Finally figured out why my kz 400 bobber run's bad

I have been fighting this little poop cycle from the devil since i rebuilt it a few month's ago. I can start it and it will idle really good. In fact it run's smooth as silk up to say 5000 rpm's.........then its like hitting a wall. It drops off onto one cylinder and run's like crap. I feel so dumb, i have replaced the coil with a green 3ohm dyna i had laying around, new dyna copper wires, new spark plugs, new points and condensor, new battery, and cleaned the carbs so many times i can do it with my eyes closed now........lol:laugh: :laugh: :laugh: . I was screwing around with it today, and my dad mentioned it sounds like a vacume leak. I didnt think much of it until later on, and sure enough i started it, and sprayed carburator jet cleaner on the left and right intake manifold, the rpm's dropped and it died.

So now i know what my problem is, i need to get new rubber intake manifolds. Does any body know were i can buy them, i thought i remember reading some wear they are a real PITA to find. I remember also seeing them on ebay some were, they were brank new and made by a company that was selling them, i will see if i can find them. Thank's Jim.

I found these on e-bay, i think they will do the trick.

cgi.ebay.com/ebaymotors/_Motorcycle-Part...C39%3A1%7C240%3A1318

oldkaws4ever wrote:

...it will idle really good...run's smooth as silk up to say 5000 rpm's.........then its like hitting a wall. It drops off onto one cylinder and run's like crap... replaced the coil with a green 3ohm dyna...new dyna copper wires, new spark plugs, new points and condensor, new battery, and cleaned the carbs so many times i can do it with my eyes closed...my dad mentioned it sounds like a vacume leak...sprayed carburator jet cleaner on the left and right intake manifold, the rpm's dropped and it died.

So now i know what my problem is, i need to get new rubber intake manifolds....

Meanwhile, if not already done, would assure there's no fuel starvation issue from such as:

clogged vent in fuel tank cap (experiment by running with cap ajar--not snapped down);

grit/crud clogging screens on petcock pick-up tubes inside fuel tank;

clog in other petcock filter screen;

clogged in-line fuel filter;

floatbowl vent being blocked;

floatbowl vent hose being kinked or blocked.

AND assure valve clearances being to specs.

AND read the sparkplugs to aid further diagnosis.

Good Luck!

Thank's patton, i have done everything you listed, as a matter of fact i just checked the valve clearences the other day, and they were right on the money. The carb holder's are the only thing i can humanly think of right now especially since it bogged down and died when i srayed them. I will keep you guys posted when the carb holders get here and i put them on.

Am uncertain of the configuration at hand, but will suggest ---

If running without airbox (or other support at intake side of carbs), it creates additional stress on the carb holders which may lead to earlier failure, as the carbs "bounce" against their grip inside the holders.

Added weight of pods increases the stress.

Running without any rear-end suspension would likely cause even more stress.

Concur with need to resolve leaks in carb holders (presumably not due to loose clamps or faulty vacuum caps), and hope it eliminates the problem. But thinking air leak more typically causes idle issues as opposed to suddenly dropping a cylinder at 5000 rpm in an otherwise smooth running engine.

Thanks for the progress reports.

Good Luck!

I an running the stock air box, although i have the rear end lowered a great deal, i mad my own supports to mount the air box to the frame. It idles pretty smooth, but every once and awhile i will have to back down the idle screw a bit. I was thinking that when the bike feels like it hits a wall and drops off onto one cylinder wouldn't the air leak in the carb holder cause it to do so?

I took the little poop cycle from the devil out for a couple mile run tonight. Although it isnt running perfect, it ran a ton better than previously, i was able to cruise at a steady 4500 rpm, with the occasional bog, but if i tried to accelerate from 4500 rpm real qiuck it would drop onto 1 cylinder. Earlier i also drained the float bowls and sprayed carb jet spray cleaner in there, and some little gold squares flushed them selves out. Then i started it, it ran stronger and had better throttle response.

Am thinking spark plug readings could be helpful to diagnosis at this point, especially upon coming to a stop from the 5000 rpm trouble spot after simultaneous throttle/ignition chop with clutch disengagement, whereby it may be determined what's happening inside the combustion chamber at the 5000 rpm trouble spot.

Good Luck!

Still pondering ---

A non-carb possibility could be a weak or broken valve spring in the affected cylinder. Could cause loss of compression at the higher rpm.

But still hoping it's something simpler, like maybe a clogged air filter, or maybe a partially blocked air filter (shop rag, tool pouch, or something), or maybe an over-oiled K&N. Tough call (for me anyhow), especially without sparkplug reading.

If not already done, is the advancer unit clean, lubed, and properly functioning?

Is there a good flow path for air into the airbox?

Good Luck!

oldkaws4ever wrote:

...occasional bog, but if i tried to accelerate from 4500 rpm real qiuck it would drop onto 1 cylinder. Earlier i also drained the float bowls and sprayed carb jet spray cleaner in there, and some little gold squares flushed them selves out. Then i started it, it ran stronger and had better throttle response.

Perhaps some little gold squares are clogging the main jet.

Clogged main jet would be expected to produce a too lean spark plug reading at the higher rpm road speed. Perhaps aggravated by the air leak.

Insufficient air supply for whatever reason would be expected to produce a too rich spark plug reading at the higher rpm road speed.

Good Luck!

Thank's patton your awsome. Ya the advancer is clean and lubed and snap's back crisply. I will try the plug chop tommorow and report with the results. When the carb holders get here i will clean the carb's real good, install new fuel lines and new inline filters. I will also check out the air filter tommorow. I am just hopping it is the cracked carb holders also.

Ok, so i dug it out of the garage today and went to town on it. I replaced the leaky seal behind the points plate, new points and condensor, and a new battery. I went to go start it and it was only hitting on one cylinder. I have kind of suspected the carbs during this whole deal, so i took them off and completley disasembled them and cleaned evey thing i could on them. I found a white substance in the left side jet. Then i put on the new intake boots. Anyway after i put them back together and back on, one kick and she fired off and ran smooth as heck. Took it for a couple mile ride.......and all i can say is for a little 2 cylinder 400 this thing hauls major ass. It run's smooth and strong up to redline. No bogging what so ever.

Pretty amazing what a small vacuum leak and a clogged jet can cause in performance & troubleshooting frustrations....

Now that I'm thinking about it, your posting (fixes & results) would help at least 10 of the carb postings recently put on the forum...

You reviewed & analyzed the situation and symptoms, tore into the carbs and cleaned them up, asked intelligent questions, listened to the advice provided and did accomplish the recommendations and low & behold, "hauls ass".....

Have to love it!

Way to go young man,

OMR