anyone with a decompression valve

Discussion in 'General Questions' started by japat100, Jul 15, 2007.

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  1. japat100

    japat100 Guest

    i have a old 49cc moped that you peddle to start ,,is has a decompression value ,,what a difference to start ,,,we read allot on this forum about back wheel skidding ,,, and i am willing to bet that starting these motors with there high compression is the worst thing you can do ,, if anything is going to break or twist this is the time ,,and the bigger the back sprocket at the back the worst it gets ,, i am planning to take the one of the old moped or is there one available you can buy ,, and if theres not then why not ,, this will make starting 10 times better and save parts and also safer
    yes the jerk starting these bikes (don't take that the wrong way)
    is 10 times more damaging then any amount of power that a motor will produce

  2. retro_racer

    retro_racer Guest

    I'm not sure if anyone person makes them for these motors,but they are avalable.They are also pretty cheap too.I had the same idea you had,and have looked around alot.There are tons of different ones avalable in Ebay.I was thinking that they wouldn't be that hard to adapt to just about any small engine.You will have to take the head off to start.Then remove a couple of small sections of cooling fins.Drill a hole,and thread it.Smooth out any rough spots,and install the valve.Connect the cable to a brake type lever,and Bod-a-Bing compression release valve.
    There were alot of the older MAC go cart,and chainsaws that used this set up.It realy helps with the higher compression racing motors,and realy large cc chainsaws.Alot of the older motorcycles both 2 + 4 strokes used them too.It made it alot easyer to pedel start the early big twin engines.I hope this helps Rod
  3. To bad we dont have dual spark plug heads on these, that would simplify a de-compression valve install.
  4. OldPete

    OldPete Guest

    Yes, this was done "back in the day."
    Smog laws kinda forbid dumping unburned fuel into the air, even trace amounts. Might find an old comp release at a M/C junk yard.

    The Robin/Subaru micro engines use a comp release that is in the camshaft. As the engines starts (via pull cord) centrifical force deactivates the device.
  5. Herrmanator8

    Herrmanator8 Guest

    our Yamaha YZ250F has a decompression lever on it so that its easier to start. the dirtbike has some serious compression though, i have to stand on the kick starter for 5 seconds to push it down a little bit then im able to kick it. it normally starts in two or three kicks but when i use the decompression lever. it doesnt give any sign of starting, its alot easier to kick but i have the feeling that it gets rid of too much pressure. yet, its also proven that starting a motor without decompression is better for it because it builds up pressure to pull more gas in then when it turns over it creates enough energy to create a spark. i know this from past experiences.

  6. psuggmog

    psuggmog Guest

  7. HI,

    Just curious but how would the decompression valve help...I mean at some time you have to recompress huh?


  8. grakker

    grakker Guest

    I have to admit my ignorance here as well. Don't you need the compression to start? Off to google I go.
  9. The compression release is to get the engine spinning easily, then when you have some momentum you drop the valve and then have compression. Whizzers use them, all the old "pedal start" motorcycles (pre 1915 or so) had them. You absolutely cant get past the compression with out them.
  10. japat100

    japat100 Guest

    i have a old moped with one on ,,what happens as i understand it that a motor will start about 60 psi so the decompression valve only lets a certain amount of air to escape because of the size of the escape hole ,, so if your motor runs 125 psi or more then you can see what a difference it would be to peddle start ,, and as soon as the motor starts you close the valve that puts you back on full power ,,
    it makes a big difference especial when you only have one gear
  11. OK....I see now...It lets you start at a low compression then you close it and have the power of max compression....Seems to make sense now.
  12. japat100

    japat100 Guest

    the faster you peddle the more compression builds up with the valve open only a certain amount of air escapes in each stroke ,,so when it hits about 60 psi it will fire ,, a hundred times easier to start my moped with decompression value ,i going to try hook one up on my mb ,,i think someone on this forum gave us a address where i could get one ,,got to make sure the air flow escape value is right size ;