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Discussion Starter #1
Hello everyone, I'm unfortunately back again with another engine issue. I've just got to about 12k miles on this engine and the other day as I got off the freeway it died (or at least it was running up to the exit and I noticed when I was stopped that the engine was dead). I tried to start it and it just turned over. I looked down and saw a huge puddle of oil under me so I jumped off the bike and pushed it to the side of the road. By the time the tow truck arrived there was another large puddle of oil under the bike. When I got it home I got it up on stands and the oil in the sight glass was just barely in the window. I wish now I had drained what was left to see how much there was but I filled it up again to see if I could locate the leak point. I tore the bike apart and found no traces of any holes or obvious signs of a leak. There was some oil on the airbox side of the throttle body but I didn't have time to go further than that. I also noticed the oil drain line (the one that snakes down the frame with the cap on the bottom it) was completely full of oil. I'm waiting on a borescope and compression tester to arrive before I go any further.

I'm just baffled as to where all the oil actually came from. At first I thought it was maybe a broken ring land/groove or ring itself allowing oil to push past the piston. But then it would just go out the exhaust not out the intake right? It also seems like that would be a far slower leak than just bleghing out all the oil at once. So my next thought was maybe the oil came from PCV system to the intake and it died because the spark plug got drenched in oil. Once I do the compression test I will have a better idea. I plan to tear the engine down if necessary and hopefully the fault is something I can replace. I'm interested in any ideas as to the cause of this issue.

The maintenance record of this bike after its first engine replacement has been impeccable. I've kept the oil level absolutely perfect and I check it every time I ride. I change the oil and filter every 4k miles. I also changed out the front sprocket for a 15t to lower the RPMs while I commute (had to remove the metal guard but I checked and its not the source of the leak). Its never overheated and hasn't exhibited any odd behavior or noises. In every way I can think of it was running perfectly up until the point that it stopped. I never experienced any moment of loss of power or weird noises/vibrations. In fact it was the absence of vibrations when I stopped that alerted me in the first place. I'm still not exactly sure where it quit but I was on the gas as I was changing lanes to get off the freeway.

Will keep the thread updated as I learn more.
 

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Might be best to get the bike up to temperature and run it up on a rear stand with the fairings off and the motor fully wiped down so you can clearly see if or where any oil is leaking from. From memory the clear tube with the plug in the bottom of it is the crankcase breather. Drain this first and put the plug back in. If a bike is over filled with oil then often it will spit the excess out the breather hose.
Check your oil filter housing cover plate and bolts for tightness. Check your sump plug for tightness. Check the inside of your airbox for oil or residue. Check your spark plug for tightness. Check around all gaskets, esp the rocker cover. That's all I can think of for now.
 

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When you tried to start the engine, did everything sound the same as normal? Was the engine turning over, or did you hear anything amiss? I'm asking because if something broke inside and flew apart while at higher RPMs.. you may be looking for a larger hole or crack in the engine that you would otherwise.
 

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Discussion Starter #4 (Edited)
After it died on the freeway it has refused to start up. I cranked it over a few times when I got it home and refilled it with oil. At first it cranked very fast like it had low compression but then slowed back down to normal speed. It doesn't sound like its even close to catching. Sounds like its slightly closer to catching if you crack the throttle a little but it wont start. No crunching or anything though and I can smell gas in the exhaust. If the compression check comes back good and I dont see anything obvious with the borescope I will try and start it again with a new plug. Will do another nut and bolt check on the engine to be sure nothing rattled loose first. Hopefully the tools arrive today or tomorrow.

