HONDA CT125 Big Bore Kit

[SLO]

Does anyone on the forum have a 181 kit installed and can share any experience with it so far? I'm trying to find out as many things about it as possible before I pull the trigger. I want to keep my reliability but also increase my speed/hill climbs by a bit so atleast im not holding up traffic/being able to keep up on 55mph roads. I do not have an interest in going on a highway as i prefer to take roads/trails less traveled but just end up on 55+ mph roads in between.

I read a bunch of positives/negatives on FB land groups depending on who's selling /buying but curious if anyone on here has it.

Also, I signed up for SCBR next year, the kit would help with some sections for sure as 400+ sections on road with low speed would be extra slow.

Thanks

If you go above a 143 cc kit you have to deal with issues that would be complex ,such as dealing with the proper AFR ,requiring a fuel controller? or a temp gauge ,with monitoring the spark plug while adding either two cycle, oil to gas or marvel mystery oil at a ratio that would vary depending on different conditions ,which I believe would probably work, but it would require experimentation with regard to actual application, (my idea) ( which is why I can't explain or recommend) varying conditions , etc, negating the ability to apply simple math in equations ,to reach a simple solution regarding the application, ( engineering ). How hot is it where you are operating? air flow rate over the engine, when will the effect of the additional heat added by larger displacement no longer be removed resulting in component failure. The ( sno ball ) effect ) ( critical mass ) ? The addition of heat increases resistance, resulting in a increase of heat ? Bigger cylinder,smaller block heats up faster, resulting in the inability to remove additional heat added in a air cooled engine. Keep it as simple as you can. I believe I have achieved the full practical potential of the bike without compromising reliability,with the frame it rolls on. I am happy as a clam with it now! It is FUN to ride, it sucked when I got it. I do not believe the engineers either ever rode it , or were not a factor in what was actually marketed. Also I went back to the 13 tooth front sprocket, big difference! It doesn't affect the top end speed,it will allow you to achieve it on hills.

 

[AZ7000']

If you go above a 143 cc kit you have to deal with issues that would be complex ,such as dealing with the proper AFR ,requiring a fuel controller? or a temp gauge ,with monitoring the spark plug while adding either two cycle, oil to gas or marvel mystery oil at a ratio that would vary depending on different conditions ,which I believe would probably work, but it would require experimentation with regard to actual application, (my idea) ( which is why I can't explain or recommend) varying conditions , etc, negating the ability to apply simple math in equations ,to reach a simple solution regarding the application, ( engineering ). How hot is it where you are operating? air flow rate over the engine, when will the effect of the additional heat added by larger displacement no longer be removed resulting in component failure. The ( sno ball ) effect ) ( critical mass ) ? The addition of heat increases resistance, resulting in an increase of heat ? Bigger cylinder,smaller block heats up faster, resulting in the inability to remove additional heat added in an air cooled engine. Keep it as simple as you can. I believe I have achieved the full practical potential of the bike without compromising reliability,with the frame it rolls on. I am happy as a clam with it now! It is FUN to ride, it sucked when I got it. I do not believe the engineers either ever rode it , or were not a factor in what was actually marketed. Also I went back to the 13 tooth front sprocket, big difference! It doesn't affect the top end speed,it will allow you to achieve it on hills.

So do you have the 143? SLO?

 

[Shoot870p]

I believe it’s safe to say not a 144 at least!

 

[m in sc]

adding oil leans the mixture, just fyi.
ive had my 143 for a while, it works very well. but you need to do something about the afr, I used an eife, you can get by w out one but it will still target 14.5-14.7 which, to slo's point, the 143 is 'ok ' at handling.

again, for the millionth time, the issue is the ecm. you can go aracer standalone, or bandaid it like i did. the early ecms are a different story.

-however-
the trail ecms, the us models, run in closed loop all the time, so they will adjust.... to factory lean settings regardless of what you are running bore wise. its that simple. But if the bore is too big, it may (to slos point) create more heat than is safe and you can get a lean seizure.

 

[AZ7000']

Less of a risk at 7-9000’? The lean condition that is???

