Decompression valve question

[jeffatc250r]

My 250cc hotsaw was giving me issues with breaking the pull rope. So i did some hunting and found this cute little guy and thought perfect! The thing i like about it most is its fully automatic since it runs off of vaccum, and its cheap.
Its a tight squeeze on a 250, but will be much easier on a 500. I plan on putting one on my trike and that should eliminate most of the comp and allow me to use that fancy forward kick contraption allowing me to start it without taking the fenders off.
Anyways, my question is this: where do i get the vacum from? Ive seen people thread in a nipple on the reed block, but id rather use the exhaust if i can so as to not modify the cylinder. Think there will be enough on the exh pipe? Where should i put it? Was thinking right before the stinger should have the most velocity.

http://i1021.photobucket.com/albums/af333/skunkhunter/2d1742c0.jpg
http://i1021.photobucket.com/albums/af333/skunkhunter/550d54ac.jpg

[KASEY]

if it uses vacuum to stay closed your in trouble.... there is NO vacuum in a 2 stroke exhaust system and not constant vacuum in a 2 stroke single cylinder intake system,,,,, there are pulses of vacuum but also pressure which would force it open while running........

[jeffatc250r]

Well, guys over on the husky forum use these on their bigbore 2smokers, and they dont seem to have any problems....so it does indeed work. This is also standard on the victa 2stroke lawnmowers and have worked for decades without changing the design. So im not gonna agree with you that it wont work. The guys that run these on their huskies run it to the intake, but id rather not drill my jug. I know some people use these on their shifter kart as well, but i dont know where they are tapping into the vacum....im just gonna weld a nipple on my pipe and try it i guess.

[HondaHarry]

if you need vacuum on the intake side and dont want to drill your cylinder you could try putting a reed spacer in and tapping into it there. I dont think you will get the proper signal from the exhaust.

[jeffatc250r]

Ahhhh, very good idea. I have a spacer on the shelf as well. I like that idea

[KASEY]

you would have to mount the spacer on the outside of the reeds.. i would like to read how they work, because you have zero vacuum in a 2 stroke when the piston is on the way down,,, unless it uses cylinder pressure to stay closed...

[jeffatc250r]

Kasey, i know what your talking about and understand completely what your thinking. Im not really sure myself, i just know people do use these and they do work. Theres a barbed nipple that goes ontop of it that isnt pictured aNd theres a smallbrass insert with a very small hole that feeds it....maybee the hole being so small doest allow the vacumed air to release fast enough to drop the valve when its experiencing no pressure...not really sure, but thats my uneducated guess

[jeffatc250r]

Heres The top, see how small they reduced the hole
http://i1021.photobucket.com/albums/af333/skunkhunter/2ec8c5a6.jpg

[jeffatc250r]

Heres The top, see how small they reduced the hole
http://i1021.photobucket.com/albums/af333/skunkhunter/2ec8c5a6.jpg

[KASEY]

i am real curious.. where can i find any info on that thing????

[jeffatc250r]

Well, its called a victa decompressor, victa is outta austrailia and i had to order it up from over there. I havent found alot of info on it, but it comes up in some kart forums and also the vintage husky one.

[jeffatc250r]

Well i took your guys thoughts and got it plumbed in on the spacer behind the reed cage, we will see if it works tomm i guess.
http://i1021.photobucket.com/albums/af333/skunkhunter/66fe25e2.jpg

[jeffatc250r]

Just fired it up! Works mint, starts easier than a 60cc saw. Im sold on this thing now....ive got a bunch of cr500's that im gonna do this too

[KASEY]

wow i have learned alot in a short time,,,looks like you have it setup.. i read several fourms,, and several have said the way you have it hooked up might even be closing the valve too soon.... crazy ,, several have then just setup between the carb and the aircleaner.. so i would say that it takes a very minimal anount of vacuum, and mainly is closed by cylinder pressure,,,, looks like you will have a full report soon,,,,,

[HondaHarry]

Nice. Any vids of that thing in action?

[motoman1012002]

I was just of ftz site and saw the cr250r saw they built that thing is wicked

[4cylinders]

hey, why not use a compression release from a saw?
why not tap the hole instead of welding it in? just my 2 cents.

[jeffatc250r]

I cant seem to get her to fire off in one pull, then again of all the r trikes ive ever had, they always fire on the second kick. Maybe i outta try plumbing it in before the carb, theres a steel 45' elbow that i can plumb into, or maybe i just gotta fatten up the pilot. Either way, here it is
http://i1021.photobucket.com/albums/af333/skunkhunter/a79453b5.mp4

[jeffatc250r]

4cylinders, you cant tap the hole because its hollow in the middle where the coolant flows through.
The saw decomp, i disregarded because they are simply too small and wont allow enough air to pass. I pulled one out of my stihl 066 and its literally one third the size of this lil guy. A saw comp would still require a welded sleeve thats threaded to accept it and i would need atleast 2 of them.

[just ben]

just a thought here but may you have been breaking ropes from friction not tensile strenth? It may be just the pic but it looks like the recoil goes straight back? seems it would be impossible to get a straight pull

[jeffatc250r]

Even if the pull start was positioned straight up, it would still break. Gotta figure, this is a VERY healthy motor with a trx rod, decked head, runnin vp c12

[tanks350x]

Nice saw Jeff, that thing is wicked!!!

[fabiodriven]

I don't know how I managed to miss this thread. That's some very pertinent info there Jeff. Thanks for sharing!

[KASEY]

Even if the pull start was positioned straight up, it would still break. Gotta figure, this is a VERY healthy motor with a trx rod, decked head, runnin vp c12

go to REI and get some 5mm climing rope,, it has a kevlar core and will not break,, i have lifted all 3 wheels off the groung trying to start my new 70 motor and no broken rope, good stuff...