Please share your experiences with the CM400 bikes.  It will be helpful to include the year and specific make (79 CM400t for example). 

Carbs:  Please update with your experiences rejetting the carbs including modifications to the bike (pods, open headers, bore, etc) and the jet size that worked for you.

  • The stock floats are NOT adjustable.  DO NOT try to bend the plastic to make even minor adjustments or they WILL break.  Honda recommends buying new floats and needles if the adjustment is off, but you can make very minor adjustments by bending the metal tabs on top of the needles.  This is delicate and you risk breaking them though so be careful.
Stock jet size:  1980-1981 CM400t - 72 Primary, 118 Secondary
1981 CM400C - 72 Primary, 118 Secondary
1979-1980 cm400e 72 primary, 118 secondary
Rejet experiences:

Stock air screw adjustment: two turns out

  • After cleaning a the carbs I have come upon a lean issue. The bike is running far too lean, and spitting back through the carbs at lower rpms about 3000-4000 and 0-1/4 throttle. I also have been getting somewhat white plugs (lean condition). So far the things I've done to fix it are as follows (hopefully this will serve as a guide to someone else)
    • Check the boots from the engine to the carburetorfor leaks using propane - Ok!
    • Check the carburetor from top to bottom for clogs - Ok!
    • Check to make sure the jets are the correct size (as listed correctly above) - Ok!
    • Check the accelerator pump for failure - Ok!
    • Check air shutoff valves - Ok!
    • Adjusted the idle screws to 3 turns instead of stock 2 - helped a little bit at about 3000
    • Since I have a Keihin Carburetor I used the following Keihin Carbuertor Tuning Guide
    • Used brass washers to raise the carbuertor needles (1 washer each) - Helped with acceleration and a little bit around the 3k-4k region. Now only backfires (through carb) once in a while. Did not change gas milage (about 48 mpg)
    • Ordered 75 and 78 main jets from [Linked to CM400 Jets] to replace the 72 slow main jets.
    • Waiting to get the jets... will update when installed.

Exhaust: Open pipes, 2into1, etc

  • Replaced stock megaphones with open turn downs ~ 5"long on 1981 CM400t.  Attached directly to collector.  Somewhat louder, but not bad.  No rejet required.  The collector does most of the baffling on this bike.

Tires: What size and brand of tires have people used on stock wheels.

  • OEM tire sizes, at least for Comstar (non-spoked) wheels:
    • Front: 3.5S18-4PR, 100/90-18
    • Rear: 4.60S16-4PR, 120/90-16
  • The spoked wheels use the exact same size, at least for the 1981 CM400E.

Note: 130 is simply too wide for the rear. Don't even bother, it rubs on both the chain guard and brake linkage.

  • Some random guy editing this article had a good experience in 2012 with the Kenda K657 Challenger front and rear for a bit over $100 for the set.
  • Another random guy editing this article also had a good experience in 2018 with the Kenda K657 Challenger tires on a 1981 CM400e.
  • Keep in mind that although the rims are aluminum, expect to buy tubes as they supposedly aren't designed for use with tubeless valve stems and may not hold air without them
  • Be VERY careful with your tire levers. The rims are extremely soft and you can gash them easily without a protector.
  • No load rating for the tires are listed in either the service manual or owner's manual. On my motorcycle ('81 CM400E), there are several models of tires listed for the front and rear on the motorcycle tire info plate located on the left-hand side. The Dunlop F11 (front) and K127 (rear) are among the models listed. Those have a load rating of:
    • 494lbs (front)
    • 536lbs (rear)

Wheels: Have you used non-stock wheels?  What kind/size and what modifications did that require?

Suspension: Any suspension mods?  Hard tail, replace fork springs?  Rear shocks from another bike?

  • Front fork pressure should be no more than 14psi or 1 Bar. CM400E excluded,

Brakes: Anybody switch brakes?  Drums up front and discs in back?

  • Used an aftermarket master cylinder from Magurra on 1980 CM400t with great success (less than 100 USD).  BUT the Magurra has a 10X1.0 banjo thread and no brake light switch so I used the stock hose by screwing the nipple end into the caliper and buying a new banjo bolt with pressure switch to screw into the MC.  Works like a charm.
  • Used an MC from CB750 (mid 70s) on 1981 CM400t.  Same issues as Magurra - move nipple end to calliper and buy new 10X1.25 banjo with pressure switch for top end.

Seats: the CM400 frame doesn't lend itself to cafe style, anybody get it to work?  Or another style? 

  • The seat will always have a rise if you don't modify the frame because the rear latching mechanism is raised about 2-3".  I cut it off and removed the toolkit holder then fabbed a custom pan out of fiberglass.  Reduced profile of rear seat by several inches (1981 CM400t).  If you do this, you will have to eliminate the fender or make a custom one.  I used the stock fender, but cut it down and drilled new mounting holes.  No tire clearance issues yet with stock suspension.
  • In general the pre-fab seats won't fit this bike without modification.
    CM400 Cafe Seat
  • I have found a cafe style seat on eBay from a seller in Vietnam. The name is "hondaclassic31". It's sold as a new seat cover with foam and goes on the original seat pan. It's tight, but a perfect fit. If you have never intalled a new seat, you need to take your time so you don't pull too tight and rip it. Taking it to an upolsterer is always an option if you are not comfortable. The foam is stiffer than stock, but that is to be expected for a cafe seat. It really changes the look of the bike and

Tanks: Have you replaced the stock tank?  What did you use?  Did it require changing anything else?