 

[m in sc]

at elevation? The ecu targets that afr regardless of elevation. So it doesn't matter i would imagine.

 

[SLO]

So do you have the 143? SLO?

YUMINASHI 143cc kit with a 238 cam was about $ 256: dollars delivered. Add a awesome exhaust from WeBike Japan $ 300:00 dollars, 150 injector $ 55.00 KNM type air filter was around $ 75.00. Remove restriction in air box. I made sure I Gapped the rings ( coating on ends of rings where the gap is ) supposed to be .010 I removed a little more to about .014 on top and second,but put them at 180 degrees apart rather than 60 degrees. I have over 500 miles on it,and it not even close to being the same.I removed restriction at end of exhaust,and it is quiet until
you open throttle. Believe it is better than using horn to notify other vehicles of your presence. Do it for safety, would be a legitimate reason to justify. Having to back off throttle when you hit 55 mph is another . I am getting about 100 mpg going up the mountain,which is better than it was, and it doesn't have to work as hard,with the throttle opened up a lot less of the time.

 

[SLO]

adding oil leans the mixture, just fyi.
ive had my 143 for a while, it works very well. but you need to do something about the afr, I used an eife, you can get by w out one but it will still target 14.5-14.7 which, to slo's point, the 143 is 'ok ' at handling.

again, for the millionth time, the issue is the ecm. you can go aracer standalone, or bandaid it like i did. the early ecms are a different story.

-however-
the trail ecms, the us models, run in closed loop all the time, so they will adjust.... to factory lean settings regardless of what you are running bore wise. its that simple. But if the bore is too big, it may (to slos point) create more heat than is safe and you can get a lean seizure.

adding oil leans the mixture, just fyi.
ive had my 143 for a while, it works very well. but you need to do something about the afr, I used an eife, you can get by w out one but it will still target 14.5-14.7 which, to slo's point, the 143 is 'ok ' at handling.

again, for the millionth time, the issue is the ecm. you can go aracer standalone, or bandaid it like i did. the early ecms are a different story.

-however-
the trail ecms, the us models, run in closed loop all the time, so they will adjust.... to factory lean settings regardless of what you are running bore wise. its that simple. But if the bore is too big, it may (to slos point) create more heat than is safe and you can get a lean seizure.

I was thinking while riding yesterday. I am beginning to realize how the ecu controls fuel AFR. The oxygen sensor does not detect oxygen or nor does the MAF sensor measure the air flow rate.It supplies an electric current to both sensors,and based on the resistance ,it varies the voltage or current supplied to the fuel system while trying to maintain a constant differential between the two sensors,which are thermistors..What is the designed delta T? and which sensor do I trick by changing the resistance from that sensor back to the ecu? Is it utilizing milliamps,or millivolts,to determine a response? My guess would be the O2 sensor,and change the return voltage or amperage back to the ecu,depending which response it utilizes to determine it's response.I just need to know the input and out values it is using,and how the return values vary the response. And I was actually thinking about the effect of adding oil to the gas before I saw your post yesterday,and the effect it would have on the AFR, It would lean the AFR if it caused the engine exhaust gases to be cooler, but what ratio of fuel mix would cause that?and does adding Marvel for example cause the engine to run cooler?and if so , what if the combustion is actually hotter due to higher compression at proper ratio? Could the exhaust gas actually be higher,while the engine runs cooler at some point . And what would be the max volume of air into the cylinder at the max rpm based on displacement and valve clearance and timing, with respect to the optimal static pressure provided by the exhaust,while not causing civil disturbance while operating? Of course we are not going to even contemplate boost yet. Not YET! I need that first cup of coffee before I get a headache Everybody have a GREAT DAY! DON"T LET THE BASTARDS GET YOU DOWN !

