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Potential problems with engine?

Discussion in 'Maintenance and Servicing' started by Meyerhoff, Sep 12, 2016.

  1. So. I have a honda cbf250. Runs fine but the valve clearances have not been checked. All valves need to be reshimmed. Never done that before but im doing it now so hopefully i dont screw up the camchain adjuster.

    What potential damage could have been caused by all valves being too tight? What should i look out for whilst im in there?

  2. Two words: burnt valves.

    IF the valves were not just a bit tight, but not sealing properly, then you might consider looking in at the valves, especially on the exhaust side. If the clearances were just too tight rather than GONE then you're probably ok. Just get them set properly. If you don't know what you're doing get a workshop to do it. Otherwise AT LEAST get a workshop manual and follow it carefully. Don't just try to wing it.
    • Agree Agree x 4
  3. How do you know the valves need to be re shmmed????
  4. i already got in there once and could only get a .02 under one of the exhaust valves. Ive got the cover off now and am now checking again. Just checking the manual since i was confused about the F and T markings on the flywheel. Man says T is TDC. IDK what F stands for.

    I saw a youtube video of a guy doing a how to for valves. He said he knew his exhaust valve was stuck open. I was wondering if there is any other potential problems like that i could diagnose whilst looking inside. Also trying to to figure out how to deal with the cam chain tension adjuster since the manual doesnt mention it.
  5. Unless it's been replaced with an aftermarket unit, the tensioner is most likely an automatic jobbie and it will be mentioned in the procedure to set the valve clearances if they use shims of any kind but possibly not if it's screw and locknut adjustment.

    The F usually stands for Front.
  6. Just check them where the back of the cam lobe is facing the valve. That will be your maximum clearance.

    And yes, I'd be worried about a burnt valve seat at that clearance.
  7. i just cant believe all my valves were that tight without me noticing major problems. Trouble is the .02 is so flimsy i cant get it under with all oil in the way.

    How would i recognize burnt valves and how can i repair them?
  8. compression test will give some indication.

    to repair you have to pull the head and get it reconditioned. If it's a minor scoring you might lap it out yourself, but then you wouldn't notice that on your compression test.
    • Agree Agree x 1
  9. Hard starting/ rough idle can also be symptoms of burnt valves.

    Looks like this may be a common problem with the CBF250's.
    There is another forum where a member from Melbourne had the exact same thing happen to him. He's 2006 CBF250 only had 12000km on it but the clearances on the exhaust valve closed up and this resulted in burnt valves.

    He repaired it himself. Hopefully you caught it in time, since you don't have any running issues.
    Here is a link.
    The Neglected Honda project...
    • Winner Winner x 1
  10. #10 Meyerhoff, Sep 13, 2016
    Last edited: Sep 13, 2016
    Is it possible for the flywheel to be out of alignment with the camshafts/sprockets?

    Ive got some measurements off all but one of the valves. But i did that by aligning with the with the I and E marks on the cam sprockets. When i looked at the T mark on the flywheel whilst the sprockets were in line, theT was just a tick before lining up with the mark on the screw hole

    the back 2 were looser than the front 2.
  11. Is that a pre unleaded engine
    Worn cam chain/sprockets, loose tensioner, turning the crank the wrong way when you bring it to the timing marks, all possible causes of what you observed.
    But since all you need is for the cam to be on the base circle of the one you're measuring it shouldn't cause any error in your measuring and adjusting.
    • Agree Agree x 1
  12. no. Its 2007. Manual says it can take e10. It oughtta since it was made in brazil.
    • Like Like x 1
  13. There's a big difference between "can take" and "actually runs well on". The former means the fuel system components won't dissolve into jelly at that concentration of ethanol. The latter means it will actually run properly. The only way to know what it runs best on is to try it. It will most likely feel best on straight petro fuel, either 91 (regular unleaded) or 95RON (premium) but not feel much better on 98. Try it and see.
  14. #14 ibast, Sep 14, 2016
    Last edited: Sep 14, 2016
    Forget about the timing marks for the time being. check your clearance with the cam lobe pointing directly away from the valve.

    The manual is just written about the timing marks to minimize the number of times you have to reset the position. It may take longer by not using the timing marks, but it will be just as accurate and less confusing, if the marks don't make sense.

    Once you are confident you have the correct clearances then check the timing marks against each other. If they don't align, you are either one tooth out on the chain or the chain is badly worn. Someone may have got them out of alignment in the past. It's not hard to do if you let the chain slip off the bottom sprocket when you back off the tensioner.
    • Agree Agree x 1
  15. I asked about the unleaded or not because I was wondering what caused the tappet clearances to close up, I wondered if it was an old bike designed for leaded petrol.
  16. Fair point and you're right to question that. Unleaded, and especially e10, is bad mojo for an old engine built for leaded fuel.
  17. Fwiw, in the car world, leaded was only really needed for cast iron heads which didn't have valve seat inserts.
    Pretty much every aluminium head has steel inserts, and don't need leaded (eh, Toyota since 1960's, but not.Holden until they went to alloy heads)

    Poor/cheap material choice for seat and valve can result in some valve or seat recession, which would.tighten up clearances. Not too surprising on a budget build.bike.
    • Like Like x 1