some info about compression release

Discussion in 'Antique Motorized Bicycles' started by Sianelle, Nov 28, 2007.

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  1. Sianelle

    Sianelle Guest

    alaska van posted something that was interesting:

    A compression release is a brilliant device and it's a shame that they don't put them on motorcycles anymore. A quick flick of the release lever and the resultant 'bang' down the exhaust pipe is teeeerrrific for getting rid of a pursuing dog. I'm hunting for the model of Villiers cylinder barrel that's fitted with a compression release for my own retro motor-bicycle replica. Not that I get chased by dogs much, but the main reason is that I prefer to have a compression release over a clutch or using non period brakes.
    Last edited by a moderator: Nov 29, 2007

  2. Sianelle, I've seen compression release kits on ebay, they fit a 14mm sparkplug hole. it could be machined into the head.

  3. ibdennyak

    ibdennyak Guest

    what's that little button for

    Many larger chainsaws have them too. Local Stihl shop had them for me.
  4. Sianelle

    Sianelle Guest

    Well that's something I didn't know :confused: Thanks ibdennyak and antique-rider :grin:
  5. ibdennyak

    ibdennyak Guest

    compression release

    Actually, I should be thanking you for the idea. (the satisfying bang) Most dogs reaction to my bikes seems to cause them to lift one of there legs. Might be nice to perform a bit of behavior modification. :grin:

  6. gone_fishin

    gone_fishin Guest

    obviously, i don't have the heart to throw good info away, so i made a topic out of this misplaced stuff. we'd appreciate a bit more help... :)

    a few clicks for you saves many for the staff :cool:

    btw-i AM interested in the contemporary applications, if any...and wonder if it's still a benefit with the modern motoredbike? beyond that neat boom for the dog, that is ;)
  7. Sianelle

    Sianelle Guest

    I'm sorry, - it's just that it's all soooooo exciting and interesting and I sometimes forget the rules (blush).....
  8. OldPete

    OldPete Guest

    On 2 smoker dirt bikes of the 1960s and 1970s a compression release was fantastic for down hill braking when the hills were steep and/or covered with loose gravel. A compression release gives a stuttering type of braking and the tire has time to regain traction.
    Many dirt bike engines came with two spark plug ports, one to the side of the chamber and one dead center. Using the center spark plug gave the best performance.
    Because strokers of that period tended to foul plugs, a spare could be carried off center and just the plug lead needed changing over. Some bikes went as far as having two ignition systems that chould be selected from a bar mounted switch (DKW).
    Aftermarket compression releases were common "back in the day" and the off-center spark plug port was used for installation. Using a 2 stroke's compression release, releases unburned hydrocarbons into the atmosphere.
    However...A compression release is valuable for push or pedal starting a reluctant engine.
  9. gone_fishin

    gone_fishin Guest

    it's all soooooo exciting and interesting and I sometimes bend the rules...wutuyugunudo, eh? :rolleyes:

    btw-azkronic, drimpact, & srdavo do this sorta stuff day in
    and out, & ne'er a complaint (to you)...

    thanks, amigo's :cool::cool::cool:

    pete...i translate "no" to contemporary benefits on compression release (?) thankee, too :)

    keep us posted? :)
  10. OldPete

    OldPete Guest

    Current large displacement, kick start, dirt-bike 2 strokes have compression releases to aid in cold starting. The engine must be slightly past TDC, then kick started.

    The Robin Subaru 35 (33.5cc) has a decompressor built into the camshaft to ease pull starting it.
  11. gone_fishin

    gone_fishin Guest

    doh! i knew that! ;)

    i see how it would be a mechanical help for starting, but i guess i'd have to ride something with one to see how it would be used for decent control at speed.
  12. smitty

    smitty Guest

    The Jacob's brake, (or j-brake, which is a compression release) on diesel trucks, provides "engine braking" without having to gear down.( Wouldn't it be nice if we could gear down?)
  13. Sianelle

    Sianelle Guest

    The Villiers engine with this feature has a boss specially cast into the cylinder barrel and the cooling fins are arranged so everything clears the decompressor. The standard barrel lacks for this feature and I don't think it's possible to bore a 14mm hole anywhere as the casting isn't thick enough. Some barrels have a small boss cast into the head to take a 1/4 inch mounting bolt for a fan cooling shroud and it may be possible to do something with one of these if I hold my mouth right.
  14. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Back in the day when I raced my 450 CC Ducati I had a 10 MM compression valve assembly screwed into the head [stock]. In the beginning I needed the release to start the big 4 stroke single, but in time I learned to start it by just going past the compression stroke and using my entire [at the time 155 lbs] body to force the kick starter downward. The compression assembly was then removed and replaced with a 10 MM spark plug attached to a second coil for more fire in the head from the magneto. During the same period of time I witnessed many, many, 2 strokes locking up at the end of the straight away when the riders used the compression release as a brake [no courage] to slow down for the corner. It always amazed me how many racers didn't realize when the release was open [and the throttle closed] and the motor was at maximum RPMs and NO OIL in the cylinder that the motors locked up. It didn't take long to watch the compression release vanish from the flat track circuit, because it made an expensive brake [broke].
  15. ibdennyak

    ibdennyak Guest

    Saaaay, maybe you can answer a question for me. Years ago I took a YZ 125 and supposedly a 500cc Ducati thumper in on a bill. I didn't realize the thumper had a compression release the first time I tried to start it. Yup, just like a cartoon...backfired and over the tank I went. Still have a scar. (Geez, another candidate for silly pics thread) Was it actually a 500, or maybe the same as yours? Thing was phenominal though idle is 2nd, crack it open, and an endless wheelie.

  16. Sianelle

    Sianelle Guest

    I found that really interesting Quenton Guenther, but for my 1920s designed low reving, soft tuned, heavy flywheeled, two stroke engine a decompressor isn't going to cause it to lock up or do anything else that a well bred elderly English engine shouldn't. :smile:
    Last edited by a moderator: Nov 30, 2007
  17. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi Denny & Sianelle,
    As far as I know the largest Ducati single was in fact called 450 CC [actually less, I think it was around 438 CC]. The Ducatis I raced were the Desmo version.
    The average 2 stroke motor with a compression release wouldn't be in danger of locking up. The problem only occured when the compression assembly was opened with the motor at maximum RPMs with the throttle closed [supply of gas with oil mixed in was stopped], stopping the oil needed to lube the cylinder & piston.
    Last edited: Nov 30, 2007
  18. Herrmanator8

    Herrmanator8 Guest

    my dirtbike has a decompression lever but it releases too much compression and doesnt help in starting it at all...the 03 model has auto decompression for starting.