« Posts under ’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.

Further Carb Work

Finally got some time to get back to the surprisingly clean carbs from the CB900.  I went through the various air and fuel passage ways with some carb cleaner and all the pathways seem pretty clear.  Piece by peice I started reconstructing the carbs replacing all original bolts with stainless socket button-head screws and also replacing all O-rings, diaphragms, and gaskets.  As this bike is already running with an aftermarket exhaust (and I plan to run without the airbox initially) I also applied the DynoJet Stage 3 (which included a new needle and larger main jet set to DJ130 from stock 105).  The Stage 3 also calls for the air mixture screw to be backed out 3.5 turns (instead of the stock 2.5); and that’s where my problems started.

I already noted that there were a number of problems with carb 4 on this bike, especially with the air mixture screw.  While attempting to install the air mixture screw I found that the carb body is stripped (looks like someone was trying to dig out the air mixture screw at some point).  The result is that I can’t install the air mixture screw anywhere near the correct back-off.Stripped air mixture hole.

In an attempt to fix the problem, I’ve filled the upper part of the hole with JB Weld and am letting it set for the next 24 hours.  Hopefully it’ll harden enough that I can drill out and thread the hole and start moving forward again.  If not, I’ll probably look for a replacement body on Ebay.

While waiting for the JB Weld to set, I went ahead and put the remaining 3 carbs togetther.  I was able to put carb 1 and 2 back together.

Tearing Down the Carbs

Finished tearing down the carbs to prepare them for a cleaning.  Found a missing washer (carb 4) on one pilot/air mixture screw and a missing O-ring (carb 1) on another.  The missing O-ring likely explains the amount of fuel build up on the number one carb.  In addition, the primary main jet emulsion tube has noticeable stripping on the head; someone was trying to play with the jets at some point.  Lastly, the bellows for the accelerator shaft was missing.  All of these should be included in my Randakk’s DOHC rebuild kit, so shouldn’t be a problem getting things back together when it shows up.


The first round of cleaning was done using a 96oz can of Berryman Chem-Dip followed by a dunk in a 1-to-2 part mixture of Simple Green and Water in an ultrasonic cleaner.  The carb bodies I placed in the Chem-Dip for 30 minutes and into the ultrasonic cleaner for 20 minutes followed by a rinse of water.  The combination of the carb cleaner and a run through the ultrasonic cleaner did a decent job of cleaning things up.  Hitting it with an air compressor I was able to blow out the passage ways and make sure everything was dry.


Doing some research about the problems the CB900 is having, it seems that with aftermarket exhausts the carbs start running extremely rich.  This was very apparent by the amount of fuel build up and the color of the spark plugs.  I had previously purchased a stage 1 & 3 jetting kit from Dynojet and realize now is the time to go ahead and put it into place.  I haven’t decided on what to ultimately do with the airbox, but either way I should have jets that are ready.  One step is to drill out the vent holes in the slides, so I went ahead and did that now.


The overhaul clean is pretty much done, the next step is to get into the smaller passage ways and make sure everything is really nice and clean.  That’ll be another day, hopefully when I get the rebuild kit in the mail.

Beginning the CB900C Carb Rebuild

It’s been a while since I’ve done some work on the bikes (winter time projects were in the way).  Bringing the CB900C back to the middle of the garage, I fueled her up to see where the project was left off.  Engine started after a bit but still running rough.  Taking some quick temps, I noticed that the header off of cylinder 4 was much colder than the other 3, so it’s time to pull the carbs, give them a clean, and see what’s really going on.

After pulling off the carb pack, I started breaking things down to single carbs.  After stripping a few of the screws on the braces I finally started getting into the single carbs but needed to break for the night.  Found a great reference for breaking these carbs down here:


Definitely will be making my way through that document in hopes of getting them back together.  Will post more and pictures as the project continues.

XS400 and CB900 tanks stripped and ready for more cleaning

Well I managed to get the XS400 and CB900 tanks stripped down. On the XS400 tank there is a bit more work to remove some of the more stubborn paint around the lip but that shouldn’t be too bad. This weekend I’m hoping the new charger arrives so I can strip the inside of the tanks next.


Digging deeper


I hadn’t really planned on doing work on the CB today, but I had a few questions plaguing my brain.  I spent a good portion of last night wondering why half the engine stays cold.  I decided it was time to check the ignition and the timing.

First I started with the ignition.  The CB900 has two ignition coils that send spark to the 4 spark plugs.

Two ignition coils on the CB900.

