KZ650-F model carbs on a B1: problems

As I buggered the original carbs while cleaning with lemon juice (the acid eats away at the metal and screwed up a few threads), I want to use later model carbs. However, this has lead to a few problems:
1 - the bike will start and idle fine, but only without choke. Adjusting the pilot screws did not help, balancing the carbs did not help. Main jet is 102.5, pilot jet is 15, all standard. Needle let has clip on groove no.4, which is standard for an F-model. The B as a needle jet with a different part no., but looks exactly the same, though it is supposed to have the clip in groove no.3.
Checked the choke circuits, and they seem fine.
2 - the bike pulls strongly, though it kinda lurges at 3k revs. Went for a ride to see if it would clear up, but after about fifteen minutes I was standing at a stoplight when I noticed a puddle under the bike. This turned out to be fuel: no.1 carb (right hand side) was overflowing. Got on the sidewalk, tapped the body to free the float, tapped carbs no.3: now both were overflowing. Saw another puddle: fuel was leaking from the airbox as well.
Since it was raining by now, I restarted it for the mile drive home. Within a hundred yards - right under an overpass with nowhere to flee - the bike gave a few loud bangs and died. Walked it home. So for question no.2: could this be just because of float levels being off, or are there other possible causes? Should I for instance use the 3rd groove for the needle jet clip instead of the 4th? I am at a loss...

Zthou1977 wrote:

2 - the bike pulls strongly, though it kinda lurges at 3k revs. Should I for instance use the 3rd groove for the needle jet clip instead of the 4th?

This may cure the lurgeing (is the clip 4th from the top or bottom?), and the fuel overflowing has got to be the floats.

Did you try to clean-up those carbs before putting them on? Are they the same mm/type, as the ones they replaced?

Sorry to hear about your bad luck... sounds like Murphy's law to me!

Bad Dad wrote:

Zthou1977 wrote:

2 - the bike pulls strongly, though it kinda lurges at 3k revs. Should I for instance use the 3rd groove for the needle jet clip instead of the 4th?

This may cure the lurgeing (is the clip 4th from the top or bottom?), and the fuel overflowing has got to be the floats.

Thanks, I'll check the floats first, because I guess the trouble at 3k could also be because of the fuel level. Given the fact that while setting the pilot screws and the balancing of the carbs it did not overflow, only after about 15 minutes of riding, I thought something else might be contributing.

Did you try to clean-up those carbs before putting them on? Are they the same mm/type, as the ones they replaced?

The are both Mikuni VM24SS, but the later model has the pilot screw on top instead of bottom.

Sorry to hear about your bad luck... sounds like Murphy's law to me!

Oh, it definitely is. First the electrics, then the points, then the coils, a leaky tank, leaking petcock, it never seems to end...but I am learning a lot and getting there, albeit rather slowly

What exhaust and air filters/air box are you using? Your jetting is spot on for factory exhaust and air box. If your needles have five notches, put the clip in the middle notch. Check your float levels and also I've heard stories of bikes sitting on the side stand and the floats shifting sideways and hanging against the carb bodies rather than floating freely.

Mellvis wrote:

What exhaust and air filters/air box are you using? Your jetting is spot on for factory exhaust and air box.

Hi Melvis, I am using the standard airbox and filter, but a 4 into 1 Marshall Deeptone exhaust. These are supposed not to require rejetting. Nevertheless I called the manufacturer (Laser, here in the Netherlands) to make sure.

If your needles have five notches, put the clip in the middle notch.

Are you sure? Last night I started thinking about it (while dismantling & cleaning the carbs), and I came to the conclusion that the requirements of the engine as far as air/fuel ratio is concerned did not change. However, the carbs did: they went from pilot screw to air screw. So my reasoning was that it was likely that the change in carb design necessitated the change from clip 3 to 4 (2nd from the bottom). Then again, I sometimes think like a corkscrew aka "it seemed the logical thing to do"

Check your float levels and also I've heard stories of bikes sitting on the side stand and the floats shifting sideways and hanging against the carb bodies rather than floating freely.

Checking the float levels today. A quick glance at the physical height (eyeballing it with the bowls off) showed maybe a 1-2 mm difference between them. As fuel was flowing to the airbox as well, but only through the left two carbs, it doesn't seem to be enough of a difference. But we'll see when I do a proper check.

Not wanting to start on choke seems to have been because the slides were way up, as in about 4-5 mm. That might explain the lurching at 3k rpm: I must have used the airscrews to get the idle down to 1100 rpm, so they were way off for the rest of the rpm range.

Not wanting to start on choke seems to have been because the slides were way up, as in about 4-5 mm. That might explain the lurching at 3k rpm: I must have used the airscrews to get the idle down to 1100 rpm, so they were way off for the rest of the rpm range.

