Shut down air cylinder sticking

richard5933 · May 06, 2018, 10:32:46 AM

The shut down air cylinder on our 4108 seems to stick if the engine isn't run for a few days or more. When this happens I can easily push it back in by hand and get going, so I am pretty sure it's not a problem with the Skinner valve.

There is a little crud along the plunger, but overall it looks ok.

Are these rebuildable? Or does it pay to just replace it?

I'm assuming that there is a spring inside intended to pull the plunger in once the Skinner valve releases the air. Or is that spring somewhere else? Perhaps under the plate where the air cylinder mounts?

luvrbus · May 06, 2018, 10:36:55 AM

If the plunger has a worn grove replace it 50 bucks. if not buy a kit from Luke and rebuild it

richard5933 · May 06, 2018, 10:50:44 AM

Do you know if the spring to retract the plunger is inside the cylinder or under the cover on the level that the plunger pushes to shut off the fuel?

chessie4905 · May 06, 2018, 11:45:17 AM

I have mine on the bench right now. It is bronze and has a label on it from Mohawk. The cap is retained with two allen head screws. Inside is a plunger with a grooved washer head opposite the rod that sticks out the end. There is a long spring that slides over the rod. The spring retracts the piston rod and air pressure extends it. There is a unique modified cup seal that fits in a groove on washer style head. I got the seals from Luke a while back. To repair air leaking. The fast idle plunger uses the same seal. I bead blasted the assembly cap and housing and see no internal wear. If you don't want to just replace seal, which is the only one in it, you can probably get a new unit from Mohawk, but might be pricey due to being all bronze.

richard5933 · May 06, 2018, 12:46:54 PM

Quote from: chessie4905 on May 06, 2018, 11:45:17 AM
I have mine on the bench right now. It is bronze and has a label on it from Mohawk. The cap is retained with two allen head screws. Inside is a plunger with a grooved washer head opposite the rod that sticks out the end. There is a long spring that slides over the rod. The spring retracts the piston rod and air pressure extends it. There is a unique modified cup seal that fits in a groove on washer style head. I got the seals from Luke a while back. To repair air leaking. The fast idle plunger uses the same seal. I bead blasted the assembly cap and housing and see no internal wear. If you don't want to just replace seal, which is the only one in it, you can probably get a new unit from Mohawk, but might be pricey due to being all bronze.

That description helped a lot. At least I know where the spring to retract it is. I'll check and see how much Luke charges for the seal and the entire assembly and then decide.

Do you know if there is a spring under the shut-off lever being moved by this cylinder?

luvrbus · May 06, 2018, 12:56:42 PM

Yes it has a 4 coil spring under the governor cover on the stop lever,they do break or jump out of the slot sometimes it's primary function is to return the engine from no fuel to the fuel position, the spring in the shut off does very little in regards to that function

chessie4905 · May 06, 2018, 03:23:44 PM

Try moving stop lever clockwise with engine off and cylinder red retracted. It should have noticeable spring tension and move smoothly about 30 to 45 degrees.

richard5933 · May 06, 2018, 04:19:37 PM

I just did a little testing...

1) Moved the air cylinder away from the stop lever, and the stop lever does indeed have spring tension pushing it towards the 'run' position.

2) Turned engine run switch to the 'run' position, hear the poof of air releasing from the Skinner valve, and the plunger in the air cylinder doesn't retract on it's own.
3) Following #2, it is possible to push the plunger in manually. However there is resistance as if the rod was slightly binding or dragging. Does not feel like there is any spring activity inside the cylinder attempting to retract the plunger.

With all that said, am I correct to assume that the air cylinder needs to be either rebuilt or replaced? Since there is some corrosion on the visible portion of the plunger my vote is to replace and avoid future problems.

Anyone able to confirm or add to my suspicions?

luvrbus · May 06, 2018, 04:30:51 PM

Just replace it some times the kits cure the problem sometimes they don't,Luke or Bus Services will have it

Scott & Heather · May 06, 2018, 04:42:45 PM

I took mine apart and cleaned up the metal
Shaft and lubed it and it worked fine for years after that.

Sent from my iPhone using Tapatalk Pro

chessie4905 · May 06, 2018, 04:45:48 PM

Replace it. It is most likely the old design and not worth fooling with. Is it all brass or bronze? In any event, remove it and take it apart to see how its made. Could be spring broken from age and corrosion. Mine worked ok, but leaked air past seal.

eagle19952 · May 06, 2018, 04:50:28 PM

Quote from: Scott & Heather on May 06, 2018, 04:42:45 PM
I took mine apart and cleaned up the metal
Shaft and lubed it and it worked fine for years after that.

Sent from my iPhone using Tapatalk Pro

What he said.

i just pull the air hose and squirt lube in there. PB Blaster is good. then Vaseline, a little dab...

richard5933 · May 12, 2018, 09:39:35 AM

Got the new air cylinder from Luke. Went in without a hitch and now works great. I opened the old one to see what was wrong, and found that the return spring was nowhere to be found. Guess that's why it wasn't returning on its own. The cylinder walls were slightly scored and the plunger was dry and a bit worn. Definitely feel better having just replaced it, but I did keep the old one to use for parts or emergency backup once I get it cleaned/lubed.