'82 XJ750J Possible Cam Chain (and Owner) Issues

Surprise update!

So I stuck the starter motor back on the bike and I just couldn't resist trying it out.

Unfortunately my nephew was asleep on the other side of the garage wall so I couldn't actually let the engine run. The sound of it would wake him up and I would never hear the end of it from my sister.

Anyway, I decided to just turn the engine over a couple times, just a quick push of the starter and no more. I hit the button, the motor went over one turn and instantly fired up! Good thing I had my hand on the key so I could turn it off right away, but for those few revolutions it sounded like it was firing pretty consistently.

Obviously I need to actually let it run in order to tell if it sounds OK, but it was the easiest startup all week. To anyone else reading this thread who hasn't already, rebuild your starter motor! It made that engine fly over more easily than I think I've ever heard it go after just cleaning up the commutator and brushes. Obviously you should replace the brushes entirely, but you get the point. A clean rebuilt starter is a happy starter.

I did find that one of the spark plug connectors (a peculiarly aftermarket looking one) had double the resistance of all the others. You'll never guess which cylinder it was on (hint: it was the dead one). I ordered a replacement for that one.

 

Super surprise update!

I fixed the running problems!

After cleaning the starter I was able to get it running today, but only barely. It still sounded like crap, but I noticed something. It would run poorly, but if I revved it just a bit it would suddenly run really nicely for a few seconds then drop back to running poorly. I don't mean it would slowly shift from running poorly to well and back again, like would happen with a fuel/air issue. It was more like a light-switch going on and off: suddenly running well, suddenly running like crap.

I killed the engine and felt the headers. Its important to remember that cylinder 4 was dead, but I had swapped the 1 and 4 plugs before this test. Now I found that cylinder 1 was cold!

I yanked the bad plug out and measured it with a meter and I got a resistance reading from the stud to the threads in the megohm range. Of course any reading at all here is a sure sign of a failed (or extremely dirty) plug. I also checked number 4 but it was fine.

Fortunately I don't throw things away. I went and got one of the original spark-plugs from before I replaced them which still tested as an open circuit and installed it in the number 1 spot, started the engine, and...

It ran like crap.

Feeling dejected I started putting my tools away, when I glanced at the engine and noticed something. Remember when I tested the number 4 spark-plug? Yeah, I never plugged the cable back in.

When I started the bike this time it ran perfectly. Just as I remembered it running. No obvious tractor or knocking sounds, no hissing or popping, just the smooth sound of an XJ750 engine.

Once I receive my parts I'll be replacing the starter motor with a rebuilt one and replacing the high resistance plug cap with a new NGK one. After all that, I think I'll go on a nice long ride and try to forget about this week.

Thanks for everyone's help. I hope this will be my last major update.

 

So I got to put some miles on the bike today riding to class. I actually made two trips (I really need to get into the habit of checking if my classes are canceled before driving all the way to campus).

My first trip (both were 42 miles over two rides each) went just fine. Everything worked well and I had no issues worth mentioning.

After I got home I received my new spark plug cap and installed it on cylinder 4. Before, the cap resistances were 4.8k, 4.6k, 4.6k 9.8k for cylinders 1 through 4 respectively. Now cylinder 4 is 5.1k with the brand new NGK cap.

My second trip went nearly as well as the first with two notable differences: first, the bike feels a little snappier. It's especially noticeable under heavy acceleration and heavy throttle use at high speeds. It's just a small improvement though, possibly imagined

The second difference, and one that I find mildly concerning, is that there is a strong buzz or vibration in the engine now. While it's pretty unnoticeable at most running/cruising rpms, it becomes painfully apparent above 6k rpm. The bike has always buzzed a little bit here and I believe that's normal, but after installing the new cap it's much worse. It feels like holding a poorly balanced high speed power tool and It just about shakes my feet off the pegs (ok that might be an exaggeration, but it's bad, trust me).

Is this buzz concerning or just a nuisance? Should I bite the bullet and just replace the other 3 caps? Would that fix the vibration?

 

The vibration is probably related to the new plug cap, but not in the way that you are thinking. Now you have all four cylinders firing as intended, so now what was a fairly smooth engine, is buzzy. Why? The carbs need to be synchronized to take into account that cylinder #4 is working properly again.

 

You're probably right.

I'll try to get to work on that as soon as I have the right tools for it. In the mean time, would it hurt the engine to continue running it in this condition? Or would it just hurt my fuel economy?

 

The extra bit of vibration won't hurt a thing, not even your fuel economy (not by much anyway).

 

Good to know.