  • There are at least two styles of stock tank with the CM400 - the one with no side badges and a raised area on the top is slightly wider than the other one by 1-2".  Both tanks fit the backbone of a 1980 and 1981 CM400t and work with stock bars, but there may be clearance issues with the wider tank and drop bars.  I have used the wider tank with a drag bar with 3" pullback, but have to rotate it up to clear the tank.
  • The coil on this bike is under the tank, so using a smaller tank (from a CB for example) will require moving electronics.
  • 'Two Types of Stock Tanks:' CM400E (Left) CM400T/C (Right). I believe the CM400A had the same tank as the T & C models.

CM400E Tank


CM400T/C Tank

I have been working on a bike I picked up a neighbour lost interest, and told me he shut it down when it made some sort of noise. 
It looks like they took it apart and tried to do something, but they put the long bolts that belong to the front cylinder head in and broke through the case inside and the short bolts in the front.. so the little piece of aluminium went through the motor cooked one Piston rod bearing.
I took it apart and put in new crank bearings, rod bearings, and its back up and running, but still dealing with dumb crap.. 
The clutch cable don't work as when it pulls the lever that comes out of the right case is too far forward and out by the edge of the case for the clutch to work so I have built a piece to bring the clutch cable out approx 2" from the cylinder and its a straight throw now and works fine, but have back fires from carbs, Hick ups, almost done all stock except has Z bars the guy I got it from had them upside down like cafe style but it didn't turn... so I put it back up right...
Any Help on the hick ups and burps and back fires ?

Wiring: Anything you noticed about your bike's electricals that other folks should know?


1981 CM400C wiring diagram

  • The wiring colors and layout change slightly by year and model, be sure you know where each part comes from before using a Clymer diagram to install switches and components from other bikes.
  • The stock switches for the 1980 CM400t include a gray wire to turn signals that is not shown in the Clymer diagram.  This provides power to the flashers when they are engaged.
  • I replaced stock switches with ones from sterlingmotorworks.  I eliminated the running light function on the front turn signals and got a little creative with the wiring but not too difficult (1981 CM400t).
  • If you would like to install a newer horn such as the FIAAM Freeway Blaster horn, You will need a relay. The wiring diagram as shown below will work well and will be switched as it should be. It will boost the +12 voltage.
    • Motorcycle Horn Diagram

Handlebars: What styles have you used?  Any troubles?

  • Used drag bar with 3" pullback on 1981 CM400t with minor clearance issues on the wider tank.
  • Clubman (Ace) bars on a 1981 CM400T clear the tank easily when rotated so that the U-shape part to bars are parallel to the ground.  I recommend rerouting the throttle cables so they don't bind, or shortening/buying new ones.  One other thing to consider is brake reservoir, the turn signals of a stock bike may interfere with the location of the reservoir.
  • Clubman Bars: I've installed these on my 1981 CM400C with the "U-shape" pointed slightly down. I st
    ill have tons of clearance with the stock tank and dispite what is mentioned above, there are no issues with the brake reservoir. It's actually on a perfect angle for clubmans. To fix the cable issue, I've re-routed the front brake hose and throttle cable over the headlight and down the left side while moving the clutch cable over and down the right. This has removed the slack and let me avoid having costly new cables and hoses made.

Engine: Bore experiences, swapped cylinders, pistons, performance parts, etc?

Below are some of the number I recorded while doing a compression test on a 1981 CM400C. All numbers should be between 171-199 PSI

Compression Test (1981 CM400C)
Left Cylinder Right Cylinder
Dry 155 PSI 150 PSI
Wet 155 PSI 168 PSI

Odds and ends: What else ya got?

  • My owner's manual for the '81 CM400E lists the following contained in the toolkit
    • Hook Spanner
    • 10 x 12mm Open End Wrench
    • 14 x 17mm Open End Wrench
    • Pliers
    • No. 2 Screwdriver
    • No. 2 Philips Screwdriver
    • No. 3 Philips Screwdriver
    • Screwdriver Grip
    • Spark Plug Wrench
    • Handle for 19 and 22mm wrenches
    • 19mm wrench
    • 22mm wrench
    • Tool bag
  • There is a steering lock, but it is located below/between the bottom of the triple tree. Turn handlebars almost all of the way to the left, put the key in the lock, turn and push in, and jiggle the handlebars. It should lock.

General Discussion: what do you love or hate?  What should somebody know before buying one?  Where do you find parts, information? 


  • - A valuable forum with plenty of experienced members.
  • - 1978-1981 CB/CM400 Shop Manual