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GPX 250 rebuild

Discussion in 'Modifications and Projects' started by Night_Stalker, Aug 21, 2006.

  1. Hey all

    So, my bike is off the road and it may well be a few weeks before it's back on. That's just what happens when the rear shock needs to be taken in and reconditioned.

    So, in the mean time I thought I'd do some engine work on the bike. I've heard that common problems with the GPX250 are the carbs and the cam chain tensioner. The bike has also done 56,000km. So I figure it's time for an engine rebuild.

    Yeah, it's a 250 and yeah I'll be moving on to something larger sooner or later. But in the mean time I want it running just as smoothly as it can run.

    So the overall plan is to remove the engine and recondition both the top and bottom end. I'll also be inspecting the transmission while I'm there (repairing as necessary).

    What's been done so far...

    Well obviously I needed to get the rear suspension out, so that's been removed, I've taken off the fairings and the petrol tank. I've only got a few more steps before it's time to remove the engine.

    The work so far...


    The minimum planned work is, rings, valves, cam chain and cam chain tensioner... Of course anything along the way that needs to be fixed will be.

    I'm gonna have fun with this :D
  2. nice one. let us know how hard you find the job, you're saving big $$$ doing it yourself.
  3. Well.

    I have the service manual. So far it's been alright. The engine was removed today and I'll begin taking the engine itself apart tomorrow morn. I hadn't the time until today to work on the bike at all.

    I did take a peek under the engine cover... it looks like I'll probably need a new cam shaft on the intake side at the very least (part of it is broken) I'll add a pic of that tomorrow morning. The damage isn't too bad, and I may need a bit of advice on whether I should re-use it or replace it (leaning heavily towards replace at present)

    Bike without the engine:


    So there we have it, the engine is out and ready to be worked on.
  4. Hey everyone.

    Time for your regularly scheduled update... or at least there would be one if there was a scheduele.

    So, the engine has been partially dismantled. the damaged cam shaft has been removed the tappers have been removed and the engine head and cylinders have been removed.

    This is what I found


    If you check the photo, it's pretty clear the pistons are badly carbonified up.

    so they'll be cleaned out, replaced, more than likely. The cylinder will be re-bored honed and new rings will be the order of the day.

    As for the top end, the valves seem to have held up pretty well adn so do the seats, that's upon initial inspection at any rate. There's not much carbon on the intake side, it'll be cleaned out.

    This is a spread of the parts as they were removed.


    There is however, one thing that's stopped me in my tracks so far....

    It wasn't in the manual and isn't in any parts sheets I've seen, but there's 3 allen bolts that hold the engine head to the cylinder. I can't move them, I don't have the leverage on my allen keys. Anyone have any tips on how I might remove those without warping the head?
  5. the memories come flooding back.
    as for the bolts try sparying some penertrin or wd 40 on top of the bolts if they look rusty and leave it soak for a few hours then get a bit of pipe to go over the and allen key to give you some more leverage. undo each bolt a small amount say 1/4 of a turn then got to the next one, keep doing this until all bolts are loose

    good luck on the rebuild

  6. Your a lot more game than i was when i did work on my EL250 (same engine). Then again you have a service manual and space to keep things spread out, i had a verandah :-(.Good luck with the rebuild, can you show us photos of the camshafts too? im just curious as to what's up with 'em.
  7. im goign to take my bike to you for a service now ha
  8. Haha. I'm happy to help if you need it.

    As for the shots of the cams, I knew I'd forgotten something, and at 2am. I'm surprised that's all I forgot.

    Latest news just in. I found it highly suspicious that those allen bolts weren't mentioned in the manual, so I took the head and cylinder in to Nathan at Caringbah motorcycles.

    It's a good thing I ddin't get them out. They're for the water jacket and aren't supposed to be removed. Also upon inspection of the head, he tells me, the reason I can't get it off is because someone glued the head to the cylinder. So I left it with him to take it off with "gentle persuasion". In other words, a plastic mallet (which I have) and a smoke wrench (otherwise known as a blow torch, which I don't have one.... yet).

    I know the procedure, but at this point the labor of him removing it will almost certainly be less than the cost of me buying a blow torch and removing it myself.

    Once that's been done I'll be measuring up the cylinders, pistons and checking the valve seats and guides for excessive wear.

    So far, the total cost of repairs has been $0 (Price from Nathan tba)

    In any case, here's the shots of the cam shafts.


  9. Just a note mate

    You make reference to the black ring (significant heating) around your camshaft sprockets.

    Thats the induction case hardening process that the manufacturers use to harden the cam chain sprockets for wear resistance.

    The reason it is case hardened rather than through hardened is that the whole gear then is not brittle and can take shock loads

    Good to see all the photos still need to work out how to do it though

    Good luck with the rebuild
  10. Wow. That's good to know, I'll take some measurements and may not get rid of that other camshaft.

    I'm wasn't worried so much about the heating but that one of the cams was losing bits inside the engine :|

    Yeah, there's a bit I need to work out how to do, but it shouldn't be too terrible organising those bits.

