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XR250 compression 110 psi

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since i got and recommissioned the 1991 project bike it's been hard to start. during the rebuild i lapped the valves and the rings and pistons were visually good but i didn't have a before tear down compression figure.

I got highest figure of 110psi having warmed the bike, with decomp cable disconnected and the throttle open.

I poured some oil down the pot to see if the figure lifted but no change, in fact it fell away to 90 psi, i think as the bike cooled as it took me a few mins to find a way to get the oil down the pit when you can barely see the hole.

I'll pull the head off, anything in particular i should be looking for ? Should I pour some petrol in it while inverted and see if it leaks ?

 

 

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Maybe i'll do a leakdown test first as i have the kit.

 

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couldn't hurt. when you did the valve grind did you make sure to wash the grinding past off properly? diamond grit will chew things to bits real quick if one speck is left.  after a valve grind I use the pressure washer to wash all parts and then petrol bath 2 times with fresh fuel and clean container. you can never be too safe when it comes to cleaning parts for a rebuild.  if you take the head off definitely invert it and fill the combustion chamber with fuel to be sure of 100% valve to seat seal. worth 5 mins for piece of mind. have you checked the valve clearance? good luck and let us know how you went.....cheers

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i think what i'll do is warm it up and then back off the clearances and retest and see if this lifts the compression.

the clearances are tiny, so i guess it doesn't take much of a change to result in a poorly closing valve.

not sure i'll bother with the leakdown as it's hard to get everything sealed up, and if the compression is low it's clear there is an issue.

 

 

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so i warmed it up (had to use easy start to get it running), confirmed compression is 110 psi, backed off all the tappets and it made no difference.

this time dropped more oil down the pot and the compression came up to 140psi.

took the head off, inverted it and filled it with paint thinners. After 10 minutes not a drop had come out. i think it's evaporating though !

does this suggest the issue is with the piston/bore ?

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If you poured a little oil in they cylinder and the compression temporarily came up that is a sure sign the rings are shot.  Bore could be completely worn and needing a bore job or just rings.  You will have to take it apart to check piston to wall clearance to determine that.

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doesn't sound good.  with the valve clearances they should be set with the engine cold. if you suspect the valves are not closing with oem clearance use the next thickness feeler gauge I think standard clearance is .10mm in and .12mm ex   so go .12mm in and .14mm ex should be plenty enough to confirm it. although moot now you have the head off.  seeing as you have the head off you may as well take the cylinder and piston off and check them out. going by your compression test id say its time for a new piston and a bore and hone. new cam chain and a re doo of the head just to be safe. do you have a workshop manual for specs and tolerances?

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i got the head off and poured spirit into it and no leaks. i've got the barrel/jug off and piston in the back of the car to get an opinion from the local engineers.

the bore is already 74.4mm so it's had a bore or two, but that isn't too surprising as it's 1991 and appears to have a racing past.

see my other post re confusing compression ratios. I can only assume the 192 psi quoted in my manual is a calculated figure rather than a measurable one which will never be the same due to cam overlap - but i'm guessing a bit here.

 

 

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yehh if the pistons been changed ???? what compression ratio is it? stock pistons have a flat crown with small indentations for valve relief. if the piston crown is raised at all it will be a higher comp ratio than stock. if the bore size is significantly bigger ie. 1mm or more will also make a difference. as for valve overlap that shouldn't make any difference to a compression reading as it is the cross over of exhaust closing and inlet opening. if it has an auto decompressor that will give false readings.

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Poor mans leak test:

  • Get motor at TDC.
  • Back off valve adjustment (just to make sure).
  • Remove carb and exhaust pipe.
  • Remove Sparkplug
  • Apply compressed air through sparkplug hole (use a rubber tipped nozzle).
  • Listen for air coming out of intake, exhuast and possibly the beather tube.

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Is anyone able to measure the compression up to 190 psi on the kick start.

I believe my gauge is approx right as I use it other things and the figures are as expected.

I'm wondering if the 192psi stated in the manual is calculated and not actually measurable.

What is the highest figure people have actually measured ?

 

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