Anyone Fab A Cheap Decompression Valve?

Discussion in 'Performance Mods' started by 5-7HEAVEN, Sep 15, 2009.

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  1. 5-7HEAVEN

    5-7HEAVEN Guest

    My HP pocketbike engine has high compression. It is also hard starting, as I have not been able to fire it up. After one second of running every 50 pullstarts, the engine quits.

    Pullstart is taking a beating. A decompression valve in the head would lessen the resistance for the pullstart.

    Has anyone ever fabbed a decompression valve for their engine? I know my Whizzer engine has one.

  2. DetonatorTuning

    DetonatorTuning Active Member

    this engine have a machined and taped landing for a comp. release ?

    might be cheapest to find one in the appropriate size from a chain saw.

    10HP is going to be a B@#*& to pull start even if it's working good, i'd look at the drill idea of Ed's pretty close.

  3. 5-7HEAVEN

    5-7HEAVEN Guest

    Steve, this engine does not have a machined/tapped boss for decompression valve. I would have to find a suitable location on the cylinder head to drill/tap/mount.

    Decompression valves for chainsaws go for $25 on ebay, should work well for a lot of motorized bike engines.

    I know what they're for, but how do you operate them? Do they have a spring-loaded button you push on? Does the valve stay open for a few seconds, or only as long as the button is depressed?
  4. DetonatorTuning

    DetonatorTuning Active Member

    the one on the Husky 350 saw i use daily pushes "IN" to activate and stays there until enough repetitive compression pushes it back out.

    makes a significant diff. on cold starts.

  5. 5-7HEAVEN

    5-7HEAVEN Guest

    Thanks Steve,

    Does your Husky valve tap into the head, near the sparkplug?

    There doesn't seem to be much room on the pb head for a dc valve. Need to inspect the engine closer:detective:.
  6. DetonatorTuning

    DetonatorTuning Active Member

    it's actually 90* away from the plug over to the side.

    i've never seen an after thought arrrangement, all the D/C valves i've seen were in heads where they were part of the oringinal planning.

    seems like with the right equipment it should be doable though.

  7. jimraysr

    jimraysr Member


    My 1971 Yamaha 175 trail has two holes for two plugs. Common then to switch the plug if your first one fowled.

    Mine has a decompression valve in the 2nd plug hole and a cable to the handlebar. It helped this bike when using electric start. Also very handy when on the side of a mountain and didn't have a spare leg to kick the engine over. I tried it on long down hill runs as a kind of "jake brake" but was never sure it did much more than make a lot of noise? )

    As an aside, Yamaha's 175 had a mag and mag lighting. The 125 had a 12 volt system and electric (motorize the generator) start. Both had the same bottom end, so for a price the dealer put a 175 head on my new 125 before I took delivery and the C/R was pretty much needed to over come the larger displacement and start electrically. The main advantage was we often rode at night after work and killing the engine on a mountain trail made you instantly blind.

    Last edited: Sep 17, 2009
  8. 5-7HEAVEN

    5-7HEAVEN Guest

    Not too much room for dc valve.

    This pb engine uses a replaceable compression dome which slips into the aftermarket head.
  9. DetonatorTuning

    DetonatorTuning Active Member

    the 2 piece head arrangement makes it even more difficult : ((

  10. Xqizid

    Xqizid New Member

    On my pb engine, same deal with the headkit with the squish set at .7mm it has huge amounts of compression.
    Usualy starts on the second pull.
    The stock pull start rope snapped but after I replaced it with Victa rope had no problems.
    The pull start is the alloy type.

    Cant see the need for a decomp valve.
  11. 5-7HEAVEN

    5-7HEAVEN Guest

    Xqizid, the new blister between my fingers tell me that a decompression valve would benefit man and machine.

    Steve, there are a few complete pb headkits, priced from $19 to $55. One seems to have a flat one-piece top near the sparkplug location.

    The standard dc valve's hole seems to be the size of a sparkplug hole. If there was one whose hole was smaller, that might work.