CT90 Overhaul Process

When I get an old beater CT90 that I decide I want to get back to where it is fully operational, I generally follow the process outlined below.

The abbreviated version:

1. Clean it
2. Get it standing on the center stand
3. Drain the oil and flush once or twice as required
3. Get it rolling and work on the brakes while the wheels are off the bike
4. Work the wire bundles and electrical system. Get a battery and get everything working
5. Clean/rebuild carb, clean gas tank, get new fuel lines and a filter for each line.
6. Work the top end of the motor
7. Do any work to the lower half/transmission
8. Put everything back together and adjust valves, point gap, then timing.
9. Put in a new plug
10. Get it started and enjoy your rebuilt CT90!



1. I always start by cleaning what I can on the overall bike by hosing it off and getting rid on most of the dirt and oil that would just make a mess in your shop.

2. As far as standing on its own, if your CT90 has its center stand and you can swing it down and use it you're all set. If you only have a kick stand your going to struggle as with just the kick stand you can't  remove a wheel and still have the bike stand up with out setting it on a milk crate or something else, so if you don't have a center stand make that the first part you hunt down and install on your project bike.



3. Once the CT90 can stand on its own, I work to get the CT90 to where it can roll around. Life is so much easier when you are working on a CT90 if you can move it around you shop or garage and then have it support itself wherever you leave it.  Overhauling a CT90 generally doesn't happen overnight and usually goes on for several months if not years, so being able to move it outside to clean the bike or move it so its out of the way of other projects you're working on makes your CT90 project just that much more enjoyable.

If you are missing any detail parts on the bike this is a good time to start a list of whats needed and start hunting things down.  I have a page of links here that has links to many great resources on where you can get parts for your CT90 project.

If the tires hold air, spin freely and the brakes work then you are probably good to go, but if not its time to pull one or both wheels and fix the tire or tube and then disassemble the hub assembly and clean the brakes and get things so they are clean and move the way the are supposed to. You should also do a check of the rubber dampers in the rear wheel and I have made a post here on how you would go about doing that check.

One good thing to remember when you are working on the brakes of an old CT90 is that the brake pads could contain asbestos.  The last thing you want to do is get mesothelioma cancer, so its always a good idea to take appropriate precautions when working on brake pads and assemblies that could contain asbestos.

With the front wheel I'll always disassemble the plate that holds the brake shoes and speedometer gear, clean and lube the small shaft that actuates the brake shoes so it rotates freely and then also clean and relate the gear that drives the speedometer cable.  This is also a good time to make sure the two tangs on the speedometer gear haven't been bent down, but are bent at 90 degrees and pointed so that they can mate with the two corresponding slot on the front wheel when the plate is reinstalled.

With the rear wheel I'll also disassemble the plate that holds the brake shoes, clean all the parts and then lube the small shaft that actuates the brake shoes and also apply a very small amount of lube to each of the points at the end of the brake shoes where they contact the actuation shaft and pivot.  If your rubber damper were found worn on during the check I described above this is a good time to replace them.

While I have the wheels off I also make it a point to start cleaning the inside of the fenders and other areas  you wouldn't normally be able to reach with the wheels installed. This is also a good time to look at the wires to the tail light that run inside the rear fender and see if they look ok or will need to be repaired.

If the rear rack is in sad shape and needs work this is a great time to pull it and clean it up and reinstall.

4. With the CT90 now a nice easy to move and self supporting project, now its time to start working through the different systems to get the bike back up and running. At this point I like to go through all the wiring and make any repairs required and check things like the coil, rectifier, lights, etc. and get to where everything works with a battery installed. As part of this effort I also go through the controls on the handle bar and get the internals clean and working.  This pays off later in the project when you are first trying to start the engine and you don't start finding all of your wiring issues at that point.

One of the most common repairs made to a CT90 is upgrading the rectifier and I have a post here on how to go about doing that.

I also have a post here on how to check to see if your coil is ok.  If you have removed your coil and it checks out ok, you still might consider replacing your condenser.  Condensers are not as robust as coils and its a pain to have to remove your tank again to get access to the coil/condenser, so I tend to just replace it as they are generally inexpensive.  I have a post here on how to check to see if your condenser is ok.

