[berto]

I recently bought a non-running 2017 CB300F with 7500km (5000mi). The bike is very clean and has been stored indoors. It's sat for the last couple of years. The person I bought it from says the bike was running fine when he parked it. I have no way of verifying that, but also no reason not to believe him PO because everything he told me and showed me (paperwork etc) has checked out so far.

I expected this to be a fairly straightforward "re-commissioning", but it hasn't worked out that way so far.

I started with a brand new battery, fresh gas (drained the tank), and clean air filter and air box. Bike wouldn't start, and didn't really sound like it wanted to.

I then replaced the spark plug, using the recommended NGK Iridium, and verified it gives a strong blue spark. Still no joy.

I then cleaned the throttle body and injector. I tested operation of the injector by cranking bike with injector disconnected - I could see repeating "mists" of fuel come out. This all looked OK to me.

From there, I checked compression and found it was really low (50psi). This was with throttle fully open, but engine was cold (obviously, since I can't get it running). I thought I might have stuck rings or a stuck valve, given the bike's been sitting. So I tried a tablespoon of Marvel Mystery Oil a few times over a couple of days. Still low compression.

Then I checked valve clearances. I found the two exhaust valves were a little tight (0.20mm), so I adjusted them to the loose end of acceptable (0.30mm). Compression is still low.

So now I'm thinking next step is that I need to drop the engine so I can open it up and see what's going on. Before I do, I just thought I'd check in here to see if I'm missing anything.

A couple of specific questions:

1. As far as I can tell from the shop manual, this engine doesn't have anything special with a decompression mechanism and I should get ~180psi from standard cranking of the engine. Is that right?
2. Based on what I've tried so far, and the low compression reading I'm seeing, am I on the right track that opening the engine is the next step? I've been trying to avoid this due to cost and hassle. The bike's got such low miles and been taken care of, so I can't see it having major engine issues. But I'm thinking years of sitting without any storage treatment (stabilizer etc) could be causing problems?

 

[kiwi rider]

Yeah that compression reading is well down eh. How does it sound mechanically when you crank it over? No nasty metallic noises? With only 7,500km on the clock the rings and bore should not be worn enough to create such a drop in compression so I'm just wondering if one of the compression rings broke when the bike was initially turned over. Working on the theory that whilst sitting for a couple of years the ring got stuck to the bore. Other than that it would need to be a valve, valve seat or valve guide issue to lose so much compression. Removing the head may revel all. I may be missing something obvious but that's all I can think of.
Are you familiar with member @lightningfrog27 's great series of clips on how to pull this engine down and put back together?

 

[berto]

Removing the head may reveal all.

It did, and it's not good

When I removed the head, I found the cylinder was scored. So I removed the cylinder, and found the piston was also severely damaged. A new top end is obviously not good, but not a disaster either. The real problem is that the wrist pin seized in place and damaged the small end of the connecting rod too. This looks a little too messy to just clean up and keep going, so I think I'm stuck taking apart the bottom end too.

As far as I can tell, I have a few options:

1. find a donor engine (I think the CBR300 and Rebel 300 are options too, having same engine?)
2. get a replacement crankshaft (online sources say they are not available. Is this a COVID supply chain thing? As far as I can tell, the same crankshaft was still used until at least 2020, so it can't really be discontinued yet?!)
3. get a crankshaft machine shop to refurbish it

What do you all think?

Are you familiar with member @lightningfrog27 's great series of clips on how to pull this engine down and put back together?

Yes, I am. Those videos were very well done, and have been exceptionally helpful!

 

[kiwi rider]

Ouch! that's ugly alright. I'm thinking a donor engine is probably easiest option esp if there's supply issues with new parts.
You could throw money at the old engine and still find there's other issues you weren't aware of.
I couldn't be 100% certain but to the best of my knowledge yes, the CBR, CB and Rebel engines are all the same.

 

[motorboy]

I believe the cams are different in the Rebel -if yours are good you can swap

 

[berto]

I've finally got the engine opened. I did not see evidence or scoring, burning, or other damage inside the crankcase. In particular, the main journal bearings look OK (my read, based on Honda's service bulletin for crankshaft replacement, is that silver=OK; copper=worn).

Photos are below, and I can add more for whatever others might be helpful. What do you all think? I know it's hard to assess from a few photos, but do you see anything else of concern?

As you all know, Honda had a crankshaft recall on this bike model (but not for this model year). It looks like they sell a kit that includes the crankshaft, base/head gaskets, every o-ring, etc for $275USD (p/n: 06130-K33-305) The top end parts (cylinder, piston, wrist pin) add another $200USD. In other words, I think rebuilding this engine would cost about $500USD in parts.

Does that sound right? Could I order these parts and get running again? Unless there's some showstopper that I'm missing here, my preference is to order the parts and rebuild this engine. Does this sound reasonable based on what you see here?

Does anyone have experience with that crankshaft kit? Do I need to make any special considerations when ordering to get the right crankshaft (for example I know the bearings have color codes to match the crankcases)? What about the piston and cylinder - do they come "matched"?

As always, thanks for any and all advice!

 

[kiwi rider]

I dont have much knowledge of engine re building as luckily I've only done one and a long time ago but I dont remember any colour matching. It was a Ducati V twin engine. I work on a different currency so I dont know if $500 is a good deal or not. How much is a donor engine by comparison?
Yes from your photos the bottom end looks OK but I guess the issue is still what caused that top end damage and until you know that then it's risky rebuilding with new parts. Oil starvation is a likely culprit at a guess?