'78 KZ 650 carb tuning

Hey everyone,

I'm new to the forum here, loving it so far.

I have a '78 KZ650 C2 that I picked up a few years back, and did a full tear down and build up on it. It's currently running a 705 Wiseco Big Bore kit, pod filters, DynaJet stage 3 carb kit (using smaller jet and needle on clip 4), and 4 into 1 exhaust off an '02 z750 with a Yoshimura can on it. Pilots are set at 17, 15, 13, 15 left-right when sitting on the bike (we'll call this cyl. 1-4 from left-right).

I've finally got the bike broken in properly (~1100 km) on the fresh engine, running Kawi synthetic oil and premium gas.

My problem is this- sometimes the bike is fussy when it's cold to start. I'm running slightly rich on cyl. 1+2 right now at idle. Sometimes #2 runs wet, other times it's ok. #3/4 run fine, burn a nice caramel. What I'm thinking of backing off a jet size on #1/2 and seeing if that clears the issue up. Does anyone have any tips here?

Another issue that comes up sometimes at 4,000-5,500 RPM is that the bike "spurts". What I'm thinking here is that it's due to the pilot issue, maybe the air screws being slightly off?

Any help here would be greatly appreciated!

Welcome.
I'm not exactly sure what you mean by: Pilots are set at 17, 15, 13, 15 ?

Pilot jets should all be the same size.

rstnick:

I've tried setting pilots all to one size, but some cylinders seem to be drawing more fuel than others, leading to some fouling, others burning where they should. When I say "pilots are set at 17, 15, 13, 15" I mean that is what the jet sizes are.

I've tried running stock pilot size on the bike, but it leans out and won't idle at all. It leans to the point where it will sputter and spit, then when it fires, it throws flames out the pipe.

I tried setting pilot size on all four carbs to one size over stock (stock 650 pilot jets are #15), and this caused cyl. #1/2 to run rich (or even wet foul). This is when I started setting individual pilot sizes, which helped my issue initially.

Agree with Rob - unless you are running different size carbs on each cylinder they pilots should be the same size. You do not mention what carbs you are running - are they the stock vm24ss carbs? Or are they some other carb set? Stock KZ650s are rather cold blooded and do require some warm up with the choke on.

Have you set the fuel level? Are there any air or vacuum leaks? Have you done a bench sync or final sync on the carbs. Do the carbs have air or pilot adjustment screws or both? What have you set them to?

The carbs that I'm running are the stock VM24's. Fuel levels in the bowls has been checked and adjusted where needed. I'm almost certain I don't have air/vaccuum leaks as I don't have surging/idle troubles. I've just done a rough sync by hand on the carbs, haven't run a gauge on them yet- I know I need to in order to ensure that they're drawing evenly. The carbs have the air set screw, and each is roughly set equally now.

By the sounds of it, I should just revert back to stock size (#15) and use a gauge to properly balance everything out on it. If I still have issues after doing this, what else could be the issue?

I've cleaned the carbs thoroughly more than once, blown every channel out, and installed new gaskets/o-rings wherever needed.

The bike isn't stock anymore- it's bored to a 705 with (I believe) 10.5:1 pistons, has pod filters, and a 4>1 exhaust on it.

Thanks guys.

I too have a 78 KZ650 C2 with a wiseco 720cc kit and the dyna stage 3 jet kit, 4 into 1 RC pipe and pod filters. My setting when I was running the stock carbs, VM24's were as follows.

Main jet was the dyno jet 118
pilot jet was 17.5
Air mixture screw was 1 1/2 turns out
needle set at the 4th slot from top

This combo was perfect. Started easily, pulled hard, ran great and got 40 MPG at 80mph.
If you are running different main and pilot jets on yours I would say there is something else wrong here. They all should be the same.
I would check your ignition system, such as voltage to coils, hight tension wires, caps ect.

Baldy:

Thanks for the tips here- I'll double check where I have everything set now. I'm thinking of reverting back to stock pilot size and working up from there. As of now, I believe I'm running the 118 Dyna main, and I'm sitting right on clip 4 (where yours is at). She pulls HARD when I crack on it, it's just my cold starting issue I would like to clear up.

Coils are brand new. What I may do is replace my spark wires, run a high flow NGK/other race line to allow the spark to flow without impedence.

Much appreciated for the tips!

I wouldn't be surprised if the different pilot jets working best was related to the being out of sync. I'd set them all the same, at whatever works best for you, then do a sync, and then change again if needed. That's a pretty decent amount of engine modification, so it'll take a little time to get zeroed in on the carbs, but she'll be fun once you do!

Seems like your pilots are averaging out to a size 15, I'd use that, then do the sync. If you're out of sync, you'll never get proper even fuel flow between the cylinders, so changing jets constantly will probably just leave you chasing your tail. One or two iterations of jet, sync, jet should get that sorted.

If you get desperate, might try finding a stock airbox, easier for the carb tuning by a long shot.

Other potential issues could have to do with ignition system. If you aren't getting a great spark, or the advancing is wrong on it, it'll make it harder to start, and could cause cylinders to not always fire, could leave your plugs looking like they're rich. Standard advice here is to check the functioning of the timing advancer, and clean as needed. Check for some posts by Patton for instructions, that pops up pretty regular.

Del:

Thanks for the advice! I'm winterizing the bike now as we're getting some snow up here in northwestern Ontario (Canada). But, the carbs are coming out, along with a few other things, and I'll for sure start with a same jet size across the board and try to sync them up.

Thanks for the help!

Snow!
It's too early.
I'm hoping to get some riding in still.
I'm on the South shore of Lake Simcoe.
Where in Northwestern Ontario are you 650mod?
Check out the Ontario Chapter link in my signature.
May be another KZR member close by.
Also the 650 link, a site just for the kz/z650s.

Rstnick:

I agree- it's a bit early for the snow. It was an early fall, colder than usual. Luckily, the snow didn't stick around

I'm up in Thunder Bay. It's a gorgeous area to ride in- tons of twisties, hills, and killer views. Living in the country makes it even better! :laugh:

I will make sure to check all that out- thanks again for all the help here

Devin

So a little update on my carb situation. I just re-synched my throttle slides, made sure they're all sitting at the same height. Tightened the lock nuts down, double checked the height. All is good.

I went a size over on my pilots (17.5), running the same size on all four carbs. Turned the pilot air screws all the way down, then backed them out 2 full turns so that they're all even right now.

I did a quick float level check, and the floats look like they're all about the same level. I did this float check with the carbs sitting in my laundry room tub, only with fuel poured into the line, not having a tank attached. I realize not having constant pressure on the line will throw the true reading off a bit.

Once it warms up a bit, and I can get the carbs back in the bike, I'll check float level again to ensure they're all sitting level.

Quick question- on these carbs, is fuel level supposed to be up to the base gasket, or at the top of the float bowl casting? If anyone has pictures, that would help me here.
I'm looking at getting a Colortune unit to tune my pilot circuit- anyone have input here?

Thanks guys,

Devin