Tools got here and I had a bit of time tonight to work on the bike. Compression test came back 70PSI which is pretty bad (tested at 200psi when it was new). I looked in the cylinder with a borescope and the surface of the piston was very black with carbon deposits. The walls were harder to make out their true condition but I could clearly see the hatchmarks around the full circumference. The valves both look pretty cruddy but I couldnt see any light coming around the valves so It seems they are sealing fine. They arent dropped or anything at least and it doesn't appear anything clapped the piston. I put a new spark plug in and tried to start it up but all it did was bellow white smoke out of the pcv hole in massive quantities. I by chance had the oil fill cap off when I tried to turn it over and you could feel a lot of air being pushed out of it. I think that indicates a lot of blow-by. Looks like I will be pulling the engine and tearing off the head here soon.
 

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Discussion Starter #5
Well the investigation has concluded and the culprit has been apprehended. I tore down the engine today and got a good look at the damage. It looks like the piston cracked in half. My current theory as to the sudden oil loss is this: The piston cracked as I was hard on the gas passing cars to get to the exit causing massive amounts of blow-by. I then went into hard breaking at the exit which may have caused the oil to slosh in such a way that the blowby pressure pushed the oil out the PCV holes. New piston and rings are going to be ordered tonight along with all the fixings to get the engine back together.

Here's the money shot: And the backside too:
I'm wondering if the cylinder bore is damaged at the top and thats what caused the initial oil burning. I couldnt find any damage to the rings or lands+grooves on an initial inspection. Will take the rings off later and have a closer look. Here's a look at the top of the cylinder bore. Those marks have me concerned. I just don't want to have to tear my engine apart another 10k miles down the road to do this again, as fun as it was :p.

Some more pictures:
 

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Thanks for the update. that's a good crack alright, good thing you didnt continue to ride or it could have been catastrophic. From what ive heard parts are cheap for these bikes at least.
 

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KC. That's awesome you were able to diagnose the culprit so well. I suspect you've already got things under control.. but there are a series of (4 I believe) how-to videos on this Forum's site posted by "Lightning Frog" that do a really nice job of showing a step-by-step of removing, tearing down, then reassembly of the CBR300s engine. The link I have may not be the 1st video showing how to remove the engine, but they're all there under "cbr300 Forum videos" Good Luck. Keep us posted.
 

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Discussion Starter #8
His videos were invaluable in getting things apart, couldn't have done it easily without them. Hopefully the reverse process is just as smooth. I went ahead and ordered the cylinder bore too since it wasnt too expensive.
 

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Discussion Starter #9
Updating with poor news unfortunately. The whole engine rebuild was one nightmare after another. And now that I've got it back in the bike on first startup it smokes like crazy and is spitting oil out of the exhaust. I screwed up somewhere. I'm going to have to drop the engine again and rebuild the top end hoping I can do it correctly this time. I had such a hard time keeping things clean inside there. We have 2 cats and 6 dogs around here and despite gloves and my best efforts there was animal hair everywhere and some was inside the valvetrain. I made sure the bearing surfaces were clean but little hairs stuck everywhere and I could only pluck out so many.

Anyway I'm just so exhausted right now after 3 days of fighting this engine. Maybe someone here has an idea where to look first for excessive oil right after a rebuild. I gather I should check valve clearances and maybe do a leakdown test. The compression test just after it was rebuild and before its first run was 175psi.
 

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I sympathise with you over the dog & cat hairs. We have a black labrador and we have black hair everywhere - on the carpets, furniture, in the bed, in my cup of coffee and anywhere else you can think of.:oops::rolleyes: Luckily so far I've managed to keep the hair off the CBR.😃
 

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I sympathise with you over the dog & cat hairs. We have a black labrador and we have black hair everywhere - on the carpets, furniture, in the bed, in my cup of coffee and anywhere else you can think of.:oops::rolleyes: Luckily so far I've managed to keep the hair off the CBR.😃
Are you sure that labrador 's a dog? And not a gorilla following you around? :D
 

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Good job she isn't a gorilla. She is with me all day, goes from room to room with me and even sleeps at the side of me on the bed.:oops: Gets morose when I go out on the bike but she can't ride pillion.:( so she goes on my bed and stays there until I come home. I suppose if she was a gorilla she could ride pillion then.🤣
 

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Updating with poor news unfortunately. The whole engine rebuild was one nightmare after another. And now that I've got it back in the bike on first startup it smokes like crazy and is spitting oil out of the exhaust.
It's not just the lube oil from your rebuild burning off? Did you fit new rings? If so they will need to bed in a bit. Did you fit the oil ring correctly? Did you have any trouble getting the piston in the barrel and broke a ring maybe.

