48 thoughts on “How a Small Engine Decompression Valve Works

  1. I have one on my craftsman chainsaw it went bad a while ago and I am waiting to get the part to fix it . I have not found a way to bypass it by replacing it with something else . My sons 1987 yamaha 350cc had one in the head of the quad and a pull starter there were only 2 years 87 88 that had pull starters and decompression valves.

  2. @IW5Industries learn the basics from my dad in my teens, had a few classes in high school and the rest is self taught

  3. good video explaining an excellent example of innovative minds employing simple and effective inventions that reap rewards multiple time over and above the investment it takes to add.

    I am sure this will save many a right or left hand the pain similar to the condition known as plough hand

  4. I have a very old briggsandstratton 6hp engine that is in a golfcart it was in a barn for 15 years i put some money into it and it was working fine. And it was smoking so i drained the oil and put fresh in but and it still smokes and now it just stoped working im only 14 but im really into this kind of stuff could it be the piston rings? and is it worth fixing? Thanks.

  5. It work's the same as compression release valve a lot of snowmobile's had them
    I seen a outboard motor that was automatic I pulled on the recoil And I could see
    Levers turning it on and off but it had a adjusting nut's for the release valve. They
    should make a RC engine for weed trimmers just to see how they turn out.

  6. That is just one "style" of decompression valve. I would like to see other "compression release" systems say on Stihl saws or on larger Honda 4 strokes.

  7. Great video as always. I have an Echo weed wacker acting as is it has too much compression to pull could it be a auto decom as it doesn't have a manual one? If so I take it it is a major strip down?

  8. hi donyboy
    I have a problem on my ts420 cutsaw the decompression valve pops open on every two pulls, i put a new decompression valve and it still pops. The starter rope is hard to pull can you tell me what the problem can be thanks

  9. hey, I have a Husqvarna 335 XPT. It's really hard to start but I don't think it has one of these valves? Any ideas? The exhaust isn't stopped up. It's not flooding. The flywheel key isn't sheered off. To crank it you have to pull it just past the "hard spot" then give it a good yank and it starts right up and runs great. I see the same saw on youtube videos but they are really easy to pull start. I'm going crazy over this damn thing, HELP!!!

  10. I have one on a 71cc 2 stroke earth auger & It's become very hard to pull-start with the valve  in or out. I feel no difference on the recoil starter either way.

    Do these things ever fail?

    How do you test them?

    Is the opening direct to atmosphere such that you can blow thru by mouth or with compressed air& actually feel it coming out the other end?

  11. donnyboy, as always, great video.

    i have an old victa corsair lawn mower and it too has a decompression valve.  however, unlike the chainsaw, it has a tube going to the carbie. 

    what i want to know is whether or not it serves the same function and is it required (meaing, can i just plug the hole with a bolt if its only function is to ease starting)?

    i only ask because i want to turn it into a horizontal mount for a four wheeler.

  12. Thank you for the informative video. I have a saw with this on it and have never really understood the purpose as it starts beautifully without using the valve. I always figured it was to assist in getting the cylinder rotated around for a full cycle to make it easier to start. Thanks again!

  13. hello, i have a problem with decmpression valve on my husqvarna 357xp, i take it off but it seem is clogg something inside there(the valve is ok)

  14. I wanted to comment after reading this that a lot of engines from chainsaws even riding mowers have that mystery lever or the valve hidden way under some cover you really have to look folks. It's important to save your saw or weed whacker tractor and mower manuals after y'all shop at Lowes and Depot's MADE IN CHINA lawn and tools bins… Read the shit people you may actually learn how your shit is supposed to work what gas and oil to use…. And then there are shops like mine where you get charged you for that re-information because we have to pay the guy $$$ that works for us to re-inform you about YOUR SAW or MOWER…. . Whereas the info in those discarded manuals you lined the cats little pan with or burned in the wood and coal stove……… LEARN ABOUT YOUR EQUIPMENT PEOPLE. BALDE SIZES MATTER SO DO  CHAIN and TOOTH SIZES as well as  BELTS AND TENSIONER ADJUSTMENTS!!!!!!!!!!!!!! HAVE A FEW CANS OF BEER CALL ME IN THE MORNING WHEN THE BEER FOG WEARS OFF…

  15. While cleaning my Pioneer P20, I noticed it has a decomp valve that is maintained open by a spring and automatically shuts closed through combustion pressure. Very ingenious and a welcomed feature in such an Old (30 years) and small machine. Built in Peterborough, they were good !

  16. Nobody can figure out why my TS500I quits running when it's cutting for 10 minutes. Is there a overheat protection temp sending unit? The local Stihl dealership has replaced all the electronics twice. They threw their hands up in defeat.

  17. I have the same saw but there is no decompression valve, but i see the place where you can put one . my question is , can you put one ? it looks like you have the option from what i can see . i never saw anyone making a video on that, would be nice to see .

  18. what about weedeaters? i have 2 echo weedeaters, my brain is rusty on this one. when i pull the chord it pulls but the chord freezes like 4 times per pull. i have a bad rib so i simply cant do this i need to fix this. i took the spark plug out and pulls the chord and it glides easyyyyyy.

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