Bad gas mileage.

Thanks I'll check for the spring in the pilot mixture screws. That might solve that problem. As far as jetting. Because I haven't gotten bigger jets I moved the needle clip to the bottom position with a small washer underneath the clip. If I get bigger jets , I can move the clip position back up and won't have to open the throttle as much which means lower rpm and better gas mileage. Am I getting that right? Lol

Schmeck wrote: Thanks I'll check for the spring in the pilot mixture screws. That might solve that problem. As far as jetting. Because I haven't gotten bigger jets I moved the needle clip to the bottom position with a small washer underneath the clip. If I get bigger jets , I can move the clip position back up and won't have to open the throttle as much which means lower rpm and better gas mileage. Am I getting that right? Lol

Might have found the gas mileage answer.... :laugh:

I don't know what exactly is the best setup. But you can't compensate for a lean condition with clip alone.

have you checked plug color after running on the freeway a while?

From your post:

60-65mph at 5,000 rpm on the highway. Won't go any faster than 80 and that's wide open.

In general, you check main jet with wide open throttle. If it pulls strong, you're good. If it is weak (sounds like yours is just running out of gas) it's probably lean but checking plug color after a freeway run is the way to know.

Once you get the main jet size right, tune the pilots and needles.

Fine tuning needles is basically to get a good off throttle response when cruising along at about 30 - 35 mph and you roll on the gas. If it balks, raise the needle but too much and it will bog down from being too rich.

One thing: make sure you have good fuel flow through the petcock/filters/lines and also carbs are clean and fuel levels set with clear tube. These carbs are sensitive to fuel level. The BS-38 need the fuel level to be 4.5 - 6.5 mm below the upper carb body's lower edge. Best to run it between 4.5 and 5 mm.

Schmeck wrote: ... 79 750 twin... uni pod filters with open exhaust headers. It has a baffle in it for back pressure because the bike will not run at all without it... 14 front and 32 rear sprockets. 60-65mph at 5,000 rpm... Won't go any faster than 80 and that's wide open... does open exhaust and pods really make mileage this horrible? I think my pickup truck gets better mileage....

Schmeck wrote: ... Because I haven't gotten bigger jets I moved the needle clip to the bottom position with a small washer underneath the clip. If I get bigger jets , I can move the clip position back up and won't have to open the throttle as much which means lower rpm and better gas mileage. Am I getting that right? Lol

Tuning CV carbs with pods is already problematic enough, even with mufflers. Absence of mufflers worsens the problem, despite adding baffles inside open headers.




If stock sprockets are 16-38, would expect lower than 5,000 rpm at 60-65 mph.

Perhaps a better approach toward resolving the current issues would be to first return the carb settings (jets and clip position) to stock conditions, fit stock size sprockets, and add a muffler.

The following link includes a free manual download:
kz750twins.com/?p=15

Good Fortune!

bountyhunter wrote: From your post:

60-65mph at 5,000 rpm on the highway. Won't go any faster than 80 and that's wide open.

In general, you check main jet with wide open throttle. If it pulls strong, you're good. If it is weak (sounds like yours is just running out of gas) it's probably lean but checking plug color after a freeway run is the way to know.

Once you get the main jet size right, tune the pilots and needles.

Fine tuning needles is basically to get a good off throttle response when cruising along at about 30 - 35 mph and you roll on the gas. If it balks, raise the needle but too much and it will bog down from being too rich.

When adjusting the needle a guy taught me to use flat washers to get a half clip adjustment. Slide the needle through the washer and put it back in the bucket and bingo. Make sure you mic the washers for thickness though, you want them the same on all carbs.

If I remember correctly, the needle slots on the stock needles for the carbs on the '79 KZ750 are about .020" apart. I did actually find some 4-40 stainless flat washers that were about .010" so I could get a half notch adjustment.

Instead of common washers of the same size that may vary slightly in thickness, look for "flat shim washers" with precise thicknesses.

Good Fortune!

Will take all advice: ) have to wait till Friday before I can order parts but I will get some new jets and different things and mess with the carbs. Till then ,

Okay guys I reset the float levels the other day and they were bout a 1/4" down on both from where they should be . I guess I used the wrong bend in the carb bowls as my line . Anyways , now that I fixted that I hope my mileage gets alittle better. Will test it when my tins get back. I finally went to a professional painter for a paint job . Hope it turns out ok.

Been alittle bit but I am almost there. Got my jets in today and I think my 55 pilot jet is too small I started it up and it didn't want to run smooth without choke on . It idles slightly under 2000rpm with choke on. If I turn it off the idle goes to 3000 and won't come down. I'm assuming because the choke "chokes off air " and when it's off there is too much air vs fuel. Is there a way I can get this #55 pilot jet to work without choke or am I going to need a bigger one?
On another note I rode around a few miles with the choke on and it seemed to ride ok other than the high idle. Can I put a colder plug in to fight the lean condition ? It would probably be a good idea to prevent possible engine overheating .
Also, for those with pod filters is there anything u can put on them for restriction ? I bought pods because of my modified hardtail and am wondering if putting cloth or something over them would help to block airflow maybe???
This jetting is taking awhile .

This is my exhaust btw . I ditched the shorties and ran them alittle longer.