JA65 Big Bore Kit 145cc

[FizzBuzz3000]

Eh, I think it's reasonable that most don't want to do it. If I recall correctly, it also requires a wideband O2 sensor further down the exhaust pipe and that usually requires cutting and welding a wideband 02 sensor bung on. Not many people can do that (like myself) and will be costly on top of the $800 aracer.

 

[ball_socket]

Eh, I think it's reasonable that most don't want to do it. If I recall correctly, it also requires a wideband O2 sensor further down the exhaust pipe and that usually requires cutting and welding a wideband 02 sensor bung on. Not many people can do that (like myself) and will be costly on top of the $800 aracer.

That wideband is kind of the whole point.

I feel you thought. You only get one shot at getting that thing lined up right.

 

[m in sc]

i agree, IMHO w out the wideband its kinda pointless. coming from the carb world as well, I even tune carbbed 4 stroke motors with a wideband. (usually) so with an efi setup... theres really no other reliable way. somebody on here was using a narrowband tuner but id be leery about it.

 

[michaelwbrenner]

Hi Guys, I'm struggling to solve my tuning problem on my NEW 2025 CT125 (JA65)

So far I have accumulated some parts that should be equivalent to a mild big bore build, here's what I intend to use and have already purchased:

• Takegawa 145cc piston/cylinder
• Takegawa N10 camshaft
• Takegawa air filter
• eBay Thai OEM+ type exhaust w/o catalyst
• Yuminashi 140cc injector
• Takegawa crankshaft support bearing
• KITACO 3 row oil cooler

Assumptions: Stock throttle body / intake manifold / air boot / snorkel, not planning to port the cylinder head

I have not installed my new parts yet, What can I do to get this bike to run well?

As of today (5/19/26) Aracer eco are sold out, hard racing could not source one for me. EFIE are sold out, contacted them and was instructed to sign up for the news letter to find out when back in stock. Takegawa FI-CON seem like they may not work on USA market JA65s (according to other posts and webike notes).

How should I effectively manipulate the fuel delivery that matches this engine combination???

 

[loganpx]

Hi Guys, I'm struggling to solve my tuning problem on my NEW 2025 CT125 (JA65)

So far I have accumulated some parts that should be equivalent to a mild big bore build, here's what I intend to use and have already purchased:

• Takegawa 145cc piston/cylinder
• Takegawa N10 camshaft
• Takegawa air filter
• eBay Thai OEM+ type exhaust w/o catalyst
• Yuminashi 140cc injector
• Takegawa crankshaft support bearing
• KITACO 3 row oil cooler

Assumptions: Stock throttle body / intake manifold / air boot / snorkel, not planning to port the cylinder head

I have not installed my new parts yet, What can I do to get this bike to run well?

As of today (5/19/26) Aracer eco are sold out, hard racing could not source one for me. EFIE are sold out, contacted them and was instructed to sign up for the news letter to find out when back in stock. Takegawa FI-CON seem like they may not work on USA market JA65s (according to other posts and webike notes).

How should I effectively manipulate the fuel delivery that matches this engine combination???

Your best bet is waiting on Aracer's new gen ECU to support the trail125 or waiting for tuning shops to release flash tunes IMO.
I had contacted Steadygarage about the same ecu not too long ago.

Without an ECU/tune, It would probably be pretty difficult to get that build running properly

 

[Harley rider]

so here is what i have and it runs perfect for 1000 miles so far

2024 ct 125

• Yuminashi 143 cc BBK
• Yuminashi n10 cam
• Takegawa high flow oil pump
• Dna air filter
• Eifi device
• outex titanium exhaust
• Kitaco 3 row oil cooler
• 150 cc min injector

no ecu tune but did have to play with injectors to find the sweet spot( i am now the proud owner of a 120, 130, 140, 150, 160, and 170 cc min injector at various build iterations), i also have a narrowband sensor inline that did help with that process, no doubt it would run even better with a tune but it runs great. better power through the low end and upper, so much so that i went back to a stock front sprocket. the 150 cc min for my bike gave the best a/f ratios

as a side note the standard pcx 150 injector that a lot of people add is only about a 130 cc min injector(regardless of what's advertised), so you need to know the actual cc min

stock injector 90 cc min
faster minis has a nice 120 cc min for mild hop ups(exhaust and intake), or a 140 will work for most big bore kits (if you haven't changed the intake and exhaust)
the pcx 150 injector is a nice low cost compromise at around 130 cc min
I found a good place of amazon to buy the others

and BTW ethanol free rec gas buys my bike another 20 miles of range, and it runs cooler