 

[m in sc]

adding oil leans the mixture, period. thats why on 2 stroke for example, you jet up when going from an oil injection system to premix. don't put that shit in your fuel, it doesn't need it.

and yes, the 02 sensor does control it by sending a voltage signal to the ecm. hence why the eife works. when you install it, you can either add or remove the signal voltage. IF you install it 'backwards' from the directions , it adds voltage to the line to the ecm, in my case, about 20 mV to get the target afr. when the afr is about 13.5, the ecm -thinks- the narrowband is seeing 14.5 or so in the 'lean' areas of the fuel map. the irony is the eife was designed ot lean out cars for hypermiling, i bet they have sold way more to people doing the exact opposite.

 

[SLO]

adding oil leans the mixture, period. thats why on 2 stroke for example, you jet up when going from an oil injection system to premix. don't put that shit in your fuel, it doesn't need it.

and yes, the 02 sensor does control it by sending a voltage signal to the ecm. hence why the eife works. when you install it, you can either add or remove the signal voltage. IF you install it 'backwards' from the directions , it adds voltage to the line to the ecm, in my case, about 20 mV to get the target afr. when the afr is about 13.5, the ecm -thinks- the narrowband is seeing 14.5 or so in the 'lean' areas of the fuel map. the irony is the eife was designed ot lean out cars for hypermiling, i bet they have sold way more to people doing the exact opposite.

Ok ,I remember seeing a previous post of yours regarding this,I will look for it. If I need to add 20 mv on the return line from O2 sensor, I can figure how to do that,if that is what I need to do. I believe I saw something from Eagle MFG that was priced reasonably. I did not want to splice into the factory harness. I can make a device, If I know which wire to send it back on,but if I can buy it for $50.00 bucks already in a sealed unit I would. The device I bought and returned would have worked,but the schematic did not coincide with the schematic in manual,and if they sold as many as they claimed, why haven't they added a plug to it that would negate need to splice into the harness? Also I do not believe that they provided specs on what return voltage should have been as it was a variable resistor I presume,that you were supposed to dial in,trial and error. If the correction is a set amount it will be simple.

 

[m in sc]

just set it between 17 and 20 (edit.. volts.. or 170-200 mv). yes, it s adjustable. i used a female/male bullet connector (Japanese style from vintage connections sinc ei have the whole kit for other projects) to do it, so i could then take it out of line. spicing a circuit in one wire is nothing. easy stuff. then i grounded the - on the unit tot eh frame, and used 12v switched off the brake switch, also another splice. done. 4 wires, super simple., took less than an hour, easily. youre going to have to tap into the harness to acess stuff, so, however you choose to do that..

 

[RustyRodder]

just set it between 17 and 20 mv. yes, it s adjustable. i used a female/male bullet connector (Japanese style from vintage connections sinc ei have the whole kit for other projects) to do it, so i could then take it out of line. spicing a circuit in one wire is nothing. easy stuff. then i grounded the - on the unit tot eh frame, and used 12v switched off the brake switch, also another splice. done. 4 wires, super simple., took less than an hour, easily. youre going to have to tap into the harness to acess stuff, so, however you choose to do that..

Hey M- Is 17-20mv correct? Ive been doing some reading on the efie's, and i keep seeing 250 and 275mv

Im considering a setup similar to yours- intake/exhaust/yuminashi 143cc and cam (witha 140cc inj), and then using an efie to bring down the afr to 13.5. The fi-con's look nice, but at over 500+the ride... ouch.

 

[m in sc]

its going to typically read .17, .20, etc on a meter, at least on mine, digital, in the regular voltage range.
but yes, you are correct, in terms of millivolts, its 200. or, .20 on the basic voltage range.

Grom EFIE Mod

docs.google.com

 

[RustyRodder]

its going to typically read .17, .20, etc on a meter, at least on mine, digital, in the regular voltage range.
but yes, you are correct, in terms of millivolts, its 200. or, .20 on the basic voltage range.

Grom EFIE Mod

docs.google.com

Thanks! Thats what I was reading through.

Have you had any problems using the Efie for maintaining fueling with your bbk?

 

[m in sc]

none. its been solid as a rock. i havent experience the voltage 'drift' some have.

