Grubee shyhawk 66 cc carb tuning question

skipS

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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?.
 
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.
 
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.

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.
 
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.
 
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.
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?
 
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.

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.
 
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
 
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
 
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.)
 
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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.
 
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