Engine Trouble Chronic 4-stroking

Discussion in '2-Stroke Engines' started by PeterG, May 4, 2016.

  1. PeterG

    PeterG New Member

    Engine: Grubee SkyHawk GT5A (66cc)

    From brand-new this engine has been chronically 4-stroking despite all efforts to solve the problem.

    Here's a time-line of what I've tried and the results and symptoms:
    All stock, out of the box. NT carb/black plastic air filter. Fuel mix (25:1 to start then 33:1) - 4-stroking much of the time, occasionally breaks into two-stroking and shows good power. If you are careful with the throttle you can often ride the two-stroking right to red-line. Backing off on the throttle will cause 4-stroking again. Going up a slight incline (putting under load) will break it out of 4-stroking and into 2-stroking. Going down-hill with throttle off will cause it to 4-stroke for several hundred yards when you get back on the throttle. Seems like it's loading up with too much fuel. Spark plug colour looks ok (medium brown). Moving needle clip up/down has little effect. Carb is spitting fuel back into filter. Will not idle when warm (too rich) but starts and idles well when cold (no choke required).

    Replaced spark plug with NGK B6HS
    Replaced head gasket
    No change

    CNS carb/red plastic air filter. Factory main jet. Fuel mix 33:1. Long "Z" intake. No appreciable change. In addition to 4-stroking, was bogging (lean) at full throttle. Raised needle one notch, solved bogging problem. Did not solve 4-stroking problem. Carb is spitting fuel back into filter. Plug colour not bad (medium brown). Will not idle when warm (too rich) but starts and idles well when cold (no choke required).

    CNS carb/SBP low profile filter/SBP main jets .72mm, .74mm, .76mm, .78mm
    z-intake. Same symptoms.

    CNS carb/SBP low profile filter/SBP main jet .74mm/BBR billet intake (short) & boost bottle. Same symptoms.

    Disconnected kill switch.
    Replaced CDI.
    Replaced Magneto loop and magnet.

    No change.

    Original NT carb re-installed with BBR billet intake and boost bottle. Use CNS needle in NT carb with clip on one notch below middle. Ran quite well for about three days. Still 4-stroking at certain points in the throttle but I can ride around it. Gets progressively worse over the next few days though I haven't touched the carb.

    Is now 4-stroking nearly 100% of the time.

    Flush out exhaust with kerosene. Replace exhaust gasket and seal with form-a-gasket. Doesn't help.

    Blow oil out of air filter. Doesn't help.

    NT SPEED Carb, New SBP Air filter, BBR billet intake and boost bottle - Same symptoms; 4-stroking 100%.

    If this were a horse I'd have been forced to shoot it long ago.

    Time for another engine?
    Last edited: May 4, 2016

  2. Timbone

    Timbone Active Member

    Time for another carb, I think.

    You are running too rich! You know, if you are running high revs and back off the throttle on a descent, the motor is gonna 4 stroke. You have to let off the throttle, though because you can let the thing rev out to infinity.

    Bogging and 4 stroking at WOT is a clear sign of running too rich. Change the carb and then, the change the jet if the 4 stroking continues. Use one of those Dellorto Clone / RT carbs with the slide. It is a huge improvement over the stock NT.

    I am using such a carb and I changed the jet to 65 (default is 70). Upside: I have a fantastic leaned out rev from 20 mph up to about 30, running so smooth it gets kinda quiet. Downside: I must use choke to start and it takes a while to get enough warmth into the engine to get her to hold a good steady idle. Maybe it's a bit sluggish at low RPM. but, boy, at the top end this thing is fun!
    FurryOnTheInside likes this.
  3. Hello Moto!

    Hello Moto! Active Member

    Yeah I have a dellorto clone type carb with a .66 jet and it's a little cold blooded but after a brief warm up, It runs excellent. I had almost the exact same problem until I rejetted to a leaner jet. In fact the bike ran best with a .64 jet but was to hard to cold start so I sacrificed a bit of performance and ran a little more rich with the .66, which really helped the starting process. I don't mess around with peddling a bike to start it. I have a pull start and wouldn't have it any other way lol.
    FurryOnTheInside likes this.
  4. PeterG

    PeterG New Member

    Thanks for the info. I've got smaller jets on order for the NT. What are your thoughts on the dellorto vs the NT-speed?
  5. Hello Moto!

    Hello Moto! Active Member

    After using the Dellorto type, I would never go back to the stock NT.

    Just a little tip for you. The NT 5mm jets will work in the Dellorto or HP carb. I bought a big jet kit and they are fully compatible for both carbs.
    FurryOnTheInside likes this.
  6. PeterG

    PeterG New Member

    Cool. Thanks for the info. Where's the best place to get the dellorto clone?
  7. Hello Moto!

    Hello Moto! Active Member

    Marine One likes this.
  8. PeterG

    PeterG New Member

  9. Timbone

    Timbone Active Member

    That'sdax.com sells a nice RT carb/cable combo. The slide carbs work much better.
  10. Hello Moto!

    Hello Moto! Active Member

    You can use your stock throttle cable if you take the angled extended cable housing inlet off the Dellorto type and put the short metal inlet off your NT carb in it's place. This will save from having to replace your throttle cable. I did it and it works perfectly. I actually had to do this because the replacement cable I bought with the Dellorto was actually about an inch too long lol. So, I improvised and it worked.
  11. jaguar

    jaguar Well-Known Member

  12. PeterG

    PeterG New Member


    Cut apart the exhaust, punched out the palladium core, welded the sucker back together, and she goes like stink.

    It appears the exhaust was malformed from birth. The passage at the top of the palladium canister was only about 1/2" in diameter.

    Thanks everyone for helping to diagnose this difficult problem.


    Last edited: Jul 19, 2016