VigilanteofLight

So I finished rebuilding the engine last wednesday, I figured I'd wait a week before putting an update to see if it would survive the first few hundred miles. It started first time which was a good sign it took a few 15 min runs before it would idle nicely, then I changed the oil and have done about 250 miles so far with not a single issue. I've only been keeping it under half throttle as much as possible, and I'll keep that pace for another week before changing the oil again and having a bit more fun from there. Here are some pics - it was definitely starved of oil is my opinion, those scratches on the bore and piston wouldn't happen otherwise. The bore was not circular any more, the piston rocked a lot more in the axis of the pin than it should have. I replaced the cylinder sleeve, piston and rings and put it back together, did a couple coolant flushes and here we are!

This version of the CBR125R is EFI, not carbed - hence why I was so convinced of an electrical issue. I couldn't check the injector because one of the screws felt like it was about to snap, so I didn't take it out. But yeah, I should've tested the compression more thoroughly before I took the sides off the engine... I am hoping the bottom end is fine as I really don't wanna rebuild it... the pics in the Haynes manual look like a nightmare. I'll put another comment in the thread when the engine is together. At the end of the day, it's probably gonna get passed on through another 10 teenagers who will forget to do their oil changes and run it into the ground, so I'm not that bothered if the bottom end craps out in 10k miles or so.

Right. So I managed to get the new throttle body yesterday, and the gasket set. Engine back together with oil and coolant in. Testing - now that the injector is happily out, I could properly see the jets - happy as anything, fuel system is all good. First crank over having drained more oil than just the usual amount - 4 bar. Compression is definitely the issue. Second crank - 7 bar, back to where we were. Third crank - oil down spark plug hole - 10 bar. It's the piston/cylinder/rings... why didn't I test this before I took the clutch cover off I think the reason it wasn't cranking was because the oil around the piston was masking the true issue, and it's been drinking oil like no tomorrow. I wonder if the previous owner thought you had to fill up on oil every time you filled up on fuel... Anyway, Engine is out and has been disassembled. The piston wobbled a lot and more so in the axis of the crank pin, so cylinder is not circular. Here's some photos of the piston and cylinder wall... I've ordered a new cylinder barrel and piston kit, so we shall see how that goes Also, yes that's an RX7 rotor, they make great paperweights

@Tinkicker So the crank pickup - I assumed it was the little sensor inside the flywheel casing - was pretty clean. the flywheel, sensor, and stator all had some very fine metal dust on them but nothing to impede function. @Fozzie I can't see any issues on the wiring, but I naturally don't have the tools to read the ECM or test it. I wanted to ask whether it's any chance of an electrical issue still? or whether I should just try an engine rebuild without buying a new ECM.

So I've done some electrical testing. Resistances on everything I've tested so far have been bang on: Ignition coil - 2.5 Ohm CKP - 115 Ohm IACV - 128 Ohm Bank angle sensor - 3.38V ( a bit low but still has voltage) I tried to take out the injector this morning and snapped a screw, can't get the bolt out with an extractor set. New throttle body coming on Monday. That's the weird thing though, no big engine noises. And while having stalling issues, no loss of power at high end. The injector, now that it's out, looks pretty good though, no obvious gummed up bits. I can't test the spray though as I can't crank the engine at the mo. I think you're right, something has failed in the general head area, and that seems suspiciously priced, yeah. I'm gonna put some new oil and coolant in when gaskets arrive then crank it, if nothing I will take the cam holder off. Cheers for all the support so far, you guys have been really helpful

It looks like it's in time, so I suspect the head has to come off. I also took some photos of the camshaft and cam gear. To me they all look pretty reasonable - apart from that weird splodge of corrosion or something on the cam gear - it almost looks like chalk. The shoulder screws had washers on and the O-rings in them have disintegrated, so maybe a bit of moisture has got in. I don't think this had ever come off before I took it off today.

I'm not sure how to check if the engine is in time to be honest. It felt completely fine riding the day before, and I didn't notice a lack of power when it was having its stalling issues. But, I only had 5 good days on the bike to get to know it... If you know a way to check timing then I will follow that, but otherwise I might just drop the engine and take the head off?

