Good News! (and questions)

I did it! Back in September I bought a 1980 kz550. I spent the winter messing around the engine and learning all about the bike. I have never owned a motorcycle so this has been a journey to say the least. After many hiccups (my favorite was installing two gaskets on the cylinder head and spewing oil like Niagara falls... I did not know one of them was a base gasket...) I am riding the bike. AND ITS GREAT. Seriously this bike is a blast to ride. I have to thank everyone who has answered my posts along the way. You all have been super helpful.

Okay. So now for my question. Lets talk about jetting the carbs. Background information. I live at 5280 ft. I dropped the jets to as follows.
Main Jet: 89 (92 original)
Pilot : 31 (32 original)
Jet Needle: Stock
Air screw: out 1 full turn.

These are the kits I have to give an idea of the wiggle room I have.
www.z1enterprises.com/keyster-premium-ne...kz550-1980-1983.html

Effective oxygen drops by about 3% and air pressure drops by about 15% at my altitude from sea level so I figured this would be a fine thing to do off the bat. The bike runs. and the plugs look fine, I think. The Rocky Mountains are right at my doorstep and I REALLY want to ride some sweet mountain roads so my plan is to run the bike SLIGHTLY lean most of the time so that when I go for longer rides and rise in elevation it goes toward even

My questions are as follows
1) how do I know where to set the air screw?
2) Are there any guides that could help me ballpark? I could go through the fluid mechanics calcs... but... ya know, that will take some time.

I want to learn this process well so could y'all point me to some good resources, tips, and points? The difficult thing is that I do not know what to look for mostly, like what should I consider important and not important and what not. I will attach some pics of the pics of the spark plugs. These were pulled from the engine after some seconds at WOT then cut power to get the most accurate look at what is happening at WOT.

Congrats on your successes and failures (aka learning opportunities).
You're riding the bike now right? So here's the thing, as another relatively new bike guy, why are you planning on taking the carbs apart?
If you just have the "I think I can make it run better" itch...when its already running well, do yourself a favor and leave the carbs alone.
If the bike is not running well...then you take action. To me, those plugs look good. WAY better than mine.

If you want to do some fine tuning, get a carb tuner. I use the Morgan CarbTune Pro. It has rubber hoses that you connect to your carbs that will read out the amount of air you're pulling. Your manual should have a section on how to use it.

Like I said, I've still pretty new BUT I made the decision to take carbs apart when the bike was running pretty strong and I wish I'd spent those long frustrating hours just riding.

Good luck

Back in the old days, you just needed a really precise tachometer (not the bike's tach) and you would use the "lean-drop" method for setting the idle mixture.

It's easier with a wideband setup that actually just gives you the air fuel ratio, but if you have to go old school, an outboard tach will work, but it takes time. I used an Actron CP7605 dwell-tach-volt meter. I think you can still get them pretty cheap.

After making sure the compression is good and the valve lash is properly adjusted, and ignition is good, and fuel levels are set properly...
The carbs should be synced.
The air-screws first are adjusted roughly so that the bike is ridable. I guess 1 turn out is a good place to start as any other.
The bike should be jetted so that it's operating correctly at all throttle positions.
You run the bike up to full operating temperature.
Put the bike on a center stand and hook up the tach.
The tach won't read properly because it's scale is for a car, but you're not interested in the actual numbers, just the change in RPM.
(To get the actual tach reading you multiply the scale by 4.)
The idle should already be smooth. If it's fluctuating a lot, there is something wrong. Maybe adjust the airscrews to get it smoother.
You blip the throttle and make sure the idle returns to the same value or near it.
Because you have air screws, out is leaner, in is richer.
You turn the screws richer a tiny bit at a time until the RPM drops.
Then you turn the screws leaner a tiny bit at a time until the RPM peaks and starts to drop.
That is where it gets the name "lean drop".
You pay attention to the turns as the RPMs go from peak to starting to drop as you go leaner.
Then you turn back rich to recover the RPM back to where the peak was.

Along the way, the RPMs may increase to where they are too high. You have to readjust the idle speed, then start over.
Adjusting the idle speed puts the needle at a different position which may be leaner or richer.

It's a recursive process meaning you go through iterations where one iteration affects the next one and so on.

This all has to be done slowly and be sure to record the screw positions so you can go back to them if needed.


The reason you use a lean-drop and not a rich-drop is because when you go rich, a tiny bit of fuel can accumulate and affect the mix even as you go leaner. By using a lean-drop, you ensure any excess fuel in the intake tract gets used up before making the final adjustment.


