Pulling decom. lever while running bad?

Is it hard on the engine to pull in the decompression lever while it is still running?

If so, why?



It will kill the engine. You need compression to drive the piston.

When it dies, it will leave unburned fuel in the cylinder and may backfire and blow your air filter loose when restarted.

You decide. Does it sound like a good idea?

You also can hole a piston or bend a valve.

You also can hole a piston or bend a valve.

Can you tell me more about this? Didn't the ol' timers used to pull it to help them go down hill? I accidently pulled it while it was running the other day, and yes it stopped running, but no aparent damage was done. Did I just get lucky? Am I right in thinking that pulling the dec. lever opens a valve, the piston might hit that open valve?



Thats Bs..Let me ask you this...Does your valve hit the piston when you pull in the decompression lever to clear it out ..or crank it up??? the answer is no.. It is so easy to start teh engine without the kickstart while go down a hill.. I have a Wr250F and use the decompression to bump start it all teh time... and it has 6000 miles on it.. Now if you pull it while the motor is revved up..you will backfire..blow flames and make noise..so it is better to jsut use the kill switch there..And I ride with a bunch of ole timers that have been doin that forever..Just my 2 cents

I know for a fact if you pulled in the compression release on a YZ400F while it was revved that it would bend the valves. Several came into some of my shops that way. I don't know about now. Now if you use it to start a dead engine bumpstarting , there should not be a problem. But you don't use the compression release on a 4stk for the same reasons you do on a 2stk. On a 2 stk you release top end compression to kill power and allow the lower end compression to slow the bike down. You release the compression on a 4stk and the bike will speed up on a down hill, not slow down. I use mine for starting purposes only. Now if I had a run away engine with stuck throttle and the kill button didn't stop it , I guess I would chance the compression release , rather than lunch the crankshaft.



i have a wr400 and have inadvertantly pulled the decomp a couple of times at high rpm with no ill effects.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Ponch
      I have a 2007 450 EXC that I recently installed a new clutch slave cylinder on. Per the 2007 KTM service manual it clearly states that the clutch needs to be filled with hydraulic oil, which I did (magura blood). That was yesterday. I went out to double check the level today and when I removed the cover, the rubber seal was all contorted and out of shape. I then looked at the cover and it's a Brembo clutch and says "DOT 4 brake fluid only".
      I flushed the line and the master cylinder about 7 times with DOT 4 brake fluid and there was no sign of the magura blood left in the system. My question is: other than the rubber boot on the master cylinder, what damage might have been done by putting in the hydraulic oil? It was in for about 24 hours.
      Thanks in advance for any advice!
    • By pumpernickelBrad
      Hi Folks!

      Sorry if there are a bunch of threads already but I used the site's search function and didn't find anything so far...

      I just found a decent deal on a '14 350 EXC-F with 11k km (unknown hours). Has a few goodies including skid plate, hand guards and it's for sale for about $2k CAD less than a '16 with 6k km on it.
      The bike is in showroom condition (appearance) with no indication it has been offroad at all.

      What would be the gotchas that would make this deal realistic? I'm coming from a WR250R where the engine lasts forever, is 11k street kms high for these bikes? 
      Are there any big defects in the '14s that I haven't dug up yet?
      With no service records what things should be done, checked, etc?
    • By Brannnt
      I purchased an '06 525 EXC some time ago, and recently pulled the motor out of the frame to address some other issues. When I removed the clutch slave cylinder, I noticed an issue the previous owner neglected to tell me about during the purchase.
      It appears that at some point, the chain broke and damaged the engine case near the clutch slave cylinder (see pics, please). I have noticed that the clutch does not fully disengage (bike lurches a bit going into gear, neutral is hard to find... its worse when engine is cold) ... I did not notice any leaks around this area (mineral oil or engine oil) before or during disassembly. 
      The previous owner (or someone) repaired it with some sort of epoxy.
      Without doing my due diligence (checking clutch fluid level, bleeding) I hesitate to state that this is the cause of my clutch disengagement issue ... but pardon my ignorance here and help me out: How bad is this?  Am I looking at replacing my engine case here or is the previous owner's jerry rig good enough?

    • By Black Sox
      Just bought a new 2017 350 excf ,will be doing 98% off road ,ordered skid plate bark busters ,
      my ? Is what's the best easiest way to eliminate rear fender light assembly.I still would like to have a break light.
      xcf fender will fit so dealer says,under $50.00 👍But xcf brake light $125.00 .👎What's the best economical way of doing this?
    • By JNV
      Hi friends from Spain.
      Mi new KTM