HONDA CT125 Big Bore Kit

[m in sc]

im around 75-85 mpg with a 143. if i really easy on it it tips into the 90s. I also weight 215 w gear, so theres that as well.

a 4 point seizure is when the piston expands, due to heat, to where it exceeds the clearance needed. as it expands evenly, the 4 points are the 1st to touch. strictly a heat issue. timing will erode the top of the piston, sometimes to the point of putting a hole in it (sunroofing a piston) , oiling will usually scuff one face or the other completely and smear the aluminum, a lot of times wedging a ring in a deformed ringland.

 

[Happy Campa]

Do you have a picture of the pin? Did the it get bigger with heat not allowing the piston to pivot or did it expand in length and the sides rubbed against the wall? I have no idea how those things fail.

Here is a picture of my old wrist pin and piston. You can tell there is some galling in the piston wrist pin journals. It ran ok until it reach operating temps, then started chugging/sputtering.

In a nutshell, it probably didn't pivot anymore.

The sides look ok to me. I pulled the rings off it to measure gap. It was still factory tight.

 

[CitizenSnips]

Are any of the Big Bore Kits out there compatible with the US green JA65 yet?

 

[dmonkey]

Are any of the Big Bore Kits out there compatible with the US green JA65 yet?

Yes, search "2022 Honda Grom BBK" in your favorite search engine to find kits for the 2022+ Honda Grom/Monkey which has the updated Euro5 engine, the same base engine as in the 2023 CT125 JA65. You will still need to sort out managing the air fuel ratio, which can be done with the EFIE device wired in reverse, a piggy back fuel controller, or a standalone replacement ECU.

 

[Samg7]

Last tank calculated 117mpg. I can be easy on the MPG, but it's still too high I reckon. 260mV on the efie with the genuine 140cc/min PCX150 injector. I am going to throw in my 170cc/min yuminashi injector and see what happens. Typical ECM break in of 40-60 miles taking it easy again I guess.

 

[Samg7]

Here’s an update on the current setup (S238 cam, 142 BBK, EFIE @ 260mV, 170cc/min injector, and Takegawa air filter)

After 60 miles easy around town riding settling in the new injector, I hit a 150 mile ride to over 10K elevation.

Multiple minutes of 55+, engine didn’t sieze…. unlike my other two attempts.

Looks like the 170cc/min injector did the trick.

 

[JPMcGraw]

To add on to the 143BBK+170cc

My last 2 failures were with the 143+PCX+EFIE@250. I was getting 115-120mi per tank with normal driving not much gas smell in the exhaust. I just finished my break in tank on the replacement 143+170cc Yuminashi+EFIE@250 and got 110mi on that tank being somewhat gentle. Now it smells like a go-kart so that's reassuring. So +1 more on a noticeable change in fueling once it settles.

 

[Samg7]

That’s interesting @JPMcGraw.
I had similar MPG with my failures, now at 112 and 116 the last 2 tanks.
I also pick up a slight gas smell from the exhaust when I get to a stoplight.

 

[Happy Campa]

To add on to the 143BBK+170cc

My last 2 failures were with the 143+PCX+EFIE@250. I was getting 115-120mi per tank with normal driving not much gas smell in the exhaust. I just finished my break in tank on the replacement 143+170cc Yuminashi+EFIE@250 and got 110mi on that tank being somewhat gentle. Now it smells like a go-kart so that's reassuring. So +1 more on a noticeable change in fueling once it settles.

Interesting. I just ordered a 148cc Yuminashi kit with a 1mm spacer and 170cc injector. The spacer will keep the compression ratio the same as factory, so I can run any octane fuel in it. My setup will be similar, but with the addition of an oil cooler. I will gap the piston rings to compensate for higher heat too.

Sept 13th 2023 - The spacer idea did not pan out. The timing chain does not have enough slack to compensate.

 

[Samg7]

148? Or 143? I would go with the 170cc/min injector either way.

 

[Happy Campa]

148? Or 143? I would go with the 170cc/min injector either way.

148CC (57MM) LIGHT BORE KITS (Z125 MONKEY / SUPER CUB 125 / MSX-GROM125)

YUMINASHI 148CC (57MM) LIGHT BORE KITS (Z125 MONKEY / SUPER CUB 125 / MSX-GROM125) (12100-KYZ-570S)

www.yuminashi.eu

 

[Samg7]

Hmm, never noticed that one!

 

[Plotus]

I suggest caution and a bit of technical abillity. The big bore changes air flow and air flow is extremly important to injector size to prevent from running lean and torching all those new components. Crank ballance is also important. Air flow then needs mods like camshaft, air filtration and exhaust.

 

[m in sc]

if you dig deep you can get them in almost any size. I mean, a 143 ish is a 17% increase, so not ridiculous. I'm staying out of the fuel management discussions from here out since I've said everything im willing to say about it here, but this has been discussed for 10 years now in the grom community, none of this is really new. If you dig around some of the forums covering the 1st gen msx, this is discussed in very deep depth. If you want alternate pro domestic advice, contact http://www.hardracing.com or https://mnnthbx.com/ or https://www.dhmotoring.com/

 

[RustyRodder]

Is there any real difference between Yuminashi's 143 and 148? Seems like they do the same thing

 

[Happy Campa]

Is there any real difference between Yuminashi's 143 and 148? Seems like they do the same thing

There is a 1mm difference on the bore diameter.