Vibration keeps breaking carbs!

Discussion in '2-Stroke Engines' started by Fletch, Feb 13, 2011.

  1. Fletch

    Fletch Member

    I've had 2 CNS carbs break where they clamp on the manifold, and an HD filter from SBP break the threads out of an NT carb on one side. The same filter on another NT carb, the bolt itself broke! I JB welded the **** thing and the bolt on the other side broke!

    I used to have rubber under my mounts but found that helped next to zero and my rear mount stud broke. I have a new Mikuni carb coming tomorrow and I think I am going to loose it if breaks too.

    Does anyone else experience this problem? The Mikuni is very similar to the CNS where it clamps on. I have a short CNS intakes from pirate that both the CNS carbs broke on. I'm afraid to use it, but I don't know for sure it was the culprit. When you use those intake manifolds you don't use the white plastic coupling inside the "mouth" of the carb where it clamps down. I'm thinking that coupling might help with vibration some and maybe the metal on metal vibration causes it to break?

  2. professor

    professor Active Member

    My 2 cents- if you tie the engine down so that it doesn't move around, I think your carbs will stay on.
    I do not recal others having the same problem. If the engine cannot move around - that moment of oscilation does not transmit into the carb.
  3. jaguar

    jaguar Well-Known Member

    Just stop the vibration

    Why don't you stop the vibration? You can do that with my CDI. It's the set ignition timing that causes most of the vibration.
  4. OOC-MB1

    OOC-MB1 New Member

    Vibration can be bad timing, unbalanced wheels, hubs not tight, or something is not aligned.

    How about inventing some kind of vibration dampener between the engine mounts and the frame?? like the ones in car engine mounts. maybe a thick rubber.
  5. MikeJ

    MikeJ Member

    Hi Fletch -

    Vibrations with these engines have contributed to many a rider's problems. Over the years, different recommendations from different builders have been offered. Some fix action that worked for one rider did not do a bit of good for another rider. If you go through the multitude of posts over many threads addressing vibrations, you will get the idea.

    Here is what I offer in my own long-winded way:
    Bicycle frames can be very different from one model to another. Tube wall thickness is a big, big factor. The thin wall tubes on lightweight bikes are a fraction of one millimeter thick from the bikes I have cut up. Thicker is better. But you don't learn that until you cut a bike up. This is usually avoided in a normal build.

    Both the down tube and seat tube will flex back and forth a little bit when these two cycle engines are running, mounted to the center of the tubes. These mounting tubes are not bedrock solid by any means, and their movements at the rate that the engine is running are the vibrations that always have negative effects, as you are experiencing. In my early riding days, I did not take my bike's two cycle engine above 4200 rpm (measured by electronic tachometer) because the vibrations just escalated out of control above that range. Vibrations were tearing my build efforts apart.

    My fix: Stop the engine from rocking fore and aft on these flexible tubes as it is running.

    Here is what I did: I removed any rubber padding from between the frame tubes and the engine mount points. I mounted the engine directly to the tubes: metal to metal. The engine still vibrated badly. Here is where I got unconventional: I got a pair of kinda small-sized turnbuckles from the local hardware store (Ace was first choice). I also got some solid steel wire, around 14 gauge or 1/16 of an inch diameter. I fashioned a "noose" with these items.

    Open the turnbuckle eyes as far apart as they will go. I looped a wire length from behind the seat tube / top tube welded joint to both sides of the engine cylinder. I ran another wire around the front of the cylinder, between upper horizontal cooling fins. The ends of the cylinder wire were looped through two turnbuckle eyes, one wire per eye, one turnbuckle on each side of the engine. Ends of the tube-looped wire were looped through the other turnbuckle eyes. Wrap the wire ends so they will not slip out of the turnbuckle eyes. (Try to position the turnbuckles so that they are not in contact with both sides of the engine. My fix does have them touching the cylinder, but there is no ill effect.) HAND tighten the turnbuckles equally, drawing the "noose" around the front of the cylinder tight. At some point, you will start pulling the seat tube and down tube out of perfectly straight alignment, but the bends will be so very small, you will not see that. When it takes effort to twist the turnbuckles by fingers only, stop tightening. (I can't give you a hard, measurable number; it is a "this seems about right" judgment.) You don't want to break the cylinder off the engine crankcase.

    By doing this, you remove most of the engine's ability to rock forward and backward while running. Hence, vibrations are minimized; the steel wire and turnbuckles will not lengthen. You "lock" the position of the cylinder and the rest of the engine to a position that allows almost no forward and backward rocking.

    This offered solution is unconventional, to be sure. I have not heard of any other builder apply the same "patch", if that is what you want to call it. But this worked for me. I have raised my top measured engine rpm from 4200 to 5800 without the nasty vibrations that were present before. Riding my particular bike is much more pleasant. I now ride it routinely in the 5000 rpm range; I never did that before.

