compression test?

[81 clark racing]

does anyone no what the compression should be on a 400ex? mines 135 ive have a adv timeing key. is that a good number or bad?

[GPracer2500]

The timing key won't affect compression.

Assuming the stock piston and cam are in use, the service manual says 100-126psi is normal. That may sound low but the auto decompression mechanism on the stock cam knocks the spec down quite a bit. Without the auto decomp it would be more like 165psi (approximately).

If your getting 135psi then either 1) you're gauge isn't quite accurate (easily possible--I've junked several that didn't read correctly) 2) you're 1000 feet BELOW sea level (next to impossible) or 3) you've got a bunch of carbon built up inside your combustion chamber (quite possible if your jetting is too rich).

[81 clark racing]

if its carbon built up inside my combustion chamber. how do i fix clean that. also will this cause any problems if i leave it alone? yes stock cam and piston i have

[250X_project]

Originally posted by GPracer2500
the auto decompression mechanism on the stock cam knocks the spec down quite a bit. Without the auto decomp it would be more like 165psi (approximately).



So if they put an aftermarket cam in a 400EX it won't have the decompression and read higher, like the 165PSI you said?

[GPracer2500]

Originally posted by 250X_project
So if they put an aftermarket cam in a 400EX it won't have the decompression and read higher, like the 165PSI you said?

Yep. To retain the auto-decomp w/ an aftermarket cam you'd have to press-off the auto-decomp from the stock cam and press it onto the new cam. Hardly anyone does that. The 400EX starting system is plenty strong enough to handle it.

What the auto-comp does is keep one of the exhaust valves from closing all the way until the engine is spinning faster than a certain RPM (~600rpm I think--a speed below what the starter will do).

The 165psi figure is a guess for what the stock 9.1:1 piston would crank if the auto-decomp was removed from the stock cam.

[250X_project]

Oh okay, then it's just like the little Briggs engines. :D