HONDA CT125 Big Bore Kit

x29Saab said:

Be careful taking the KOSO AFR values as actual, they are more relative. With mine, 250mV set on the EFIE results in the ECM targeting 13.0-13.1:1 on my KOSO Mini3 AFR meter. My understanding is that the ECM is actually targeting closer to 13.5:1 at 250mV. I would be very careful if your reading is mid 12's with 200mV (which is leaner than I run under closed loop) to accept anything over that, like 12.7:1 at WOT. You might cook something. Watch for stuttering or detonation. If you have even the thought that you felt something, immediately back off the throttle and allow the piston and cylinder to cool.

Click to expand...

Thanks for your thoughts. Surely the reading on the AFR is the result if whatever the ECU has put into the fuelling mix by us manipulating the signal through the EFIE ? If we put an AFR on a stock bike we'd see something like 14.7:1 I get that the Koso might not be as precise as more expensive units and with no way to check readings and calibrate, but can't see how the reading is relative. What values at WOT do you set as the maximum you would accept ? I guess accurate cylinder head temps would be useful but like yourself I've got an Opmid which doesn't measure them. Happy to learn from your extensive testing so thanks for taking the time to chip in. On another note have you put a different spec oil in your CT to cope with higher temps ?

selly said:

Thanks for your thoughts. Surely the reading on the AFR is the result if whatever the ECU has put into the fuelling mix by us manipulating the signal through the EFIE ? If we put an AFR on a stock bike we'd see something like 14.7:1 I get that the Koso might not be as precise as more expensive units and with no way to check readings and calibrate, but can't see how the reading is relative. What values at WOT do you set as the maximum you would accept ? I guess accurate cylinder head temps would be useful but like yourself I've got an Opmid which doesn't measure them. Happy to learn from your extensive testing so thanks for taking the time to chip in. On another note have you put a different spec oil in your CT to cope with higher temps ?

Click to expand...

First the easy question, yes, I use a 10W 50 Motul 300v2 oil that can handle the higher temps without breaking down as much. Now for the harder questions. My thinking is that the 250mV value of the EFIE (whatever the AFR reads that as) is safe (at my level of tune). This was originally based on others' experiences with the Grom and other fuel injected cycles. So anything that reads richer than that is good to go. My testing over 2000 miles has repeatedly shown that any reading leaner than that will eventually experience overheating (so in support of others who have used the EFIE). These AFR values are relevant only when I am certain the ECM is running in closed loop only. That would be partial throttle at speed after it stabilizes to an afr value (usually in 2 seconds or so). I have tested the AFR values without the EFIE and it does target 14.7 on the mini Koso. I have quick disconnects on everything so I can test different configurations easily. Now, using the closed loop targeted value for the EFIE 250mV, everything should be relative off of that. For me, the WOT value is much more dependent on the fuel injector flow rate than the EFIE setting even after 100+ miles where the ECM should have stabilized if it actually learns. Others on this forum will disagree with this statement but it is my belief that WOT uses original map values from the factory and never get changed. In other words, I can use one injector for 100s of miles and always overheat and a different injector of just a slightly different size and never overheat . . . and the afr meter does also show the difference and the same difference during the test regardless of miles piled on. In theory both should either overheat or not if the thing can learn. It may be that the very fact that we are targeting so far away from 14.7 that the true afr values bounce so much that it ignores any new data and just keeps original. Either way, if you are willing to risk it (I am) you can test these things repeatedly with various injectors. Final thought, the narrowband O2 sensors are only really stable at 14.7 or stoic depending on your fuel. Therefore these O2 sensors themselves vary between each other as you move away from that stoic value. Caveat emptor applies. Know your bike and the way it feels. If it missfires, change what you are doing immediately. Build in a safety margin. For me, that is a reading at or below 13.0:1 on the display of my particular (and known to be inaccurate) narrowband afr meter.

OK thanks, a lot to unpack there. I guess the EFIE adds a fixed voltage addition to a dynamic signal returning from the O2 sensor. It could be that the voltage added to the O2 signal should be greater at WOT but you are getting round that by addressing this with injector size. I guess there's a lot we don't know and a lot of empirical data is shaping what folks are doing. You mentioned that my AFR readings at WOT of 12.7 were risky for overheating yet you say you look for 13.1 as a maximum on your bike, why is that ?

Sent from my SM-G965F using Tapatalk

selly said:

OK thanks, a lot to unpack there. I guess the EFIE adds a fixed voltage addition to a dynamic signal returning from the O2 sensor. It could be that the voltage added to the O2 signal should be greater at WOT but you are getting round that by addressing this with injector size. I guess there's a lot we don't know and a lot of empirical data is shaping what folks are doing. You mentioned that my AFR readings at WOT of 12.7 were risky for overheating yet you say you look for 13.1 as a maximum on your bike, why is that ?

Sent from my SM-G965F using Tapatalk

Click to expand...

