decompression lever

   / decompression lever #1  

stumpfield

Gold Member
Joined
Dec 7, 2005
Messages
455
Location
Sierra Foothills
Tractor
2005 MT265B
Being a new chinese tractor owner, this may be a real dumb question. What does the decompression lever do? In what situation would one use it? I pulled the lever and the engine shuts down.
 
   / decompression lever #2  
I think the decompression lever is to help starter spin motor faster, by not letting motor build full power untill it starts, especially in cold weather. You use it until engine fires, then release lever.
 
   / decompression lever #3  
Not exactly...
The decompression lever opens up the valves (usually exhaust) so the engine can spin without the resistance of compression. This allows the starter motor to spin the engine (rather) freely and also allows some oil fluids to get into the bearings and journals before any load from compression occurs and also in cold weather where oil fluids may be thicker due to being cold. Saves wear and tear on the starter and the moving engine components to a degree. With the valves held open, the fuel mixture won't ignite (it is exhausted) so you have to release the lever BEFORE you quit cranking. Then it ignites and your engine is running.
My Kama starts easily without the decompression lever. As a matter of fact, it was shipped with it disconnected. After reading about it here on TBN, it made sense to hook it up and use it. I thought a little extra TLC wouldn't hurt. I'm sure someone else will list added benefits...
 
   / decompression lever #4  
My Jima 284 wont even turn over without using the lever. I am pretty sure it is the crappy battery that comes with the tractor causeing this.
 
   / decompression lever #5  
Rob hit most all the high points. The only thing keeping metal from touching metal in the bearings is the oil pressure. The rod journals on the crankshaft are the last place in the engine to see oil pressure at startup. Unfortunatly, they are also the ones under the most load as they force the rod/piston up to full compression. By removing the compression load and allowing the oil pressure to build during the first few seconds of engine cranking(pre-lube), you keep the rod bearings from comming under heavy load with no oil pressure and allowing the bearing surfaces to possibly touch.

Another potential issue is hydraulic lock. The starter motor is very high torque. The engine is very high compression. A very small ammount of fluid in a cylinder could cause the piston to be unable to complete a cycle because of the incompressability of fluid. If this happens, that spinning energy in the starter, crank and flywheel has to be absorbed somewhere. It can bend rods, twist crankshafts, blow headgaskets, fracture pistons and crack heads. On the marine diesels I have operated, we had to do whats called a "blow-down" prior to start if the engine had not been run in the last 4 hours. A blowdown is basically cranking the engine with compression relieved to purge any fluids that may have found their way into the cylinder from leaking injectors, leaking head gaskets, cracked heads or even water from the exhaust or intake. All of those are bad, but not as bad as having to change out a half a ton of crankshaft that has been twisted:(

Electric starter motors draw more current and make more heat at lower RPM's than at higher getting an engine up to speed. Compression release allows the starter to accelerate to a more efficient higher speed faster, particularly in colder weather.

The way I figure, they put it on there for a reason. In my opinion, it is better to use it than not to use it so I might as well. In my shipboard machinery experience, I have never had a marine diesel that I operated that we did not "blow down" and use pre-lube prior to startup. I always use the compression release.

My routine startup procedure warm or cold is as follows:
1. Advance hand throttle slightly above normal idle position.
2. Turn key to glowplug/preheat position for about 20 seconds or untill I see the amp gauge come off the -30 amp peg(I skip this step if I am restarting a warm engine)
3. Turn and hold decompression lever.
4. While holding lever, turn key to start and crank engine till I see the oil
pressure needle start to move(about 2-3 seconds on my 284).
5. As soon as I see the needle move, I drop the decompression lever followed quickly by the starter key. The engine starts as you drop the lever and compression is restored. As the engine RPM increases, I reduce the hand throttle to not let the RPM exceed 900 IDLE. Then I let her warm up a bit before I start work.
 
   / decompression lever
  • Thread Starter
#6  
Thanks for the explanation. So, it's a good thing to use the decompression lever for starting even though the Nortrac manual said "use of the decompression lever is not neccessary in warm ambient conditions"
It also warn "Do NOT use the decompression lever to stop the engine!". What kind of damage could happen if you pull the decompression level while the engine is running?
 
   / decompression lever #7  
The decompression knob can also be used when you run out of fuel and are trying to prime it again. Long story on how I found that out, but sure was better than trying to loosen an injector.
 
   / decompression lever #8  
stumpfield said:
Thanks for the explanation. So, it's a good thing to use the decompression lever for starting even though the Nortrac manual said "use of the decompression lever is not neccessary in warm ambient conditions"
It also warn "Do NOT use the decompression lever to stop the engine!". What kind of damage could happen if you pull the decompression level while the engine is running?

No it is not "necessary" to use it when warm, and the tractor will(should) start right up. It is just easier on the starter and rod bearings and eliminates the potential of hydraulic lock to use it. It is probably not healthy to use it to shutdown the engine though as the way it interfaces with the rocker arms might not work well or have the potential to cause damage when trying to engage the moveing rocker arms of a running engine.
 
   / decompression lever #9  
The only time one would use the decompresssion lever to shut down the engine is if the engine runs away (uncontroled rpm).It is possible for a hydraulic pump seal to blow out and pump the engine oil pan full of hydraulic oil, if this happens the engine can run away, no control and not able to shut it down with the engine stop since it is running off of the over full oil.
If this ever happens pull the engine stop and hold it and apply the decompression lever at the same time until engine stops.
 
   / decompression lever #10  
fishmasterdan said:
My Jima 284 wont even turn over without using the lever. I am pretty sure it is the crappy battery that comes with the tractor causeing this.


That is not normal what so ever.

Check the battery and it's fluids.

And make sure you have changed out all your fluids.
 

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