Grubee shyhawk 66 cc carb tuning question

Discussion in '2-Stroke Engines' started by skipS, Jun 8, 2010.

  1. skipS

    skipS Member

    I have a new Grubee 66 cc skyhawk . I think it has the new carb with the fuel shut off on the carb itself. Just ran through my first break in period (two tanks) at 16 to 1 now im on 20 to 1. bike runs great up to 3/4 throttle but when I twist past 3/4 it bogs down. is this common or should I look for and air leak, or rais the needle to richen it up?.
     

  2. asfazrq

    asfazrq Member

    you don't need 20:1 after the break-in. I do 32:1 and it runs great. Checked the spark plug at that mixture and it was slightly tan/brown. that's exactly where you want it to be. in my opinion 20:1 is too rich.
     
  3. AussieSteve

    AussieSteve Active Member

    Your opinion may not be right.
    As I mentioned in the other thread, my engine's instructions recommended 16:1 for break-in, then 20:1 thereafter.
    I've stuck to that for 510km and never had an oily plug or any problems, once the carb was tuned correctly.
    Although, again, as I said in the other thread, 32:1 won't do any harm and is OK to run, it is really too little oil for these badly manufactured, poor tolerance engines. There's a good reason that the manufacturer suggests 16:1 then 20:1. If the plug oils up or is black, the fuel/air mixture is at fault, not the fuel/oil ratio.

    The 'slightly tan/brown' plug that you mention tells you that the fuel/air mixture is about right, not the fuel/oil ratio.

    Rich/lean is always used to refer to the fuel/air mixture, not the fuel/oil ratio. By saying '20:1 is too rich', you're confusing the issue.

    skipS, it sounds like it might possibly be running a bit rich at WOT. Most of these engines do if there are no air leaks. If this is the case, the answer is a smaller main jet. The slide needle only has effect up to 3/4 throttle, then the main jet takes over.
    Is it 4-stroking at WOT?
    What colour is the plug?
    You might need to do a WOT plug chop.
    There's a bit more info in NT carb tuning basics.
     
  4. skipS

    skipS Member

    ok so I sealed off the carb against any leaks with some permatex ultra rtv. and I raised the needle one clip. it runs so much better. problem is I dont know which was the real culprit the needle being to lean, or the seal to the manifold tube. It does four stoke a little around mid throttle now but as soon as I give it the gas it clears out. I can also get to full throttle now if i roll it on slowly. but it still bogs a tiny bit if I try to get to it to quick.
     
  5. skipS

    skipS Member

    thanks for the advise but because its still new I am going to leave it on the rich side for a while till I am sure it was broken in good.. I figure it cant hurt. On my next gallon of gas ill go 25 to 1 . What octane do you run?
     
  6. AussieSteve

    AussieSteve Active Member

    Personally, I wouldn't have done both at once. Too confusing. Now you don't know which way is up.
    Still, if it's 4-stroking at mid-throttle, it's too rich so drop the needle back to where it was and see if the original problem returns. If not, you know that originally you had a leak and a lean mix.
    Incidentally, most RTV silicon sealants are not recommended for use around fuel. I used to use Permatex, before I fitted the 'O' ring.

    Since it also won't rev out properly at WOT, like many of us, your main jet might need reducing in size. I had to go down in steps from the stock 0.79mm to 0.72mm, with the slide needle at it's lowest setting, before I could get my engine to run properly.
    'Sick Bike Parts' now sell a range of jets to suit. (They're Dellorto PHBG 5mm main jets.)

    On the fuel/oil subject, there is absolutely no advantage in running less oil. It won't make the bike go faster, the engine will just wear out faster.
    Also, if your fuel/air mixture is already rich, reducing the oil in the fuel/oil mixture results in a richer fuel/air mixture due to the higher percentage of fuel, which will exacerbate your current problem.
     
  7. skipS

    skipS Member

    I lowered the needle back and it bogged again but not as bad as it was. I think the richer needle setting is masking a small intake leak and thats why it runs better. ill fit the O ring and spend some time sealing the carb and intake tube, The permatex ultra held up in my motorcycle head after being exposed to tons of oil, thats why I chose it but ill clean it off and do the o ring
     
  8. skipS

    skipS Member

    I did the carb mod with the o ring, and i changed the clamp screw to a bolt instead of a screw. No intake leak. Here is the dilemma,at even throttle it four strokes. as soon as I dial on the gas it clears up. When I raise the clip to lower the needle ( lean it out) the four stroking gets better, but then it Bogs when I go to full throttle.
    I have some hills to climb on my commute and the way it runs now it climbs them no problem but the four stoking at even throttle is driving me crazy and killing my mileage. Any help is appreciated
     
  9. AussieSteve

    AussieSteve Active Member

    The only real answer to 4-stroking at constant mid-throttle openings is to lower the needle as you did.