I am saying 13.0ish:1 = 250mV for my EFIE & narrowband AFR combination in closed loop. The 250mV being the (considered safe) important number and the 13.0ish:1 is fixed to that and is relative and rather unimportant except it gives the reference number to know if WOT is running richer or leaner than 250mV in closed loop. The same can be said on your bike. Mid 12's:1 = 200mV (considered less safe) in closed loop. Mid 12's now being your somewhat less safe number and you are running leaner than that value at WOT (12.7:1). So leaner than your somewhat lean 200mV. If we were both running (known to be accurate) wideband O2 sensors with afr meters, then we would not be having this conversation. Wideband O2 sensors + meters are accurate and relatively close to actual afrs. Our narrowband versions are precise (gives the same reading each time and can discriminate) but inaccurate (not necessarily giving a value that is actual).

I adjusted to 250mv and just completed a 60 mile run. Here are my observations - the bike was a bit less torquey on hills until about 50 miles in when it seemed to settle in. Mid throttle 45 to 59 mpg AFR readings 12.3 1 and 12.4:1, WOT on level road 12.5:1 so it seems the EFIE is still making a useful change at this throttle position, top speed 63 mph for a 20 seconds or so which is about 3/4 more than before fitting the bbk. The AFR readings were noticeably lower when coming off the throttle, changing down etc- generally in low 13's most of the time.

I've got about 200 miles on the engine since fitting the 143cc bbk so it should be broken in now over about 8 /9 heat cycles. I dropped the oil after today's ride and found a small amount of ferrous metal stuck to the magnetic sump bolt, some of it looking like fibres 10mm long and very thin, I hope that's nothing to be concerned about. The Opmid seemed to be showing slightly lower oil temperatures today compared with a few days back running at 200mv and getting 12.7:1 AFR readings at WOT. Any thoughts ?

Sent from my SM-G965F using Tapatalk

sounds right to me. anything under 14.0 is safe though, i wouldnt sweat it.

I do plan on refitting my wideband autometer AFR gauge, which is 100% the way to test and see where mine has landed a few thousand miles in ont he trail. im also out of big projects for the moment so its always fun to revisit.

She's running well but is that an unhealthy sound ?

if the valve clearances are in spec, run it. digital cameras can amplify/enhance mechanical noises.

Yep checked the valves and all in spec. The actual noise is a bit more tappety than the video clip. If I hold a dowel to my ear the exhaust valve area is where it seems to come from. Guess I'll just ride it.

Sent from my SM-G965F using Tapatalk

I got another heat related failure this time with the yuminashi 143. I was just doing a mid day joy ride, it was probably 90-92 out and humid, just cruising along at 45, no traffic, on a flat, not beating it up. Then i got power loss and that beautiful metal on metal sound until i lifted throttle and then this luxurious tapping sound at certain throttle positions started. Great. I was able to ride home no problem other than the tapping sound.

I bought an endoscope that was luckily on sale and free overnight delivery, took a look through the spark plug hole and i have vertical markings in 3 spots that i can see. I don't have a spare SD card handy so ill get pictures when i tear it down. I double checked my EFIE and it was where it should be.

I got a new yuminashi 143 on the way, im switching from the PCX150 injector to the yuminashi 155cc injector, im switching to 10w50, im going back to the stock airbox and probably going to do a 15t front sprocket.

Its hot out and even hotter in my garage so im not sure when ill get around to doing the swap.

yeah if you were running an open pod the injector probably wasn't able to keep up. good plan on the bigger injector, sorry you had the trouble.

I got lucky with rain this weekend so i woke up early on Saturday while it was cool and got it knocked out.



I tried to get good pictures of the cylinder scars but it was overcast and 7am.



Oil is changed to 10w50, I have a 155cc and 170cc injector on the way (Ill probably go with the 170 just for the safety) and im going back to the stock airbox, probably with the NK pipe.

Happy Campa said:

Yes. I checked the voltage shift too. What was being fed back to the ecu was telling it the engine was in a leaner condition an actual. (Add fuel) That's definitely something to good to double check. Any advice on resetting the ecu? I assume disconnecting the battery for a while will make it remap to fuel tables.

Click to expand...

Following up... It turns out that the wrist pin on the piston was toast. Replaced the piston and wrist pin. I also gapped the new piston rings a little wider than stock while I was in there. The originals fit very tight. I don't think the efie had anything to do with it running awfully.

I ordered finned cam cover and valve covers, as well as an oil cooler kit plus performance oil pump. I'll probably put the efie back on too.

4 point seizure is definitely heat related. see it on 2 strokes all the time, 99% of the time when jetted too lean. probably what toasted the wristpin.

m in sc said:

4 point seizure is definitely heat related. see it on 2 strokes all the time, 99% of the time when jetted too lean. probably what toasted the wristpin.

Click to expand...

Could you define 4 point seizure? I agree it a heat problem and yes it was on the side of lean. I was running a cam and air filter. At least a stock 125 cc piston is cheap.

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.

the 4 point seizure happened to me twice. Most likely from running too lean. I posted about it a few pages back.

Even with my new injector and EFIE @ 250, I am still getting 113mpg. Doesn’t seem right.

Stock 125cc I used to get around 125mpg.

Unless youre running full tilt all the time, i dont see the mpg changing a ton. My mustang only lost 2-3 mpg when i slapped on a blower compared to before. I dont see a 125 losing 30+mpg going to a 143 or 164.

With the increase in bore and the increase in fuel, I would expect a bigger decrease.

My Tacoma actually increased MPG with a supercharger!