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Low Compression

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Wypuk, Sep 20, 2005.

  1. Gday fellow netriders,
    my beast (83' gpz250) has finally began turning over and im o so close to gettin mobile so i figured i'll give it to the mechanic to look over
    apparently the compression is really low and it needs to be debored and
    large pistons have to be put in, considering i only paid a bottle of dimple for the beast it seems fair i shuold splurge the "sub 1000" to get it preped up for roadworthy, but its botherin me, WTF should i pay so much for something that hasnt bothered me when i rode.....
    and wat is a barrel :oops: koz he said that he will look at them to try locate the source of the problem
    thanx alot

  2. the old girls also suffered from a soft head which means the low compresion could be due to valve problems.

    Either way your gonna be looking at around the $1000 to get it fixed. A good mechanic who is after the work should be able to test the engine and tell you what the problem is for at most $50, that's definately worth while.

    Coz simply one day soon it's gonna die and if that happens the price to fix will double or even write off the bike!
    your choice
  3. Bye barrel I assume he' talking about the cylinder.

    It could be cyclinder wear or it could be a valve not seating properly.

    The later is cheaper to fix

    How many k's has it got?

    The good news about doing this is that you'll probably get 50,000 out of it without touching the engine, provided he does the right thing and replaced bearings and cam chain etc whilst he's there.
  4. If it's going to cost you that much to get the repairs done I'd be checking with the wreckers for a complete engine. I know 83 is a long time ago, but you might luck into a motor that hasn't done a lot of miles for a fraction of the money that a rebuild could cost.

    After all, putting a brand new (effectively) engine in a 22 year old bike is probably false economy anyway.
  5. Sounds like you've been off the road for a while so the low comp. may be a product of the bike sitting for a while. The bores go dry and old rings don't seat so well, it may pick up after running a while and getting the oil to where it needs to be.

    The other common cause is valve problems: burnt seats, bent valves or badly adjusted/worn valves not closing

    By looking at the barrel your mechanic means looking at the cylinder walls for glazing, scuffing or scoring which the compression will leak around so give a low reading. He sould also be able to tell if there are any broken rings.

    If the bore isn't too bad a new set of rings and a hone will help but not as much as a full rebuild. Can't see any value in a full rebuild on a bike that cost you next to nothing.

    A more useful test is a leak down test, much easier to diagnose the source of the low comp.
  6. Assuming your on a license restriction, how long till you graduate and are allowed to ride a bigger bike?

    Chances are the money you sink into fixing it will not be recouped when you up grade, so I'd nurse it and put the cash towards the ride of your dreams.

    AAlways hated Kwaka 4 stroke twins...
  7. It could be nasty as others have said, but given the lack of use the bike seems to have had recently, it could also be as simple as a stuck compression ring....
  8. thanx for the responses im gonna call him tomorrow
    i only need the bike to last for a year then i want the 600 ninja (mmm ninja) so i just need it for the year
    the bike only set be back 50 bux so im not too worried bout recouping the cost of it ;)
    im just worried that it wont pass roadworthy koz of the engine
    o btw is it possible to repair a tank thats rusted on the inside or do i need to replace it koz finiding a tank for the beast has been a pain...
  9. the bike has done 37k koz its been sitting in the prev owners garage for the last 5 years
    apparently it had its heads redone so thats why i was surprised that the mechanic mentioned it
    is it possible to ride it the way it is for a year, assuming i dont thrash it too much
  10. yep, I'm with inci, stuck piston rings are what I'd suspect first
    Get it serviced and have the engine flushed and see what happens then..
  11. Sounds like you'll be right with fresh oil and fluids and some miles under the wheels. Might be worth checking the carbs. aren't gummed up with dried fuel.
  12. carbs have been done by yours truely :)
    the mechanic doesnt want to start working on the engine until i get a petrol trank koz it wont pass road worthy with the current tank, so im hunting around for a tank,
    if any1 knows where i might be able to pick up a '83 gpz250R petrol tank PLZ PLZ PLZ let me kno koz im really desperate to ride :(:(:(:(
    ive called most wreckers on the east side and no one has it and mech reckons it cant be repaired :(:(:(
    im desperate enough to get one from interstate :S