KZ1000 jetting advice

I've been searching for about two hours with no luck so I'm just going to ask. My 78 KZ1000 starts to run poorly at about 5000 rpm. It has a 4-1 Kerker and a K&N stock replacement filter. I ordered a stock OEM filter (as 650ed suggested) from Z1 and will try tomorrow to see if it helps, but my real question is what main jet should I go to if my air filter change doesn't help? My old Clymer manual says the stock main is a 107.5. Roy, you had recommended 112's or 115's thinking the stock jets were 105's. Would you still recommend those sizes? Also, can these be changed w/o removing the carbs? Are these jets available from Z1? I put four new manifolds on a couple of years ago and swore I would never take the carbs off again! I just hope I don't regret anything I do because overall the bike runs well, just not as good as I remember when it was completely stock. I believe everything else is in order, so it would seem that the jetting is the likely culprit. Sorry for all of the posts and questions…..I just can’t remember how to do this stuff anymore.

Thanks again!

you will have no choice but pull the carbs because you have to change your mains up to 115's at least and have raise the needles up at least one position so get ready to be frustrated hace a heat gun or a hairdryer handy when you put the air box boots on. to make it easir on your self try sliding the boots back inside the air box until you get the carbs on the manifolds then work them on the carbs from in side the airbox it will make it go alot easier.you will have to richen up the midrange as well that is why you will have to raise the needle up

78KZMAN wrote:

I've been searching for about two hours with no luck so I'm just going to ask. My 78 KZ1000 starts to run poorly at about 5000 rpm. It has a 4-1 Kerker and a K&N stock replacement filter. I ordered a stock OEM filter (as 650ed suggested) from Z1 and will try tomorrow to see if it helps, but my real question is what main jet should I go to if my air filter change doesn't help? My old Clymer manual says the stock main is a 107.5. Roy, you had recommended 112's or 115's thinking the stock jets were 105's. Would you still recommend those sizes? Also, can these be changed w/o removing the carbs? Are these jets available from Z1? I put four new manifolds on a couple of years ago and swore I would never take the carbs off again! I just hope I don't regret anything I do because overall the bike runs well, just not as good as I remember when it was completely stock. I believe everything else is in order, so it would seem that the jetting is the likely culprit. Sorry for all of the posts and questions…..I just can’t remember how to do this stuff anymore.

Thanks again!

Carb removal and replacement is a piece of cake with new soft pliable sensual? airbox hoses from Z1E

If not already done, would assure proper operation of ignition timing advancer unit (watch it move back and forth under a strobe-type timing light while varying rpm by blipping the throttle).

Also check/adjust valve clearances.

Good Luck!

I'd have to second Patton's advice about new boots, makes things much easier. If however you really don't want to do this you can change main jets with the carbs still on the bike. I bought a little tool from Sears that works pretty slick for removing float bowls while they are still on the bike. The Craftsman number is 41380 it is a little knurled knob that you can fit a #2 phillips bit into. The bits I use I got at Harbor Freight and they are 2" long with six sides on them. The entire setup will measure 2.5" long which is just perfect for popping off carb bowls. You can also adjust the height of the needles without pulling the carbs as well. It is a PITA but it can be done.

I am told the boots get very soft if you soak them in boiling water before you need to fit them.

Update: I put the stock air filter back in today and the bike ran much better! In fact, I was able to run up to redline with almost no issues. When I pulled the plugs, they were kind of white, not nice and tan. So, I'm sure it is running a bit lean. All things considered, I think I'll leave it the way it is for now. Thanks everyone for all of your help.

78KZMAN wrote:

Update: I put the stock air filter back in today and the bike ran much better! In fact, I was able to run up to redline with almost no issues. When I pulled the plugs, they were kind of white, not nice and tan. So, I'm sure it is running a bit lean. All things considered, I think I'll leave it the way it is for now. Thanks everyone for all of your help.

not for nothing, but i would nt run it lean for too long....unless you ride real easy and slow ,below
4k, you will grenade a piston eventually....just my .02.....

For what it's worth, here's what worked for me. I have the same bike, '78 KZ1000 all stock except for a 4-1 kerker with street baffle. It was always cold-blooded to start up and a little bit weak on top end. The stock carbs came with 115 pilots and 105 mains. Last year per Wiredgeorge's suggestion I changed to 117.5 pilots and 110 mains. This made a huge difference in startup and riding. I couldn't be happier. Wish I had done it years ago. Of course a valve adjust check and carb sync and all the other items mention by others are equally important.

keith1; is the possible piston damage due to the lean issue or the hot plug? Would I lessen the potential problem by going to a cooler plug?

Gman; Prior to your jetting change were your plugs kind of white like mine? Did they turn tan after the jetting?

Thanks.

78KZMAN wrote:

keith1; is the possible piston damage due to the lean issue or the hot plug? Would I lessen the potential problem by going to a cooler plug?

Gman; Prior to your jetting change were your plugs kind of white like mine? Did they turn tan after the jetting?

Thanks.

due to lean condition....damage isnt a certainty, but the potential is there.....i m not sure about going to a cooler plug.....

78KZMAN,
Yes my plugs were whiteish looking before I changed jetting.The factory just jetted these bikes too lean for optimum performance. I think they were more concerned about exhaust emission standards.