Lost Compression, really?

5 replies to this topic
  • JSTheJack

Posted June 12, 2002 - 11:07 AM


I've been reading a number of y'alls (Texan) posts about losing compression, that later comes back.

What in the world is going on here? When you say "lost compression", do you mean like it only has like 50 psi of compression or something. With these things having compression ratios of 12.5:1, I'd imagine that a new bike would have on the order of 185 psig or greater. I've never checked, and cam timing can affect this.

However, it sounds like the compression release is hanging open if the compression comes back later. If you've got worn rings, compression will always suck, but not change. Might be easier to start if the bike doesn't cool down too much between ride sessions. If you've got poorly adjusted valves, compression will always suck, but not change.

So, does anybody have a definitive answer for this gremlin? I'd like to know.

  • sirthumpalot

Posted June 12, 2002 - 12:33 PM


My only guess would be a valve temporairily stuck open or something. It happened to me once after shooing a pile of carb cleaner through the carb (and into the motor). When I kicked it I had about the same resistance as you get when you pull the comp release all the way in (could not tell where the compression stroke was at all). A few kicks and it fired and has been normal since. That was several months ago, and I now remove the carb before shooting a lot of cleaner through it. :)

  • eddiec103

Posted June 12, 2002 - 01:15 PM


I lost my compression this weekend for the first time. I didnt ride my bike for 3 weeks{Garage move and work} and when i got to the track i could not start my bike.It had no compression I could kick my bike with no resistance back from the kickstarter.I put a new plug in and it started up 1st kick.It usually starts first or second kick all the time,never a problem for me.The old plug was about two months old.I ride/race usually every weekend but this 3 week layoff and no compression.Has this ever happened to anyone else before?

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  • Shawn_Mc

Posted June 12, 2002 - 03:34 PM


I think its happened to just about everybody that has let the thing sit for 2 weeks or better after a wash. Mine did it too.

Now after washing and fiddling, I kill it, and bump it around to the compression stroke and leave it. I also got an rubber expansion plug for the exhaust.

  • zdylanh

Posted June 12, 2002 - 05:12 PM


If your valves or rings are worn, when the bike is cold it will have less compression. When the metal heats up it will expand and it will have more compression.

  • Boit

Posted June 12, 2002 - 08:06 PM


zdylanh. What we are talking about here is a mysterious loss of compression followed by an equally mysterious return of compression. It happened to me with with two thumpers. One is a Stroker built KLX and the other, of course, is my 426. Both circumstances were the same. Both bikes sat up for a couple of weeks and when I went to start them, the engine had very little compression. After a few kicks, the engine started and once I stopped the engine to check compression again, it was normal and has remained normal ever since. My theory is that there was a small deposit on a valve seat or valve face that prevented full closure and sealing of the valve to the seat. After kicking it through a few times, the deposit was dislodged and the valve sealed normally. At any rate, both engines have over 170 pounds of compression now and it has never happened more than once on either engine. It IS a little stange, plus, it bothers me not knowing exactly what caused it. Some attribute it to the oil being "washed" off the cylinder walls. I dunno. Could be.

[ June 12, 2002: Message edited by: Boit ]

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