Hot cams and smoothbores - how should it idle?

If the head had to be clearanced for the cams,you have some whopping cams in there. There will not be much vacum signal.

I would pull the cam cover and see what cams they were. There should be a hint to thier make on the ends of the cams. You could use a dial guage to measure them in the engine. Roy

Kapahulu wrote: Well Patton, I cleaned the pilot circuits per the pics above and saw a steady flow of cleaner coming out of #6 for each carb. Also re-sprayed cleaner into the air jet inlet.

Result - the same, doesn't idle well.

Next I guess I'll check the ignition timing and hookup the carb sync. And eventually I'll get around to checking the valve clearances.

BTW - when I had the carbs apart, I measured the float levels and they were 23mm from the gasket surface.

Would recommend the following:

Before further addressing carbs, would first assure valve clearances are within specs.
And that the timing advancer is properly functioning.
And that the F mark aligns with the case mark at low rpm (do a static check to confirm timing if having difficulty with dynamic testing).
And remove plugs to inspect for visible fat blue sparks (not wimpy orange sparks).

Would bear in mind that measured float height is only a trial effort toward achieving the correct fuel level, as fuel level -- not float height -- is the crucial setting.
Essentially, correct float height is whatever height it happens to be when the correct fuel is achieved.

The clear tube test is used to measure the fuel height.

It is possible that the carbs might require new pilot jets of the same or a different size.

Basic smoothbore carb settings and jet sizes are suggested at Z1E under the carb info as follows:.
29mm smoothbores Z1 / KZ900 / KZ1000
Main Jet 120
Air Jet 0.9
Needle Jet O-6
Jet Needle 5DL31-3
Throttle valve (slide) 1.5mm cutaway
Pilot Jet 17.5 or 20.0 (or 25 in the later versions)
Air Screw setting 1 1/2 turns out

Even perfect carbs cannot compensate for deficiencies in the other components, and that's the main reason carbs should be addressed last in the tuning process.

The stuck-in-position air jets may yet need to be removed for proper cleaning and/or replacement.

Good Fortune!

Fab up the needed drain connection to enable the clear tube test.

I use a conical shaped rubber grommet with the clear tube pressed snugly inside, and just hold the grommet firmly in position during the test.

Others use different methods, such an appropriate size outer hose that fits snugly into the carb drain hole.

The more fancy rig is a barb fitted into a modified drain plug with the clear tube attached to the barb.

A slight leakage at the drain hole fitting won't invalidate the clear tube test result. Because incoming fuel enters the bowl faster than fuel lost by the slight leakage.



Good Fortune!

Have you read through this thread of Godfrey's?
www.kzrider.com/forum/3-carburetor/44799...kunis-wont-idle-down
Also the Net is full of associated Info, mostly V8 stuff though, but the principle is the same. Here is just one; www.hotrodgenius.com/html/carburetion1.html

Kapahulu wrote:

I picked up this KZ900 this past summer. The previous owner had it for about 15 years and it looked like a mess. I've been cleaning it up haven't opened up the motor though.

According to the previous owner it has an 1105cc big bore, hot cams and 29mm smoothbores. I can confirm it has the smoothbores.

Well I rode it once before buying it, the bike was much faster than my stock KZ and it didn't like to idle much. It would either idle high or barely idle at all. Now that I've gone through the carbs and rejetted them, then bench synced, the problem is still there. When it's warm it will idle around 2,500 rpm or if I move the idle screw down it will idle at 1,000 for awhile and then die if I don't blip the throttle.

I'm kind of wondering if this is what to expect with these cams? I don't have the specs but the guy said the head and cam cover had to be clearanced for them to fit, so I think they're lumpy. And I've also read that smoothbore idle circuits aren't as refined as 26 and 28mm Mikuni's.

Can I get opinions from other owners who have built motors and smoothbores?

Still some work to do, maybe one or more of these tasks will help:

Sync the carbs

Check the ignition timing

Check valve clearances

Thanks, Mike

Mike,

In addition to the basic things already mentioned, a badly stretched cam chain can cause problems like that by making the valve timing late.

Generally, the problem you describe sounds like carb / ignition. Fuel level is everything. That should be verified when you go through the carbs.

