New jets on KZ750?

Hello, this is my first post in the forum.

I recently picked up a 1981 KZ750E (the 4, not the twin) as a project. One of the things I'm setting out to do is some carb work. I've done some reading on here, but wanted to get a second opinion before I get to work.

The air box on the bike is broken. The previous owner intended putting POD filters in it, although I realize this will need some jetting. A #70 primary main and #140 secondary seems popular with this configuration? Do I need to adjust the pilot jet too, and are the pods 54mm?

P.O. also had a Vance & Hines 4-1 exhaust installed and he said it was running lean because of it. If I do a #70 and #140 on the jets with PODs, will this be adequate for the exhaust too?

I appreciate the help.

i have a mac 4 into one pods and i went with a stage 3 kit from dynojet it makes me happy

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rhb629 wrote: Hello, this is my first post in the forum.

I recently picked up a 1981 KZ750E (the 4, not the twin) as a project. One of the things I'm setting out to do is some carb work. I've done some reading on here, but wanted to get a second opinion before I get to work.

The air box on the bike is broken. The previous owner intended putting POD filters in it, although I realize this will need some jetting. A #70 primary main and #140 secondary seems popular with this configuration? Do I need to adjust the pilot jet too, and are the pods 54mm?

P.O. also had a Vance & Hines 4-1 exhaust installed and he said it was running lean because of it. If I do a #70 and #140 on the jets with PODs, will this be adequate for the exhaust too?

I appreciate the help.

Assuming you have Keihin CV34 carbs like me
I have a Mac 4 into 1 exhaust on mine, a 1980 750/4 and when I had K&N pods I had the 70 and 140 jets. You should not have to do anything with the pilots. I dont think different pilot jet sizes are available anyway. I cant remember how many turns out I had the air screws. Either 2 or 3 turns out. Having the correct fuel level in the carb bowls is really really important for either lean or rich running. If you are going to do anything with the carbs measure and I mean measure not eyeball the fuel level and adjust to spec. Hsrd to imagine a different brand of exhaust would make that big a difference. Besides. Remember. The previous owner always lies.
Finally. I bought an air box in a boneyard and I prefer the low end performance. I put all jets to stock except the seconday main which is 130... I think

I do still have the air box from the PO. It is missing the top screw cover and 2 springs around the carb tubes. The tubes also feel really stiff... not sure if they'll crack if I try to install them.

Are the PODs that much better? I like how they look and it seems like if I put the air box on instead I'd have to take the carbs off anyway.

JR, why did you keep the secondary at 130 with the box?

Hi there rhb629 and a belated welcome to the forum.

You might find an airbox top screw cover on ebay. You can get the springs and new supple airbox hoses from Z1enterprises.com

When I bought the bike it had K&N pods which do look cool and I believe I got the jetting correct and the bike ran well. However I decided to go for a more stock look and bought an airbox on impulse at a recyclers one day. I like the look and I prefer how it now runs. Pods may be better at high speeds. I kept the secondary main at 130 to allow for the Mac 4 -> 1 exhaust

Hmmmm. From my calculations it is going to be more expensive to fix the airbox and put it back on. I'm also not sure how the box would attach to the rest of the bike... there are two slide indents on the back (far side from the carbs) on either side. Is there supposed to be something on either side of the frame that slides into the back of the air box to secure it in place?

If I end up doing PODs, does anyone know the tube diameter? Looks like 54mm, but some sources list my bike ('81 KZ750-E 4 cycl) as 42mm.

At work right now so I can't go out and measure it

Yeah there are slide indents for a bracket on either side of the airbox and there is a part of the frame that a nut and maybe a bolt thread through and into the bracket but you need to remove the fuse box and other stuff to get at it. It becomes intuitive once you clear the area.

Here's an old pic of 2 of my old my old K&N pods with a ruler. I can dig them out of the shed in the morning when the snow has stopped and take a more accurate measurement if you need. If you do go with pods, have fun. Dont be discouraged by the naysayers here, but keep the airbox in case you ever want to go back.

Thanks, JR. I'm going to keep the air box on the shelf and try the pods out. Was looking through the shop manual and rejetting it doesn't look too bad.

I've been tearing the bike down and have easy access to it right now:

I've got some of my parts in and will doing some work over the next couple of weeks. One question though:

I noticed the P.O. took the overflow tubes off the carbs, probably when the airbox was removed. Do I necessarily need these, and if I do, where should they flow to if there is no airbox?

rhb629 wrote: I've got some of my parts in and will doing some work over the next couple of weeks. One question though:

I noticed the P.O. took the overflow tubes off the carbs, probably when the airbox was removed. Do I necessarily need these, and if I do, where should they flow to if there is no airbox?

Would route overflow hoses to exit underneath the bike where fuel discharge won't get on the engine.



Good Fortune!

One thing I would recommend about the pods is make sure the shoulder where the seal is made doesn't block that port at the top of the intake throat. I think some people have had issues with this.

Also, if you are going to clean the carbs, make SURE you get all those tiny little pilot/transition holes and passages clean. Mine ran somewhat crappy until I got these squeeky clean. Also, be very very gentle with your diaphragms. They are fragile (I never even made a dirty look at mine out of fear they would tear) and are expensive and not that easiest to find.