The Miracle of Anti-Decompressants

Discussion in 'Thumpers' started by BillyD, Mar 25, 2013.

  1. BillyD

    BillyD Been here awhile

    Jun 17, 2007
    San Mateo County Coast, CA
    As some of you know, I’ve recently reconfigured the 640 to reduce engine vibrations.
    (See: )

    In the process, I also took the opportunity to upgrade to a Rallye Cam, which has slightly more lift, duration and overlap compared to stock, but it wasn’t enough of a change to mandate a significant change in jetting.

    Nonetheless, I was having extreme difficulty in getting the jetting right. I could either get it to run great at WFO, but then it would not idle and would abruptly die when coming to a stop. OR, I could get it to idle roughly, but then it would be extremely blubbery in the mid-range and the RPMs would hang on deceleration.

    I tried at least 30 different settings on two different Keihin FCR 41mm carbs (MX and Single verions). Rechecked all ignition components. Still, I could not get a steady idle, good mid-range and WFO performance. There must be something else at play…all the basics were good.

    The Three Stooges (Mikuni CV, Keihin FCR Single, Keihin FCRMX)

    That’s when I started to suspect the automatic decompression mechanism. Quick checks with ThumperRacing and DJH reinforced this suspicion. So I decided to remove the AD weight and spring and see what happens without changing anything else.

    Stock Cam with Auto Decompression

    Well, guess what…removing the auto decompression made all the difference in the world. The engine now purrs like a kitten at idle and pulls strongly and cleanly across the entire load range using the jetting set-up recommended by DJH. Before, it couldn’t even idle.

    Rallye Cam with Auto Decompression Weight Removed

    For those of you really interested, here’s a before and after video…
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  2. Kawidad

    Kawidad Long timer

    Jul 9, 2005
    Central Coast, Cal
    Nice :wave
  3. lamotovita

    lamotovita Ageing Adventurer

    Oct 5, 2007
    WA/AZ, USA
    Are you using electric start and does it work ok?
  4. XDragRacer

    XDragRacer Long timer

    Mar 25, 2009
    Congratulations at the results, but . . . any explanation of HOW they were obtained?

    Understand a functioning automatic compression release is only operable at cranking speeds, of no consequence at idle or higher rpm.
  5. larryboy

    larryboy Chopper Rider

    Jan 9, 2005
    Über Alles,California

    I bet the cam he got had been over revved at some point, it'll stretch the spring slightly and cause all sorts of problems.
  6. Beezer

    Beezer Long timer

    Oct 26, 2004
    Anchorage, formerly Spenard (hub of the universe)
    stretched springs could screw with the low speed part, but the flyweights are supposed to pull the decomp out of the system at anything over starter speed. removing the springs should disable the system. it's not like an old flyweight advance for ignition timing. mass on the off side of the pivot pin pulls the finger out of the path of the valve. very curious, since the cam timing does not change... the system only holds one ex. valve open when its working correctly right?

    is the cam lobe worn??? making it possible for the c/r to hit the valve?
  7. Seikkailu_R

    Seikkailu_R Been here awhile

    Jun 18, 2007
    Piikkiö, Finland
    Both decompressors open both exhaust valves.
    Forked h-shaped rockers, one end on cam and two on valves.

  8. VictorADV

    VictorADV n00b

    Jul 12, 2014
    A few months ago I came across this thread since I had broken my auto decompression from over-revving during misshifts whilst practicing stuff. At over 2500rpm my autodeco was still clacking away at my exhaust valves.

    I read this thread and thought what the heck I might as well just remove the autodeco, and so I did (the spring that's supposed to be under the autodeco cam wasn't there; explains why it was broken). To E-start the bike you now had to hold the manual decompression lever JUST right so you DO decompress, but not so much that it wont start. You really have to get a feel for it but after a while you'll get the hang of it.

    Unfortunately this is where my problems began.
    So with no autodeco, every time you turn the engine off is really hard on the entire engine: in particular the starter system. The engine actually stops like it's hit a wall, and is quite noisy at that. Every time you shut the enigne off the starter clutch is engaged. Because the inertia of the rotating mass slams into the massive 625cc compression which pushes the piston and whole engine back backwards, the starter clutch is engaged and turns the whole starter assembly backwards. The starter clutch, starter gears and starter motor get a real punch in the face this way and they really dont like it. So much in fact that first some teeth broke off one of the intermediate gears and a week after fixing that my starter clutch died on me, making a ton of awfull noise. After taking it apart it turned out the starter clutch was absolutely ruined.

    This problem might be relieved by manually slightly releasing compression when shutting off the engine. I never got to try that thought since have now happily reverted back to auto deco :clap

    Further more the autodeco keeps the bike from stalling when doing stoppies, or for instance when you have misfires, or when you have too much of a bog because you'r AP is off. In other words its really helpful.

    In the video posted above you don't hear the tell-tale CLACK of the autodeco killing the engine, sounds more like it just misfires and dies from incorrect jetting as the OP said himself: "The engine now purrs like a kitten at idle ... using the jetting set-up recommended by DJH. Before, it couldn&#8217;t even idle.". In fact any added rotating mass (autodeco in place) should actually make the idle smoother. And any unbalances should not cause idle fluctuations, just mechanical vibrations.
  9. dnrobertson

    dnrobertson Big Bike, Slow Rider

    Jun 30, 2008
    Frankston, Australia
    My auto-decomp is currently not functioning (just stopped on a new cam after less than 2000 kms).

    I have got into the habit of always stopping my engine using the manual decomp lever. I just pull it ALL the way in and bike stops.