2003 XR100.. Little/no Compression

Discussion in 'Mini Racing' started by rcarson15, Mar 24, 2018.

  1. rcarson15

    rcarson15 Well-Known Member

    Hello! So I picked up a 2003 XR100 and can't get it to run.. we've tried to clean the carb, replaced the carb, new spark plug, air filter, adjusted the rocker clearances on the valves, checked the timing, and kind of out of options at this point. It has spark, but also has little compression (I dont have a compression/leakdown tester, but can pull the starter down with a finger or two) - some kicks have compression, some dont.

    I've talked it over with a few people and they suggest a potential top end rebuild, or it might be cheaper to replace the motor. What will I need, and what will it cost?

  2. TurboBlew

    TurboBlew Registers Abusers

    they are pretty easy to work on.
    Did you spin the cam in the tower? It only rides on a small film of oil and can wear the journal if run low on or out of oil. You would see some scoring and feel the cam loose if this were the case. Also on the throttle side is the oil galley plug(8mm)... you can check to see if you get oil pressure when you kick start it. I wouldnt judge compression by how easy the kickstarter operates. I would also check the timing. Some people partially disassemble the motor and get the timing wrong.
    Also Ive seen them have the wrong plug in them and the piston hits just enough to waller the threads. Amazingly it will still run with the electrode beat to shit! :D Also check the key connectors.
    noles19 likes this.
  3. noles19

    noles19 Well-Known Member

    Buy a cheap compression tester and check it, they aren't expensive and should give you an idea.
    And I agree with turbo, the cam journals wear really bad on some of them if they are run low, thats something to check and keep an eye on.
    TurboBlew likes this.
  4. TurboBlew

    TurboBlew Registers Abusers

    I think some autoparts stores actually lend out compression gauges. Just need the right fitting.
  5. PistolPete

    PistolPete Gone sailing...

    Better yet, find someone with a leak down tester. More important than your compression number, is knowing where it’s leaking. Listen for air at the carb (intake valve), exhaust pipe (exhaust valve), or oil fill hole (rings).
  6. Dave675

    Dave675 Well-Known Member

    Sounds like it's time to upgrade to the 150R
  7. joec

    joec brace yourself

    Takagawa dohc!!!
  8. speedracer170

    speedracer170 Well-Known Member

    Mine was doing the same thing and I had a bent valve. Pull the head and use a flashlight and see if you can see light thru where the valve seals.
  9. rcarson15

    rcarson15 Well-Known Member

    Almost have the motor pulled, hope it's nothing too serious lol
  10. rcarson15

    rcarson15 Well-Known Member



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  11. rcarson15

    rcarson15 Well-Known Member

    Hokay so observation.. the exhaust valve is bent or has a bad seal as you can shine light through it and see some beams (picture 1: top valve is exhaust) - the intake valve flat out looks awful (picture 1 - lower valve, and picture 3). It also looks like it was hitting the piston if you see the mark on the piston in picture 2. The intake valve also doesnt look like it hadn't been moved in a long while. Thoughts of what is wrong and what direction to go in?
  12. TurboBlew

    TurboBlew Registers Abusers

  13. TurboBlew

    TurboBlew Registers Abusers

    Just the parts for the stock head are about $80... or so. Aint worth putting any $$ into it.
  14. rcarson15

    rcarson15 Well-Known Member

    well im certainly not putting a big bore in if I dont have to lol, What parts would I be looking at for the head?
  15. TurboBlew

    TurboBlew Registers Abusers

    the valve guides look terrible. I think the last time I bought stock valves it was like $37 for the intake a $23 for the exhaust. Youll need a valve spring compressor tool to get the collets out of the keeper.
    Youll need some compound to lap the valve if the seats are serviceable. If you can find a complete 80 head... it will bolt right up and raise the compression a hair because of the smaller combustion chamber. Otherwise that kit I posted plus a stock carb with a 108 main will make about 9hp on pump 93.
    Sell your stock parts for ~$50 to recover some money... Also that kit has the preferred big valve head.
    If you want to make over 10hp... get this carb
    and this manifold
    Make sure its a genuine mikuni...lol.
  16. rcarson15

    rcarson15 Well-Known Member

    Ok so finally got the ball rolling on this thing.. a guy cleaned up the valve guides and lapped the seals for the new valves. Putting it back together, I'm finding that sprocket will not go nicely on the came, as if the chain is too small. I've check the cam guides for proper installation, removed both adjusters, checked the chain (no kinks or binding), the jug and head sit flush on each other, and I'm coming up like 1/8th of an inch short no matter what I do.. what am I missing here to get this thing on? Shouldnt there be enough slack in the chain to be able to put it on?

    Last edited: May 10, 2018
  17. AshdontcrAsh

    AshdontcrAsh Member

    Put the rocker cage on and tighten slightly, it will hold the cam in place and compress the head to the cylinder to help installing the timing chain.
  18. rcarson15

    rcarson15 Well-Known Member

    I've also tried that.. but at that point, the chain is incredibly tight and there isn't any room for adjustment? There isnt any more slack to take out
  19. AshdontcrAsh

    AshdontcrAsh Member

    For me, the way it's pictured in post 16, I can never get the sprocket/cam to stay in place and timed correctly without the rocker cage somewhat tightened. You may want to torque the cage down to spec ~14ft/lbs and then bolt the sprocket on. There shouldn't be much if any play in the chain, it usually only needs fine adjusting from there- in my experience.
    rcarson15 likes this.
  20. rcarson15

    rcarson15 Well-Known Member

    Update: Got the sprocket back on and the bike back together, got the valve clearances adjusted, and we have compression!

    It still won't start though, this thing has given me fits the entire way.. So I got everything back together, but now it seems as though it isnt getting gas. I also put on a new carburetor just to be sure it wasnt an old carb problem. I've sprayed in starter fluid, and itll start for a brief second, then immediately die. I have gas through the petcock and spark as well. Is this just a carb tuning issue now? What other options do I have?

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