Each of the ignition coils has two lines that come off of it.  Properly wired coil 1 (left hand side) is connected to cylinder 1 and 4, and coil 2 is connected to 2 and 3.  The bike was all connected properly, so that’s a good start.  Next comes checking the resistance between the two outputs from each coil.  This was all well within spec for the two coils.  Using a spark tester on the spark plugs, I ran down the light and tried starting up the engine; all seem to fire without a problem.

So, then I thought about the timing.  Maybe something is off that’s not quite firing on the second side.  I pulled off the timing cover, adjusted the cam shaft to top dead center and took a look.

View of the timing layout for the CB900. Cam shaft at TDC.

I had to do a static timing as the bike isn’t in much shape for using a timing light.  In this way, setting the bike to TDC, the pointer on the cam shaft should be at a marker on the left-hand pulser generator.  The alignment is a little bit off, but quite close.

With a lack of knowing what else to look at, it was time to get into the engine.  Taking the cam cover off, I was able to take a look at the valve clearance for all the cylinders.

Left-hand side; cylinders 1 and 2.

The process is a bit dirty and a little time consuming, but not too bad.  Using feeler gauges, you rotated the cam shaft to different positions allowing you to take proper readings for the different valves (distance between a non compressed cam to the valve).  The clearance was all over the board:

Intake (mm) Cylinder Exhaust (mm)
.28 1 .15
.13 .18
.20 2 .18
.15 .18
.18 3 .13
.13 .23
.20 4 .20
.13 .18


According to the manual, the intake and exhaust clearances should be (cold) 0.06-0.13 mm, so things are definitely a bit off.  I might double check again in a little bit just to see that my numbers are right.  In the meantime, it looks like it’s time to find a shim kit and a valve lifter tool.

More on the CB900

It’s been a little while since my last update.  I had some time today to see if I might be able to get some bugs worked out on the CB900.  A few days ago I decided to take the bike around the block to see what the current status is.  While it sounds quite mean, there’s a problem with the power.  About half way through the ride the engine died, and though it started right back up it couldn’t even make it up my drive way without stalling.  Basically this leads to the following list in the hopes of getting the CB900 back on the road:

  • Clean and check the carbs.
  • Check carb synchronization.
  • Check engine compression.
  • Check valve clearance.

A few days back I took a look at the carb synchronization, and surprisingly everything looked good.  So, today I decided to start the day with checking the compression across the 4 cylinders.  Making my way down the line, everything seemed a little bit low, but not too bad.  The next step was to start tearing down the carbs and see if there’s anything out of the ordinary there.

Digging into the four carbs and checking the jets took a better part of the day.  Looking closely at every jet, everything actually looked fine.  I noticed that carb three had a bit of rust color in the float bowl, but everything seemed pretty solid and a quick clean fixed that.  No major problems were found so everything seems good in that realm.

I popped the carbs back onto the bike and started the engine to see if maybe I just got lucky.  Alas, it was not so; still the same problem.  In addition, I took some temperature readings at the headers.  I’ve always suspected that cylinder 1 and 2 seems to be doing everything and the temperatures backed that up.  Cylinder 1 and 2 were running above 200 degrees F, and cylinder 3 and 4 were down around 90 degrees.  Somethings not quite right.  In addition, I noticed while doing the compression test that the spark plugs on 1 and 2 looked pretty good, but 3 and 4 were wet.

It’s pretty clear, next step is to open the engine and start really figuring out what’s wrong.

Bobber Restoration – Some Progress

Took a bit of time today to install the helicoil.  I’ve been worried about the lack of clearance getting to the cam chain tensioner lock nut hole looking for all sorts of ways to get a drill bit in there.  I decided today that it was time to skip a step and just get to installing the helicoil.  I used the tap and got the hole prepped, and then screwed in the helicoil.  The bolt now fits in there quite nice, just like it should have.

As I was going to return to adjusting the cam chain, I found that the bike no longer turned on.  After some searching, I found the “fuse” in the solenoid was bad, swapped the wire for a new one and everything started right back up.  I believe I’ve got the chain tension set now, but the bike is still sounding like crap.

Not only is there still the tapping, but I’ve noticed hesitation from the engine and a lot of popping in the exhaust (even at idle).  Looks like I’ve got to keep digging into this engine.

I received the new front master cylinder in the mail today.  Got it installed and it looks pretty nice on the bike.  Attempted to bleed the brakes, but it’s looking like there might be a problems with the right side bleeder valve.  At this point, I don’t recommend using the front brake–of course, I also don’t recommend trying to go down the road.