ok i know its not a kz butttt

my girlfriend just inherited a 81 honda cm400t and shes really excited to ride, this weekend we spent time cleaning it up, storage since 92

her dad got it "running" earlier this summer but running was all, it started barely with choke and then died immediately after choke was off.

we got a new battery, new plugs, great spark to both cylinders, great compression 165 on both.

carbs i figured were the issue, so we dove in together and her taking direction from me did almost all of the work taking the carbs off.

anyway, after about 2 hours we had them SPOTLESS all jets cleaned with spray cleaner and air. bodies soaked in cleaner and then spray and air.

they look great and it started up first second on the starter! wooo

now the question!

bike runs pretty well it idles strong and drives ok but around 5k it bogs down REALLY bad and you just have to throttle through, then you hit 7.5 and it RIPS really hard but its pretty annoying and shes a beginner and i want it to be as close to perfect as possible

the airbox is hooked up correctly, the intake boots are solid with no cracks, BUT the RH exhaust is rusted through at just after the header, its basically not attached. could this cause the midrange engine bog?

any help would be great, i know its not kaw but there is some serious knowlege here so i figured i would ask

Is this the carbs?


Did you check for holes in the diaphragms? How about setting the fuel level? Having one side with more back pressure than the other won't help. Can you connect the rusted out area, weld or replace the metal?
Did you check that the ignition timing is correct and advancing smoothly?

it looks to be these carbs, there is no diaphragms it is a metal top...odd to me having just re-built kz1100 carbs with rubber diaphragms. also the floats are plastic and non adjustable...odd little carbs if you ask me. i am working on the exhaust now, gonna try jb weld first... then if i have to take them off and bring them somewhere. i havent checked the timing... i have to find the service manual for this bike i dont even know where to look for the timing!

What are those in the breakdown then, the cap on top, and the spring, then? Looks like it would have a diaphragm.
Manuals can be had, they are on the net for free download if you search. I just got a Concours full service manual, off of a Russian server, free pdf file.

ignition advancer
garreneteed

i have gotten the shop manual just tonight, i joined hondatwins.com another sweet forum with a wealth of knowledge.

@jeffasaki ignition advancer you say? perhaps i can just check it out and lube it up before buying a new one! thanks for the tip ill check it out tomorrow

correction it looks like i have electronic ignition and there is no mechanical advancer... hmmm

bearshawk wrote: correction it looks like i have electronic ignition and there is no mechanical advancer... hmmm

There may still be a mechanical advancer. My kz 750 has electronic ignition, with a mechanical advancer that hides behind the ignition plate.

Motor Head wrote: What are those in the breakdown then, the cap on top, and the spring, then? Looks like it would have a diaphragm.
Manuals can be had, they are on the net for free download if you search. I just got a Concours full service manual, off of a Russian server, free pdf file.

Those carbs look just like the Kiehin carbs on my CB750F. That thing in the diagram that looks like it should be the slide with the attached diaphragm from a Mikuni CV (#8 points to the cap and piston) is actually a metal piston that fits closely inside the cap. There is a diaphragm but it's small and goes into the side of the carb body. In that diagram it's marked as #14 for the assembly. It's a small diaphragm about the size of a quarter with a spring on top, and then the cap covering that.

BTW I should add that when I tore into my Kiehin carbs from the F'er those small diaphragms were hard as a rock. It had been sitting since the mid 90's. Also the accelerator pump diaphragm was similarly hard, and I wouldn't expect it to work either. The accelerator pump is marked as assembly #7 in your diagram.

So those are Vacuum Pistons, Sans Diaphragms. I can't tell from the drawing what the #14 is, but mounted up on the side seems that it would be a Coasting Enricher.
The Flat Spot described should rule out the Accel Pump, as it is for up to about 1/4- 1/3 throttle. I saw the CDI in the parts section. Those vacuum pistons may well be effected by the low back pressure on the one side. Making it lean.

Thanks for all the insight guys, im working on getting new mufflers as we speak, JUST the rt side rusted through the left is nearly perfect with no rust to be seen

i will still look for the mechanical advancer and see what i can do about lubing it up, i should probably just take the carbs out again and clean them again and check the rubber on the side that i didnt know existed!

either way ill try everything out that was mentioned here and see what happens

i will also post pictures of the little beauty, it really does look good and everything is original