    Regarding the bog at full throttle, the best way to check out the situation is a WOT plug chop with a new plug.
    It could be 'lean bog', but I suspect it's too rich at WOT, but not rich enough to 4-stroke. I get similar symptoms if I go up a couple of sizes in main jet.
    I'm not familiar enough with the new carb to know the main jet size, but about an 0.72mm works best with the older NT carbs on a 66cc engine.
    Do you know what size jet is in your's?

    Incidentally, to double-check that there are no air leaks, spray some carb cleaner toward the suspect joints with the engine running. If there's a leak, you'll hear the change in rpm as the carb cleaner enriches the mixture. (Avoid spraying it directly into the air filter or you'll defeat the purpose.)
     
    Last edited: Jun 13, 2010
  10. Al.Fisherman

    Al.Fisherman Active Member

    I'll also stand by Steve's assessment. Myself I use 6 ounces of Amsoil per gal which runs out to between a 21:1 and a 22:1 ratio. I'm one of the lucky ones, and never had any fuel/carburetor issues.
     
  11. AussieSteve

    AussieSteve Active Member

    Thanks Ron. It's hard to tell if this one is rich or lean, but most are rich when the leaks are sealed.
    21:1 sounds good, too. I've never had a problem with an oily or black plug since I got the carb tuned properly, even when I was running 16:1.

    The extra-high fuel consumption is another indication that the main jet is possibly too large, causing a rich WOT mixture. (Still, that could just be due to a rich mid-throttle needle setting.)
     
  12. skipS

    skipS Member

    I will do a plug read during the week but I am leaning towards the main jet is too small or lean. That explains why the richer needle setting makes it run better going from 3/4 to full throttle . I also removed the air cleaner for a two minute ride to see if the 4 stoking and full throttle bog or hesitation got better or worse. The 4 stroking got a little better as the extra air in the intake leaned out the mixture. The wide open throttle bog or hesitation got worse. If it was running rich at wide open throttle, I would have thought the extra air from the air cleaner being removed would have helped it, not hurt it. Next up ill pull the main jet then send out for a larger one.
     
  13. AussieSteve

    AussieSteve Active Member

    Can't argue with your assessment. I should have mentioned using the choke to help diagnose - must be getting even more absent-minded.

    It's still a good idea to do plug chops at mid and full throttle.

    The only other point is that it's well worth while using carb cleaner to double-check for leaks before jetting up. It is unusual that you'd need to go up in jet size and not down. You don't want to rejet just to compensate for a slight leak.
    Your's might be one of the carbs with an 0.7mm main jet, instead of an 0.79mm like most current NT carbs.

    Anyway, sounds like you're on top of it. Good luck. Keep us posted, Skip.
     
  14. skipS

    skipS Member

    I will surly do the sealer check just in case there is leak. You are a big help Steve, and I appreciate your input. Are the jets marked clearly on the newer "speed" carb?
     
  15. AussieSteve

    AussieSteve Active Member

    As I think I mentioned earlier in the thread, I don't have a 'Speed' carb, so I'm not sure, but probably not.
    The standard NT carb's jets aren't stamped with their size, so the 'Speed' carb is probably the same. Makes it hard to work out the size of the original main jet.
    Possibly the vendor you bought it from could help there.
     
  16. Vistaman73

    Vistaman73 Member

    Well, 16:1 is way to rich to run these engines on. I don't care what the instructions say, we have better oil here in the U.S. and don't need to use a much. I broke my engine in at about 32:1 then I added a little splash of oil. I run it at 32:1 now and itsseems to be the perfect ratio. Never listen to the instructions, it's mostly wrong.
     
  17. AussieSteve

    AussieSteve Active Member

    How did you determine that 32:1 is the perfect ratio?
     
  18. butch100

    butch100 Member

    I've been following this thread as my new build is running rich as well, ( 4 stroking most of the time, black, sooty plug). I put the carb clip on the top notch (closest to the top of carb) to try to lean it out (that's right, isn't it?). Running 16:1 mix for now, still haven't finished my first gallon yet and as already pointed out it's not a mix problem anyway. I've actually considered loosing up the intake tube to get it to lean up some but have decided to wait until my first gallon of gas is gone to see if it leans up by it self..
     
  19. AussieSteve

    AussieSteve Active Member

    Don't loosen the intake to induce a leak to lean out the mixture, for a number of reasons.
    First, the carb might fall off.
    Second, it'll be near impossible to get a good mixture that way.
    Third, due to #2, you could potentially overheat and/or sieze the engine.

    Best answer is to fit a smaller main jet. Sick Bike Parts sell a range of sizes.
    Also, it's a good idea to differentiate between 0-3/4 throttle and wide-open throttle, since they're controlled by the slide needle and the main jet, respectively.
     
  20. butch100

    butch100 Member

    Thanks for the fast response Steve, yea, it's a kinda whacked idea, it mostly runs bad idle-3/4 throttle, I was thinking about checking the float level but not sure if that would make a difference anyway, Worst case scenario is I buy an aftermarket carb that allows mixture setting. Did I have it right adjusting the clip like I did??
     
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