1980 CB750

[USCGMK30]

Hey everyone, this is my first post here. I recently purchased a 1980 cb750k that runs but needs some work. The first thing I did when I got it home was change the oil and went through it with a magnet. I found some little specs of metal which I'm thinking is normal (please correct me if I'm wrong) and two 2" pieces of what looks like a shredded o-ring. I'm hoping someone can shed some light on how serious this is, where the o-ring came from, and what my next step should be. Thank you in advance.
P.S. here's a pic of the bike.

[FastCletus]

OIl filter has oring around inside of it

[USCGMK30]

It wasn't from the oil filter. I replaced the old one and it was still intact. My only guess is it could be from the wet clutch but I'm not real confident in my knowledge of motorcycles.

[icerigger]

There aren't any o-rings in the clutch or the crank/transmission case that would leave multiple 2" fragments. The o-ring from the filter that you took out may have been OK, but what about the ones before that? That's probably what they are.

[motogrady]

Nice bike, welcome to da club.

The valve cover has 8 bolts with rubber gaskets that get brittle,
there's a valve cover gasket that's rubber,
the cam chain tensioners are a kind of rubber where the chains hit.......
I'd pull the valve cover and look around a bit.


Is that a stock tank painted flat black?

[dyelawn]

The larger valve cover itself should have a paper gasket, however there is a large rubber o-ring around the pulser generator cover, sort of within the valve cover.  Beats me how that could have been shredded off though if that is indeed it.

[dyelawn]

I mean the crank case cover, not the valve cover.

[USCGMK30]

Thanks for the info and making me feel welcome. I'll pull the valve cover tomorrow (gotta adjust valves anyway) and let you know what I find. I also have to adjust the cam chain tensioner (the engines knocking at idle so I thought I would start with that and bench sync the carbs). Thoughts?

I didn't get much info of work already done to the bike and I don't know enough to say whether its stock painted or not but the paint isn't quite black, almost a really dark purple/brown color.

[USCGMK30]

So I decided to ignore the o-ring in the oil and clean and sync my carbs instead. I spent a good bit of time on them yesterday and got them back on only the have my engine backfire (sounds like a fire cracker right in my tailpipe).  I feel like I screwed something up. Any advice would be greatly appreciated.

[seestheday]

Did you mess with your air/fuel mixture screws?  Backfire in the exhaust happens because of incompletely combusted gas getting into the exhaust and then igniting in there.

If you're running lean then this often happens on deceleration, especially if you've bypassed your air cut off valves.

[Hoosier Daddy]

Puzzled as to why you would pull the carbs to sync them, but regaurdless...
Did you check your intake boots for cracks? You might not have got the carbs seated into them properly or if they were hard and brittle, damaged them by disturbing the seal. Are all the clamps tight? Also it would help to know when the backfire occurs. Idle, certain RPM range / throttle position, or WOT.
  For your o-ring question, I'd bet a 2" long strip of black rubber would be from one of the cam chain tensioners.

[USCGMK30]

My engine was knocking at low RPM and I read out of sync carbs and the cam chain tensioner could do that. Thought I would give the carbs a good cleaning and bench sync um before I adjusted the cct.

All the boots appear to be fine and clamps are tight.

Back fire occurs at low RPM (under 2000). When I give it gas it lags but runs better(smoother) at higher RPM's than it did before I pulled the carbs. If I let it idle it backfires and bogs down pretty much as soon as its under 2000rpm.

[USCGMK30]

I did and then set them all the 2 turns out to see how it ran. changed it back to how it was and it was still backfiring.