Can a HT have a plastic intake manafold?

Discussion in '2-Stroke Engines' started by autobo7, Aug 9, 2008.

  1. autobo7

    autobo7 Member

    I have a new build progressing and I wont be about to attach the carb directly to the stock intake manafold. The frame of the bike is a little smaller but I want to stick with it.

    So i am going to extend the intake manafold about 6" so the I can move the carb to a different spot on the bike(I have read that a longer intake manafold can help tune the bike a bit?). I am thinking of using a plastic tubeing for ease of construction, but a 2 stroke expelles some of its exhaust through the intake and that could heat things up.

    So my question is will a plastic coupler between the carb and manafold work or do i have to make some thing out of copper?

    here is the bike;
    you can see in the engines picture that the carb is mounted behind the clutch cover

  2. Simon_A

    Simon_A Member

    It depends what plastic you use, dont forget this is an air cooled combustion engine. The top of your engine gets extremely hot.

    Most plastics cant withstand the temperatures the cylinder gets to.
  3. HI,

    FWIW - I was one of the people tinkering with intake runners...You can do a search to see some of the results...I have been fairly busy and haven't had a chance to tinker with this further but it can really make a difference....(I used an intake that was was too long and had too small a diameter tho).

    As far as plastic....I am not too sure that would work because the engine can and does get pretty hot...

    For my intake build I went to a shop nearby that uses/bends hydraulic tubing (standard size pieces so a shop near you could probably duplicate it) it was nearly a perfect fit for the stock HT carb...Plus they had the bending machine and could weld too if needed....All I had to provide was the metal flange (cut out to match the intake port of the cylinder and pre-drilled for the mounting studs....This may be the easiest non plastic way to go ....

    Hope this helps you


    PS - Soldered copper may work but I can't say for sure...I think somebody here tried it for an exhaust but I think it didn't work....But then again the intake side is probably somewhat cooler than the exhaust side....
  4. Add a 90 degree bend from even a PVC pipe that fits over your existing manifold then get a short PVC to the carb so it faces to the side. The PVC bend shouldn't melt because the original intake is taking most of the heat from the engine.
    If you get the carb to exit to your left then handling the choke could be done with your left hand while your throttle is on your right.
    Epic Win.

    Here's an example of a PVC elbow. Bring that manifold with you when you go to the store.

    I guess it could be glued into place?

    Attached Files:

    Last edited by a moderator: Aug 9, 2008
  5. BigDaddyT

    BigDaddyT Member

    Id get it up to temp and laser it to see how hot it is. If you dont own one I bet you could con a quicky lube or someone like that into lasering it for you.
    My brothers hemi dodge has a plastic intake manifold. Doh!
  6. machiasmort

    machiasmort Active Member

    I'd stick with steel, or better yet, if you could find it in that diameter, a conduit elbow!
  7. Mountainman

    Mountainman Active Member

    Yes, over the years I have seen plastic used for intake manafolds several times. I must say though - they all seemed to be rather special looking - very strong and I am sure - made to withstand heat. Would our so called common PVC work ? At my job sites - repairing broken water lines and replacing valves - I have seen direct heat - torch - applied to PVC so as to make bends - never did like to use this method - but it is done occasionally.

    Note: Andrew's idea above -- MUCH BETTER.

    Happy Riding from - Mountainman
  8. autobo7

    autobo7 Member

    Thanks for the input all,
    Andy's idea is definitely tops, but i don't know of any one in the area that will do that for me(at a reasonable rate). I think I'm going to J-B weld some bent copper tubeing to the manifold as the extension, and then just clamp the carb to that. I am waiting on a few links of chain to complete the drive line, and if they are in tomorrow then I'll tackle this too.
  9. silverscreen

    silverscreen New Member

    The problem I had with PVC is that it does not melt or bend when the engine is running, however it starts to melt and sag after the engine is off. This happens because there is not any air going through the intake to keep the PVC cool, and the heat from the engine migrates back through the intake to the carb. I ended up using a 90 degree copper elbow an sealed it with black high temp RTV gasket maker. There are a few pictures in the picture gallery under the heading "First build... almost needed a shoehorn!"
  10. autobo7

    autobo7 Member

    oh ****...

    I got it running today, and although it looks great and is super confy to ride, I have idle and fuel ratio problems. I used 2 street elbows to attach the carb, but the elbow next to the manifold would quite fit in inside so i notched it much the like the carb is, and i crushed it down a bit to make it fit.

    Now it wont idle if i open the choke, and if i keep the choke shut the engine loses power between 1/3 throttle and WOT. I am guessing that this means that i made the extension to narrow?

    Attached Files:

  11. silverscreen

    silverscreen New Member

    Not necessarily. Sounds like you did not geat a good seal between your carb --- and your --- manifold. There is probable an air leak somewhere in that area. try to seal every joint between your carb and you manifold air tight.
  12. justinsidethegates

    justinsidethegates New Member

    yeah an air leak will give you all kinds of idling problems. you would have been fine with only using 1 90 or even a 45. was there any reason you wanted facing the usual way but off to the side? Hope you got ir running better since then.
  13. Neon

    Neon Member

    Never heard that. No way it can happen. A small amount of gas is expelled through the exhaust. Look at the port arrangement on a two stroke and you will probably see why it doesn't happen. It would be an extremely bad scene if that did happen.
    Last edited: Sep 9, 2011
  14. Big Red

    Big Red Active Member


    This is gonna hurt, but BGF(ouch!!!) sells an extended offset intake manifold for about $10.00. Bolts right on and gets the carb away from the downtube.
    Warning: If anything goes wrong with "Boygoslow", (bad part, lost in shipping,ect,) Do not expect him to make it right couse he wont. 99% of the time everything is good with him. But when it goes bad it stays bad.
    Big Red.
  15. cloud_2901

    cloud_2901 Member

    Neon, you never looked hard enough then :p

    A 2 stroke can have blowback from the pressure wave generated by the exhaust and the previous combustion returning to the cylinder at not exactly the right time, a rather high possibility with these chinese engines I think, unless the inlet port is blocked by the usually present reed valves or a rotary valve system. We don't have them, so we get blowback, hence people selling boost bottles, that I don't think would have a huge effect, but I'm sure they would have some small amount of blowback entering them.
    Last edited: Sep 10, 2011
  16. motorpsycho

    motorpsycho Active Member

    if you want to make an intake extension out of plastic, it can be done.
    you can't mount plastic right to the cylinder, but you can add an extension to the existing metal intake manifold.
    You need to look for nylon plastic elbows which are used on car engines for some of the emissions parts.
    It may be hard to find the right diameter but they are out there.
    the nylon will handle the high heat better than pvc.
    look for pcv (positive crankcase ventillation) nylon elbows and straight peices made for car engines.
  17. justinsidethegates

    justinsidethegates New Member

    Hmm very interesting but i think i'm going to try a piece of ABS plastic if i can find it in that small of a diameter or go back to the drawing board with a copper 45. they wont be touching each other directly but a piece of 1/2" fuel line as a coupler to the intake tube and another piece to the carb. Also just realized how old this post was that i accidentally brought back to life but it shows i used the search function AND managed not to start a new topic over something gone over so much. still need to figure out the best setup for this tho.

    Another thought is has anyone thought about making an intake tube with both a long spout and a shorter one with a butterfly valve inside the longer one so it only kicks in when you go lets say 75% throttle +?