Only running on Full Choke... 1982 KZ750 M1 CSR

Looking for some help in getting my bike FULLY operational. I had her out for a 15 mile ride last night, all side streets, and on full choke ran fine. The only major issue is holding an idle at a stop sign/light as it will tend to die after fluttering around 1200 RPM.

If I run it off the line, get into second or third, I try to depress the choke to half or no choke. At this point the bike will hesitate through the power band A LOT. Maybe from around 1500-4500 RPM then smooth out on the run up to 6500 or 7000 RPM.

Any recommendations on how to either,:

1) Live with running the bike on FULL CHOKE but iron out the idle issue, or
2) Help me understand how to fix running the bike on 1/2 or NO choke.

Any input is great!

carbs need cleaned at the very least :S ..... your bike is running on the choke circuit and main jet circuit ....... :huh: the idle (pilot) jets are more than likely plugged :pinch: ...... remove your carbs and clean them. :dry:

not good for the bike to run around on the choke circuit as it kills MPG and can't run well like that........ :ohmy:

Welcome aboard!

Assuming you have the stock airbox and filter I think a good starting point would be to thoroughly clean the carbs. The carbs on these bikes are very sensitive to clogging and if they sat for more than a month or so with gas in them they are probably dirty inside. Also, if there is any rust in your tank it can cause carb problems. I recommend cleaning them, putting fresh fuel in the tank, and installing a quality inline fuel filter before turning the petcock back on, otherwise any rust or dirt in the tank may contaminate them again.

Your original carbs are the CV type, so while you have them apart for cleaning check the diaphragms and other parts for any damage. Be sure to keep each carb's parts in a separate container; you don't want to mix parts between the carbs.

Hopefully you have not removed the airbox. If you have you will need to become somewhat of a carb guru to be able to experiment with different jets, needles, setting, etc. to try to find the magic combination to make the CV carbs work well throughout the powerband. Ed

Thanks for the advice. The bike had been sitting for over two years. I took the carbs off and did a light cleaning but perhaps I missed some gunk. Think I will be taking them apart tonight. and using a few cans of carb cleaner on them. Thanks again.

Oh and just to be clear.... The idle issue would be caused by this as well?

"Oh and just to be clear.... The idle issue would be caused by this as well? "

It could be, it depends on which little orifices may be dirty. Ed

Sounds like you are running lean in the low to mid range and only getting enough fuel when she comes onto the main jet at WOT.
An air leak can be causing your erratic idle.
More details needed;
Do you have pods or 4-1 pipe? (rejetting)
What state are the carbs in, have they been dipped and cleaned properly?
If you are running a stock airbox, is it sealing properly?
What condition are your intake rubbers in (air leaks)

Once you have attended to the above, then and only then, can you start with dialling in the carbs, otherwise you will just be chasing your tail, and get nowhere fast and the frustration will drive you nuts.

Once you have attended to the above, and spent some time ensuring all is well, then the tuning can start.
First you have to establish where you stand as the bike is, are you running rich? lean?
The most accurate way of doing that is by doing plug chops.
When the mixture strength is dialled in correctly, the plugs should have a light brown/tan colour.
Sooty black means too rich, white means too lean.
First thing we need to understand is the carb and how it operates.
The carb has three circuits;
1. Low speed circuit - Idle to 1/4 throttle - Idle jet
2. Mid range circuit - 1/4 - 3/4 Throttle - Needle and emulsion tube
3. Wide open - WOT - Main Jet
Once you break it down to it's basics, the mystery and dark art of carb tuning disappears.
So the idea is run the bike on each circuit in turn, read the plug colour and change jets or adjust accordingly, simple really, no voodoo magic involved.

on full choke ran fine

Choke adds fuel, so this points to a lean condition....maybe.

the bike will hesitate through the power band A LOT. Maybe from around 1500-4500 RPM then smooth out on the run up to 6500 or 7000 RPM.

