Engine Trouble Knocking at Higher RPMs

Discussion in '2-Stroke Engines' started by Marine One, Apr 18, 2016.

  1. Marine One

    Marine One Member

    Hello all and great site yall have. Long time lurker, first time poster.
    I recently bought my first 66/80 cc motorized bike, it came with 1 spare engine and most of the parts for another kit. The bike had a large crack in the frame near the bottom where you pedal but I already had a better bike for the transplant. Anyhow. The motor runs fine and starts up without a hiccup except when its cold (I have to pedal a little longer, no big deal) but my big problem is that it knocks on the high end. I have a 36T sprocket and I just put a speedometer that is calibrated for my 29" rims and I am hitting about 20-25 normal with a top speed of 29.1 registered on it. I don't know what kind of RPM's I am actually running because I don't have a tachometer yet. I have the stock NT carb it looks like. When the knocking starts it robs a LOT of power and it stops accelerating and actually decelerates a little. I can tell that it really wants to give more but its just cant for some reason. Under reasonable load it seems to work much better but once I hit a downhill or get up enough speed on a flat stretch is when it really happens. The chain isn't hitting anything, I have a fairly heavy duty spring tensioner on it. I don't ride the bike for long periods of time, typically 20 minutes or less.

    Wondering if it is because the ignition is firing too soon and I need to get a CDI with a delay at the top end or maybe its not balanced and there is too much play in the bearing at the piston, but if it was the bearing wouldn't it do it all the time?

    Keep in mind that I have no idea how old this motor is, what brand it is, or what the old owner put it through before he sold it. He probably weighed about 250 but we live near the beach so its pretty flat.
    Trying to include everything I think might help but I'm sure that I left something out. Any help would be Greatly appreciated. Thank you and Semper Fi.

  2. butre

    butre Well-Known Member

    Is it knocking or four stroking? spark knock is typically catastrophic in 2 strokes. have you tried leaning out the jetting?
  3. Marine One

    Marine One Member

    I have used the needle and the D clip to try and lean it out as much as I can, I'm not sure what 4-stroking is. I have heard other people talk about it but never understood exactly what it is.
    I think that I can lean it out one more click on the needle though.

    EDIT: I have noticed that it only seems to start the knocking after its warmed up.... but it still only does it at high RPMs.. if something was actually hitting wouldn't it do it at ALL RPMs after the engine warmed up?
    Last edited: Apr 18, 2016
  4. KCvale

    KCvale Motorized Bicycle Vendor

    Put an NGK 5944 (BPR7HIX) Iridium spark plug in it.
    ~$9 at http://SickBikeParts.com or of course NGK.com
    Worth every penny and you'll fell the difference.

    One note on that plug numbering...
    The 7 in the middle is a heat index number.
    The higher the number, the more heat it takes out of the engine.

    The 7 is great for Phoenix, but if you live in the cold go down to 6 or even a 5 to keep enough heat in the engine to warm up quick and run well.

    If that doesn't help your high rev knocks check your fuel flow.
    If your fuel system can't get all the fuel it needs you will get that symptom.
  5. sbest

    sbest Active Member

    Hey my unit's motto was Semper Fi as well, but I wasn't a Marine.
    The WNSR retained the Lunenburg Regiment (formed in 1870) motto when they amalgamated.
    The Marine Corps picked up the motto about 13 years later.

    Your problem is near and dear to my heart and I think I may have the answer.
    If the sound is detonation, I had the same problem.
    Detonation sounds like rattling marbles or popping corn, a bit erratic.
    Detonation is usually worse as the engine warms up or under prolonged load.
    Bearings are very regular sounds, that vary with load more than rpm.
    Bearing noises will often vary inversely with load, noisiest with the least load, but not always.

    The classic answer to detonation is:
    1) less compression,
    2) less timing,
    3) higher octane,
    4) cooler plug,
    5) cooler head

    To which I'd add;
    6) better cylinder head chamber shape,
    7) ideal squish size and gap to raise chamber mixture swirl velocity

    Thousands of these engines run on regular fuel with the same timing and the standard NGK BR7HS plug.
    So what is different with your engine? Fuel mixture and chamber dimensions are likely.

    First, is your oil right? about 32:1 seems like enough for these engines.
    Second, what is your plug telling you?
    So, look at the tip end of the insulator to see if you have the right heat range. 8 is colder, 6 is hotter.

    You might have to invest in several new plugs and even cut the threads off to see the mixture smoke ring.
    Here is a range of cut plugs (lookup "Plug Chop"). Rich on the left, lean on the right:
    Cutting the threads off is easier than looking down into the hole or reading the shell.
    Buy the jets you need to get the Wide Open Throttle (WOT) mixture right.

    This is what detonation in our HT engines looks like, note the pockets in the corners:

    Very little squish area and mixture movement or velocity.
    Here is how it should look after you fix it:
    Subtly change the chamber shape and squish area and gap to get a turbulent swirl that will discourage detonation. As the piston gets close to the head it creates a jet of mixture that keeps the fuel in suspension and spreads the flame front fast and smoothly without detonation.

    This is what the modified chamber shape looks like in real life (on the right) :
    The squish area goes right out to the gasket, comes in closer to the plug, and matches the angle of the piston dome.
    How do you get it machined that way? I used coarse sandpaper on the dome of an old piston.