A key part of the electrical system are the switches on the handle bar controls. If you do remove and take part these assemblies to clean them I have a post that details how to reassemble the left turn signal switch and horn button here and the right headlight switch here.

If your speedometer assembly needs some care and attention, I made a post here on how to rebuild a speedometer that is for a 1969 CT90, but the process would be the same for other models.

I also make it a point that anytime I remove a detail part I try and remove any rust and clean up the part and paint it if appropriate. I have a post here on removing light rust on chrome and here on removing heaver rust.

5. I next will focus my attention on cleaning and rebuilding the carb and flushing and cleaning the tank.

With the carb I usually end up with a rebuild kit as the idle jet is next to impossible to clean and unless your gaskets are in good shape you'll end up with a gas or air leak later in the process, so I find it easier just to do everything once with a rebuild kit instead of having to pull the carb later when you are first trying to start the bike.  If you do rebuild your carb and have issue getting it back together, I have made a post here that will help walk you through the assembly process.

I have a post here on adjusting and jetting your CT90 carb.

This is also a good time to go through the tank and get it cleaned up and ready to go.

To get access to the carb you will have to remove the rubber boot that connects it to the air filter assembly and on old CT90's these can become rock hard which makes them difficult to remove.  I did a post here on a short term method to soften the rubber parts so they can be removed and I also did a post here that outlines a method to restore the compliance of these rubber parts almost back to when they were new.



6. If the engine had relatively good oil in it and it turns over and has good compression I usually leave well enough alone and count my blessings. If the oil looked bad, the engine won't turn over or there is very little to no compression, I'll pull the top end when I'm pulling the carb. If the cylinder and piston look ok I may just hone and get a new set of rings, but it usually doesn't so I just send the cylinder out to Beatrice Cycle for one of their great clean-up deals which can't be beat for the money. If you do need to dig into your engine I have done a post here on how to reassembly a complete engine.

I'll completely disassemble the head and clean the valves and valve seats and cut and lap the valves to the seat so there is a solid seal.  Here is a post on a simple check that I do to verify I have a good fit between the valve and the seat.

7. I then would work the lower end of the motor/transmission as required and for the eight bikes I have gotten running to date I have been fortunate enough to only have had to flush out the oil several times to get everything acceptable to go, but I do always change the oil on any new bike I pick up.  If I have the gasket for the cover on the clutch side of the engine I'll pull the cover and check and clean the oil filter screen and also remove any sludge I see within that cavity of the engine.  Sometimes I have been able to pull the cover and reuse that same gasket, but for the most part I try to have a gasket on hand before I pull the cover.  If you do pull this cover this would also be a good time to check your shift drum stopper as I describe in the post here and once you have the cover off its not to much more effort to rebuild you clutch pack and I have a post here on how to go about doing that.

8. I then spend some quality time getting everything back together and really focus on getting the vales, points and timing nailed down.  This is the point to take your time as if you really focus on getting things right you'll be amazed at how easy you're bike will start, and if you're sloppy you'll just end up wasting a lot of time kicking to bike over for no good reason.  Its also advisable to replace your points as they are not really that expensive and if you are going through adjusting them its a good time to replace them so you know that they are not an issue.  The spark advancer assembly is located right under the points and if your bike has been setting for a number of years it should be removed, cleaned and reassembled and I have a post here on how to do that assembly.

If you need help with adjusting your timing I have made a post here to help explain that process.

9. Don't be cheap and try and reuse an old plug.  The first thing you should do is through away any plug and get a nice fresh new plug and set the gap as it really does make a difference.

10. If you did everything right up until this point all it should take is a few kicks and you'll have a running CT90!

Also if your CT90 was in rough shape when you first picked it up you may be able to salvage any of your broken or cracked plastic covers as I describe in a post here.  If your seat has seen better days you can make your on seat foam as I describe here and actually sew your own seat cover as I describe here.  If your CT90 is an older model with a vinyl down tube cover you might try making your own as I show in a post here.

I hope you enjoy working on your CT90 project as much as I enjoy working on my bikes!

Helpful Links  (Shop Manuals, Wire Diagram, Model Information, etc.)