Monday, May 11, 2015

Master Mechanic Journal: CB750 Edition

To prove my mechanical aptitude, I decided to try to alleviate the low idle "chunk" sound on the lower left of the motor. In my eyes, it was 1 of 4 possibilities: cam chain, clutch, starter clutch, or carb sync was needed. I'd already tackled the cam chain and pulling in the clutch didn't eliminate the sound, so I figured it one of the other two. Since the bike is kick only, I decided to just rip out anything to do with the starter. I pulled the starter out and plugged the hole in the case. Then I pulled the rotor off and removed the starter clutch barrels. And while I was there, I removed the big starter gear off the crank and put everything back together. I was so proud of myself.

Well now I had a leak at the cover. And then I discovered that the starter gear's bushing actually covers a hole in the crank the oils the gear. And now that hole had open access to spray oil everywhere, thus reducing my oil pressure! So I got to rip back in a put the gear back, and replace the cover's gasket. And I still have a leak.

Well that didn't fix the chunking sound, so I ponied up and bought a MotionPro carb sync. I calibrated the tool and spent 2 days trying to get the damn thing to sync. My readings were all over the place! I'd blip the throttle and 3+4 would jump up, then come back down, and 1+2 would be higher. So frustrating. I went through everything: checked for air leaks, tightened the clamps, slacked the throttle cable as much as I could. Nothing helped.

I was getting ready to sell all my bikes and become a peanut farmer. I started going through all the bolts on the carbs and discovered that the 3 bolts and 1 nut on the connecting shaft for the carb lifters were loose! They either worked themselves loose or I forgot to tighten them 2 years ago... oops. That also explains the horribly wonky idle I've always had. Basically the lifters could move a little on the shaft and that would severely affect idle and sync. Once I tightened those up it became much easier to sync.

After all this, the low speed chunk is still there. I'm starting to think it might be inside the gear cluster/final drive sprocket area.

Finally, I had that horrible wobble of death on grated roads to figure out. I've gone through everything and added a fork brace, so it has to be the square profile front tire. I recently got that Duro and decided to put it on and try. You can see how much rounder the profile is.

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