KZ750 Keihin CR Special Carb Tuning

Posted this over at kawasakimotorcycle.org a little while back and didn't get any hits on it, so just wondering if anyone over here has any insight for me. Since posting last, there have been a couple developments.

I've never tuned carbs before aside from playing around with the idle mixture screws. Don't really have any idea what I'm doing, but I do have some troubleshooting materials from a post I found on kzrider made by satanicmechanic.

Carbs are 33mm Keihin CR Specials on a 1982 KZ750 engine, running pod filters and an open exhaust.

Standard Jetting Specs for 33mm are:

Main Jet: 120
Air Jet: 230
Throttle Valve: 3.0
Needles: YY8
Clip Position: 4
Slow Jet: 58
Starter Jet: 50

My Jets are:

Main Jet: 112
Air Jet: 240
Needles: YY8
Clip Position: 4
Slow Jet: 50
Starter Jet: 50

And here are my issues:
I originally thought the starter circuit was useless. If the bike was off and I tried to start it with the starter circuit engaged, it wouldn't start. If the bike was on, it would instantly die if I engaged the starter circuit. Now (after fixing a slightly leaky float bowl o-ring in carb #3), the bike will start cold with the starter circuit, but if I leave it on for more than 5 seconds the bike will die. Shutting it off makes the bike run smoother, but it won't keep idle until it warms up unless I'm on the throttle.

My mixture screws are all the way in, and when I experimented with backing them out a turn the idle got rougher and slower, so I'm assuming that's bad?
Bike doesn't pull very hard up until after about 1/8th throttle, and then there is a surge of power that is reasonably consistent through the rest of the range.

I bought these carbs on a parts bike, with the same engine and exhaust that they're currently attached to, but I'm not sure if the PO ever had it running. So I don't know if he jetted it that way on purpose, or if this is simply whatever he picked up from someone else.

My (potentially wrong) diagnosis is that since the mixture screws are all the way in, the carbs are running as rich as possible... and since it's got a starting issue, that probably means the mixture is still too lean?

As far as I know the way the starter circuit works is to just bypass all the other stuff in the carb and pump a bunch of fuel and air in from the side port. When I start the bike cold with the starter circuit engaged, some fuel sprays out the left side of the #1 carb.

Anybody have any ideas of where I should start? If my diagnosis is correct, does that mean I need a larger slow jet? I'm assuming there's no way to test this stuff without dropping a bunch of money on different jets. A shove in the right direction would be hugely appreciated. Sorry about the huge post.

I am no expert on CRs but 33s seems too big for a std 750 engine. The actual recommended size is 29s.

I was wondering about that. Does this mean that in general the carbs pull in far too much air for the engine? Is re-jetting going to make a difference at all or should I not bother?

Wow, those CR33's are way to big for a stock 750. You are going to have some major headaches trying to tune them. If you are eventually going to modify the engine then keep them otherwise I would look for something smaller say, the Mikuni VM29, VM28, VM26. For example My ELR clone has a hopped up GPz1100 engine with big bore kit 1174cc, high com piston, performance cams, porting, ect and I am running the Mikuni RS36's. Those are almost to big for my engine.
The general rule of thumb is, the larger the carb throat is the slower the air velocity is going into the intake which is fine if you have a large displacement engine. A Large carb on a small engine is almost impssible to tune to run effectivly throughout the RPM range. You will get it to run OK in one area but runs like crap in all others. You are constantly trying to tune around a huge hole which moves around the RPM range as you tune it. Very frustrating.
The upside is those carbs are worth some money around $450.00 if they are clean and not boogered up.

Not great news, but not exactly unexpected I guess. I'm going to remeasure them to make sure. This is a quote from an email I got from Sudco when I was first trying to figure out a rebuild kit for these carbs and was wondering what size they were.

"You would have take out the machined aluminium threaded intake adapter on the motor side of the carburetor and measure the inner diameter of the thread side. If measures 31mm then you have a 31mm carburetor, if it measures 33mm then you have a 33mm carburetor, etc....
I would imagine you are running a 33mm or smaller."

From this I got the idea that 33mm wouldn't be a problem, but I guess that's not the case?

I'm no carb expert either, but whats the difference between the Keihin CR 33's or the Keihin 34's that came stock on a 1982 LTD 750 ?
P.S. if you want to trade those CR specials for a set of 29 smoothbores, let me know
Pete

Just filled up my tank to day, and I'm getting about 10.5L/100km (about 22mpg). Pretty sure those carbs are waaaaaay too big now.

I could be very interested in that trade, if the consensus is that my bike would run a lot better with the 29s. The problem is that I don't know anything about carbs, so I've got to do a bit of research into how well they would fit the engine.

The 33mm's that I have right now are in great shape, save the starter circuit piston on carb body #1. The plastic guide/keeper there snapped, so it's not sprung properly anymore. Sudco sells new ones for about $2, it's just a question of getting the threaded portion of the old one out of there.

What kind of shape are the 29's in?

1982kz750 wrote: Just filled up my tank to day, and I'm getting about 10.5L/100km (about 22mpg). Pretty sure those carbs are waaaaaay too big now.

I could be very interested in that trade, if the consensus is that my bike would run a lot better with the 29s. The problem is that I don't know anything about carbs, so I've got to do a bit of research into how well they would fit the engine.

The 33mm's that I have right now are in great shape, save the starter circuit piston on carb body #1. The plastic guide/keeper there snapped, so it's not sprung properly anymore. Sudco sells new ones for about $2, it's just a question of getting the threaded portion of the old one out of there.

What kind of shape are the 29's in?

They look to be in great shape, I bought them about 4 yrs. ago for my 900 project that still hasn't gotten off the ground
Pete

Just to be clear, are we talking about Mikuni VM29 Smoothbores, or the Keihin CRS 29 Smoothbores? Remember, I don't really know anything about this stuff...

1982kz750 wrote: Just to be clear, are we talking about Mikuni VM29 Smoothbores, or the Keihin CRS 29 Smoothbores? Remember, I don't really know anything about this stuff...

Mikuni

33CRs are too big for a stock 750
I've got em on my 1100
It take time and knowledge to set them up
Unlike the 29miks you will go though needles and jets to set them up
Ended up cutting my slides in the end to correct a rich issue out of the hole
29 are easier, start with proper idle jet,buy some mains and be ready to raise and drop needles
You can not run those pods,first the rubber shoulder covers all the CR air intake ports,
The stacks that come with it have holes cut out to bypass air intake , Google some pics
That will make it rich also tiny cheap filters will also richen them up,try tuning without
Filters

This is all good stuff to know, thanks.

Possibly dumb question: I thought a big part of the reason behind putting pod filters (or any kind of filter really) over your intake was to prevent crap from being sucked into the engine. Aren't velocity stacks just open to the carb throat?

Sounds like 29s are the way to go. My hesitation with the Mikunis would be being able to find parts for them - anybody have experience sourcing them, and whether or not they're readily available?

It looks like these CRs are worth quite a lot of money, which is a nice surprise. Got them with a parts bike that I bought for $300 last year.