Low compression '98 400f

my 1998 yz400f has fairly low compression. the valve clearance is good and the timing is right. the rings are fairly new too. the compression release isnt stuck open either. its still good enough to ride but i can kick it over as fast as a two stroke. what could it be?

May have a scar on the cylinder wall. so it only gets compression above that point.

Have a shop do a leak down test on it and find out for sure.

can i just do a simple compression test with a generic tester? the kind that screws into the spark plug hole with the gauge on it?

Yes, if all you want to know is whether it's high or low, but you'll notice there is no specification given for that.

A leak down test shows precisely how much is leaking, and lets you find out precisely what is leaking as well.

ok thanks. i know its low, im just curious how low. so there is no given psi level it should be at?

It isn't specified because when the bike is healthy, you usually can't kick it through the compression stroke anyway.

yeah, thats the only way i figured out something was wrong.

i found this home made leakdown tester on the web. seems like it would work to me.


could i make it into a dual gauge by placing a second gauge between the pressure regulator and the damper valve?

Yes, depending on the damper valve, but by the time you get done with it, it won't have been that much less than buying one ready made.

Also, applying air pressure to the cylinder will blow the piston down to BDC, and no matter which BDC it goes to, valves will be open. Because of that, the engine must be locked at or near TDC on the compression/power stroke to do this test. DO NOT attempt to hold it by hand or with a wrench by hand.

so how do i "lock" it at TDC?

The simplest way, if it works, is to use a breaker bar and a socket on the flywheel nut. Lash the handle of the bar to the foot peg with a hose clamp or 2-3 zip-ties.

could i just put it in gear up against something so it doesnt roll? maybe push the rear brake too?

That likely won't work because the engine will probably be able to move more than the 120 degrees or so required to open one of the valves.

ok so i will try the way you suggested. but where can i get a spark plug fitting for the gauge that will fit my bike?

From a compression tester, or by making on from an old spark plug.

It would really be a lot simpler just to go and have the test done, or at least just buy the tester.

10mm. I should mention that the tester will not be accurate unless the orifice in the buffer valve between the gauges is of the right size.

so i found out i have leaky intake valves which doesnt surprise me as they have never had anything but adjustment and the exhaust valves have only a season or two on them. is there any way to tell if rings are bad? when its hooked up to the leakdown tester, where will bad rings leak? i know a bad head gasket bubbles the coolant.

when its hooked up to the leakdown tester, where will bad rings leak?
Into the crankcase. Air would escape through the breather, and through the timing plugs, if they are out.

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