TOPIC:

Anyone know the correct float bowl height for the stock 34mm CV carbs from an '82 KZ750-N1 Spectre 4-cylinder shaft drive?

I don't have a service manual and can't find service specs on the net as of yet.

I've searched this site, maybe I'm not searching right but I can't find it.

I'll also do the fuel level check afterwards with a clear tube to make sure it's at 2.5mm - 4.5mm below the carb body/float bowl seam; but I want the correct float height measurement first as I reassemble the carbs, to get it close.

The carbs are sitting here on my desk, waiting for this one bit of info

Thanks.

From the FSM I have access to it indicates the design level for the float is 36.5 +/- 1 mm- this is measured from the center of the carb bore to the level of the fuel in the bowl. The service level is noted as 4.0 +/- 1 mm this is measured via the clear tube method.

Having said all of that there is a note in the FSM that indicates " if is impractical to measure the actual design fuel level. Service fuel level is defined as the vertical distance from the bottom edge of the carburetor body to the surface of the fuel in the float chamber. Measuring the service fuel level is an indirect method of inspection for correct design fuel level".

Thanks for looking. That FSM seems sort of hard to understand....

I'll keep looking and asking around. Some other boards have referenced an FSM that says 21mm is the correct float height and 4mm under the seam is the fuel level using a clear tube. I checked my floats and each of them were already pretty close to that measurement.

That's the tough part about this bike: I don't know if I want to keep it, so I don't want to put any money into it until I'm confident I can get it to run well. I plan to NOT buy parts to make it run well or (for example, a factory service manual) things to learn how to tune it. If I get it running ok and like how it feels, then I'll be willing to put some money in it, including a manual.

Still trying to sort this one out, but here's what I've found so far.

36.5mm measured downwards from the center of the carb bore (approximated using the underside of the choke butterfly... I know, it's not exact, but it's within a couple of millimeters...) comes to about 7mm below the bottom of the carb body's mating surface to the float cover.

21mm float height results in about that same 7mm (+/- 1mm) fuel level as measured with a clear tube.

The design fuel level of 4mm doesn't seem practicable to reach; at least not with the float bowls in my carbs. The absolute highest I can adjust the floats to is about 18.5mm, then they bottom out ("top"-out) against the carb body.

So something's out of whack. If the manual said that the "service fuel level" were 7mm (vice 4mm) below the mating surface, then all would make sense, and 21mm would be the correct float height because it causes the fuel to be at 7mm which is about 36.5mm from the center of the carb bore.

I'll try that tomorrow. I'll set them to 21mm (7mm fuel level) and reinstall on the bike and see what it does. If no good, I'll set them to their highest practical height, about 19mm, and see if that's any better.

Sure didn't think it was going to be this difficult to set the float height on these carbs. That job usually takes me about a minute per carb: Set, measure; bend the tab a bit, re-measure; bend once more, re-measure, good to go, on to the next carb. This has taken days, and days.... and it still doesn't seem right.

Something is definitely out of whack and please dont take this the wrong way but is it possible you have the floats upside down ? I have no problem setting the floats so that the fuel level is 4mm below the seam. I have Keihin CV34 carbs on my 1980 750/4 E1. If you end up @ 7mm below I think it would be way lean.




I found the attached pic from a couple of years back which shows one of my floats at 25mm height and I know I had to adjust most floats a few mm either way to arrive at a 4mm fuel level. I measured the fuel level first by holding the same ruler alongside the clear tube. I noted the measurement. I opened the carbs, measured each float height and did the math required to end up with the correct level and bent tangs accordingly. The pic should give idea of ballpark

Don't worry, i won't take it the wrong way. Never worked on these carbs before (though I've worked on many others) so I'll try flipping the floats the other side up in the morning. If that fixes it, I won't be embarrassed -- I'll be thankful that you suggested it. I don't get embarrassed when I'm working on a machine I've never touched before. I'm no mechanic, never pretended to be. I just figured things out like most dads while trying to get my kids' old motorsports toys to work. I've learned by trial and error (mostly error), and by asking, searching the net, and just plain looking at the parts to figure them out.

The way this thing is frustrating me I'm tempted to flip the whole dang bike upside down if tomorrow's try doesn't work. I bought it because it was cheap and had papers, and I'm on a tight budget. I'm reaching the point where I'm getting ready to throw my hands in the air and put it back on Craigslist where I found it.

Based on your photo, it doesn't look like I've put the floats in upside down. Here are two photos. First one is the floats the way i originally had them in; where I can't get them much past 21mm.

the second photo is of when I flipped one upside down after reading your reply; i put a text box in that photo.

Just thought of a couple things.

I never had to think about changing fuel level in a carb. i always just set the float height to factory spec and that was fine & dandy.

Maybe I have the whole fuel level adjustment picture backwards in my mind.

I can easily set these carbs' floats to 21mm. But that's with the carb upside down on my workbench. Setting the float to a larger number, to my brain, says that when you flop the carb back to right side up, the higher float height number will actually cause the float to sit lower in the bowl, DEcreasing the fuel level in the bowl.

In that mindset, I'm thinking I need to set the float "height" to 16mm to reach the required fuel level of 4mm. Maybe my thinking is upside down instead of the float being upside down....

I'll go set them to 25mm and recheck the fuel level. Maybe it's that simple.

Nope, didn't work. I just set all four floats to 25mm and rechecked the fuel level.

25mm float height = 10mm fuel level

Worse, not better, assuming 4mm is the correct fuel level.

I'll go back and set the floats to 18mm again (about 7mm fuel level) and will install the carbs and give the bike a test start. The four carbs were all at about that float height when I got the bike from the previous owner.

Yep
Looks like you have the floats in correctly. And yes you have to think up side down

In my picture above where the ruler shows 25mm... if that equated to a fuel level of 10mm below the seam of bowl and body then I would say 10-4 = 6 and I would bend the tang until the ruler showed 19mm for the float. When reassembling the carbs then the fuel level should be 4mm ..... ie 6mm higher than it was.
good luck

Setting the fuel level to about 4 - 5mm helped a lot. The bike starts right up now and will actually hold an idle.

It still runs rough, but no more of that automatic surging or rpm increasing. I can rev it up and it'll hold the rpm's that i twist to; and when I let off the throttle it goes back to idle right away like it's supposed to.

Also should mention that I removed and thoroughly cleaned the air mixture screws -- three of them were pretty dirty, glad you folks talked me into that.

I bought a 4-gauge vacuum synch kit off Craigslist for $60 and I'll try my hand at that dark art of carburetor synchronization next. But first had to set the bike aside to work on my son's dirt bike -- school's out now and he's itching to ride. Almost done with the dirtbike, so I'll get back to the Spectre soon.

Thanks for the advice and help. And thanks for the photos -- they ALWAYS help me -- guess I'm a "visual" type.

-Rob

The carbs at hand may not have an overflow circuit. But if they do have an overflow circuit, the clear tube test requires fuel from the "drain" function and not from the "overflow" function (which would keep showing a higher fuel level).

When the fuel level is already pretty close to spec, it's best imo to proceed with the tang adjustments, and not mess with trying to achieve some upside-down measurement of float height.

The carb doesn't care about the "measured" float height.
What the carb does care about is the fuel level.

Whatever float height happens to produce the correct fuel level is the "correct" float height, regardless of what the upside-down measurement happens to be.

Good Fortune!