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  1. #11
    Wonders How Big It Is piratius's Avatar
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    Compression at 120psi (throttle fully open) seems low for a motor with a freshly rebuilt cylinder head. When you did that, did you use all new gaskets? HD or Cometic? Do you know what thickness you used? Did the cams get adjusted/moved at all? Pushrods put back in the right order? (one is longer than the other!)

    I still think the jetting should be 45/175 for a bike with the intake/exhaust combo - running lean will feel really down on power, especially when you get up into the higher RPM's/throttle positions.

  2. #12
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    Jet-Lee's Avatar
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    120psi does seem low. IIRC, a couple Blasts on here were at 130psi and required rebuild (Stang and Piratius?).

    If it's a fresh rebuild, I would look at the cam timing.

    I would also suspect the valves not properly seating, especially if it's not a new head.
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  3. #13

  4. #14
    Wonders How Big It Is piratius's Avatar
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    Ok - so Nyghtfall has a modified stock intake (probably with a K&N) and an exhaust, but he's still at 45/170 with NO SHIMS. Yes, the compression is a little low - but he'll probably get a lot of power back if he puts in the 175 jet and shims the stock needle up .040".

    -Now back to the low compression!-
    • If your battery is weak/low it will make compression numbers come out low, but wouldn't have any effect on the performance while running. Did you put any oil on the compression tester O-ring to ensure it was sealing? 20psi difference from closed throttle to fully open during testing is NOT very much :-/
    • If you removed the cams during the work, I agree with Lee and EZ - check the cam position.
    • If you didn't touch the cams, then I would verify static timing. The fact that adding oil didn't raise your compression means that the rings are most likely OK, which is good!
    • Where did you get the cylinder head from? Someone local, or from the internet?
    • Did you have a shop verify it was in good shape and check the valve job? Did they do any other work to the head at that time?
    • When you reassembled the rockerbox and pushrods, did you put the long pushrod towards the back, or at the front of the motor?
    • What gaskets did you use? For a stock rebuild, hopefully you got new Cometic gaskets rather than the paper HD style.
    • Did you check squish clearance when you did the rebuild?


    With these thoroughly answered, we can keep going on helping you get it figured out!

  5. #15
    Gearhead crazysycodude's Avatar
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    well the HD gaskets wont give bad results because i rebuilt mine with HD stock gaskets and mine is at 200 PSI or 250 with oil. but i also closed the squish clearance and have a few other head related fun stuff.

  6. #16
    Wonders How Big It Is piratius's Avatar
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    How did you change the squish clearance with HD Gaskets? Did you omit the bottom gasket completely and use gasket maker/sealer?

    It's just my personal preference for the cometic gaskets - they're multi-layer-steel, so they come off with no mess, and no need to scrape off old gasket material over and over every time you pull down the engine. The stock papery gaskets were a massive pain to clean off well enough to satisfy me, and if I never have to do it again, it'll be too soon!

    -B

  7. #17
    Gearhead crazysycodude's Avatar
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    You are right, no base gasket and smaller head gasket, I actually didn't have any cleaning of old gaskets when I rebuilt. I think HD redid their gaskets because none of mine were paper but I don't think they were metallic either. They just seemed like normal automotive gaskets so I didn't really think to far into it lol

  8. #18
    Super Moderator ezblast's Avatar
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    The new HD gaskets are an almost direct copy of Cometic gaskets - Lee and I were comparing them - lol
    EZ

  9. #19
    Gearhead crazysycodude's Avatar
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    Well that explains alot lol.

  10. #20
    Wonders How Big It Is piratius's Avatar
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    Yes it does! The stock ones that came off my 03 were a super dense paper type (tar paper? Who knows!) that stuck to everything - I tried different solvents on a rag, and ended up just very carefully scraping it all with a fresh razor blade. The cometic gaskets (and new HD style, I suppose) are all MLS - multi-layer steel, and those things are awesome in comparison!

    -Brad

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