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Inlet valves not seating.

Discussion in 'Technical and Troubleshooting Torque' started by daewoo, Mar 16, 2008.

  1. My XR250 had a problem in that it would loose all compression, so when you were trying to start it, it was like you were holding the decompression lever.

    The problem was intermittent (of course :mad: )

    I rre-ringed the piston, and paid to have the head serviced and the (valves and valve seats re-cut), thinking that it was just old, coked up and worn out...

    Not happy when just after the rebuild the problem came back a couple of weeks later :mad:

    Today I pulled the rocker cover off, which means that there is nothing to activate the valves, so they should all be fully closed. The problem was still there...

    So I put compressed air through the spark plug hole, and can hear it at the air box.

    Assuming that the guy who did the head, did actually re-cut the seats and valves (I have no reason to think that he didn't), why wouldn't they be seating??? The only thing that I can think of is a bent valve or a cracked seat...

    Anything else before I pull the head off???

  2. :?
  3. i did the same thing when i read that....unless it's not OHC and has some fancy setup that i'm not familiar with :?
  4. Why am I getting the :? smilie???

    I pulled the cover off, because I thought that maybe the decomp mechanism was fouling or something... actually, I had sort of forgotten that all of the rocker gear is in the cover...

    What did I do wrong for the :?

  5. You need to make sure the piston is at top dead centre to apply the "compression" test.

    The rocker cover is just that. It keeps oil in and dirt out.

    Next question? :roll:
  6. But if all of the rocker gear is in the rocker cover (it is), when you take it off there isn't anything to active the valves. The rocker over is basically also the cam journals and holds all the rocker gear. All that you can see on top of the valves are the colettes )???) and caps. So it should mater where the piston is, because all of the valves should be closed... am I right or am I missing something???

    I know a little about car motors, but this is the first bike engine I have played with (and I am not enjoying it :( )

  7. Assuming the valves were machined and seats cut .. it could be lack of spring pressure. I'm also assuming the have the cam timing set correctly ?

    :? :?
    Intake valves begins to open just before TDC, closes ABDC.
    Both Closed nearing TDC Compression Stroke, Plugs ignite, Combustion takes place,Exhaust begins to open ABDC to expel burnt gases, Intake opens ( whilst exhaust is still open ) to assist in scavenging and helps to provide a cleaner Air/Fuel fill.
    Intake and exhaust valves DO actually open when the piston is at TDC during overlap.
    This, of course is 'roughly' the process.
    so YES it Does matter where the piston is in relation to valve operation .. ( ie Valve/Cam Timing ). If this timing is out .. She wont fire .. possibly backfire.
    Even worse, if your valve timing is that far out, you have possibly BENT the valves :eek:hno: Hense ZERO compression

    EDIT : Have you set valve clearances??? zero clearance = zero compression
  8. I understand that, but if no rocker gear is attached, just the springs holding the valves shut, and a cam sitting there without anything running on the lobes, there isn't anything to activate the valves...

    I am assuming thatmost motorbike engines don't have the rocker gear bolted to the rocker cover like mine does, so it is removed with the rocker cover... most motorbike engines have a 'dust cover' like Grey Gentry described...

    Just to clarify, treat this as if in a normal engine I had removed the cover, and all of the rocker gear... nothing left but valves, springs, and a camshaft without anything running on the lobes...

  9. Valves

    If the rocker gear is in the rocker cover there's nothing to push the valves down as that is how the valves are activated. Therefore something is wrong such as valve seat problem or bent valves. When it was assembled was the valve timing correct? It is very easy to bend valves as the stems are very thin. Rotate the piston to BDC (bottom dead centre) & see if you can push the valves down & see if they they move freely.
    Doesn't sound good though. :cry:
  10. [quote="daewoo]
    I understand that, but if no rocker gear is attached, just the springs holding the valves shut, and a cam sitting there without anything running on the lobes, there isn't anything to activate the valves... [/qoute]
    With all valves closed, you won't get much, if any, compression reading as there will not be any cylinder filling. However if after applying compressed air, you can hear it leaking out the airbox, then chances are the Intake Valve is NOT seating .. SPOT ON !
    Bent Valve ... or poor machining is my guess.
  11. Thanks for the Help...

    Thanks for the help... sorry for any confusion, not knowing bikes, I didn't know that the rocker gear isn't typically part of the rocker cover...

    looks like it is time for the head to come off and see if there are any marks on the top of the piston... if not, I guess I talk to the bike shop about some sort of warranty...

  12. Re: Thanks for the Help...

    No way.

    You have forgotten something.

    In the cam cover should be two inspection plateswhich allow you to check the valve clearances.

    You haven't checked valve clearances, have you. That must be done with the rocker gear intact and the covers off. If they were too tight when you reassembled the head then no wonder the valves are leaking now.

    You could have also bent a valve through over-revving...

    If the clearances are too tight - that is an adjustment on the end of each rocker, should be a screw with locknut - then of course the valves will leak eventually.

    I can't see any mention in any of your posts about who refitted the head and adjusted the tappet clearances.

    All the best

    Trevor G
  13. Re: Thanks for the Help...

    Actually 4, two for inlet, two for exhausts, it has the RFVI (or something) head. I have adjusted the tappets a few times, and it didn't help... but I can tell you it is a shit of a job...

    and remember, even with all of the rocjer gear removed (the tappet adjustment is in the rocker cover as well)... no compression and noise in the airbox when I put compressed air into the cylinder...


    I pulled the head off last night, with the engine still in the bike... I filled the inlet ports up with petrol, and it runs past both valves, one worse than the other... so I am guessing bent valves (who me over reving???)... I guess there isn't really any way to know, but I think this is still the original problem that I rebuilt the engine for... There doesn't appear to be any marks on top of the pistons...

    Thanks again everyone for your help...

  14. Where are you located ??
    If you have any trouble getting the bike shop to help you out .. PM me
  15. Thanks for the offer... I'm in Sydney, southern suburbs...

    I dropped into the bike shop this afternoon, and he said bring it back and he would have a look at it no problem, so we'll take it from there...

  16. Re: Thanks for the Help...

    There doesn't need to be a mark on top of the pistons - the valves will frequently hit each other when you they bounce.

    Possible reasons:

    1) Springs too soft, or not preloaded the correct amount.

    You need to go to someone who has a spring gauge. Measure the installed length of the springs, then remove them and check them on the gauge at that same length.

    I don't know what Honda specify for your bike but a seat pressure of 50 lbs (22 kg) is often specified for valves around 22 - 28 mm in diameter. It could apply equally to smaller and larger valves - I just don't have knowledge (yet) in those areas.

    Some springs lose their tension really quickly. Sometimes repairers replace with the wrong springs. I am just in the throes of repairing some Moto Guzzi 4 valve heads which had 20 lb seating pressure springs.

    When they finally bounced once too often at 5200 rpm for the previous owner the heads came off the exhaust valves as a result of clashing with the inlets, and destroyed the cylinder head and piston. I have 2 bent inlet valves to show for it. The normal redline is 7800+ rpm.

    Make sure that there are spring seats fitted as well.

    2) Valve timing out. I believe it doesn't have to be out a great deal on some engines to cause a problem. Check those timing marks again!


    Trevor G
  17. I think the potential problem has been suggested, whether its the valve, guide, seat or spring, but i just wanted to say that for the record, it used to be quite common practice to have the rockers installed in an alloy 'rockerbox' rather than have the mounts cast into the head.