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xr400 - another one without compression

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well i went and lashed out on paying for fixing the head, valves ground and relapped by a professional. put it back in the bike and next to no compression. what the hec? i disconnected the decompressor cable, but no difference. What the hell is going on? I can turn it over by hand.....

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dave... dont panic yet!

you need to give us some more information before we can help though.

give us a detailed step by step of what you have done...

finding youre lost compression shouldnt be too hard(it doesnt just get up and run away)

cheers

a

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hi, thanks. ok, all i have done is remove the head, get it sent to get fixed. then replaced it, making sure the timing sprocket lines lined up with the top of the head (as per the manual) when the T mark lines up with the mark on the inside hole of the stator cover. I have another xr which has comjpression. sometimes when kicking over slowly it doesnt have compression, but then i hear the 'click' then it gets tight. i think the click might be the auto decomp, which gives me a hint. is there a way to see if it is doing its job (or in this case stopping to do its job so theres compression).

I have another cam, maybe i should try that? another hint is that there was about 90 pounds psi compression before i took the head off, you're right it doesnt just disappear so what the(*&(*&^*&^*&^ is happening?

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ok youve had the head off and then reinstalled.

start with the basics... and take your time. we do our own work because we enjoy it!!

new gaskets?... yes i have seen people forget to fit a head gasket!!!

valve timing?... check all your timing marks again( dont just glance at them..check them properly)

valve clearances?... if you have had everything apart the clearances will have changed, check them again(watch out for the auto decomp).

logic... compression doesnt just dissapear.

there are only a few places it can escape

past the piston..bad ring seal

between the barrel and head... bad gasket seal

past the valves... poor seal on valves, valve clearance wrong, valve timing wrong

past the spark plug... poor seal on crush washer

other.. cracked head barrel or piston

now you should be able to start eliminating possibilities.. consider them properly before discarding them

let us know if you still cant find where it went.

cheers

a

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ado thanks for your help, how silly of me not to do the valve clearances. of course that was it, nice amount of compression now. Trouble is still cant start the sucker, i've so far done the head, swapped the carby, the ignition has got a nice blue spark, i'm running out of options. maybe inlet manifold has a hole in it or something, what u think.

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see, i told you it couldnt just run away!!...lol

easy to get lost and start running around in circles.

now for the next problem.

to make an engine run..

compression... youve fixed that one

sparks.. youve checked that??.. and its hard to get the ignition timing wrong on xr400

fuel... start with fresh fuel. our fuel in aus has a shelf life of about ten seconds.(seriously.. its about two weeks max before the fuel has a noticable drop in performance).

unless you have a serious manifold leak, the bike should still fire.. it may not run, but it should at least pop a few times.

make sure the carb is clean, if it has been left sitting around with fuel residue in it, it may have clogged jets and or galleries.

cheers

a

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thanks will do that, could simply be the fuel, will drain and try again.

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pull the plug after a few kicks, is it dry? no fuel is getting to it, check the flow from petcock to carb. Float needles love to stick if left dry for a while.

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