The metal fragments were there when I did my oil change, before I took anything else apart. It was fairly sparse so I thought it wasn't anything to worry about. The oil strainer had about one or two thumbprints worth of bits in it, and the fingerprint (x3 fingerprints) were from a small pool of oil in the bottom of the clutch area, after taking the clutch casing off. This sounds like I might need to take the head off then, the camshaft looked OK visually but it could have been starved of oil. the level was below minimum when I checked it, after having stalling issues. One thing I read in the manual said about doing a compression test, which came out as 7 bar, then doing another after pouring some oil through the spark plug hole - if it increases, then it's the piston/rings. if not, valves. I stupidly didn't read this before draining the oil, but I will do it again once my new casing gaskets arrive. with regards to the clutch cover marks, I have a suspicion my clutch was done at some point, maybe a bearing failed? the ball bearing on the outer face feels brand new on mine.

I wasn't sure if this was possible - I guess it is, but one thing that made me think otherwise was that when checking the valve clearances, they were both nicely closed at top dead centre on the compression stroke - if the cam was one tooth out, maybe this would still happen but I don't know. I suppose I might see extra wear on the tensioner/guides perhaps?

I am not too worried about the fuel filter for now as I couldn't get it to start with brake cleaner in the intake. This generation doesn't have an oil filter afaik, just a weird little tray thing at the bottom of the engine, which I've cleaned out... it was also speckled with sparkly bits

Hi All, I bought a 2016 CBR125R last Monday. I'm well versed with cars but this is my first foray into bikes. It had a few issues I knew about, such as leaking fork seals, front brake pads, loose shifter linkage. I've ridden it every day to work and back since buying it, no mechanical issues other than clutch wrongly adjusted. After about a week the bike started struggling to cold start, and stalled at pretty much every opportunity unless keeping the revs above 2k. After two days of that, I had time to have a look and the bike wouldn't even start at all. The tachometer was showing around 5-600RPM while cranking cold. The next day, not a peep from the tachometer needle while cranking. No warning lights on the dash. Before the issues, it had absolutely no sign whatsoever of any issues. it started very suddenly on the first day of stalling. The previous owner seemed pretty genuine when he said he hadn't seen any issues while riding, and he had been using it most days. He gave me his mechanic's number which he said he had taken the bike to once a month - I believe him on that but I don't believe the mechanic did a lot in these instances. Stuff I have checked: Engine and chassis codes - match V5C. Original. Battery voltage - happily sat at good voltage >12.6V. Cranking at 11.8V. Spark plug - sparking happily while outside engine. replaced with new one and tested the new one, still sparking happily. - (this would also indicate to me the crank position sensor is fine, because it knows when to spark?) Fuel lines - with injector disconnected, had fuel flow out of the supply line. When supply line connected, it visibly pushed itself tight on the connector, showing it had pressure when priming. The fuel pump sounds great when priming too. Just to be sure, sprayed a bit of brake cleaner down the throttle body before/while cranking and no luck. Also had a camera down the throttle body while cranking, saw a spray of fuel from injector. Compression - 7 bar. Not great, but a car engine would probably start OK with this compression. Should be 11:1. Note- while testing compression this morning, the tachometer needle moved a bit, but I didn't see what it did properly as I was watching the compression dial. I think it sort of flickered but did not stay at a constant RPM as it should while cranking. The engine sounded like it was cranking fine. Coolant - brown. not good. New coolant waiting to go in. Oil - black, very black, sparkly bits. Oh dear. See photo. That is new oil I put in since I bought, and it has not done a lot on that oil. Pretty sure it's a petrol engine, not diesel... Also there was a lot more sparkle than just what was on my finger. (the drops of red are from taking off the coolant pump) Clutch-side Engine casing - see photo. WTF are those heat-scarred gouges. Clutch assembly looks untouched though. Clutch exploded and then replaced? Also the gasket is ruined and someone tried to use goo. Oh dear. Valve clearances - Checked using Haynes manual procedure. Slightly out, but now adjusted to spec (I'm pretty sure at least). Haven't been able to try cranking it as I need a new gasket for the clutch-side engine cover before putting oil and coolant in. Inside the valve cover in general looked pretty good, no dirt or sparkles. So it has fuel. It has spark. it has air. It has a bit of compression. I don't know if its doing the spark, fuel and air at the right times, but it has them. I get the impression this engine has been treated pretty poorly. But that shouldn't stop it from dying so quickly over a couple of days. So I am a bit stuck. I think it might be an electronics issue, but the lack of warning lights is strange. What else can I try short of taking the engine apart or buying Ebay's finest motor? I don't want this bike to defeat me.