The other area where oil may be getting into the combustion chamber is via the valve guides. I've never had a head off one of these bikes, do they have an oil seal on the guides?
 

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Updating with poor news unfortunately. The whole engine rebuild was one nightmare after another. And now that I've got it back in the bike on first startup it smokes like crazy and is spitting oil out of the exhaust. I screwed up somewhere. I'm going to have to drop the engine again and rebuild the top end hoping I can do it correctly this time. I had such a hard time keeping things clean inside there. We have 2 cats and 6 dogs around here and despite gloves and my best efforts there was animal hair everywhere and some was inside the valvetrain. I made sure the bearing surfaces were clean but little hairs stuck everywhere and I could only pluck out so many.

Anyway I'm just so exhausted right now after 3 days of fighting this engine. Maybe someone here has an idea where to look first for excessive oil right after a rebuild. I gather I should check valve clearances and maybe do a leakdown test. The compression test just after it was rebuild and before its first run was 175psi.
wonder if u should have done an overbore to the cylinder.
If u did then disregard my comment.
 

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Discussion Starter #16 (Edited)
Sorry for no update in a bit, had to take a break and clear my mind for a week. But I finally have an update. I got the leakdown tester and found excellent seal in the cylinder at multiple positions where the valves were closed and another compression check showed 190PSI (first check was with a mostly dry bore so a little oil after the first run probably helped there).

It's not just the lube oil from your rebuild burning off? Did you fit new rings? If so they will need to bed in a bit. Did you fit the oil ring correctly? Did you have any trouble getting the piston in the barrel and broke a ring maybe.

The other area where oil may be getting into the combustion chamber is via the valve guides. I've never had a head off one of these bikes, do they have an oil seal on the guides?
Yeah I was just a big baby it turns out lol. I mean I've never rebuilt an engine before I had no clue a bit of smoke and spitting was normal. After letting it idle up to temp I gave it a few small bursts of revs up to about 6k and then let it slowly come back down. After a few times doing that its now smoke free. There was a slight amount of oil weeping out of the small hole under the exhaust but it seems to have stopped now. Going to just go ahead and start the break-in procedure and see how it goes. It seems normal though, no rough running and it sounds excellent. Also can't smell any oil burning and with the muffler off its easy to see no smoke in the exhaust either.

So yeah, I probably could have been riding last week if I wasn't such a scaredy-cat. Things are definitely looking up now.

edit: forgot to add, I used new rings along with the new piston and cylinder.
 

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Discussion Starter #18
Just got back from a quick 70-mile test ride through the mountains and the bike feels fantastic! Probably just in my head but it feels peppier and smoother revving than before. As much trouble as this was I'm pretty confident I could do it again with only a quarter of the mistakes :p.

Thanks again everyone for the support and help. I honestly don't know what I would do without this site and its resources. Here's hoping this is the LAST time I make a thread about a blown up engine lol.

Also decided to dig out the bronze side covers from my original blown up engine. Its nice to have them back on, lookin fresh!

34839
 

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Just got back from a quick 70-mile test ride through the mountains and the bike feels fantastic! Probably just in my head but it feels peppier and smoother revving than before. As much trouble as this was I'm pretty confident I could do it again with only a quarter of the mistakes :p.
Good on you for the success. Next time you update your employment resume... add certified and proven motorcycle engine mechanic. Otherwise, you may want to keep it close to the vest. You'll have "friends" coming out of the wood work asking for, and expecting, free repairs. :giggle:
 
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