I also used it on my ktm390 years ago, worked well on that too. .02

 

[SLO]

Hey M- Is 17-20mv correct? Ive been doing some reading on the efie's, and i keep seeing 250 and 275mv

Im considering a setup similar to yours- intake/exhaust/yuminashi 143cc and cam (witha 140cc inj), and then using an efie to bring down the afr to 13.5. The fi-con's look nice, but at over 500+the ride... ouch.

I am not sure if it is required, I will find out I am sure,but 600 plus miles and no issues with a Honda 150 injector,,free flow exhaust that is quiet until you open up the throttle,with a KM type air filter with the snorkel laying to the side,and that tube type thing in the air box cut out. I put a 143 cc YUMINASHI big bore kit with the recommended 238 cam and checked the temp with an infrared thermometer, and it was well within heat range all over engine after running hard for hours. It recommended using synthetic V-TWIN 20W-50W for heat dissipation,which I was running 10-40 W. I also was running a 13 tooth sprocket in front as I live and ride in the mountains and pretty much believe it won't rev up enough to use 3rd and 4th gear going up any incline to maintain speed. I keep checking the spark plug as I believe that is ultimately what you should go by. But I will admit I only know what I know since I got the bike in October of 2021,and knew nothing about motorcycles or tuning prior. They make and sell a oil plug cap type oil thermometer,but the plug will indicate if you need to do all that other shit. Go any bigger and you will. Mine is perfect for where I live,and I bought a Dr 200 ( 22 years old ) that was garaged with 2000 miles on it for other applications. 3.5 gal fuel tank ,and 90 mpg should take care of other applications. Of course I will need another bike for others. There is NO UNICORN. Some asshole would kill it as soon as it showed itself......... SLO SEZ!

 

[m in sc]

well, if you want to aim for stock afrs, no, its not required. but, more hp = more heat and fuel IS a cooling agent. will it survive? probably.

best way to verify is with an afr gauge. .02

 

[SLO]

I am absolutely sure that the condition of the plug shoes not lean, if anything it is running rich.I am wondering if right after i got it, and was messing with the battery,for whatever reason,maybe after injector install, I hook up the battery backwards !! Duh ! But anyway, it runs strong,and that was well 3000 miles ago. It didn't damage it,so that's good ?Maybe it isn't the 150 injector,it said genuine Honda on package.

 

[SLO]

well, if you want to aim for stock afrs, no, its not required. but, more hp = more heat and fuel IS a cooling agent. will it survive? probably.

best way to verify is with an afr gauge. .02

I did by the way unhooked that tube from the crankcase going back to the air box,and put a inline fuel filter on the end of tube,and tucked it under the cover by the battery. I haven't checked it to see if there is any blow by due to my over gaping the rings,since I am thrilled with it,and was afraid to do so.Not smoking plenty of WOOMF !

 

[RustyRodder]

I am not sure if it is required, I will find out I am sure,but 600 plus miles and no issues with a Honda 150 injector,,free flow exhaust that is quiet until you open up the throttle,with a KM type air filter with the snorkel laying to the side,and that tube type thing in the air box cut out. I put a 143 cc YUMINASHI big bore kit with the recommended 238 cam and checked the temp with an infrared thermometer, and it was well within heat range all over engine after running hard for hours. It recommended using synthetic V-TWIN 20W-50W for heat dissipation,which I was running 10-40 W. I also was running a 13 tooth sprocket in front as I live and ride in the mountains and pretty much believe it won't rev up enough to use 3rd and 4th gear going up any incline to maintain speed. I keep checking the spark plug as I believe that is ultimately what you should go by. But I will admit I only know what I know since I got the bike in October of 2021,and knew nothing about motorcycles or tuning prior. They make and sell a oil plug cap type oil thermometer,but the plug will indicate if you need to do all that other shit. Go any bigger and you will. Mine is perfect for where I live,and I bought a Dr 200 ( 22 years old ) that was garaged with 2000 miles on it for other applications. 3.5 gal fuel tank ,and 90 mpg should take care of other applications. Of course I will need another bike for others. There is NO UNICORN. Some asshole would kill it as soon as it showed itself......... SLO SEZ!

Good to know, thank you! How many miles have you put on it? And what exhaust did you use? I am looking at the MiniMoto exhaust