Need help finding leaner main jets for my NT carb!

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Feb 2, 2024
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Alright so a few months back I bought a pawn shop beach cruiser and bought this 50cc 2 stroke kit off of amazon to put on it and it has been mostly running great with my highest flat land speed record at 26mph (42kmh) after removing the exhaust baffle.

I noticed after leaning the fuel-oil mixture to 40:1 from 16:1, full throttling 4 strokes all the time at mid-high RPM and does it much sooner than at 3/4 throttle so I suspect my main jet is probably too rich given I live at ~5000 feet altitude.

I have my carburetor pulled apart and measured my main jet and tried to find a set online but it seems like my main jet is a unusual spec. Calipers say the threading is M6x0.75 (6mm diameter x 0.75mm pitch) with a 5.5mm head diameter and a total length of 9mm with the head being 4mm long and the threading portion being 5mm long. The hole diameter is ~0.6-0.7mm I think but it's hard to tell even with calipers.

I've already tried playing around with the needle clip (even using tiny shimmies to fine tune it) but I've basically perfected all but wide open throttle. I've also drilled some holes into the stock air box but it's still pretty rich at full send.
Anyone know where to find a jet that fits the specs above? Any help you guys can provide is greatly appreciated. I really want to break 30mph!

Here's a picture of the jet for reference. Most listings I see, the head diameter is 7mm while the threading is typically M5 or M6 but with 0.8mm pitch.
IMG_0593[1].JPG
 
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I noticed after leaning the fuel-oil mixture to 40:1 from 16:1, full throttling 4 strokes all the time at mid-high RPM and does it much sooner than at 3/4 throttle so I suspect my main jet is probably too rich given I live at ~5000 feet altitude.
I also live at a high altitude of 4500 ft. in the high desert of New Mexico and I would NOT go any less oil in that than 32:1 ratio with that motor, (4ounces oil to one gallon gas), unless you want to burn that motor out in your lean high desert/mountain altitude...lol.

And check out if it's a BoFeng carby like @ImpulseRocket mentioned in his post, if so, you have the same carby that both him, I and many others on the forums also have...They come stock with a #70 jet.

Here is where you can get the proper sized jets if it is a BoFeng...Like @ImpulseRocket previously posted...If is is a BoFeng, you will see the name embossed on the upper round chamber on the carby where the barrel slide with needle valve slides into.

 
I also live at a high altitude of 4500 ft. in the high desert of New Mexico and I would NOT go any less oil in that than 32:1 ratio with that motor, (4ounces oil to one gallon gas), unless you want to burn that motor out in your lean high desert/mountain altitude...lol.
Probably a good thing full throttle was rich then eh?
And check out if it's a BoFeng carby like @ImpulseRocket mentioned in his post, if so, you have the same carby that both him, I and many others on the forums also have...They come stock with a #70 jet.
Does the carb say Bofeng on the side? If yes, then it's not an NT, which is why the jet is weird. If yes then the main jet is based off of Keihin jet 99101-124, aka Honda jet 99101-zf5.
On the carburetor on the airbox, it says NT on the part that faces the back of the bike, but it also says BoFeng on the side of the carburetor body. The jets for Bofeng are pretty close and likely close enough to fit anyways but man, it might be more economical to buy a carburetor on amazon that comes with different jets than to spend over $8 each on a jet and spend possibly over $50 to find one jet that will work for my bike at this altitude.
 
I dont know where you have been setting the C clip on your needle valve, but I would start off at the second groove from the bottom, (the rich side), and if that is too rich, go to the middle notch, and so forth and so on.

Slide Needle Grooves.JPG
 
Also make sure your head bolts are tightened, they loosen up during heat cycles and can cause the effects you are describing.

Just wait to adjust them and do so only when motor is completely cold...also check that the carburetor hold down bolts and exhaust bolts also haven't loosened,
 
Also make sure your head bolts are tightened, they loosen up during heat cycles and can cause the effects you are describing.

Just wait to adjust them and do so only when motor is completely cold...also check that the car bolts and exhaust bolts also haven't loosened,
I've thread locked my critical bolts that weren't my head bolts and I haven't had anything go loose yet from checking every month and just now. I actually snapped one of the head studs from trying to tighten to what the almost useless manual said to torque them at (10 ft/lbs) when I was putting the engine on my bike so I have them at 8 ft/lbs but they're all very tight still and haven't had any signs of head leaks from when I've taken the head off.

Running the C clip at the neutral setting, my bike would 4 stroke above 4-5k RPM so I set it a notch higher. The engine ran a bit hot for my taste (375 - 400 F) so I put a shimmy made of aluminum foil with a hole in it to just barely make the needle richer but the change is probably negligible.
Look up gas bicycle carb jets on ebay or amazon you will find what you are looking for in sets of different sizes. Only 10 or 11 bucks
I've looked for the life of me with different searches on ebay and amazon but all I get are mikuni and oko jets and what looks promising is actually just a predator 212cc main jet set. If you could reply with a link to BoFeng jets if you know of a listing that would be awesome.
Solder up the OE jet and redrill it smaller.
Honestly, I'm starting to consider that given I already have a soldering iron and some leaded solder that I used to hardwire SLA batteries in a razor scooter. All I would need is the micro drill set and very steady hands lol.
 
Honestly, I'm starting to consider that given I already have a soldering iron and some leaded solder that I used to hardwire SLA batteries in a razor scooter. All I would need is the micro drill set and very steady hands lol.
Those micro-drill bits just drill in by hand. A kit should come with the little bit holder.
 
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