I guess the title pretty much sums it up but I will go into more detail in case someone might be able to help me.

I know this has been covered a little in the past but I think my compression problem is getting worse. At first it seemed like a valve was sticking open but after I washed the bike yesterday, it wouldnt start so I just brought it home and decided to mess with it tonight. So I went out and kicked and kicked and kicked. The compression was really weak. Then I pulled the hot start and it fired on the second kick. After letting it warm for about 3 minutes I tried to blip the throttle and it died. So when I tried kicking it again there was hardly any compression again. Now it wont start at all.

I can actually crank the bike over by hand. I rode it this weekend and it ran great. Wasnt down on power at all. What should I do next? Ive got a race this weekend and need some advice quick! Help! :)



Darin from Missouri - 1999 WR 400F

You checked obvious things like valve adjustment and made sure there was plenty of slack in the comp release cable right?

If you had to use the hot start to get it running then that tells you that it was really flooded or something is causing it to run real rich. Is there black smoke when you run it? Does it run better with the hot start on? Sounds really rich to me. I really oiled my airfilter good one time and then tried to start it right away and it wouldn't, that makes it too rich and then you try to start it a few times and it floods. When it floods the gas washes the oil coating off the cylinder wall and the rings don't seal and you can kick it right over past the hard spot easily.


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Yes. I checked the obvious stuff.

And the plug was right on. It hasnt been running too rich.

I couldnt stand it anymore so I went out and tried kicking again and opened the hot start (with no compression) and it fired on the second kick. Let it run for 10 minutes shut it off and now it has all the compression in the world. Im really confused, SCREAMA.gif but glad its running good again.


Darin from Missouri - 1999 WR 400F

Hey Milkman, I had the same problem a couple of weeks ago, remember? I tore the bike completely apart, checked valves,compression release, etc. Everything perfect. I left the fuel petcock on when it happened. I think the cylinder got washed with fuel. I'll bet that's your problem too. If you had to pull out the hotstart the engine had to be flooded. After washing the bike maybe the ignition, or the spark plug lead got wet? If it's only happening every so often, that would be my best guess. The only other thing could be the comp. release cam is sticking. If you examine that setup, it's pretty simple, and you should be able to see any damage.I still think it's a fuel problem though.


That is strange. If I turn the idle all the way down I can crank my 426 without the compression release, with my foot, haven’t tried using my hand, but it feels like kicking a bike with a blown head gasket.

My accelerator pump got dirty and the bike would die when you blipped the throttle. Maybe some grunge clogged it and then dislodged itself. With a race upcoming you may want to check that.

A motor with a dry cylinder will probably have less compression than one that has just been warmed up.

I dunno, chalk it up to finicky four-stroke syndrome. I kicked my YZ one day for half an hour. My two-stroke only buddy climbs on and roars off, comes back and says “Hey, whats this little black lever for?”

Go figure…

Along the same line as MotoGreg - I've had cars that would not start because of being flooded and shot a couple of squirts of oil down each cylinder to restore compression and they started fine.

Thanks for all the input guys. I did wash the bike prior to this happening and when I pulled the plug there was wetness at the base of the plug but not on the tip. I smelled it and it wasnt gas.

But I dont know what that would have to do with no compression.

Im pretty sure its not the accelarator pump because I just cleaned that out 1 month ago when I got back from Colorado and changed my jetting back.

Should I do a leakdown test next time this happens? Do you think this would tell me anything espically if a valve is sticking?


Darin from Missouri - 1999 WR 400F


now are you listening my good fellow.


then i will begin.

pull the plug out, hold the threaded bit at the end in a pair of pliers. hold it over a gas cooker flame for 5 whole minutes. when it's burning the fuel off the flame will be orange, when it isn't (the first 2 minutes) it will remain blue.

your plug is so wet you're losing your spark.

now go start your bike.


[This message has been edited by Taffy (edited 09-19-2000).]


If you can turn the engine by hand, I think you have a more serious issue than flooding.

Most likely a sticky (bent) valve or the compression release valve, cable, or some internal componet of it is not closing properly. Remove the cable and try moving the arm at the head by hand. Make sure you get full resistance all the way in and out. Or force it close and try kicking the bike again to test for greater compression.

Other possibilities would be valve timing. But in either case, you'll need to pop off the head cover to investigate such issues.

Other than that, make sure you don't have a broken or improperly installed sparkplug, (have you taken a compression test?).

Sometimes the ceramic cone can break venting air out the top.

In either event, I think the flooding is simply caused by the fact you don't have compression and you're not burning off the fuel that is being introduced during all the attempts to start it.

Let us know what you find and perhaps we can get closer to what the problem may be.


Mine did the exact same thing about 1 year ago.. was ideling.. blipped and stalled.. no comp..

Mine had a tiny piece of metal stuck under one of the inlet valves.

Thought it was throttle slide but checked and checked and can't see where its come from..

Bikes been raced and thrashed alot since and is fine!! :)



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

Discounts for ThumperTalk members.

Excellent responses guys. These are all good ideas.

I changed the plug right away with a new one so that elimnates the wet, cracked theory.

Matt, thats an excellent idea. Im racing on Sunday so mabye if theres a piece of metal in there do you think it might pass?

If not, then I will have it checked out shortly. Ive only got 3 races left and I want to get through the season before a top end teardown is started. :)


Darin from Missouri - 1999 WR 400F

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