    Thanks for the info mate, I'm a little less worried about that now.
  11. was your motor making a bad clunky clunky bang bang noise with that busted up cam?

    on friday i rebuilt a cb250n motor, only sohc so it was relatively easy. the pistons looked similar to yours, just scrubbed up the tops and bobs yer uncle. this wasnt a new cb either, an 81model and still had hone-marks in the bore.....i probably didnt need to rebuild it.
  12. I can't be certain if that's the origin of the noise I was hearing, but it was making some ugly ones at low revs. It did disappear (or the engine got too loud) at higher revs.

    It's interesting you should say that you've still got hone marks there. It is in my opinion that what's happened here was that the rings have been replaced not long ago and the pistons were re-used afterwards.

    The service manual has a .12mm cylinder to piston gap and the clearance on the pistons to cylinder I'm running are at .25mm

    I'm thinking it's time to re-bore and replace the pistons. Irrespective of that though, I have cleaned them up and they don't look to bad, it's just that measurement I'm really afraid of that's the problem. I'm losing a fair amount of compression and I'm thinking that's the primary cause (along with a busted head gasket)

    I am however, taking the pistons and cylinder block to a machine shop tomorrow to verify my measurements.

    I don't want to buy parts needlessly, but I also don't want to put it back only to have to replace them again later. I'll also be checking to find out whether the engine head is warped while I'm there.

    I was beginning to think that maybe I didn't need to rebuild myself too, but the proof is in the measurements, which will be done professionally tomorrow. If I did pull that apart needlessly, I still cleaned the pistons of all that carbon. and the cost of a replacement gasket is arount $20 anyways.

    The cars I've worked on are mainly sohc as well, truth be told, I thought they were more complex :|
  13. .25mm does sound like you are getting a fair bit of slop, though as you said, get it measured properly (by properly i mean get it done at the shop) that will give you two things - the best possible measurement and also it will give you a good guide as to how accurate your measuring devices are.
    sounds like a re-bore and OS pistons is the order of the day for you though.
    good luck!
  14. Thanks mate, I'll continue to post pics and progress reports here, so you can all laugh at me if I make a killer mistake or share in my success... whichever comes first :p
  15. And for the latest update.

    I took the pistons and cylinder block to a machine shop (Peak re-bores, great engine reconditioning place in at Penshurst, I have their address if anyone wants it)

    And it turns out that my feeler gauge is quite accurate, it was a .27mm gap. Way more than the service limit.

    So they'll be re-boreing and honing the cylinder block as soon as I have replacement pistons and rings. I also picked up a replacement cam shaft this morning.

    In addition I've also ordered the following...
    Cam chain
    Cam chain tensioner

    I'm considering shaving the head and checking the valve seats and guides (I need to buy a valve spring depresser first)

    Hopefully I have the money to get all this done this week, otherwise it's looking like it'll be finished next week.
  16. Good luck are you going to spanner day , ill get you to look at something on my bike .
  17. please please replace the cams, the rockers/ajusters and the cam chain... these ware out on them on regular basis and with them new it will be like a new engine!

    Also they are cheep parts.

  18. Just a side note, almost all Gippies do that and it doesn't mean certain death... It gets worse as the cam chain gets streched... When my clunking got real bad i opened up the bottom end to find the auto-tensioner as metal filings lol
  19. What he said. I would also remove the valves, check the valve guide clearances, and if all was good, clean up valves and lap them in. Be sure to use moly break in lube on any new parts you assemble.
    You are already there, it will cost you nothing but time to check the valves. Also check valve spring free length, and check that coils aren't cocked( bent over one way). This is easy with a set square. Be aware that some motorcycle cylinder heads need a special valve spring compressor that can reach into teh valve pockets. It may pay to have someone disassemble the head if this is the case.
    Have you found the missing camshaft parts? Do not button up the engine till you do!
    Shame someone went dodgey and honed out the bores and didn't install new pistons, unfortunately, a very common and quick fix to get rid of a smokey engine at sale time.
    If ANYTHING looks suspect or is at marginal limits, just replace it. I know it costs more, but it saves you disassembling the engine later.

    Regards, Andrew.
  20. I agree.

    Fortunately, I believe the engine has had top end work done before. The valves seem to seal correctly. Though I am going to take some proper measurements on those tomorrow or the day after.

    The reason I think it seals correctly is because, when a pool of WD40 is sprayed on the valves it doesn't leak.

    You're right though, it is worth taking it out and checking on the valves and springs. The tappers also look good.

    However, since it's already openned, it's well worth checking out I know. So I will be checking those, but so far on my inspection they look pretty damned good also with a lot less carbon than the pistons.

    About the spanner day. I work on saturday morning and my saturday afternoon may be booked out, If I get the parts for my bike, I'd love to get it up and running, if not I've got my sister's 21st that evening. I'd also feel somewhat out of place if my bike isn't up and running at that stage.

    However, I'll make an appearance if I can. It'd also be great to meet a few of you all!