TOPIC:

I need help with setting the air/fuel mixture on my 1981 KZ750 LTD. History at the bottom.

I took it to the local bike repair shop today. He doesn't work on bikes this old, but was nice enough to spend a few minutes listening to my story and bike, then gave some advice. He said it runs like crap (even though it seems pretty good to me), stating the idle mixture needs to be adjusted. He advised to start the bike and idle low, feeling the exhaust to see which carb gets warm the slowest so I know which cylinder is not getting the right mixture, then adjust that idle mix screw.

I browsed the Clymer again when I got home. Is this the pilot jet idle mixture? There is nothing that I can find in the manual to describe the adjustment process.

How do you recommend I balance the idle mixture? I thought that was what the carb balancing was, though that reads vaccum pressure.

I asked my wife if she wanted to come outside to feel how hot the pipe was, and she jumped on it!-).

History:

kzrider.com/forum/3-carburetor/595399-ar...nce-kz750-ltd#645529

Just ran the cold start temperature test. Cylinders (exhaust pipe temps) 1&4 were hot, 2 seemed really hot, and 3 was cool to the touch. The bike tech/shop owner said to drill a small hole in the aluminum cap and pop it out, then adjust the screw underneath, opening it up until it sounds and runs better.

I have new idle mixture screw, spring, and o ring from the K&L rebuild kits that I bought, so I will finally replace the whole set. We didn't before since the friend who helped me with the carb float levels needed to get the bike out of his garage.

I could not find a factory setting in the Clymer... do you know what it is?

Can I do this whole shebang with the carbs on the bike? Finding that gently seated position is difficult. I need a torque driver that reads micro or nano newtons of force. Carb #1 was set at 2 1/4 turns, and when I removed it the tip was blunted. I was pretty frackin' careful. Does that damage and dent the soft aluminum more than the brass needle?

Where is everyone at? It is only 23:45 central time! This is project time!

FaultedGeologist wrote: Just ran the cold start temperature test. Exhaust 3 was cool to the touch.

I have new idle mixture screw, spring, and o ring from the K&L rebuild kits that I bought, so I will finally replace the whole set. We didn't before since the friend who helped me with the carb float levels needed to get the bike out of his garage.

After confirming the same screw depth for all 4 cylinders, #3 was still cool to the touch... Frackin frustrating!

Picture below is #2 next to #3
On a side note, #3 had no needle on it!!!

Shart! #3 on left side in pic, #2 on right side and same as 1 & 4.

FaultedGeologist wrote:

FaultedGeologist wrote: After confirming the same screw depth for all 4 cylinders, #3 was still cool to the touch... Frackin frustrating!

Bike ran same/worse/same as before after finally replacing all the air/fuel mixture needles. Grrrr....

The Morgan colortune is the best way to truly dial these in...Google it.

The FSM procedure will tell you to run the screws in and out one at a time, and stop where the highest idle is acheived. The generally accepted starting point is 1.5 turns out, but your model may require something else.

As I stated I. The other thread...inspect your ignition advancer for proper function. Also make sure #3's pilot circuit is clean. All of them for that matter.

I just swapped the cables for 2&3 as suggested in another thread, and it seemed like 3 was still not getting hot. After a minute I backed out the idle mix needle and it warmed up, but it seemed to run worse.

When I set the new needles last night, in order to avoid dulling the needle tip, I backed one out the 2.25 turns that the original needles were set at and measured the depth with a micrometer. That depth was 5.25mm, so I matched them all to 5.25mm. Since having to back out #3, its depth is probably around 4mm.

A few things here...

The tip of that one pilot screw may be stuck in the carb body, blocking all flow though the pilot circuit. You best tend to that. It's critical to not bottom out the screws too hard or stuff like that happens.

It's critical that the carbs are 100% clean before attempting to tune them. After 33+ years the rubber O-rings tend to get hard and brittle too, and not seal too well. Z1 Enterprises sells the various O-rings for those carbs so you might consider ordering a full set. I would NOT use aftermarket carb kits when you have high quality OEM jets already installed, just update the rubber parts.

You can't tune an engine that is in need of general purpose maintenance. In particular, things like valve adjustment and vacuum carb sync. The intake system is sensitive to air leaks too and having a proper air filter in the airbox. Do all maintenance before attempting to fine tune the bike.

General process is to tune pilot screws for the highest idle setting. Again, you can't do this until the other maintenance is tended too though. At any rate, adjust each screw until the highest idle is achieved. Sometimes the idle doesn't respond to screw adjustment, in which case just leave the screws at about 2.5 turns open. This is a safe setting and shouldn't cause issues.

Good luck and hope this helps.

Nessism wrote: A few things here...

The tip of that one pilot screw may be stuck in the carb body, blocking all flow though the pilot circuit. You best tend to that. It's critical to not bottom out the screws too hard or stuff like that happens.

I checked for the tip of the pilot screw. The hole originally did not show light through it. I ran a pin through it from the bottom, up through to the top, and it cleared out. I never found the tip of it. I was uber gentle with the needles when screwing them in before backing them out so I find it hard to imagine I did the breaking, but the break point did look fresh.

Nessism wrote: It's critical that the carbs are 100% clean before attempting to tune them. After 33+ years the rubber O-rings tend to get hard and brittle too, and not seal too well.

I replaced all the rubber o-rings on the carbs with the rebuild kits, and used the new pilot screws just last night.

Nessism wrote: You can't tune an engine that is in need of general purpose maintenance. In particular, things like valve adjustment and vacuum carb sync. The intake system is sensitive to air leaks too and having a proper air filter in the airbox. Do all maintenance before attempting to fine tune the bike.

The guy at the bike shop said I needed to get the air fuel mix set before doing a vac carb sync. Valves would be nice to check, but bike ran great before digging in to the carbs when they were pissing gasoline. Do I need a new valve cover gasket if I check valve clearance? A new gasket is what is holding me back on checking... Have original air filter, new airbox boots.

Nessism wrote: General process is to tune pilot screws for the highest idle setting. Again, you can't do this until the other maintenance is tended too though. At any rate, adjust each screw until the highest idle is achieved. Sometimes the idle doesn't respond to screw adjustment, in which case just leave the screws at about 2.5 turns open. This is a safe setting and shouldn't cause issues.

They should all be set around 2.25 turns now, with the 3rd set around 3.5ish just to get it to warm the exhaust. I am wondering why it seems to run worse with cylinder 3 firing now (at least heating up the exhaust pipe). It should be running much better with all 4 firing...

Yesterday I timed the bike using the Dyna recommendation of advance tuning at 2,500-3,000 rpm and setting the timing mark to the right most hash (T F II) on the advance mechanism. I kept the timing light degrees set at zero (is this correct?). The bike seemed to run great, even though I had set it previously as per how it rode best, but the new setting was at least 5 deg or one cm different.

I took it for a ride and it ran pretty damn well, though it didnt seem extremely peppy. Since doing the Dyna S and coils replacement, the gas tank light hasnt worked. I ran out of gas on my return to the city, and now it runs like crap again. I need to get a fuel filter as there isnt one on the bike now, so maybe I am getting gunk from the tank in there.
Today I sprayed carb cleaner around the carb holders and vac tubes. It only kicked up in rpm in the very slightest. New carb holders necessary?