Motor gets weak when hot...

Discussion in '2-Stroke Engines' started by XenonDream, Nov 29, 2009.

  1. XenonDream

    XenonDream Member

    I couldn't find this anywhere, so I'll post my first question. But first, a little info: I'm running a SkyHawk 66cc motor, everything stock, with an NGK B5HS plug. (The plug is a replacement, but was not beneficial to performance.) The motor has about 450 miles on it. Engine break-in was 2 gallons of gas at 20:1, run no longer than 10 minutes at a time, and no faster than 15 mph. It is still being run at 20:1 per dealer recommendation, and I have only taken it up to its top speed once. Generally, I baby it because I want the motor to last as long as it can.

    When I first start the motor, it's VERY punchy and I have to be careful on the throttle. It almost seems as though the bike would rip itself out from under me if I turn the throttle too much. Once it warms up to operating temp, I find pretty much the opposite is true. It will still cruise comfortably at 19-20 mph, but getting there takes a good while. From a stop, it will 2-stroke up to about 12-13 mph, and then start to 4-stroke during acceleration regardless of throttle position. Half the time, I have to check my cyclecomputer just to verify that I am in fact accelerating. Once I've gone a block or so, the bogginess goes away. I hit another red light, and we start all over. And only when it's hot. Before it reaches operating temp, it will rip me out of my socks. Any ideas? Sorry to be long-winded, but I wanted to give as much info as I can to make responding as easy as possible. Thanks in advance.

  2. AussieSteve

    AussieSteve Active Member

    Last edited: Nov 29, 2009
  3. Flapdoodle

    Flapdoodle Member

    What color is the plug?

    With so few miles on it maybe the piston is still a bit tight. It gets bigger when hot=more friction. If so, making it leaner might make it run worse.

    Does the sound of the engine change when is is performing poorly?
  4. XenonDream

    XenonDream Member

    I always make sure the choke is completely down a few seconds after the motor starts. The plug that is currently in it only has about 25 miles on it, so it may be hard to tell at this point. The stock plug that I removed, however, had a fair amount of sludge on it. When the engine was hot and had been run for a block or so (which clears up the problem), it would have very clear, crisp sound, every power stroke having a distinct pop. When the symptom is present, the engine sound is somewhat dull and quiet by comparison, but still smooth. Also, I should note that the engine response when the throttle is snapped feels somewhat delayed as well.

    A carburetor adjustment was mentioned... Are the stock NT carbs adjustable that way? I haven't taken mine apart yet. Or would you recommend a CNS carb?
  5. Flapdoodle

    Flapdoodle Member

    I can think of only one simple way to tell if the piston is tight.

    Get it up to speed and kill the engine but leave the clutch engaged.
    If the engine stops you much sooner when the engine is hot than when it is cold, the piston may still be tight. Not to worry though. It will break in in time.

    Since it runs great cold I would hesitate making changes in the carb just yet.
  6. AussieSteve

    AussieSteve Active Member

    That's not it, Flapdoodle. It's done 450 miles, (720 km) - that's 1 1/2 times the recommended run-in period.
    Also, XenonDream said that the plug had a fair amount of sludge on it - another sign of a rich mixture, so that's where I'd be looking

    Yes, the stock NT carby is adjustable in this way. Mine was very rich straight from the factory and needed the slide needle lowered and a smaller main jet.

    25 miles is plenty to get some colour, (color), in the plug. Whip it out and check. It will tell the story.

    ... Steve
    Last edited: Nov 30, 2009
  7. Skyliner70cc

    Skyliner70cc Active Member

    You've blown your chance to break in your engine by babying the motor.

    Re-ring the engine, and rehone the cylinder lightly or get a new cylinder.

    Use 32:1 mixture and run her by varying engine load and rpm. Rings seat with combusion pressure and light loads won't push the rings firmly enough against the cylinder to break in. Full throttle is fine but do it going up hills and on flats (varies load and rpm).
  8. XenonDream

    XenonDream Member

    Okay... Thank you, Steve. I finally pulled my cable out. Found that precious little e-clip and moved it up (moving the needle down) one notch. The plug was blackened and wet. So we'll see what happens.

    Amazing to find so many different schools of thought here.

    The break-in process (and fuel mixture) I'm told is incorrect was advocated by Roland at Spooky Tooth Cycles, where my bike was purchased. They've been getting 5-10,000 miles out of engines.

    What to do...

    I'll post the results of today's ride later on. Thanks again.