Once you have a baseline for what the bike should be set at when hot and idling for a long time, you can make intelligent adjustments to the mix. For instance, if it's colder out, you may want to adjust an 1/8 turn richer. Or if you go up an extra 5000 feet in elevation you can turn leaner a 1/4 turn, and so on. The important thing is to write down what setting you have and how it ran and in what conditions like temp and elevation.

Typically, when done this way, depending on the needle and other variables, I find you end up lean because the motor and carbs get hot and the gasoline is heated in the tank. So the next time you start the bike cold, once you get off choke the bike will still tend to be lean. You will then increase the idle speed. But then the idle speed will creep upward as the bike warms up further.

Instead of immediately increasing the idle speed, I tend to like to adjust the air screws a bit richer. This tends to reduce the RPM creep up. There still will be some, and these bikes did that even with factory jetting and airbox, but it won't be as dramatic.

loudhvx wrote: Back in the old days, you just needed a really precise tachometer (not the bike's tach) and you would use the "lean-drop" method for setting the idle mixture.

It's easier with a wideband setup that actually just gives you the air fuel ratio, but if you have to go old school, an outboard tach will work, but it takes time. I used an Actron CP7605 dwell-tach-volt meter. I think you can still get them pretty cheap.

After making sure the compression is good and the valve lash is properly adjusted, and ignition is good, and fuel levels are set properly...
The carbs should be synced.
The air-screws first are adjusted roughly so that the bike is ridable. I guess 1 turn out is a good place to start as any other.
The bike should be jetted so that it's operating correctly at all throttle positions.
You run the bike up to full operating temperature.
Put the bike on a center stand and hook up the tach.
The tach won't read properly because it's scale is for a car, but you're not interested in the actual numbers, just the change in RPM.
(To get the actual tach reading you multiply the scale by 4.)
The idle should already be smooth. If it's fluctuating a lot, there is something wrong. Maybe adjust the airscrews to get it smoother.
You blip the throttle and make sure the idle returns to the same value or near it.
Because you have air screws, out is leaner, in is richer.
You turn the screws richer a tiny bit at a time until the RPM drops.
Then you turn the screws leaner a tiny bit at a time until the RPM peaks and starts to drop.
That is where it gets the name "lean drop".
You pay attention to the turns as the RPMs go from peak to starting to drop as you go leaner.
Then you turn back rich to recover the RPM back to where the peak was.

Along the way, the RPMs may increase to where they are too high. You have to readjust the idle speed, then start over.
Adjusting the idle speed puts the needle at a different position which may be leaner or richer.

It's a recursive process meaning you go through iterations where one iteration affects the next one and so on.

This all has to be done slowly and be sure to record the screw positions so you can go back to them if needed.


The reason you use a lean-drop and not a rich-drop is because when you go rich, a tiny bit of fuel can accumulate and affect the mix even as you go leaner. By using a lean-drop, you ensure any excess fuel in the intake tract gets used up before making the final adjustment.


Once you have a baseline for what the bike should be set at when hot and idling for a long time, you can make intelligent adjustments to the mix. For instance, if it's colder out, you may want to adjust an 1/8 turn richer. Or if you go up an extra 5000 feet in elevation you can turn leaner a 1/4 turn, and so on. The important thing is to write down what setting you have and how it ran and in what conditions like temp and elevation.

Typically, when done this way, depending on the needle and other variables, I find you end up lean because the motor and carbs get hot and the gasoline is heated in the tank. So the next time you start the bike cold, once you get off choke the bike will still tend to be lean. You will then increase the idle speed. But then the idle speed will creep upward as the bike warms up further.

Instead of immediately increasing the idle speed, I tend to like to adjust the air screws a bit richer. This tends to reduce the RPM creep up. There still will be some, and these bikes did that even with factory jetting and airbox, but it won't be as dramatic.

That's some fine, old-school experience talking there. Detailed with supporting explanation. That kind of knowledge is at risk of being lost as FI becomes ubiquitous.

Good Ridin'
slmjim & Z1BEBE

Thank you for such a comprehensive explanation. I will be looking into the carburetors in depth later this week. I went for a long ride this weekend and I have a few ideas to try to smooth out the throttle response so I will work on that before going for the final touches. If this bike lasts me a while (which I see no reason as to why it will not), you will have played a large part in maintaining it. Its crazy how valuable this information is