horrible engine noise - suddenly metallic pinging

[eugencbx]

Dear experts,
I need help. I hope your experience can help me here. Brief history: it is a CB1, 7,000 miles, in very good condition. The only thing I ever did on it was a carburetor overhaul. I drove it for a nice ride in the fall. In winter I started it once and let it warm up. In spring it started normally, we loaded it on a trailer. The owner started it on arrival and drove a few meters. When he wanted to go for a ride after a week, she made a terrible engine noise. Now the Honda is back with me.
At https://youtu.be/aPqkyfcwxJI you can find the video with the metallic knocking. It always makes that noise, only when starting without ignition you don't hear it.
What I have done so far:
- removed the alternator (did not help)
- measured the piston height, if a connecting rod is bent, everything should fit (we never had them longer on the side stand)
- Compression measured - everything so far o.k., only cylinder #5 is about 20% worse
- with an endoscope I also see quite a lot of oil deposits on the piston, the others look much better. Otherwise I can't see anything in the cylinders
- and here it comes: when I turn the crankshaft with the ring spanner, the engine stops at the same place again and again (360 degrees rotation). Cylinders 2 and 5 are at top dead center here. I made a video here: https://youtu.be/SeRzUtpGJb4
With a lot of power I can turn even further, with the starter motor it is very easy. It feels as if the piston is in contact with the valve.
- I have checked the cam timing according to the manual - it all matches.
- the valves all move normally

What could be the cause here? Thank you for your help!
best regards from Salzburg/Austria

[tevan]

Have you watched the valve movement turning the crank by hand?

[daves79x]

I would say, without all the tests you've done, that you have a slightly bent rod, with the crank flyweight hitting the bottom of a piston at BDC. That's just about what they sound like. But you would have seen that in your measurements. Did you measure each pair of pistons at TDC? Any rod can bend, not just #1.

Whatever the case, the head must come off to see what's going on. May as well get a head and base gasket.

Dave

[EMS]

You may have a broken valve spring in piston No5. That could explain the lower compression and also the noise. The top of the piston hits the valve as the broken spring does not completely reseat it. If the valve shaft is slightly bent because of this, you also may have oil leaking into the cylinder through the shaft guide. Check your "Ventilspiel".

[eugencbx]

Thanks, yes, everything looks normal to me!

Have you watched the valve movement turning the crank by hand?

[eugencbx]

Thanks, I measured the #1 and #6 with a caliper, I will try to make something to measure the rest of the cylinders more precisely than a screwdriver

I would say, without all the tests you've done, that you have a slightly bent rod, with the crank flyweight hitting the bottom of a piston at BDC. That's just about what they sound like. But you would have seen that in your measurements. Did you measure each pair of pistons at TDC? Any rod can bend, not just #1.

Whatever the case, the head must come off to see what's going on. May as well get a head and base gasket.

Dave

[eugencbx]

Thanks for all your help! Yes, that could be - I'll measure the valve clearance!

You may have a broken valve spring in piston No5. That could explain the lower compression and also the noise. The top of the piston hits the valve as the broken spring does not completely reseat it. If the valve shaft is slightly bent because of this, you also may have oil leaking into the cylinder through the shaft guide. Check your "Ventilspiel".

[eugencbx]

Dave, I think you (unfortunately) nailed it. I made a tool to measure the inner pistons more precisely. And voila, the #5 piston is 1/8 of an inch more inside than #2. Maybe on the trailer transport one carb ran over (maybe petcock was not closed) and filled #5. Shit! Now the fun begins!

Thanks for all your help!

I would say, without all the tests you've done, that you have a slightly bent rod, with the crank flyweight hitting the bottom of a piston at BDC. That's just about what they sound like. But you would have seen that in your measurements. Did you measure each pair of pistons at TDC? Any rod can bend, not just #1.

Whatever the case, the head must come off to see what's going on. May as well get a head and base gasket.

Dave

[daves79x]

This explains the sooty #5 piston - much lower compression with a bent rod. I'd first get the engine out, remove the head and carefully inspect all the piston pairs at TDC to be sure all the other ones are OK, then flip it over and split the cases. You will find #5 rod bent visibly, plus a small nick in the underside of the piston at the wrist pin boss where flyweight contact was made. It likely did not hurt the piston, so all you should need is a replacement rod of the correct codes. Not a cheap fix labor-wise, but relatively few parts should be needed, especially with the low miles.

Dave

[eugencbx]

Thanks, I removed the engine already, that was the easy part! Parts-wise it will not be much, that's the good thing. I am not sure, if I will repair the engine though (enough other projects in my garage), I am checking for companies right now able to fix these magnificient 6-cyl-engines. There is one in the Netherlands https://www.sixcenter.nl - they should be able to do this.

Eugen

[NobleHops]

Yes, SixCenter is an excellent resource, they will repair this perfectly.