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Discussion Starter · #1 ·
Hey everyone,

I used to be a fairly active member of the forum with my CM400 a couple years ago but have since sold the bike and haven't been on the site as frequently. I recently picked up a 1972 CB350 K4 project because I hear they are excellent bikes. So now I wanted to re-introduce myself and ask for a little advice as well. The 350 was sort of a "barn find" or at least has sat for some 10 years before my acquisition. Supposedly the previous owner used to race these 350's and had gotten too old to ride anymore. When I bought it, it had a stuck partially disengaged clutch cable which wouldn't let me kick it over to check for compression or anything that I normally would. I got it for the right price so I continued with the assumption that it may need a top end rebuild.

When I got it home I pulled the sprocket cover and the clutch returned to the engaged position but the engine was stuck and wouldn't budge. I initially thought the bores had rusted but I now think it was the cam chain binding on something. After some gentle movements I got the engine free and spinning okay. Applied some oil to the top end because it was dry and thought I'd try to get it running just to see if the engine was healthy. It fired up and ran well enough after some mild carb cleaning but the top end was making a horrible noise. I immediately shut it down and found that the tensioner had (I think) slipped past the contact point on the idler wheel and lost chain tension. I re-installed it and the top end sounds quiet now. I went on with the typical maintenance items for a bike that sits for any lengthy period i.e. chain, oil, cables, electrical, etc.
When I pulled the oil filter apart I found it full of metal. Mostly aluminum, and lots of it. I changed the oil, and after just some running in my garage (It still has original tires) it appears as if the engine has stopped adding more aluminum to the oil, but I am sure there are still bits floating around in the crankcase.
My question is this: will the engine be okay with a few short rides and a couple oil changes or am I better off tearing it down and cleaning it out? I know the bigger bikes have journal bearings, which I'm certain would be damaged in this situation, is the 350 a journal bearing engine as well? What would cause the tensioner to loose tension like that?

One more piece of information I will add is that I think the engine has been modified. With the carbs off it looks like head has been ported and it has a Mac exhaust, is there any way to tell if it has a re-ground cam?

I have heard the CB350 is a blast to ride, if I like it enough I will probably restore this one.
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Congrats on the purchase.
I'd suspect the metal in the slinger is from the loose cam chain tension.
Do a thorough tune and take it wherever it'll go.
Carbs look stock so I'd assume your cam and rest of the engine are too. What makes you think it's been rebuilt?
 

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It not unusal to find the Oil Slinger Filters packed full of crap.

Many owners never bothered with cleaning that as it required a special tool to do the job.
They just changed the oil and rode. Or at least we hope they changed the oil.

I would recommend an immediate Oil Change with a filter cleaning.
Then if you continue to run/ride I would clean the filter every 50 -75 miles until it comes up clean after that interval,
 

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Discussion Starter · #4 ·
Carbs look stock so I'd assume your cam and rest of the engine are too. What makes you think it's been rebuilt?
Yes the carbs are stock but when I had them off there was some fairly obvious port work done. It looks like the rubber manifolds where port matched to the head and I can see where some material has been ground away further up the port just above the valve. I did have the jets out but forgot to note the sizes, I would guess they are larger than stock.

As for the metal in the oil, it seems like the filter is just picking up whatever is floating around in there and not much more. I ordered the gaskets to pull the side cover off and clean the oil pickup/pan the best I can. After that I'll ride it and make some decision accordingly. It seems like it may smoke a little but this is just from me riding it up the street once or twice. I really don't trust the old tires that are on it so that's as far as i'm going till I get some new rubber.
 

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What you describe sounds like a lot of heads I've come across. Cant speak to yours, but I suppose if you set up a dial gauge and measured the valve stem movement then you could measure travel and duration against stock. Other members will have more to say on the matter. Why not check compression to see what it comes up with?

Btw, if you start seeing black hardened rubber bits in your oil, then it's a sign your cam tension rollers are coming apart and will need to be replaced(via top end rebuild) and you can confirm cam suspicious at that time.
 

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Discussion Starter · #6 ·
Well I've now got the bike running great. Sounds great with no abnormal noises, feels as powerful as it should, no smoke, etc. I got my gasket kit in today and started pulling the side cover to clean out the oil pump screen.

What I found when I pulled the oil filter was the cup once again filled with metal. This is after maybe 80 miles of riding with a fresh oil change and filter clean just prior. What could be causing this much metal? I ran a magnet through and found that most of it must be aluminum as it didn't get picked up. Any ideas?
 

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It may also just be a case of never been cleaned and it is still pikcing up crap from the sump.

As I said before I'd ride 75-100 miles and keep changing the oil at that inteval until it comes up clean.

If after 3X it is still a mess do a rebuild and find the problem.
 

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^^^agreed... very possible you're just catching up from years of neglect and a big buildup of junk in the bottom end. Shows that the centrifugal filter actually does its job as Honda intended
 

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Discussion Starter · #10 ·
Seems like quite a lot of metal for it to be still picking up chunks from the sump? Another note is that the oil pump screen looks brand new, no metal whatsoever. Maybe the pieces are just too small to be caught there. I'll ride it some more and keep checking the filter. I would think if the cam chain was contacting the case somewhere it would make a noise
 

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I would be tempted to pull the cam chain tensioner to check the condition of the small wheel.

Today I happened to have time and was able to photo the intake of 2 350s. Both had stock cam.
 

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Discussion Starter · #12 ·
I would be tempted to pull the cam chain tensioner to check the condition of the small wheel.

Today I happened to have time and was able to photo the intake of 2 350s. Both had stock cam.

Hmm that does looks pretty similar to what I remember mine looking like. I guess Honda may have done come cleanup work after casting? I'll get a picture of mine next time I pull my carbs off (won't be long) to compare.

When I pulled the tensioner out previously, it looked like the small wheel was worn, but not shedding pieces and was still rolling fine. No side to side play, anything else to check?
 

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Discussion Starter · #13 ·
Well I think I've decided I am going to pull the engine. The metal seems to be decreasing but I took it for a long ride today and noticed the crankcase breather is blowing quite a lot of oil/smoke after about 45 minutes of riding. I did a compression test and found ~100 in both cylinders (admittedly my compression tester has like 4 feet of hose so the numbers may be a little low) but I added some oil and they jumped up to around 110. The bike only has 9000 miles so I'm thinking the bores should be okay to just hone and put in a new set of rings.

What is the consensus on gaskets? I have a Suzuki GS and those guys swear that the aftermarket gaskets don't seal at all. I see some really cheap gasket sets on ebay ($14), I assume those are junk? How about the common motor sets? Looks like cases take just sealer and no gasket? I figure those should be pulled apart to clean out all the metal. Any other tips for a rebuild? I've worked on a lot of different stuff but never rebuilt a 350
 
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