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  1. #1
    sniper300's Avatar
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    Strange compression with (29) cam

    I have been having trouble with my cam timing for a while now and found something interesting.Recently rebuilt my engine with new pistons (8.4) 29 cam, valves etc and found static compression to very low around 110-112.If i advance the timing comp comes up but engine runs rough and won"t rev so the more i retard the better it goes but compression gets lower.I have two mates here with fresh engines aswell , both with 29 cams and fresh pistons ( of different makes aswell!!!) and sure enough their compressions are the same as mine. The one with the lowest (105-110) actually idles and runs a little better than the other two. We have tried different comp gauges and all read the same.So my question is has anybody else found static compression to drop after installing a 29 cam in place of a 51 or have all three got our cam timing wrong.!!! I will be putting my 51 cam back in in a few days to see if compression lifts just out of curiosity !!!!


  2. #2
    Moderator Snoman's Avatar
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    I would say you have your cams off a tooth..I put a 29 cam in my ski and am running/seeing 145 psi of static compression in all my cylinders...I seen it go as high as 175..Did you guys lock both the cam and the crank???

    Check this thread out, Vern did alot of research and there is alot of info that might be able to help ya..

    http://www.greenhulk.net/forums/showthread.php?t=143312

  3. #3

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    29 cam in my stock engine w/ stock head gasket is at 145+ retarded and 175 advanced-- no problems spinning it to 8600 rpm-- haven't ran it advanced thou.

    What stones does your mechanic use?

  4. #4
    Banned User
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    29 cam riva stock cr pistons 150ish retarded 175+ full advance

  5. #5
    SLASHER's Avatar
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    Well I have the same problem as Sniper and so another guy ...

    All the lock in pins where in and set up as by a Seadoo Dealer ...

    But my ski is a 07 RXP , does the 04 cam need to be set up different on the latter motors ?

  6. #6
    sniper300's Avatar
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    Click image for larger version. 

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    PIcs of timing before put my engine in!!!

    Snowman i have read Verns thread a few time and still can"t figure this one!!! When you are testing compression are you just using the battery in the ski or adding something to make it spin a bit faster when cranking.I noticed VErn said he hooked his battery charger to his and did it that way. I tried this and made no difference.We have spoke to LES Cooke and he said he has seen this before with low compression readings from not being able to crank the engine fast enough.My RXP used to run around the 81mph mark at 8500 with my Skat 14/20 setup.LAst weekend it ran 78 mph with same setup but at only 8100 . It got there pretty quick too but then started to nose over about 8000 so am thinking to retard the timing a bit more and see if rpm"s raise back up!!!

  7. #7

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    Your static compression is not important , to may variables eg.. battery power , compression ratio , cam specs , boost pressure . Every motor is going to be different unless they are all built the exact same way . If youve got a performance cam in it its going to have lower comp. until it comes on song , thats y they idle rough because manifold pressure will be lower . If your going to run high boost you should be running lower comp. If your not your going to lift the head off the block at revs because the comp. increases . Is that a decompress piston youve fitted thatll be part of your answer . As for the cam if the rings are all sealing properly and the valves are seating right just measure your TDC intake valve lift figure and if its right you cant go wrong ( it should be slightly more than exhaust on overlap ) Hope this helps

  8. #8

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    just read the thread . Some of you guys have got too much time on your hands , compression pressures are not important so long as they are even you would do better to spend your time checking valve lift @ TDC is what it should be . If this is right you wont go wrong , you should get a spec sheet that gives this figure and if you dont ring them they should be able to give it to you . If you still dont know what it is measure the intake and exhaust lift @ TDC on overlap and just make sure the intake has slightly more lift . It sounds like nobody is manufacturing vernier cam sprockets if they were im sure the conversation would change to lift figures and not comp. pressures . In all my yrs of engine reconditioning this is the first time ( no rudeness intended ) that i have ever heard of someone compression testing and engine with the plugs in and running let alone at WOT . Happy travels

  9. #9
    sniper300's Avatar
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    Hellman13 , no one is testing these engines for compression while running i can tell you that!!!! and the reason we hold the throttle WOT is you have to before you crank the engine over so it goes into limp mode and won"t try to start!!! The 3 engines i am refering to here are all running the same comp pistons ,same cam and valves on almost identical superchargers so there is relevance here to all having the same static comp. Until recently we all had stock internals and only bolt ons so we are new to building these Rotax engines but we are not new to building supercharged engines i can tell you that!!!! 1 of the owners of one of the 3 ski"s i am refering to is responsible for building some of the most powerful Small Block Chevs on the planet and still holds the Record for Worlds fastest Class legal Small Block sedan in the WORLD down th 1/4 , so yes we know about engines but we don"t know these ones that well yet which is why i come to this forum.I have been reading this forum for a long time now and there are some people on her who have been playing with these things for a long time and a lot longer than i have so i value their input and any advice they give so i wouldn"t be saying they have to much time on their hands!!!!

  10. #10
    Eric Ocman's Avatar
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    Are these skis running fine and not using any oil?

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