    Wow,what a **** day for riding. Despite the tree-bending wind, I was able to find a few places during my 1.5 hour errand run. Cold, the engine was the same. Warm, it ran better. Still bogged down a bit during acceleration (intermittently), but it it wasn't as bad, and cleared up a lot faster when it did. It appears that WOT floods the engine after a few seconds. So, Steve, it sounds like you're dead on about my situation here. Where does a guy get a smaller jet for a stock NT carb?
    Last edited: Nov 30, 2009
  9. AussieSteve

    AussieSteve Active Member

    Jet Supplier

    Your run-in sounds OK to me. My kit instructions recommended a run-in of 500km, no faster than 25kph and rides under 30 minutes duration. 15:1 oil during run-in, then 20:1.

    I got my jets from Rock Solid Engines. In my carb, an 0.79 jet was standard, so I bought a #76 and #73. The #73 works best for me.

    Also, in the end I had to move my slide needle to it's lowest setting. Use plug colour as a guide when carby tuning. blackish is rich, light to whitish is lean, medium tan is good.
    Keep in mind that the needle setting controls mixture at low to 3/4 throttle and the main jet controls 3/4 to full.

    To check main jet mixture: Start and warm the engine, then fit a clean plug and ride for several seconds on full throttle, then hit the kill switch and pull in the clutch at the same time. Now check the plug colour.

    For the slide needle setting, do the same at about 1/2 throttle.

    ... Steve
  10. XenonDream

    XenonDream Member

    Well... I've obtained a new CDI box and installed it (warranty item) since my last post. It helps some, but the engine's still boggy unless I lean it out. However, I've come to a conclusion that has caused me to adopt a new attitude toward things. I've spent hours reading forums, and I've found that 10 people will give you 10 different answers to the same question. MBing doesn't seem to be an exact science by any stretch. Based on my own learning, logic tells me that leaning the air/fuel mix out will give me more performance but will wear my engine faster. By the same logic, a richer mix will push more oil into the engine resulting in lower performance, but provide for better cooling and lubrication, meaning a longer life. And, well, let's face it...I'm not racing around a track. I'm commuting to work. I think I'll just live with the boggy low-rpm take-offs if it'll ensure I get enough oil at higher rpms.
  11. Bent Spoke

    Bent Spoke New Member


    20:1 is an insane mixture

    I broke mine in a 32:1 and hammered on it to break it in. babying a new motor does more harm than good.

    if this works on a $1200 chainsaw running at 14,000+ RPMs, then it's good enough for a cheapo HT motor.
  12. AussieSteve

    AussieSteve Active Member

    Why do you say that?
    16:1 is the recommended ratio for run-in, then 20:1.
    Surely the designers/manufacturers must have had a reason for that ratio. They were smart enough to design the engine. I'd assume they were also smart enough to know what fuel/oil ratio to recommend.
    I'm running mine at 16:1 during run-in - no plug fouling, no smoke trail, plenty of compression, (115psi w/ stock head).

    ... Steve
    Last edited: Dec 9, 2009
  13. Bent Spoke

    Bent Spoke New Member

    Cause they listed that ratio to be used with 30w oil, in fact with too much oil it will prevent proper break in resulting in excessive blowby.

    There are hours of reading when in comes to oil:gas ratios and it seems everyone has their own opinion on it, so I say go with what works for you.

    As for compression ratios, I'll post up what I have, using 40:1 WoodlandPro 2-stroke mix, (my chainsaw/weedwacker premix) just for comparison sake.
  14. AussieSteve

    AussieSteve Active Member

    Yep, we all do that in the end anyway.
    I understand your point about weedeaters, but they aren't (usually) loaded as heavily as these little HT motors.
    Also, it wouldn't shock me too much if the weedeater is better engineered.
    With 2-strokes in general, keeping the mix a little oil-rich is important for compression. More so with these engines, due to bad engineering. (Good design, poor engineering/manufacturing)
    The more oil, within reason, that you can put in and get them to run cleanly, the better. More often than not, an oily/fouled plug is due to a rich fuel/air mix or too cold a plug heat range rather than excessive oil.
    As I mentioned, mine runs very cleanly on 16:1. (When I pulled it down, though, there was a little bit of blow-by evident, so I am looking forward to dropping to 20:1.)

    I personally wouldn't run less than about 25:1 or thereabouts.
    Too much oil is infinitely preferable to not enough.

    ... Steve