« Posts tagged ’81 CB900c

Shimmed and Ready

Got the new shims in the mail today; ordered them from Thunder Valley Powersports and they were good about keeping me up-to-date on where my order was at.  With parts in hand I set about correcting my valve clearances.
Using my spreadsheet from before, I was able to easily decide which shims to reuse and which to replace with new ones. While relatively easy, the entire process is extremely messy.
To keep the bike somewhat under control, I continued to have it supported mostly upright by my ladder. This bike has no center stand (one of the previous owners ripped it off) so this sufficed in the meantime.
After all was done, I went through and re-measured my clearances. Things definitely look better than they were.

Cylinder Original Clearance (mm) New Clearance (mm)
1 Intake A 0.25 0.08
B 0.13 0.06
Exhaust A 0.15 0.10
B 0.18 0.08
2 Intake A 0.2 0.10
B 0.15 0.10
Exhaust A 0.18 0.08
B 0.18 0.10
3 Intake A 0.18 0.08
B 0.13 0.10
Exhaust A 0.13 0.13
B 0.2 0.10
4 Intake A 0.2 0.13
B 0.13 0.06
Exhaust A 0.2 0.13
B 0.2 0.13

Prior to putting the cylinder cover back on, I inspected it and found a ton of residual silicon all over where the old gasket was.  I’m guessing someone didn’t quite know how to get the gasket back on, so they went with adding more gunk.  I cleaned it off as best I could, installed a new gasket and closed things off.  One issue is that around the opening where the tachometer cable plugs in, there’s no gasket (and none seem to be available).  I’m going to have to silicone that as soon as I can.

I was able to test fire the bike after buttoning back up.  Things seem to actually be running a little bit better, but I can’t really tell until I take care of the opening near the tach cable outlet.  It was spewing oil all over from that location, so I need to fix it before I can tell what’s going on.  The engine definitely seemed happier than it was, though.

Calculating New Valve Shims

Went back into the cylinder head and took some measurements. Dirty work, but had to be done. I used metric feeler gauges to find the clearance and a digital micrometer to find the thickness of the current shims. The manual mentions that the shim thickness should be between 0.06mm and 0.13mm. I calculated for middle of the road (0.095mm) to find the new shim thickness. The current layout of the bike is:

Cylinder Clearance (mm) Current Shim Thickness (mm) New Shim Thickness (mm)
1 Intake A 0.25 2.589 2.75
B 0.13 2.693 2.75
Exhaust A 0.15 2.585 2.65
B 0.18 2.581 2.65
2 Intake A 0.2 2.49 2.6
B 0.15 2.544 2.6
Exhaust A 0.18 2.593 2.7
B 0.18 2.586 2.65
3 Intake A 0.18 2.765 2.85
B 0.13 2.767 2.8
Exhaust A 0.13 2.634 2.65
B 0.2 2.592 2.7
4 Intake A 0.2 2.568 2.65
B 0.13 2.7 2.75
Exhaust A 0.2 2.61 2.7
B 0.2 2.616 2.7

The result of this is the need for the following shim sizes:

New Shim Thickness Count
2.6 2
2.65 5
2.7 4
2.75 3
2.8 1
2.85 1

I have placed an order for the shims I need (reusing some of the old shims where I can). Hopefully the new shims will get here soon and I can see how things go.

Clean Carbs But Still No Love

Finally had a chance to button up all the carbs and get the pack rebuilt.  My attempts to fix the stripped air mixture screw all went poorly, so ultimately I picked up a replacement off of Ebay and cleaned it up.  Things are looking pretty clean and came together quite well.

Clean Carb, Bottom Clean Carb, Throttle Clean Carb, Choke

Placing the carbs back on the bike went pretty well and I just had to test it.  I put some fuel in and checked for leaks; happily everything seemed to be holding pretty well.  Started up the engine (with my fingers crossed) and things fired up, but still having issues with carbs 3 and 4.  Doing a temperature test at the headers, cylinders 1 and 2 read around 250+ degrees F while cylinder 4 was at 83 degrees F.  Not good.

After stepping back and trying to figure out the next plan, I decided to go ahead and revisit the compression and ignition.  To rule out “no spark” problems, I went ahead and did an ignition test as well as a spark test; both seemed to pass for all for spark plugs.  Pulling them I did notice that cylinder 3 and 4 were both “wet” compared to 1 and 2.

Spark Plugs

Next up was the compression test.  The Clymer manual says to try a normal compression test (crank the engine a few times) and a wet compression test (add a tablespoon of oil to the cylinder and retry the compression test).  I found the following:

Cylinder Dry Compression (psi) Wet Compression (psi)
1 120 210
2 97.5 270
3 105 150
4 97.5 165

The low compression readings seem to be within reason (the Clymer manual says that ~100psi is good enough to get the engine going), but the worry is that with the additional oil, cylinders 3 and 4 didn’t significantly change.  This could point back to needing to finish the valve shim adjustment I mentioned in an earlier post.  Perhaps in doing that I can correct some of these compression issues.  To do so, I’ll need to order the following 25mm valve shims:

Shim Thickness Number
2.6 2
2.65 5
2.7 5
2.75 1
2.8 1
2.85 1

After putting all the spark plugs and re-wiring, I decided to fire the bike up one more time (this time with the oil still in the cylinders).  I expected quite a bit of smoke, which is what I got, but was also pleasantly surprised to notice all 4 cylinders seemed to be firing.  Seems the added compression from the oil has helped a bit; furthering the idea that the next step is to finish the valve adjustment.