TOPIC:

Hey guys,

First of all please read the ENTIRE POST before giving recommendations. Also this is NOT a question about carb jetting. And, although this is my first post i have read many threads on this site.

Quick FYI, the KZ750-F1 has the single main jet & open pilot jet configuration that Kawasaki used in the later years of the KZ's. Also, i am running the stock air box, with a new filter, and the stock exhaust. Everything is stock except for what is mentioned below.

Here's the story.
Got a KZ750-F1 a couple of weeks ago. It ran good aside from that it wouldn't rev above about 5k-6k rpm with throttle open more than 1/4. I knew this had something to do with the main jet so i took apart and cleaned the carbs, and set the float heights to 21mm. While doing this I discovered that the main jet was a #150, and that at that 5k rpm it was running very rich. Not surprising, the stock main jet is only #100 (56% smaller than the current #150). After I cleaned, set float height, and put it all back together it still ran as it did before.

Now, I had trouble finding the stock configuration at first so i got confused and ordered a #125 jet, and set the float height to 21mm. By the time the #125 came in the mail I had discovered that i really needed a #100, and that the float height should actually be 14mm. So i pulled the carbs and tried the #125 just for grins, and set the float height to 14mm. I also recleaned the pilot jets, making sure to blow out the three small holes out by the throttle. Put it all back together, and now it idoled super high (like 5500 rpm).

I though this may be because i had changed the float height and now since i still didn't have the right main jet it just wasn't working right. After scolding my self about how one should only change one thing at a time, i pulled the carbs and put the #150 jets back on. Still had the high idol. so i pulled them again, and put the float height back to 24mm, and tried the #125 jets. Same high idol. So I pulled the carbs again and put everything back to how it was when i last had the bike running normally (#150 jet, 24mm float height). To my dismay it still idled around 5500 rpm.

The carb boots are a little cracked, but not very bad, and using carb spray i identified that the one going to the #3 cyl., has a very small vacuum leak. Also when its running if i stick a rag in the air box intake hole it promptly dies, so i don't think there is much of a vacuum leak.
I also turned the idol screw all the way out, made sure there was play in the throttle cable, and double checked the throttle is all the way closed.

The video below is me demonstrating the issue, Its a warm start, but the bike behaves much the same from a cold start.
www.dropbox.com/s/pvuigs6dta9cv6l/VID_20140722_093828.mp4

This picture is of the #1 cyl. spark plug, Just after shooting this video, looks like its running rich.



This picture is of the #3 cyl. (the one with the slight vacuum leak) spark plug just after shooting the video. looks like its rich but not as rich as #1, which fits the vacuum leak theory.


So i guess the question is, what would make a bike run with high idle like it does in the video.
Again all that was changed from running to not was, the carbs were pulled on and off and opened up 5 times, and the pilot jet passage-ways were verified as clear.

Hello and welcome

I'm one of the new guys here with some grease under my fingernails. My guess is your carbs aren't fully closing to where they should be. If you bench synced your carbs perhaps a proper carb synch would fix the high rev issue, although I'm having trouble in my mind connecting the 2. It's like you are running on a high choke.

Is the throttle cable correctly adjusted and moving freely?

The throttle cable is relaxed right now so there is a little play in it.
When i cleaned the carbs i looked at the throttle butterflies. They are all approxamately syncronized. and there is a little less than a 1mm gap under them. I also pushed down on the external parts of the throttle to make sure they were all the way closed.
This is what confuses me so much. If the throttle is all the way closed, and there isn't really a vacuum leak, how is there enough air to run at 5000 rpm?

Valve clearances within specs?
Clear-tube tested?
Ignition advancer functioning properly?
Spray tested for carb holder leaks?

Good Fortune!

Hey guys, thanks so much for your input.
After LineArtist's recommendation that i check the bench syncing, i pulled the carburetors and took a very good look at the butterflies.
I realized that the butterfly valves were not completely closed, open a little less than a mm as i previously noted. I then realized that the idol screw was not all the way out like i had thought it was. turns out that the choke actually opens the throttle a little bit, and when i had turned the idol screw out all the way before i did it while the choke was on. so it actually wasn't all the way out.

So problem solved,

As a note, i put the 125 jets back in and its running alot better than it was with the 150s. I think the stock 100s are coming tomorrow so i'm really excited to try them out. In response to Patton, Valve clearance, clear-tube, and ignition advancer checks are planned for next week when i get some seals in the mail, and a timing light.

reindel, good to know you got a handle on the issue. 5000 sounds great but burns too much precious fuel on idle.