Compression gauge drops quickly

[game04]

What would be the reasons the compression gauge needle would drop quickly when performing a compression test? I am trying to check the compression on my Stihl BR420c blower. My compression gauge does have the schrader valve in hose. After about four pulls the compression builds up to about 125 psi but immediately start dropping down. Within a few seconds it drops down to about 30 psi and then the drop slows from there. I checked the Schrader valve and it does seem to be sealing properly. Any ideas? Could this be a sign of a stuck piston ring?

 

[Whiskers]

I would focus on the compression tester, sound like there's a leak somewhere, air should go past the one way valve on the tip of the hose and be held in the hose creating pressure as you crank. It shouldn't go down until you push the release valve which has nothing to do with the saw itself. The fact that the compression tester climbed to 125psi with a leak is a pretty good sign.

 

[Roanoker494]

I agree that the gauge must have a leak, the needle should not drop until you push the release button. I recently picked up my compression tester to find the gauge still showing 150psi and the last saw I tested was the Makita 6401 about a month ago.

 

[game04]

I would focus on the compression tester, sound like there's a leak somewhere, air should go past the one way valve on the tip of the hose and be held in the hose creating pressure as you crank. It shouldn't go down until you push the release valve which has nothing to do with the saw itself. The fact that the compression tester climbed to 125psi with a leak is a pretty good sign.

That's a really good point. The problem has to be with the gauge. I will try spraying the guage and hose with soapy water and see if I can pinpoint the leak. The weird thing is I used this compression tester on the same engine just a few days ago and the reading held at around 150 psi. That's why I started thinking a ring had become stuck.

 

[galde]

Check the comp. release valve.

 

[JustinM]

Yup to the advice so far. Sounds 100% like the tester, not the saw. I would start with Galde's advice of the release valve.

 

[blsnelling]

This is not a function of your engine at all. It can't hold combustion pressure. Also, you must have a valve at the very tip of your hose where it screws into the cylinder.

 

[Scooterbum]

Just try the tester on another engine and see if it does the same thing.

 

[nick 55]

Before you start turning over the saw any more, check the tester hose for a valve. Usually they look like a schrader valve in a tire valve stem. If its missing, look inside the cylinder of the saw you used it on last as you don't want foreign object damage from it being in the muffler on startup where it could roll back into the jug.

Nick

 

[surfincr]

i had the same issue. i was able to resolve it by pulling the schrader valve cleaning it and reinstalling. worked fine after

 

[game04]

i had the same issue. i was able to resolve it by pulling the schrader valve cleaning it and reinstalling. worked fine after

Yep, thats what the issue was with mine too. I cleaned the schrader valve and its working fine now and the PSI is reading 155. A spec of dirt must have gotten in there.