XR400 Huge issues!

Hello everyone!

I got a 98 xr400 that i bought not to long ago which supposedly had a blown top end. I broke the engine down, had the cylinder bored and bought a new set of piston/rings. Put those in, set the timing as stated in manual, kicked it over and it has little compression. I rechecked the timing again(piston at tdc cam lobes down sprockets lines parrallel with head).I also checked the valve clearances multiple times (.004 in and .005 for ex). With the rocker assembly removed, compression tests show around 90 psi(which is low but probably not very accurate reading considering guage/conditions). This leads me to believe the problem has to be from the to of the motor.Many have said auto decomp but i dont know what to check on it to verify its lined up right or what. It was very dissappointing to put this engine together and it no run after slapping 450r motors together like its nothing. Any help would be greatly appreciated.Thanks

There is a quick way to eliminate the auto decomp mechanism from the equation. Take off the head cover and remove the rocker arm that sits on the RH exhaust and auto decomp cams. Grind off part of the foot that runs on the auto decomp cam.

When I did this to my own bike, I compared the two exhaust rocker arms side by side and aside from the wide foot on the RH arm they seem to be identical. I bet if you put a new LH arm in the RH position you would not need to do any grinding to get rid of the extra foot area.

Although the grinding method is destructive (you will have to replace the RH rocker arm if you want your auto decomp cam back), these items do not seem to be too expensive. In Aus a RH rocker arm goes for about AUD $50, so in the USA you'd probably get one for about $35. Given what you have already spent I'm sure this is no big deal.

You should start by checking for the basics. Is it getting fuel? Is it getting spark? It sounds like this bike hasn't ran since you bought it so if the carb has a lot of gunk in it, it won't run either. Don't dive in too deeply until you check the easy stuff. Good luck...

Hey dethloff04,

Do not forget you will not get full compression until the rings are seated properly. Only a good ride will break them in so as to get 100% compression.

Clean the carb, put fresh gas in it, kick and go for a ride !!


Btw, mine felt like that after a rebuild too until broken in. Then it was full compression.

To get an accurate compression test reading, back off the adjustment screw on the RH exhaust rocker so that there is excessive clearance. This will ensure that the auto decomp does not engage and hold the valve open.

Thanks for all the suggestions..I bought a new carb, and fresh gas/new air filter and im getting spark. It honestly feels like it has no compression fully assembled(30 psi or so). So adding gas and such doesnt make the engine feel like it gonna start.It has to be something in the auto decomp area.I just have no idea how to check, service, or understand the function of the autodecomp on the cam in order to service it. With the cam removed, compression shows around 90 psi or so.cam installed, no compression.valve clearances are spot on too.

quit overthinking it and break the bike in :applause:

it wont start and feels like it has the compression of an xr80.

The auto decompresion is just that. When you kick the motor over it releases the compresion so you can get it past top dead center. Be sure you have some slack in the decomp cable and it is not engaged all the time. Did you put in an aftermarket cam? Some cams do not work with the decomp. Try bump starting it.

IIRC, if you pull the header off (and disconnect the manual decomp) and rotate the engine, you can watch the right exhaust valve open up and close. If you are at TDC and turn the opposite direction slightly, the auto decomp will open up the valve. Does this happen?

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