Cold start issues

Hey all. 1981 KZ750-4 with CV carbs. Bike runs and idles great once its started, but starting is an issue. It does almost nothing when using the choke, no popping, like it wants to start. It will only start with a shot (or two or three) of starter fluid.

• app four flaps are closing when I choke it
• cleaned the carbs along with the idle circuit. It is running pods, however, once started and warmed up it idles fine with no hesitation; so I say that to say, I think the idle circuit is free and clear
• adjusted the air/idle screws on top. Turned this out only 1/2 turn and it made no difference

Any ideas/suggestions? I was thinking of opening up the idle jets a bit (I have a set of micro drill bits). Do you think that would help? I am dubious about this due to the fact that it idles fine once its warm.

If it starts with starter fluid that suggests the fuel air mixture is no rich enough when choke is applied. 
Only three things that can affect that are the seals/o-rings on each choke plunger are not stopping air being sucked in through the top of the plungers so not getting sufficient vacuum to suck fuel up the choke pickups. 
There is a blockage in the choke pick up and or bowl feed tube to the choke pick up. 
Or the vacuum created by the pistons falling is insufficient to suck suffient fuel via the choke and pilot Circuits. This would be very evident by a compression test.  Once the engine is running the heat generated will expand the piston rings increasing compression / vacuum. 

My first checks would be the choke pick up tubes and bowl feed tubes as they are easiest to check and most likely as it runs okay once started. 

Two things, first, the choke flaps don't look to the the best fit. There are some gaps when I engage the choke. I'll try and loosen the two screws holding each of them on and if there is any wiggle room to get a better seal.

Second, where is the choke pick-up tube exactly?

Do you have vm or CV carbs.
I assumed you have vm. 

lumbee wrote:

Two things, first, the choke flaps don't look to the the best fit. There are some gaps when I engage the choke. I'll try and loosen the two screws holding each of them on and if there is any wiggle room to get a better seal.

Second, where is the choke pick-up tube exactly?

You don't want the choke flaps to "seal" but simply to cause a restriction (the motor still needs to be able to pull air or it won't start)

Oh, sorry, I fat fingered that on the initial post. CV carbs, NOT VM.

The choke flaps should fit snugly in the carb opening.  There is a small hole through the flap to allow air in.

Also, when the choke is engaged, the linkage opens the throttle butterflies a small amount.  This creates the fast idle feature under choke.  It should not be necessary to crack the throttle at all when starting.  Hit the starter, but don't open the throttle.

Hit the starter, but don't open the throttle.

Yes, that is how I have been [attempting] to start it.

I will take a closer look at the choke and see if it's cracking the throttle when its engaged.

Since new, our '93 CB750 Nighthawks were reluctant to start if they sat unused more than 4 - 6 wks., and very reluctant to start if the carbs were drained.  We discovered decades ago that they start immediately if the carbs are primed by applying vacuum to the vacuum port of the petcocks for a half-minute or so, to fill the float bowls.  There is no 'prime' position on these petcocks like on some vintage petcocks.  Once running, they start cold very easily for many days afterward.  They're jetted lean from the factory, so it doesn't take much of a drop of fuel level in the float bowls to make cold starts cantankerous.

Looks like your KZ750 has a vacuum petcock.  If it doesn't have a 'prime' position, consider trying this:
With the petcock in the 'RUN' or 'RES' position, apply vacuum to the petcock's vacuum port for 15 ~ 30 seconds to make sure the float bowls are full.  Doesn't take much  - 3" ~ 5" Hg should be plenty if you have a vacuum pump.  You may be able to accomplish the same thing by temporarily removing the vacuum hose from the petcock's vacuum port, sliding a clean piece of vacuum hose on to it, and applying vacuum by sucking on the free end, like trying to draw thick milkshake up a straw.  Make sure the vacuum supply hose is reattached before starting.  Agreed it sounds sounds weird, but works for us.

Good Ridin'
slmjim & Z1BEBE

@simjim, the petcock does not have a vacuum port, its just a standard gravity feed.

I pulled the carbs this w/end and just slightly oped up the pilot jets with some mini drill bits (maybe 001 or 002") and also blew carb cleaner and air through the rear holes (see image below). I was mostly just spitballing as I'm at a bit of a loss at this point. I reinstalled the carbs and a couple of rotations of the starter yielded nothing but starter wining. Again, just spit-balling I placed my hand over a couple of the carb rear openings (pods removed) and with a couple of rotations of the started I got a couple of pops. I tried this again, and it rumbled to life. Not sure that that tells me much other than a lean condition, but I think we already know that so :shrug

 

Randomly spraying chemicals and high pressure air into places without knowing where it goes can cause more damage. I hope you have not destroyed the slide diaphragms. They are fragile, and may not be replaceable.

Just wanted to button this thread up. I ended up drilling out the slow jets from .014" (35 jet) to 0.024" (#74 drill bit) and the bit starts pretty easily now.