funny choke behavior

I picked up a 77 650B that is in great shape, but the choke seems to behave strangely.
With full choke on, I can start the bike and let it warm up, and it idles about 1500.
Then when I slowly turn the choke down, the rpm blast up to 5 or 6K. (At about half choke). Then if I continue to turn the choke full off, the engine chokes, boggs and dies!??
Im thinking A/F mixture? But not really sure.
The carbs are stock Mikuni's (not sure what size) But Im quite sure that they are stock for this bike.
Any ideas out there?

Thanks

Welcome to KZR.

I start my '78 650 like this on cold mornings:
Turn fuel valve to ON or prime if available.
Kill Switch and Key ON.
Pull Choke all the way Up, Full Choke.
Attempt to start with Electric Starter or Kick Starter.
Depending on warmth of engine or time since last being run, it may take up to 4 or 5 tries to start bike.
Once bike is running somewhat smooth with FULL Choke ON, I will start to lower the Choke Lever slowly, Revs building. I do not let the RPM build to a High roar, but will let them build to 2000~4000 rpm for a little while.
If the engine is running smooth at this point, I will try lowering the Choke Lever all the way.
If the engine wants to die with the Choke Off, I will bring the Lever back up quickly to keep her running.
The time of year will determine how long I need to warm her up.
In cold weather it could take 5 to 10 minutes of warm up before I can get going. And in the Summer, just a few minutes without using the Choke.

With full choke on, I can start the bike and let it warm up, and it idles about 1500.

Then when I slowly turn the choke down, the rpm blast up to 5 or 6K. (At about half choke). Then if I continue to turn the choke full off, the engine chokes, boggs and dies!??

Sounds about right. These bikes are cold blooded. They need a little warm up first.

If your carbs havn't been cleaned in a long time they may need a good cleaning/adjustment.

Post edited by: rstnick, at: 2006/01/02 11:07

Thanks rstnick, I catch your point. But shouldn't the rpms be the highest with the full choke, and the rmps decrease as the choke is turned down?
Instead of going from 1500 at full choke to a peak of say 5k at half choke, then down to bog at no choke?
And this behavior seems not to change even when the bike is warm. (running for 10 min or more in CA air temp 65F)
Actually I cannot get it to idle at no choke, even after warm, and adjusting the idle speed adjustment on carbs.

Thanks again for any ideas.

But shouldn't the rpms be the highest with the full choke, and the rmps decrease as the choke is turned down?

No, not on mine anyway. Highest RPMs in the middle.
After a few minutes of Full Choke, you have to move your Choke lever down, through the High rpm range, quickly if needed, until the revs come to comfortable 800~1200rpm. All the way down on the Choke lever if the engine will handle it without stalling. Otherwise raising the lever to keep the rpms going.

Post edited by: rstnick, at: 2006/01/02 11:48

Hi mjkawa,

My 78 650 has the same behavior, I think it is normal. I start mine with full choke when the engine is cold. It will start and rise to 3500-4000K then slowly settle down to about 1500-2000K. It will start dropping more and from this point I have to lower and raise the choke lever to keep it somewhere around 2000K. It is a constant up/down motion on the lever and throttle blipping until she idles smooth enough to take off, usually 3-5 minutes later.

As far as your engine not idleing at all when warm.. It could need a good carb cleaning like Rob said, and it sounds like the idle jets/circuits are dirty. Maybe a tank or two with Seafoam will clean it out. If that doesn't help then you might have to tear them down and do the whole cleaning bit.

It is good to let it warm up for a few minutes. My friend has a Yamaha 650 Custom, the v-twin. He starts his bike and can leave in about 2 seconds, no kidding. I wince every time he does it. Not because I can't do that, but because his poor engine has no warm oil in the head!

Is this a common practice on the newer bikes??

Post edited by: OKC_Kent, at: 2006/01/03 11:15

Post edited by: OKC_Kent, at: 2006/01/03 11:23

Im still not sure that this is normal? On my kz1000 the choke affected the rpm liniarly from open to close. (but these are different carbs).
Is the expert WG around?
Im sure he can give us some guidance.

Thanks again.

My experience is pretty much the same as rstnick and OKC_Kent.
This is probably a dumb question but I just wondered if you need turn up the idle a tad once the bike is warmed up. On my 750 the idle adjust is a large thumb screw between carbs 2 & 3 and is easy to reach from underneath.


.......................................................
Woodstock ON Canada
1980 kz750E1, 4->1, K&N pods

Thanks All for the help.
Ill try messing with the idle speed, and some carb cleaner.

My 95 kzp ran basically like that, hard to control the idle with choke on and basically ran rough even when warmed up. I was having to adjust the idle constantly as it would idle faster and faster the more I rode it. Once WiredGeorge balanced the carbs and we got the air mixture adjusted, it has changed everything.
Now when I start it, I put it on full choke on COLD mornings or half choke on warmer days. It starts right up and idles up to about 1700 rpms. Then when it starts to drop in rpms, I take off the choke and it idles smooth as a sewing machine. You can count the pistons firing and it never needs idle adjusted, no matter how far I ride. WG is the MAN! He knows carbs!

I would guess that the carbs need cleaning and balanced and adjusted.