely (including the impulse hole to the carb) then drill a hole in the center of the flat bar so that you can attach the Mityvac. This hole is centered over the carb intake area.
Click to expand...

Good one. Thanks ! I was trying to figure how to make a plug hole adapter. Your way will be easier.
 
I'd be happy too, but, I won't be able to do it till monday. (I'm at work, camera's at home) I'll do my best to describe it though.

It's a coupling with 2 thread forms. The outside is the same thread as a spark plug. ( I wanna say it's a 14mm, but don't remember) The inside is a 1/4" NPT. I screw my part into the cylinder head, and then screw a quick releas into it. (You can also use a hose barb or anything else that has the same thread)
 
FATGUY said:
I'd be happy too, but, I won't be able to do it till monday. (I'm at work, camera's at home) I'll do my best to describe it though.

It's a coupling with 2 thread forms. The outside is the same thread as a spark plug. ( I wanna say it's a 14mm, but don't remember) The inside is a 1/4" NPT. I screw my part into the cylinder head, and then screw a quick releas into it. (You can also use a hose barb or anything else that has the same thread)
Click to expand...

Thanks for that. I think I can visualize what you're describing...but I'd still like to see a pic Monday. I would be using a hose barb. Did you get the coupling (w/the 2 threads) at someplace like Lowes or Home depot?

Kevin
 
FATGUY said:
no, I made it. (I'm a machinist)
Click to expand...

LOL, okay. I guess I'll scrap that idea, and go and see if I can find a fitting that will fit in the plug whole with a hose barb end. I know of a local store that has a good stock of different types and sizes of fittings. If you could confirm the 14mm thread size, that would help. Thanks.

Kevin
 
KMB said:
LOL, okay. I guess I'll scrap that idea, and go and see if I can find a fitting that will fit in the plug whole with a hose barb end. I know of a local store that has a good stock of different types and sizes of fittings. If you could confirm the 14mm thread size, that would help. Thanks.

Kevin
Click to expand...

Somewhere in the "search function" I recall someone taking an old sparkplug, taking off the porcelain and making what you need. I'll see if I can find it. Plugs are 14mm.
Bob
 
redunshee said:
Somewhere in the "search function" I recall someone taking an old sparkplug, taking off the porcelain and making what you need. I'll see if I can find it. Plugs are 14mm.
Bob
Click to expand...

I have that thread bookmarked (http://www.arboristsite.com/showthread.php?t=94407 - it also has where a member bought a brass fitting for the plug hole)...I just thought I'd see what FATGUY had. I don't have any old spark plugs around, so I figure the easiest way is to get the fitting with the hose barb.

Thanks for the confirmation on the 14mm.

Kevin
 
The Husqvarna fitting part number it 503 8440 02. it retails for $7.63.
any husqvarna dealer should be able to get this for you. if not let me know i work for a dealer. because i work on soooo many different engines i make my plates the shape of the exhaust gasket so im not fumbling around with alot of rectange plates with different bolt spacing. i make them out of 3/16 steel so it wont flex and use 1/8 rubber. its frusterating when your trying to test an engine and your own plates are leaking. thats why i hate the rubber wedges that you put between your muffler and exhaust port. another good trick is when ever you have a rubber intake (like stihl and newer husq) dont use the rubber gasket to help seal the intake. just use the flate plate then you can see if your carb flange leaks.

steffan
Zombie Performance
 
Zombie Perf. said:
The Husqvarna fitting part number it 503 8440 02. it retails for $7.63.
any husqvarna dealer should be able to get this for you. if not let me know i work for a dealer. because i work on soooo many different engines i make my plates the shape of the exhaust gasket so im not fumbling around with alot of rectange plates with different bolt spacing. i make them out of 3/16 steel so it wont flex and use 1/8 rubber. its frusterating when your trying to test an engine and your own plates are leaking. thats why i hate the rubber wedges that you put between your muffler and exhaust port. another good trick is when ever you have a rubber intake (like stihl and newer husq) dont use the rubber gasket to help seal the intake. just use the flate plate then you can see if your carb flange leaks.

steffan
Zombie Performance
Click to expand...

To make sure I'm understanding you correctly, this fitting is for the spark plug hole to attach an air supply hose?

Kevin
 

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