« Posts tagged ’81 CB900c

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.

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.

Bobber Restoration – Another Surprise

I didn’t get a super long amount of time on the bike, but wanted to see if I couldn’t figure out where the tapping was coming from.  One possible occurrence can generally be from the tension on the cam chain needing adjusting.  So, I set out to remedy this and found the front lock nut and the two rear lock nuts in charge of adjustment.  The instructions for this bike is pretty simple:

  • Start the engine, let it idle
  • Loosen then tighten the front lock nut and bolt
  • Loosen then tighten the two rear lock nuts

Got the engine started, loosened the front lock nut and bolt, and then realized that I couldn’t tighten it.

Stripped hole for the front cam chain lock nut.

Might be a little hard to see, but hole that the lock nut came from is stripped clean.  Kind of hard to tighten it down when there are no threads to tighten it with.  Set out to get a helicoil set to fix this, but it turns out that the clearance to drill out the hole is interrupted by the frame itself.  Have some ideas to fix this, but it’s going to have to wait.

Bobber Restoration – Surprise!

Had some time today to do some work on the bike.  After cleaning the carbs and airbox and lots of degreaser everywhere else, I put the parts back on the bike.  Like I mentioned before, I’ve learned a little bit more about DOHC engines, so putting the airbox and carbs back on was the best solution.  In addition, I decided to hook back up the emissions control run-off box (it was dangling under the battery not connected to anything).  I was worried about some of the connections the previous owners did, so this at least gets the bike back to what it used to be.

I’ve also decided to put the previous bars back on.  The drag bar was by far cooler looking, but with the air-based front suspension ran right into the controls.  Pushing the original bar back looks better than it did, but I’m keeping my eye on something to do with those bars.

Then came the surprises of the day.  After put everything back on the bike I decided to start it up and see how it goes.  Nothing.  Turns out a wire connected to the key ignition disconnected.

Broken ignition wire.

The fix was pretty quick, I just didn’t expect this issue.  After I patched the cable, the bike was able to start up.  Even took it for a ride down the road and back.  Still work to do, but good to know some parts of it are working.

The next surprise came when I decided to check the final drive and subtransmission oil.  Normally you set the bike level and check the amount via the side mounted check bolt.  Normally this doesn’t have too much of a problem.  Normally oil doesn’t come spewing out of these holes.  Todays seems to be a different day.

Draining the final drive oil.

Oil draining out from the subtransmission level checker.

Draining of the subtransmission.

After the initial shock, I drained them both and refilled with Hypoid Oil; this time to the correct level.

Bobber Restoration – Removal of the skull

The CB900c has a subtransmission which allows you to select between Hi and Low gear.  Effectively this means that the bike has 10 gears total (though most don’t really use it this way).  One of the previous owners thought it would be cool to swap the subtransmission selector with something a little more snazzy.

Skull based selector switch

Now I’m sure some will like it, but I personally thought it was more in the way than anything else.  So, I found a standard one and did the quick swap.

Standard selector switch.

I think it looks much better (and the skull doesn’t keep bumping into my leg).

In addition, I noticed some issues with the readout on the speedometer.  So I pulled the handle bar off and the speedometer/tachometer.  I’ll disassemble it later and see what’s wrong.

Bobber Restoration – Stripping off some parts

Needed to remove and drain the tank to get better access to the bulk of the bike.  From the year of sitting outside, the bike’s pretty dirty–leaves, grime, etc.  I was able to clean some of it off, but this is going to be an ongoing process.

My plan is to make my way through most of this bike, and the most obvious place was to start at the carbs. After a little bit of effort, I had the airbox and the carbs off the bike.  When removing the carburetor pack, I found that the throttle push cable was snapped.  The other cables (throttle pull cable, choke cable) seem alright, but I’ll need to inspect them a bit more as well.  The airbox has a bit of oil in it, but at the moment it doesn’t seem to be too much to worry about, mostly just blow-by from the crankcase and subtransmission.  Likely the accumulation is from this bike not be able to actually get warm enough to ride.

Removed carbs and airbox

Another view of the removed airbox and glove box.

My original thought was to completely remove the airbox and replace with pod filters. Looking at some examples online, I really like the cleaner look this would give. But, as I research a little bit more, it seems that these DOHC Honda’s don’t particularly like not having their stock airbox. I’ve got the parts for the pod conversion, but I think I’ll shelve them for a later time and focus on just getting things working again.

As time was pretty limited today, I decided to also tackle that handle bar. I’m not a fan of the big handle bars, preferring the much sportier look of cafe racers. As such, I needed to get rid of the bars that came on the bike, and opted for much leaner looking drag bars. Since I needed to pull all the controls off the bars, this gave me a chance to also yank out the bad throttle cable.

Drag handlebars installed

Carbs pulled, airbox pulled, and handlebars swapped.

New bars in place, I think the bike is going to look a lot better. Storms are moving in and other errands are needed to be done; so it’s time to see what all has been pulled off so far.

Parts removed from this venture.