Grubee shyhawk 66 cc carb tuning question

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.)
 
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.
 
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.

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.
 
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.
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?
 
Are the jets marked clearly on the newer "speed" carb?

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.
 
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.
 
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..
 
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.
 
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.

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??
 
Back
Top