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77 Posts
Discussion Starter #1
Ok here we go. 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:

  • Starter circuit is useless. If the bike is off and I try to start it with the starter circuit engaged, it won't start. If the bike is on, it will instantly die if I engage the starter circuit.

  • This may or may not be contributing to a hard/no start problem I'm having when the bike is cold and the air temperature is 10 degrees celsius or less.

  • 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?
What I don't understand is that shouldn't the starter circuit solve that problem for starting the bike? As far as I know the way it 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.

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...

51 Posts
I have the same carbs, not sure if they are the stock bore but the original rubber boots attach them to the heads. I also do not know the jet sizes as I have not had to delve into the carbs...yet.
The carbs do run rich to the point if you do not rev the bike you can get a bog when you open up the throttle. The other thing is the cold start is a procedure where you to a quick crank with the choke closed then open it up right away. That is one point where I would like to put in a smaller jet if possible.
I have not been able to find easy to read information or videos on tuning of these carbs.
cr owners manual
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