This would point to the bike running lean in the mid range = Needle
Try lifting your needle a notch or two, this will richen up the mid range, and should get you in the ball park.
However, this is not good enough, it is a bodge fix, and we want to do it right, so we need to go through the whole plug chop routine.

Find a quiet, deserted piece of straight road, the longer, the better, pack a plug wrench and a wrench to remove your tank if you need to, to get to the inner plugs, writing pad and pencil, gloves (plugs get hot) and a camera.
There is no guessing involved, so mark your throttle rip with some white marker paint or fine line tape.(Something you can remove or wash off again)
Mark, closed, 1/4, 1/2, 3/4, WOT

Head out to your road.

The idea is to run the bike on each circuit in turn, reading the pugs and making notes.
The main jet influences the needle circuit as well, so we need to start there.
WOT run first, then mid range run, and lastly low speed run.
Pointless dialling in the needle and then changing the main jet, as it gets its fuel from the main jet, so you will throw it right out again, so we start at the main jet.
Remember
1. Low speed circuit - Idle to 1/4 throttle - Idle jet
2. Mid range circuit - 1/4 - 3/4 Throttle - Needle and emulsion tube
3. Wide open - WOT - Main Jet

First run, run the bike at WOT for 3 - 4 min, 5 even better if you have the room, DO NOT CLOSE THE THROTTLE hit the kill switch, pull the clutch and coast to a stop.
If you move the throttle, the mixture strength will change and your run will be worth nothing, you need to kill the motor at the required throttle setting.
Once you have coasted to a stop, pull and read the plugs, make notes, and take pics of the plugs for uploading and future reference (we want to see too)and it is better than memory.
Replace plugs, turn around and do the second run at half throttle, 3-4 min, 5 is better, hit kill switch, pull clutch, coast to a stop, pull plugs, read, make notes, take pics, replace plugs.
last run is done at 1/8 throttle on the slow running circuit, repeat as above.
Head home and adjust accordingly.

Lets assume on the first run, the plugs were a nice tan/light brown, that means your main jet is dialled in correctly, on the second run, the plugs were white with near no colour, that would indicate a lean condition in the mid range, lift the needle a slot, on the third run the plugs were black, indicating a rich mixture on the idle circuit, go a size down on the idle jet.

Head back out Rinse and repeat, you may have to do a few runs to get it all dialled in properly, but it is worth the time spent, once it is dialled in properly, you will not recognise your bike, it will be smooth, have power and torque you never imagined was there, and will be a pleasure to ride.

Sorry for the novel, but I hope it helps, let us know how it goes.

EvanXU wrote: I took the carbs off and did a light cleaning but perhaps I missed some gunk. Think I will be taking them apart tonight. and using a few cans of carb cleaner on them.

using the carb cleaning ON THEM sounds a bit suspicious..... i.e., doesnt sound like you really took them apart in a way that will let you clean the all important fuel and air passages.

real cleaning is decribed here:
www.thegsresources.com/garage/docs/vm_carb_rebuild.pdf

you could skip dismantling the bodies off the rack but its essential that you do all the rest - properly.

and BTW, a few pics of your bike and a close up of your carbs could help, both as an itnroduction and for problem solving

Agreed, you need to strip the carbs right down and dip them in carb dip, like Berrymans for 24hrs.
A spritz with carb cleaner is not going to do the job properly.
These are wonderful motorcycles, but tey are 30 years old and do need a lot of maintenance to get them up to scratch, you are dealing with 30 years of neglect, just when you think you have seen it all a PO will surprise you every time.

It's a 750 twin with BS34 Mikuni carbs and electronic ignition.

Just so people will know what it is.

steell wrote: It's a 750 twin with BS34 Mikuni carbs and electronic ignition.

Just so people will know what it is.

good point, thought it had slide carbs....

so this manual is even better

www.mtsac.edu/~cliff/storage/gs/Mikuni_B...Rebuild_Tutorial.pdf

Anyone still have that mikuni bs34 manual around?