speed problems with my KZ750

My KZ750 b4 which is a twin is only getting up to about 70 to 75 mph, after narrowing it down im pretty sure its something to do with the carberator, im thinking not enough air is getting in, making a weak explosin. So i jsut looked in to see if i could see if the butterfly valves opened all the way, but instead of seeing those i saw these to big cyclinder things that are blocking about 85% of the air flow going into the carbs, i thought maybe they regulated the air flow and raised when the throttle was opened up but they dont, they only raise when i push up on them, but then they fall right back down. So im wondering what these are and if they are the problem and if not, what else could be wrong with the carbs that would cause the engine to be so slow(im thinking some valve clearence or the rocker arms ?), and how hard is it to fix.
thanks for reading and any help would be realy appreciated

Those things that are blocking the opening of the carb bodys raise with increased engine vacumn.

How long has the bike been doin this? Was it a sudden loss of top speed or gradual?

I just experienced the same problem with a twin I got that hasnt ran in five years, and I got er goin within two days. It would rev over 5K. I knew the carbs were fine. So I started looking at electrical things. The plug caps were so loose that at high rpms the spark energy was lost. So the plugs were not firing as they should.

Strible wrote:

My KZ750 b4 which is a twin is only getting up to about 70 to 75 mph, after narrowing it down im pretty sure its something to do with the carberator, im thinking not enough air is getting in, making a weak explosin. So i jsut looked in to see if i could see if the butterfly valves opened all the way, but instead of seeing those i saw these to big cyclinder things that are blocking about 85% of the air flow going into the carbs, i thought maybe they regulated the air flow and raised when the throttle was opened up but they dont, they only raise when i push up on them, but then they fall right back down. So im wondering what these are and if they are the problem and if not, what else could be wrong with the carbs that would cause the engine to be so slow(im thinking some valve clearence or the rocker arms ?), and how hard is it to fix.
thanks for reading and any help would be realy appreciated

Speaking of twins, my bike is the "twin" of yours (KZ750 B4). BTW, they don't have rocker arms it is shim over lifter design (cams sit right over the valves).

First thing you should do is check cylinder compression and see what's up.

Valve clearance is not that hard to check.

You will need to get a manual for the bike, check ebay and some posters here have links to manuals online.

I suspect that little if any maintenance has been done on this bike and the suggestion to purchase a manual and do some basic maintenance is likely the proper one but to answer the question at hand...

OK, you have vacuum slides sitting in the middle of the venturi. These are lifted when you twist the throttle and give the bike more gas because more engine vacuum is produced. The vacuum pulls the slides up because there are rubber diaphragms in the tops and above the slides. With the airbox off the bike, turn the throttle and see how the slides rise. At full throttle, they should come almost all the way up and if they don't there is a dandy possibility that the diaphragms are damaged/compromised. Have you checked the diaphragms?

It is also possible that you are running out of gas... if you have modified the stock airbox by putting pods or not using any air filter, more air will be drawn in and there isn't a corresponding increased fuel amount, the ratio between air and fuel is hosed up... in other words you will be running lean and the bike won't get sufficient gas at higher rpms. What airbox/filter are you using?

I bought the bike about two months ago from a guy who said it was sitting in a gargage for 15 yrs. At first it didnt run but then he took apart the carbs and completely cleaned them, then put in new spark plugs and spark plug wires. and it runs great, except for this speed problem. Now that you guys have clarified what these vacuum slides should be doing im thinking these are the problem. I opened the throttle up in neutral and the slides didnt move at all, but in order to see them i had to detach them from the air filter. Luckly i do have an extra set of carbs that i can swap for parts, i also have a manuel but im not a mechanic or anything, in fact i didnt really know how an engine worked untill i got this bike and i really got intrested in it. But i have learned alot since then but i could really use some help in what i should do with these vacuum slides. I was wondering how harmful it would be to take these out? i mean im sure they are there for a reason but dosnt the butterfly valves regulate the air flow into the engine?

It's pretty simple, take out the vacuum slides and the motor won't run

Guess what controls the main jet needle, the slides do.

Take off the little round cover on the front right side of the motor, behind that cover is the points, grab hold of the points cam (the shaft that opens and closes the points) and see if you can rotate it clockwise a small amount. If you can't, then you just found the problem (and it's fairly common), you will have to remove the points plate and the small bolt that goes in the end of that shaft to remove it so you can clean and lube it.


It's called a timing advancer and it's probably stuck, and that will cause the problem you have.

Strible wrote:

Now that you guys have clarified what these vacuum slides should be doing im thinking these are the problem. I opened the throttle up in neutral and the slides didnt move at all, but in order to see them i had to detach them from the air filter. Luckly i do have an extra set of carbs that i can swap for parts

If you are ripping the throttle open and the vacuum slides (we used to call them pistons in SU carbs) don't rise at all, they are definitely not working right. They MUST lift as the throttle opens.

I agree with WG on that point. If your B4 is stock, you have Mikuni BS38's. Those slides are very easy to get out:

1) take off the fuel tank.

2) take off the "cross brace" on top of the carb set between them. It has four screws you will need a "right angle" phillips screwdriver.

3) take the other three screws out of the top cover on the carbs, be careful there is a spring and it may pop up.

4) take the cover off and the slides lift right out, be VERY CAREFUL not to tear the rubber diaphragm around the edge of the slide.

You can inspect and clean them, check for tears in the rubber.

NOTE the screws are in three different lengths (depending on what they go through), keep them straight.

Thanks alot, you directions seem very easy to follow. Im going to try and fix it this weekend and ill post the results Thanks agin

oo and if it turns out to be the slides what exactly do i need ot clean and lube, i was looking at this peice in the space carb i have, so the only thing i can thinkof is to clean the rubber thing and lube the needle.

You wrote that you have a manual... is it the "Kawasai Shop Manual"? I hope so. It is an awesome manual.

I think you should seriously consider following WiredGeorge's advice above, and just do all the basic checks outlined in the manual (timing, valve clearance, float levels, pilots, bla bla bla) before you tear everything apart. If you say it runs well but does not go over 75, then I'm sure something is just mis-adjusted, and you will find that by running through a thorough set of system checks....

...and you might just want to spend a calm evening or two reading the manual from cover-to-cover.... especially since you're new at this like I was when I got mine. The Kaw manual was soooooooo helpful in teaching me motorcycle "theory", and is a perfect reference at any time...

It's a whole lot easier to test the slides and diaphragms by following the instructions in the manual for doing so, the only thing you need to remove to test them is the airbox to carb boots.

If you think they are stuck, then why not stick your finger in there and lift them up to see?


By the way, you're wasting your time and energy, if the slides don't open then your bike might run 30 mph, but there's no way possible for it to run 75 mph without the slides opening.

The slides control the main jet needle, if they don't move up then the needle don't lift up, if the needle don't move up then the bikes not going to get enough fuel to run 75 mph.

And yes the butterfly valves regulate the airflow, but the slides regulate the fuel.

Strible wrote:

oo and if it turns out to be the slides what exactly do i need ot clean and lube, i was looking at this peice in the space carb i have, so the only thing i can thinkof is to clean the rubber thing and lube the needle.

Be real careful with the rubber. If they have not been out for a long time, there will probably be scum on the outside of the slide. take some carb cleaner and a soft tissue to it. I wouldn't hose the rubber with carb cleaner, it isn't good for rubber.