Installing bigger jets

Hey,

My 1977 KZ650 has had bigger jets put inside, although Im not sure it was done properly.

When upping jets, what jets do I have to replace?

Main Jet? Pilot Jet? Jet Needle? Needle Jet?

I understand once you have pod filters (individual filters) you need to increase jet size about 25%

I have the specs in my manual for the stock jet sizes

Any direction what needs to be replaced?

you usally only have to change main jets and usally only 3 or 4 sizes. 25% would be a massive increase if you haven't increased engine size dramiticly, pilots usally can be adjusted to compensate in stead of replaced. same with the needles usally onlyraised up one notch but only if needed.does your motor bog, snap and pop on acceleration or decell. these would be reasons to play with jetting.

www.oldbikebarn.com/Chrome-Pod-Air-Filte...sc=12&category=74707

I had read at that link that a 25% increase would be needed. I beleive the stock size is 102.5 for the main jets, and mine are at 122

Is that too much? The bike seems to run fine with the current configuration, although it doesnt like it if I crack the throttle wide open, sometimes itll bog out if its just idling.

that is apretty big jump in size i'd probably go down to 110's or 112.5's it will be a lot crisper and you might have to go evenlower than that what is your elevation.

238 Meters, or 781 feet.

How big of a deal is elevation? Would I be able to travel up into the mountains for example?

And your saying my jet doesnt need to be adjusted?

When I crack off the float bowl, there are two jets in there, the main jet and the pilot jet right? (different from the pilot screw)

Yes, usually its the Main jet and Pilot jet.

You can ride up into the mountains, but the higher you go, the bike will seem to become less responsive/less power. It will get too rich from the thinner air.

Jetting is a PITA!

But the proper results can be awsome.

with each 1000 ft in elevation change up you lose 3-3.5% of your power due to less atmospheric pressure available. so if your going to be at certain elevation for any lentgh of time it is a good idea to jet accordingly.

How much bigger should my pilot jet be than stock?

i'd see how it run's first before you change them out. these carb's are off a bigger bike so they may be to rich for your application as it is.

With pod filters and 4-1 with stock carbs on my 77 b1 I was running 17.5 pilots (15 is stock size) with the needle clipped at the #4 slot and 115 mains. With no baffle in the pipe I had to up the main to 117.5. This set up gave me perfect plug chops on all three circuits. As was stated, jetting is a PITA and to get it right you almost have to do a plug chop on all three circuits to make sure that you're not lean somewhere and rich somewhere else. The idle circuit is the easy one to get right, because if it idles and doesn't hesitate when you crack the throttle you're pretty much right on. The needle and main jets overlap eachother and it can be a bit of trial and error to get those right. You might very easily be lean on the needle (mid range) and too fat on the main and never really know unless you spend some time tuning. With the previously mentioned jets my bike didn't hesitate at all, she jumped to life when you twisted the throttle from idle and screamed like a raped ape all the way to redline.

When I changed from the stock vm24 carbs to vm26's my jetting didn't change at all. I assumed it would and tried many different configurations before realizing that the same jetting was what my bike wanted even with the bigger carbs.

Make sure you sync them good too! You'll have a hell of a time getting rid of sputters and hesitations if the carbs aren't synced properly.

Here are the readings for my stock carburetor setup. Can someone give me specs on the proper size jets?

Just a reminder, I have some nice headers (4 into 2, 1-4 goes out one side, 2-3 out the other).

Type: VM24SS
Main Jet: 102.5
Needle Jet Bade #: O-8
Jet Needle: 5DL31-4
Pilot Jet: 15
Throttle Cut-Away Valve: 1.5

The current jets in there that I can identify are

Main jet: 122.5
Pilot Jet: 50


Somehow the pilot jet seems like its way too big.




When I am idling, even after the bike has warmed up, if I crack the throttle the bike will die. I always have to be pretty gentle with it.

Also, I may be mistaken

When I take my float bowls off, I see two jets there, the main jet, and another one that has 50 punched onto it, which I suspect is the pilot jet. When I remove what I suspect to be the pilot jet, there is another little peice in the passage that unscrews that appears to be a jet too.

My manual points to the small jet in question and calls that the pilot jet. If that is the case, what is the jet I had to take off to get to the supposed pilot jet.

Any clarification on this?