    Try this, all parts will cost less than $10. It is not difficult to install, but it is a snap to remove if this does not work for you.

    I hope this helps.
  6. Fletch

    Fletch Member

    Thanks 2 everyone for the advice... Especially MikeJ. I appreciate such a long detailed reply. I'm trying to visualize your fix and I think I've got it, but I'm not positive. Do you have any pics? Honestly though I really have to draw the line on the amount of energy I put into this HT, because I think the thing is going to drive me completely insane if I let it! :sweatdrop:

    One thing I suspected was that my spring chain tensioner wasn't tight enough and the whipping from the chain was contributing to extra vibration. So I was installing the stock tensioner when I noticed that LOTS of rollers were completely GONE from my chain! I couldn't believe how many were missing. There were some chipped away and cracked ones barely hanging on for life too.

    Now I have to figure out what caused this to happen? I know these stock chains are cheap quality, but this seems extreme. I suspect the sprocket because if I roll the bike backward the chain pops like its not aligned. At first I thought it was a truing issue, but came to realize that it was probably a few teeth on the sprocket that don't seat right. I didn't do anything about it because when the chain moves forward there is no popping and it is completely smooth, and I didn't want to ruin the sprocket with my dremel somehow. After so much time working on this thing and so little riding it, it becomes really frustrating and I don't fully investigate everything like that.

    I was about to start a new thread on this chain issue, but since I've got some knowledgeable guys following this one, I'm hoping you can help me figure out this chain deal. The chain has about 350 miles on it, and I'm hoping you'll tell me it' just a crappy chain, and I can just replace it?

    My new carb is coming tomorrow but it looks like I won't he riding.
  7. MikeJ

    MikeJ Member

    Hi Fletch -

    I found one picture of a turnbuckle I pressed into service. I don't have current pictures of my "noosed" engine (another long story). The attached photo shows the 14 gauge wire and a turnbuckle about the size I used on my bike. This turnbuckle helped control my engine (with jackshaft). But the later-added two turnbuckles, one on each side of the cylinder, made a huge difference. My later wiring work was much cleaner. Just keep in mind that if you don't like the way your effort looks, disassemble and try again.

    Don't let this project get to you; I assume you don't have a deadline to complete it. I have been working on my latest build a few hours each week for the past four months. Take your time. I use between-build time to dream up "What if I do this?..."

    And for what it is worth... I read in another thread that is over a year old that those CNS carbs have been recognized as easily separable from the intake manifold. The culprit? Vibration. Other reader's carbs have developed cracks and fell off.

    The chain breakdown is a good question. I have never heard of what you are describing. Go ahead and start a new thread asking about that. Post pictures if you have them. Maybe someone else can explain.


    Attached Files:

  8. Dave C

    Dave C Member

    You have the CNS carb adapter from Pirate Cycles, right? This goes to preventing breakage.

    This may sound crazy but cut your manifold in half or there abouts. On the end that bolts up to the block leave about 7/16ths of an inch of straight section. Get two 3/8ths inch wide stainless steel screwclamps and a short section of fuel proof hose 1" ID. Put the pieces together with a 1/8th to 3/16ths gap between the two cut sections. You have just made a vibration isolation mount for the carb. The shock of going over bumps and vibration from the motor is greatly lessened on a weak point on the carb.

    If your feeling ambitious then chamfer the edges of the passage to smooth airflow.
  9. Fletch

    Fletch Member


    Thanks a lot, the picture really helps. I've been working on the 2 HT bikes I've had almost everyday since day one. I don't think they are ever "finished". With near 400 miles on this engine now I'd think it would be stable, but something is always needing to be tweaked or fixed. Most of the issues are with the carbs and the bike itself I admit, like kick stand, fenders breaking/rattling, coaster brake, etc. The engine itself runs fine most of the time. It's the little annoying stuff like air leaks you know? Just when I think everything is dialed in, something unexpected happens or is noticed (chain last night is perfect example!) It definitely keeps me busy though, and it's that much more rewarding when it does run because of the added challenges. I think my next build (after this shift kit one I'm about to start) will be a more "reliable" and hands free engine/system. I'm thinking Gebe maybe? I'd like to try that Harbor Freight 4 stroke, but I don't know anything about 4 strokes.


