Stihl 028 Carb Pressure Test

[nos351wcm]

If you get er going with the clone and want a OEM carb PM me and I'll steer you to a guy that may have a non-clone or know of a lead on one.

He don't like China carb clones on Stihl saws and may have something.

Give me a heads up notice here later if you have PM'ed me and I'll watch my junk mailbox?

Thank You for the offer! I will contact the customer and see what he wants to do.

[nos351wcm]

Customer declined OEM carb. NExt step was to pressure test the crank case. I sealed off the intake with a cork. I use a small amount of dielectric grease around the cork to aid in sealing.. I next removed the muffer assembly, and covered the exhaust port with duct take, then re installed the muffler. I use a mity-vac pressure/vacuum tester connected to the impulse line and pump the crank case psi to around 15 psi. I spray a water simple green solution on the crank seals, bottom of the cylinder.. Held 15 psi, saw no bubbles.

The "new" carb, however took close to 30 psi to unseat the needle! I am going to adjust the lever and re test.

Ill post the results later

Thank you for looking
-dan

[bgsengine]

I use a mity-vac pressure/vacuum tester connected to the impulse line and pump the crank case psi to around 15 psi. I spray a water simple green solution on the crank seals, bottom of the cylinder.. Held 15 psi, saw no bubbles.

really, though 15 PSI is a bit more than it should be.. should not be more than 7-8 PSI.. maybe 10, but not more than 10.. as a rule of thumb (too much pressure, can blow out the seals) Also, it should hold pressure for 1 minute (though with your duct tape and cork rig-up it may not) ..

and also you want to do a vacuum test right after, since you have a vacuum side to your pump, see if the seals can hold 14 inches vacuum for 1 minute.. if they can't it's still a crankcase leak,, but that test only tests crank seals (assuming nothing else leaks)

.. better way to block off ports, get some impervious rubber (such as a thick inner tube) cut into a wedge shape (Trapezoid) then you only need to just loosen muffler enough to slip the block off between the exhaust bolts (tightly) and tighten muffler back down to seal it off, and between the carb bolts/studs and carburetor, then tighten down carburetor, everything should be blocked off good then.. but whatever works for you, long as you get a tight seal on the ports, and crankcase can hold pressure (7-8 PSI) (and vacuum - 14 inHg ) for 1 minute...

[KE4AVB]

Customer declined OEM carb. NExt step was to pressure test the crank case. I sealed off the intake with a cork. I use a small amount of dielectric grease around the cork to aid in sealing.. I next removed the muffer assembly, and covered the exhaust port with duct take, then re installed the muffler. I use a mity-vac pressure/vacuum tester connected to the impulse line and pump the crank case psi to around 15 psi. I spray a water simple green solution on the crank seals, bottom of the cylinder.. Held 15 psi, saw no bubbles.

The "new" carb, however took close to 30 psi to unseat the needle! I am going to adjust the lever and re test.

Ill post the results later

Thank you for looking
-dan

If it is taking 30 psi to pop off the needle then the impulse isn't pull it off either. Most cubes I have test usually pop off at no more than 10 psi. Apparently the metering needle spring is wrong for the carburetor you have.

As BGS suggest you need to do both pressure and vacuum test on the crankcase. The crankcase seals may seal under pressure but may not under vacuum.
If it is taking 30 psi to pop off the needle then the impulse isn't pull it off either. Most cubes I have test usually pop off at no more than 10 psi. Apparently the metering needle spring is wrong for the carburetor you have.

[Arkie]

Keep a heads up for this on the chainsaw diaphragm carbs:
On some of them little chainsaw carbs you can check the pop-off with the carb all together then take the carb apart and check it again. If it's close to still same, it's the needle/spring area. If the pop-off drops quite a bit the pump diaphragm is too stiff or if someone installed (replaced) the diaphragm that has the wrong size lever button that connects to the needle lever. The depth (height) of the little button on some pump replacement kit diaphragms is slightly different (actually the wrong one) and will upset the carb needle lever pop-off and can cause the carb to either flood or starve and the installer will not notice such when kitting the carb. All the other gaskets etc are ok but some kits have the wrong size little button on the diaphragm. (wrong diaphragm and usually from CHINA)

[bgsengine]

(wrong diaphragm and usually from CHINA)

Careful, though at rejecting just because it is from China - OEM Zama is from China too... But yeah - cheap clone parts will often try to increase their sales by cross referencing their kit to fit models and replace part numbers that it should not... simply because something bolts up, doesn't mean it is going to work correctly... so it's always a case of buyer beware when going to aftermarket parts (and EXPERIENCE with OEM parts to be able to tell the difference even without packaging)

[nos351wcm]

I put a coil on the saw, because i was second guessing myself. Saw ran well for a few minutes, then started showing its normal low power at WOT symptoms. I also pressure tested the crankcase and found it to be sealed.

I am still thinking the replacement card is the issue. Arkie, i am going to PM you.
THanks for looking.

-Dan

[Arkie]

PM answered:

Watch your junk mailbox if you do not see my reply.

[nos351wcm]

Thank You!
-Dan