A whipped cam chain is easily discovered by turning the motor with the valve cover off. Look for slack showing up at the top idler sprocket. I've seen chains so whipped they would actually drop down away from the sprocket teeth. That usually occurs in sections of the chain.

If you get totally shut out after having don the carb clean and normal tune up procedure, get a picture of the cam lobes on cylinder #4, with that piston at TDC, compression stroke, looking right across the edge of the valve cover surface and post it.

What you're looking for is the relative position of the lobes to the head's valve cover surface. That will tell a great deal about the valve timing. Would be best to identify the cams too.


BTW...the air jet in Patten's little picture [BS3097] has absolutely nothing to do with idle quality at all. That air jet feeds the needle jet for emulsification of the fuel. At idle, you're not using it at all.

What are the air screw settings? I believe there are 2 types of slide cutouts for the 29's. I've seen some 29's idle well with air screws turned out 1/4 turn. Then others idle well with the air screws turned out 1 1/2 turns. I would play around with the air screws to see if one of these 2 settings will smooth out the idle. Of course the rest of the tune up needs confirmed first - valve adjust, carb sync, plugs, points, timing, etc.

You're correct Gman. There were A7 and A9. The 9's were for Suzuki and had 2.0 slides. The 7's were for Kaw and had 1.5 slides. Personally, I think the 2.0 slides work better on a built KZ.

I looked at the carbs and they have 1.5 slides and there's an air screw on the side and a fuel screw on the bottom (maybe the same with all 29mm's?). I have the air screws at about half a turn out, have tried them from no turns to two turns out and no matter what it won't idle smoothly and consistently at 1,000 rpm. Don't recall where I set the fuel screws but they were taken out, cleaned and reinstalled with the same number of turns as when I took them apart.

I'll take a look at the cam chain soon. The bike has some great power, however I did expect it to be a bit faster with an 1105cc and cams. Maybe it's the cam chain.

Will get back to you on this in the near future. I have three KZ's in various stages of repair and of course family and work come before the bikes.

Kapahulu wrote: I looked at the carbs and they have 1.5 slides and there's an air screw on the side and a fuel screw on the bottom (maybe the same with all 29mm's?). I have the air screws at about half a turn out, have tried them from no turns to two turns out and no matter what it won't idle smoothly and consistently at 1,000 rpm. Don't recall where I set the fuel screws but they were taken out, cleaned and reinstalled with the same number of turns as when I took them apart....

air screw on the side and a fuel screw on the bottom -- the 29 mm smoothbores with which I'm familiar have only the side-located pilot air screw and don't have a bottom-located pilot mixture screw.



Good Fortune!

Kapahulu wrote: I looked at the carbs and they have 1.5 slides and there's an air screw on the side and a fuel screw on the bottom (maybe the same with all 29mm's?). I have the air screws at about half a turn out, have tried them from no turns to two turns out and no matter what it won't idle smoothly and consistently at 1,000 rpm. Don't recall where I set the fuel screws but they were taken out, cleaned and reinstalled with the same number of turns as when I took them apart.

I'll take a look at the cam chain soon. The bike has some great power, however I did expect it to be a bit faster with an 1105cc and cams. Maybe it's the cam chain.

Will get back to you on this in the near future. I have three KZ's in various stages of repair and of course family and work come before the bikes.

Exactly

They sound like a hybrid rack that was occasionally found on some 76 models. Not very common. Infinitely adjustable. Not 29 Mikuni Smoothbores like I've ever seen.

Float bowls on OEM's have a screw drain. 29's have a plug drain [17mm].

The odd thing is you say it had removable air jets like in Patten's picture. No OEM Mikuni had those. So it's not really clear exactly what you have there :S

You should post some photos of the carburetors so they can be accurately identified.

If you pull the valve cover and get some cam pics relative to the position I described previously along with some carb photos, it will certainly cut the typing down. Guessing gets nobody anywhere.

An earlier related thread:

www.kzrider.com/forum/3-carburetor/48034...-hybrid-carbs#480351




Good Fortune!

Patton wrote: An earlier related thread:

www.kzrider.com/forum/3-carburetor/48034...-hybrid-carbs#480351

Good Fortune!

Those are the odd OEM carbs I was referring to. You don't see them very often. You have quite a collection Patten