    SRPM, FurryOnTheInside and Comandoriv like this.
  6. Marine One

    Marine One Member

    I live on the Gulf Coast so the cold is Not an issue during the summer here.
    I currently have a 3 prong plug in it. I have the parts needed to convert to a twin fire head but have been trying to holdoff until I get the knocking issue figured out.
  7. crassius

    crassius Well-Known Member

    if the motor is new, this may be normal for a while till it runs in enough for rings to seat - there is often a sound like popcorn popping in new motors at high revs that goes away when motor runs in enough to develop full speed

    not sure what the cause is, but have thought it to be a bit of piston slap from rings dragging to create a good seat
  8. Marine One

    Marine One Member

    crassius, I am for sure that it is not a new motor. there is far too much wear on it. plus I know that I have put about 3 tanks through it so far (about 2 gallons)
  9. Marine One

    Marine One Member

    I was looking at your spark plug examples and mine looks like the one all the way to the left, which means that it is far too rich. Thanks for the help. I am still trying to figure it out and play with it until I get it right. Your post and diagrams have been a huge help, I am a very visual person.
  10. sbest

    sbest Active Member

    Crassius, your description of that noise was so accurate that I laughed in agreement when I heard it.
    The erratic pap---pap-pap------pap of popcorn in a pot. It was detonation in my bike.
    Made worse when I was climbing a hill or head turned around for poorer cooling.

    3 tanks should have it pretty much broken in. Mine was rich and knocking too.
    Usually rich helps protect from detonation, but not if the head shape is poor.

    I think the engine being new does make for a hotter piston temp until it and the rings seat in.
    I know piston slap and bearing noises, they are far more consistent, rhythmic.

    I would not recommend a twin plug head, but I would try a NGK BP8HS (or BRP8HS) plug to see if the noise goes away.
    Here is what a perfect heat range plug looks like (from a cleaner 4-stroke, but 2-stroke similar with hints of oil):

    Here is too hot a plug heat range:
    Note the blistering on the electrode and you will see it on the tip of the insulator as well.
    The shell base ring and what I think I see at the base of the insulator tells me this plug was run with rich mixture.
    Running too lean will overheat the electrode as well, regardless of plug heat range.

  11. Marine One

    Marine One Member

    image.jpeg This is what I meant when I said that I have the parts to do a twin plug. image.jpeg image.jpeg Gentlemen, this is what my plug looks like.
    I do have a single prong plug that I could try.
  12. sbest

    sbest Active Member

    That chamber looks beautiful, if it goes out to the full bore diameter (inner gasket diameter).
    You can run it with only one plug wired up, but the dead plug needs to be no hotter than you are running.

    Plug has too many miles on it to read for sure other than it is not too hot or hugely rich. You are in the ballpark.
    Synthetic oils "burn" cleaner and make plug reading easier too.
    What we see on the shell base ring is oil residue. I cannot see deep enough to read the insulator base.

    The NGK plugs are slightly longer than the chinese 3 prong, but typically not a problem.

  13. Marine One

    Marine One Member

    image.jpeg image.jpeg

    The base of the insulator is the same color, burnt honey brown all the way down to the bottom.
    Why wouldn't you use both plugs for ignition on the new head?

  14. crassius

    crassius Well-Known Member

    if detonation is suspected, use a new plug, clean any carbon deposits from head, and try high octane gas
  15. Marine One

    Marine One Member

    I used high octane the first 2 tanks (+/- 1 gallon) It seemed better Ill admit but it was still there if I remember correctly. I have thought about using Marine fuel.
    What do you mean by Detonation? I am fairly mechanically inclined but I'm still trying to learn all of the terms used by this community.
  16. sbest

    sbest Active Member

    If it is not jet black or blistered white, you are in a workable range. Need new plugs to fine tune jetting.

    You know I mean the insulator inside the engine, inside the threaded shell?
    I think you know this, but where you include a picture of the exterior plug insulator I thought I'd make it clear.

    You could fire both plugs, it makes little difference.

    I had a 1974 CR250 Elsinore many years ago that would foul plugs constantly because:
    1) I used crappy oil,
    2) I used crappy plugs
    3) I jetted too rich (cuz more is better, right?)
    4) I used too much crappy oil (cuz more is better, right?)
    5) too much low speed trail riding

    My answer, like most of us, was to MODIFY!
    I stuck on a twin plug MT250 head with a cold plug for running hard and a hot plug for trails.
    Didn't work. Hot plug runs hot even if no wire to it and when one fouls, they both foul.
    Try out your ideas. Even when they don't work you can learn something.

  17. Marine One

    Marine One Member

    Thanks, I will keep all of that in mind. I'm using Tractor Supply oil right now. I think that I am actually running it pretty lean at 40:1
    Sorry about the mix-up with the insulator.
  18. crassius

    crassius Well-Known Member

    detonation (called 'ping') is when fuel burns too fast due to high compression or prematurely due to a hot spot in chamber acting like a glow plug

    note, back in 70s when dual plug harley heads came out, we had to retard timing a bit because flame front had less distance to cover - so it does make a diff if you fire both at once, but I doubt these POS CDIs will put out enough to do both at once
  19. Frankfort MB's

    Frankfort MB's Well-Known Member

    My bike is actually doing the same thing but my plug looks good to me, I like to run just a tad rich to keep my stuff together ;)

    Attached Files:

  20. butre

    butre Well-Known Member

    needles aren't jets. get a jetting kit, the needle only affects idle and part throttle mixes
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