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YH compressor questions/request for help

Just ran my new YH, 240V and settable stop version today.

Problem/Issue: Unit switches between stumbling/stuttering and running fine when it gets above 4000 psi. Unit barely makes it to ~4300 psi before is shuts off.
Suggestions?

My initial start up went as follows:
1. Checked for loose screws, bolts, nuts, fittings. Unit only needed low pressure relief nut tightened.
2. Filled compressor with compressor oil, Powermate Px, to just above red dot, capped with breather cap.
3. Connected water lines and pump.
4. Placed pump in 5 gallon bucket of water.
5. Plugged unit in and checked water flow.
6. Placed stopper plug in hose.
7. Set shutoff to 1000 psi.
8. Started compressor.

Results, compressor pressurized line to 1000 psi, alarmed at 1000 psi and shut off. Good.
Raised shut off to 2500 psi and powered unit on. Unit ran, alarmed at 2500 psi and shut off. Good
Raised shut off to 4500 psi, powered on, ran, alarmed at 4500 psi and shut off. Good

Attempted to top off 75l tank
1. Set shutoff at 3200 psi, tank was at 3000 psi.
2. With tank valve off, turned compressor on, when it got close to 3000 psi, tank valve was opened.
3. Filled to 3200 psi, alarmed and shut off. Good
4. Upped setting to 4500 psi.
5. Tank valve off, confirmed water flow, started compressor.
6. Compressor temperature at start, 20C (its cold in the garage ~40F, water ~50F).
7. When compressor reached 3200 psi, tank valve was opened.
8. Compressor ran fine up to about 4000 psi then stuttered at little, then smoothed out.
9. Compressor got pressure up a little more, then stuttered multiple times, so shut compressor off.
10. Compressor was just under 50C.
11. Waited until temperature dropped to 25C then ran again.
12. Compressor increase pressure to maybe 4200 psi then stuttered.
13. Repeated 11 and 12 a couple times, got tank up to 4200 psi then decided to stop for the day.
 
Second stage was dirty. Cleaned up
piston and cylinder. Lowered oil level from top to middle of red dot. Ran only loaded with filter which came with compressor and no load on filter output for 15 minutes. Waited a few hours and ran loaded with 2 filters in line and capped cable. Ran much better but after a few more short runs, less
than 5 minutes each, notice a stutter now and then. Bet I’m going to have to disassemble and clean again. 
 
Nvelkhunter- Yes, I cleaned the 2nd stage piston maybe three times before it finally stopped wearing the seals. The good thing is once that's done, the semi-metal seals should be good to go for a long while. When I first got my YH, it didn't turn over at all. I took the motor case off and could *barely* turn the shaft by spinning the fan by hand. Lots of stiction - primarily due to the carbon from dieseling in the 2nd stage. Now I can spin it easily. A trick is to put the 2nd stage cylinder back on but don't tighten the 4 bolts down right away. Instead, turn the shaft by spinning the fan blade while sequentially tightening the bolts. That way you align the cylinder to the piston travel as best as possible. Good luck!
 
I have the 110V auto shut off model. Mine did the same stuttering then smoothing out until it finally stopped. Ended up being the relay that is the problem, I found this out by taking the cover off, plugged it in, turned it on and noticed it arced inside the relay and tried to start, so I pushed on the relay and the compressor started up and ran. Compressor is back running but I know the relay is on borrowed time, contacted seller and they are sending an "upgraded" relay. It seems to be a common problem for the auto stop compressors. You can bypass the relay by removing the brown wire from relay and unplugging the green wire and plugging the brown wire were the green wired plugged on the end away from the relay or just connect the two wires together off the relay. Compressor will work but without auto shut off function.