Sudden loss of compression

16 replies to this topic
  • ryster

Posted August 03, 2004 - 06:12 PM


I was getting my bike ready for the next ride (tomorrow)and went to fire it up and noticed I could kick it through without d-comp lever :thumbsup:.I can not figure out what what is wrong as it ran perfecly last trip.The only things that I did was wash,change oil,coolant,clean and oil air filter, replace throttle cable,and new plug.We checked the valve clearances and all are in spec.We put the old plug back in and checked for leaks around plug and top end gasket and no leaks.The bike has been running fine other than some slight smoke out the tailpipe for a couple seconds on first startup
Time for rebuild? Any ideas appreciated. Thanks. :devil:
The bike is a 98 wr400

  • Hamish

Posted August 03, 2004 - 10:33 PM


Is it real easy to kick over, or does it take a bit of effort? can you get it to start? If it wont kick start try bump starting it. Sometimes a bit of carbon gets dislodged from the chamber and holds a valve partially open. After it fires up it will generally 'fix itself'. otherwise do a leakdown test and listen for the escaping air. Breaher = rings, exhaust/carb = valves. If you dont have a leakdown tester an air compressor down the spark plug hole should suffice. Just have sure you clean out the plug hole before you do it.

  • don319

Posted August 04, 2004 - 04:43 AM


I had the exact same problem. Water is probably in the cylinder from washing.
Pull the plug and kick it over to see if you get a little water mist from the plug hole. Then drop a few teaspoons of oil into the cylinder. If it's water, this will fix it.

  • ryster

Posted August 04, 2004 - 05:10 AM


It does take a little effort to turn it over,but you can do it with your hand.It will not start with kickstarting.
I will go try both of your suggesions and and see if it clears up.
Thanks for the replys :thumbsup:

  • ryster

Posted August 04, 2004 - 07:47 AM


It runs!!! :thumbsup:I put a little bit of oil in top end and voila!, compression. :lol: I would have never thought it would have been so easy.I was thinking of a valve hanging up somehow and was glad I did not have to tear down and check.luckily a teaspoon of medicine worked :lol:.Thanks Hamish and don319 for the advice :D :lol: :D :D :devil: :awww: :lol: :lol: :thumbsup:
The knowledge and help here is awesome.I will take extra precaution from now on when washing and taking out the plug.
Now off to riding. :snore: :awww:

  • WR_Dave

Posted August 04, 2004 - 09:28 AM


After washing always run the bike for a few minutes. On my '02 I always set the engine at TDC when it is in the garage, that way the valves can't hang in the open position. Just some thoughts to improve the weekend enjoyment. WR Dave. :thumbsup:

  • kxf320

Posted August 04, 2004 - 10:42 AM


250 kxf--I was one of the early ones. My compression dropped seemingly overnight. I did not notice a let down in power--although it is 'weak' anyway. I can take 3 fingers a pull starter fully. I am only able to roll start. I have never checked valves---about 70-80 hours on bike--Vet B rider. Suggestions for this issue...Leakdown test---drop of oil into cylinder...? kxf320

  • ryster

Posted August 05, 2004 - 06:23 AM


So after getting my bike fixed and loading up for the ride ,my nephew gets his bike out (99 wr400) and--- :devil: unbeliavable, same problem, absolutely no compression
What the !%@$? Luckily we knew what fixed mine and did the same to his,a little oil on top of cyllinder and good as new.This is the first time this has happened to us and we are at a loss of what is causing it.We cross the occasional stream but nothing to deep.we are not using a pressure washer to clean. The only conclusions I can come to are
1.Water getting sucked up vent breather tube
2.Excess gas washing oil of top of cyllinder
3.Water seeping in past spark plug
First thing I am going to do is route the breather hose to air box.2 Try not to use choke to start, and shut off gas and run bike to burn off gas in carb and keep at tdc to keep valves closed.Try not to get any water near engine when washing.Hopefully this will take care of it :thumbsup:

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  • jchantzWR400F

Posted August 05, 2004 - 07:00 AM


This happened to me once before (99WR400). My situation was one of changed grips on the bike (throttle and clutch side) While changing the throttle side and checking to make sure that it was not sticking I flooded the hell out of it. I thought the gas was off. Probably turned the throttle 50 times before realizing it wasn't off. When I tried to start the next day, viola, no compression. I was able to get it to start and then the problem took care of itself.

Conclusion, gas probably washed down the cylinder wall.

  • R_Little

Posted August 05, 2004 - 11:29 AM


Mine did the same thing last year.

While starting in am I noticed I could kick it over by hand.

I kicked a few times and it started and the problem never came back????????

  • 5valve

Posted August 19, 2004 - 11:52 PM


currently I have the same problem, for the second time!!!
both times it happend when I checked for valve clearances and playing with YZ, WR exhaust...first time the bike started with no problem and the compression came back, second time is still to try, today...hope it starts

I was told it has sth to do with oil lines through the cams?

  • Hamish

Posted August 20, 2004 - 01:52 AM


I was told it has sth to do with oil lines through the cams?

:thumbsup: Thats a new one to me. If anyone has the theory behind this i'd like to hear it.

I'm of the opinion that it's caused by carbon or forign matter getting onto the seat and holding the valve partially open. I've just started using MotorEx ValveGuard to prolong the life of the valves and seats, and switched to a fuel that has a built in cleaning agent so time will tell if this helps the situation.

  • 5valve

Posted August 20, 2004 - 02:45 AM


in your theory the only problem is, why would the not_wanted particles build up only when the valve checkup is being done (in my case) to result in a non compressing of the valves, because none of the debry felt into the cylinder and the bike ran OK the day before

Im also doubtfull about what I was told, but I just cant imagine what could result this behaviour

hmm, maybe it just has to do sth with the valves to sit again in their place forced by the power of explosion

  • Hamish

Posted August 20, 2004 - 05:57 AM


I don't have an answer to your question.
Another case for the X files :thumbsup: We can only guess what the cause is. My theory is based on;
1) happened after head removal, where it is possible I disturbed some carbon in the chamber
2) titanium valve engines have lower valve spring pressure, making it easier for something to hold a vale partially open.
Both times it happened to me it was immediately after I had the head off. Bike started 1st kick, ran for a few minutes the stalled, and had no compression. Stuck some 2 stroke fuel in the carby, ran it for 5 min, problem gone.

  • jwriott

Posted August 20, 2004 - 06:24 AM


I was thinking maybe your compression release was slightly stuck keeping a valve open.

  • firestarter

Posted August 20, 2004 - 08:04 AM


Everytime I flood my bike (with gas), it feels like it has no compression at all (i can move the kickstarter all the way with my hand), the first time it happened to me i thought that it was something wrong with the valves or decomp,, but it wasnt,, it just needed another spark plug to start burning the excess of gas in the chamber. An the compression came back.
I hope this may help you a little. :thumbsup:

  • 5valve

Posted August 21, 2004 - 02:41 AM


I started the bike yesterday, it jumped after the first kick even though the kick starter was soft, and the compression came back instantly - this time I was only reversing it from YZ back to WR, but previous time the bike lost compression I also had removed the head


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