Yea, your screwed with that much gap. I don't have gauges but used this method with good results. Just do it slowely and carefully and it should make a huge difference in how bike runs.

This MUST be done first...



Then do this carefully...



Try it and let us know how it turns out!

Contains VM Tuning Manual...

kzrider.com/index.php?option=com_docman&...d&gid=461&Itemid=108

This afternoon I checked and adjusted the float heights, and installed new seats and float needles. After checking once again that they were clean and all passages were free, I did a mechanical sync of the carbs like OMR suggested. Problem is however, that there is a small square cutout of about 0.3-0.4mm right in the middle of the engine side of the slides. My factory manual does not show it, nor does it mention it.
So I put a piece of wire of 0.7mm in this cutout, and did a mechanical sync.
As it needed to be done anyway, and I was worried about gas in the oil, I changed the oil as well.

Results:
The bike now starts on choke like it is supposed to, but will run rough and slowly die. Adjusting the idle with the idle screw does not help: I can get a stable idle at 2k rpm, but when I back off to 1k, the engine stops. Twisting the throttle will raise the revs, but they fall back slowly, and will sometimes stay stuck at 2 or 3k rpm.
Using the throttle and idle screw to stay at about 1k rpm, I can get nowhere near the 22-24 cm Hg that I am supposed to have on the carb balancer.
So: unstable idle, rough running, revs will get stuck, Hg-level insufficient. My guess at this point is that there is a vacuum leak. But spraying with WD40 during running had no effect. There are cracks, but they seem to be surface cracks.
A few days ago, when the carbs flooded, they were very difficult to sync and the idle started to wander a bit after some ten minutes. But the engine ran very sweet, while now it's bucking and lurching, without any power.
Tomorrow, I'll put on some spare carb holders that are in very good shape. If that doesn't help, I am out of ideas.

Any ideas anyone?

Edit: the advancer is in good shape, with good springs.

My thoughts:

With a 4 into 1 pipe, run #110 main jets, #15 pilot jets, set the needles one notch from the bottom, back the air screws out 1.5 turns from their seats. Make sure you don't have a vacuum leak at the fuel valve or the hose going to it from the carbs. Check both diaphragms in the accelerator pump on the bottom of the number 2 carb (if the carbs are 1979 or newer). Lube your spark advancer, check for leaks at the carb manifolds, check the vacuum hoses going to the switch for the exhaust air injection system. The switch is over the cam cover with two large hoses going to the cover over the exhaust cam. Not all bikes have this system, but as far as I know all of the USA models after 1978 had it.

I hope I haven't bombarded you too much! Let us know if any of this stuff makes a difference.

Zthou1977 wrote:

Problem is however, that there is a small square cutout of about 0.3-0.4mm right in the middle of the engine side of the slides. My factory manual does not show it, nor does it mention it.
So I put a piece of wire of 0.7mm in this cutout, and did a mechanical sync.

Finally found it, here is what you need. (click pic to enlarge)

Bluemeanie wrote:

Zthou1977 wrote:

Problem is however, that there is a small square cutout of about 0.3-0.4mm right in the middle of the engine side of the slides. My factory manual does not show it, nor does it mention it.
So I put a piece of wire of 0.7mm in this cutout, and did a mechanical sync.

Finally found it, here is what you need. (click pic to enlarge)

Thanks! Tonight I'll do a new bench-sync.

I don't believe the bench synch is absolutely critical, since there's a good chance you'll change them afterwards when you synchronize the carbs w the carb synchronizer tool: you just need to get them close.

A handy way to do the bench sync is to use drill bits (credit to MurphyAU):
(1) Remove the throttle pulley spring temporarily so that the slides don't get buggered, particularly the notches.

(2) Loosen the locknuts & eyeball all four slides real quick-like so that the gap is about the same, then quickly set one using the wire gauge - it doesn't have to be perfect - tighten the locknut - you're done adjusting that one which is now the reference carb.

(3) Raise the slides in unison w the throttle pulley and place & keep one drill bit in the reference carb, then lower the slides in unison until the reference slide touches the drill bit.

(4) Next, use a second identical drill bit to adjust the second carb so that the slide touches the bit - it should be the same as the reference carb w both slides resting on the drill bits. Tighten the second locknut - you're done w the second carb.

(5) Do the other two carbs the same way - one drill bit stays in the reference carb, while the second drill bit is used to set the others.

(6) Remove the drill bits, then lower the slides. All four should be about the same & the gap should be whatever size gauge wire you used to set the reference carb. Reinstall the throttle pulley spring.

(7) Please note that this method does not set the gap to the width of the bits - you can use any size drill bits. Also, you could use 4 identical drill bits. And beforehand, make sure there's plenty of adjustment available in the idle adjusting screw in either direction.