78 KZ400B Tuning Help

I'm working on a KZ400B for a friend of a friend. I worked on the stock carbs on my KZ400 for about 2 years before I finally gave up and went 2-1. Anyway, this bike is completely stock so he wants to stick with the stock carbs if possible.

Here is where I am at after about 4 hours of tuning, and $150 in parts...

Exhaust: Stock
Intake/filter: Stock
Jets:
Pilot = 35 (stock)
Main = 70 (stock)
Secondary = 95 (stock = 90) Note: This was jetted up by someone else. Not sure why.

The stock petcock is leaky, so the first thing I did was replaced it with a aftermarket non-vacuum type. At the same time I capped off the vacuum nipple on the carb. I replaced both diaphragms because one was sticking ope (Wrong part installed by some idiot). Carb holders were leaky, so I replaced those two. Checked float level and everything looks good.

All of this said you would think the bike would run like stock. Wrong.

Bike starts up, runs fine off of idle for a bit but bogs majorly at anything above full throttle. Maybe that jetted up secondary is the issue here?

Then if I try taking it for a spin it just bogs, bogs, and bogs some more. Acts like it's getting fuel but not any air.

Like I said, I haven't messed with CV style carbs in a while. Could the capped off nipple be forcing the slides to not slide with increased throttle? I forget how the diaphragms actually operate.

My only other thought is replacing the petcock with a stock vacuum style replacement, and jetting that secondary back down. Just don't want to waste my time if neither of those are likely the issue.

Any input at all appreciated.

I think your instincts are good. I wouldn't mess around with the fuel system, assuming the inside of the tank is clean, you have a low-pressure in-line filter, and there;s nothing clogging up the works.

Check the spark.

If you haven't done so, check the timing.

Make sure the mechanical advancer is working correctly.

If not already done, would perform the clear tube test to assure fuel levels are within specs.

Good Fortune!

When is the last time the advancer was cleaned/lubed and new points/condenser installed? If the ignition points, advancer, and timing are not up to snuff the engine will bog. Ed

Thanks for the input guys. I did check spark before I started, but the points and condenser looked like they could use a replacement. Maybe I will start there.

Should I put the gravity petcock back on and cap off the vacuum nipple again, or would you advise a replacement OEM style petcock?

That's a personal preference question. Both have their obvious advantages.

Cynjut wrote: That's a personal preference question. Both have their obvious advantages.

I prefer the gravity feed. Just want to make sure that capping off the vacuum nipple isn't causing issues. I've done it before without issue, so I don't think that is where my problem lies.

Ok did you check the pilot system? When you pulled the pilot jet did you remove the brass pitot tube that feeds gas up into the carb ? The early carbs may different, the factory changed that pilot system a few times. But that little tube has a bunch of tiny holes cross drilled in it and often can gunk up and starve the pilot circuit.
I found with my 440 that those CV carbs are very touchy, but the run very well if you are thorough in cleaning them. 90% of the time the pilot circuit is problem. Also is the air filter intact? If that has been messed with the bike won't run for $hit.

Oh yeah one more tricky point to the carbs, some of them have a plastic plug over the pilot jet with a small cross hole drilled to pick up fuel from the near by main jet, if those carbs were supposed to have the plugs over the pilot jets the bike will flood out. Again Kawasaki changed that a few times too...

Lastly capping the vacuum port for the pet cock will not affect the operation of the diaphragms.

Here is picture of the type with the capped pilot jets... don't know when this change was made

My 79 KZ400 had the caps on the pilot system. These do not. However if they are SUPPOSE to have that cap, that would explain a lot wouldn't it?

I left those off on purpose once, and any throttle would immediately flood the bike. If you see a small hole drilled from the pilot jet casting over to the secondary jet, then chances are it was supposed to have them.

According to the book I think it is suppose to have them. Now the hard part is finding replacements.