    Awesome man...thanks! I can see how that would work well. Yes, I have the short pirate CNS manifold (2 of them actually). I just want to make sure that one is what you mean to cut in half... It sounds like it? This carb is a Mikuni which is almost identical where it clamps on, but I'm hoping that it is built stronger than that "piece" CNS.
  10. Dave C

    Dave C Member

    Yah, that's the one. It's just long enough to do this. You have like a inch and an 1/8th to play with. Use the narrowest blade you can find and cut very carefully, a goof up is a $20 mistake. The usable part of the intake is about 1 1/8th inch. Measure from the small open end 3/4 ths of an inch, that would leave you enough to be gripped by the screw clamp. Use about 1 1/4th inch of 1 inch ID silicone tubing, like the stuff from sick bike parts. The black is really, really thick so I'd use the blue. I've been useing a clear tubing I got a ACE and it seems to hold up well to gas.
  11. Fletch

    Fletch Member

    Good deal, thanks. I'm going to try it without cutting for now and see how it goes. Did you have CNS carbs brake on you too, or do you just do this as a precaution? I just got the mikuni 18mm and it looks a lot stronger than the CNS. I had to sand down the white collet to fit the intake. It would have been too big without it. Do you think a dremel would cut it too wide? What is the clear tubing used for at ACE? Maybe I'll pick some up. It doesn't get too hot right there, so the silicone is probably not needed.
  12. Dave C

    Dave C Member

    The second good thing about silicone is that it's one of the best vibration dampeners there is:cool:

    The clear tubing is on a roll in the back of the store, usually :p I had to use something because the frame barely fits the motor despite having 28" tires. The frame is contorted to shorten it up. When I build my second bike it won't be a Mongoose Paver again :poop: The tubing is so the carb actually sits right besides the seatpost on about a 4" clear tube. My heel just clears when I'm pedeling.

    I don't really like the clear for this. It doesn't like to bend so I think I'm trying a pre bent auto heater or polution control hose. I need one with about a 45 degree bend then cut to length.
  13. MikeJ

    MikeJ Member

    Hi Fletch -

    I took some photos of my bike with the "noose" around the cylinder; they should be attached. I remembered I ran a 1/4 inch bolt, about 6 inches long around, slightly bent at the center to form to the cylinder, in the front of the cylinder so to avoid "pressure points" on the side of the cylinder. In reality, I don't have any proof it is needed nor if it does any good at all. The metal-looking muffin-shaped things bolted to the frame are lead (as in the heavy metal) "muffins". They were intended to prevent excessive vibration of the tubes. They did not work; I need to remove them. The photos should better explain my previous wordy description. I hope this helps.


    Attached Files:

  14. Dave C

    Dave C Member

    Mike, I'm going to use your idea with slight modification. I'll be using steel flat stock to tie it to the frame. I'll have to have 4 turnbuckles to do what I have in mind. What I envision is 2 pieces of flat stock with 2 long bolts with the bolts running like you have the on top. 4 holes in each bar. Bolt on the turn buckles to the cross bars then enought flat stock to reach the frame to be bolted on there. I just don't like the wire but if it works for you it's all good:cool:

    Actually the lead dampers are a good idea but I think it might take a lot more than you have. So much it could serve as a boat anchor in a pinch:ack2:
  15. MikeJ

    MikeJ Member

    Hi DaveC -

    I am betting your idea works. I will be looking forward to the photos. Stiffer supports almost always mean better control over engine vibrations getting into the frame. If you like the results, I may follow suit. I used wire as a quick "proof of concept". It was easy to work with and did not need any machining. It worked well enough for me that I was satisfied and moved onto test rides.

    After this, my next move may be to upgrade to a better carb and arrangement.
    Last edited: Feb 21, 2011
  16. Dave C

    Dave C Member

    Time to update this a bit. When I replaced the stock headstuds with SBP studs I added a Puch Hi-Compression head. After torqueing the nuts down there was about 3/8-7/16 of an inch of stud exposed. I went looking for a coupler nut to bolt on all 4 stud ends. There is no M8 1.00 coupler nuts made in the world, M8 1.25 sure... I ended up welding two M8 1.00 nuts to a small diameter 1/2 pipe 1.25 inches long. Made 4 of them. I now have a place to bolt on attachment points for extra frame mounting using the 4 head studs.:dunce:

    Not only do I hope it helps to cut vibration but also prevent the motor from twisting in the frame breaking more back motormount studs. The 8.8 metric bolts will help with that. I had to pedal home after the last 2 broke :ack2:

    I got to learn the photo thing, I'll work on it tomorrow:helmet:
    Last edited: Mar 26, 2011
  17. Lazieboy

    Lazieboy Member


    Too much to read, but I replaced frame mounting bolts with stronger ones at Fastenal. I also broke off a stud inside but got it out.
  18. Irish John

    Irish John Guest

    Fletch, post a good close-up picture of your engine in the frame and I will probably be able to tell you why it is vibrating.
  19. adb140275

    adb140275 Member

    The more solid the engine is mounted to the frame, the less it will vibrate. I got more vibration when i used rubber than i do now with NO rubber. I hardly have any vibration. Granted, i have a puch head installed which effectively lowered (yes, lowered) the compression. get it bolted on tight and solid, you will be surprised. this is the only picture i have where you can see both mounts, but you get the idea.