KZ750B2: One Cylinder Rich, the other lean

First off, I have had a lot of success with my troubles thus far from this board, so I'd like to take a quick moment to thank everyone for their help. Since setting the timing, replacing the ignition coil, point, condensor, battery, I'm now running and riding and discovering new issues.

I've figured out my 1982 KZ750 twin is actually a 1977B2 (or at least the engine is). However, the left cylinder is running very rich and the right cylinder is running lean. I have pod filters and the stock exhaust was cutoff just before the mufflers, but cross over is still in place. I have no reason to believe these are not BS38 carbs.

If I leave the fuel line disconnect open from the tank to the carbs, I will overflow gas out of the left cylinder. Is this normal? I opened the float bowls and they actually look really clean. Anything to consider here?

When starting cold, I do not need any choke. Usually one kick or a few pulses of the starter will get it fired right up. After a few minutes, the left is still pretty cool, but the right is warm.

When idling, there is some gas coming out of the left exhaust pipe. Once warm I am experiencing an occasional afterfire that sends a flame out of the exhaust. Opening the throttle and riding at speed it is ok, but slowing down to stop and it pops.

With the pods I know I need larger jets. Any suggestion on size or where I can get them?

sounds like you have abad float valve assembly on the left side

If determined not to be a leaking float valve (a likely suspect), might possibly be a leaky or sticking or mal-adjusted float, whereby float action is failing to close the float valve (i.e., failing to push the float needle into the float needle seat at the prescribed fuel level).

Good Fortune!

I shouldn't even admit this... but I actually had the "choke" on the whole time. That partly explains the rich mixture and easy starting in the left, but I will investigate the float valve as well. Engaging the enriching circuit wouldn't cause the overflow, would it?

The right is going to be explored, I'm HOPING to find a plugged jet or passage... I needed new guitar strings anyways.

I ordered larger jets today for the BS38 from www.islandracing.net/mikuni.html I was told main in the BS38 is an N100.606 so I got a few different sizes to try out (135,140,145). KZ750 twin forum suggested 140 main 50 pilot, but others recommended only swapping out main and adjusting pilot screw. Anyone else have suggestions?

The carbs are off and the floats removed, I will begin cleaning them as soon as possible. I can't get back on the road until I get the rear caliper rebuilt, so it's better to get this done now anyways. Oh, and it's only 31F outside right now(comfortably warm in my garage at least)... can you tell I'm a little excited to get this going?

jimmyfinski wrote:

...but I actually had the "choke" on the whole time....

D'oh!

...That partly explains the rich mixture and easy starting in the left, but I will investigate the float valve as well. Engaging the enriching circuit wouldn't cause the overflow, would it?

Nope. Like the others, I suspect something amiss with the float valve. Could be sticking on the axle, could be debris or clogged old fuel crusties, etc., etc., etc... basically anything that could be keeping the float needle from seating int he seat.

...The right is going to be explored, I'm HOPING to find a plugged jet or passage... I needed new guitar strings anyways.

Be gentle... don't scrape up the carb with the string tips...

...I ordered larger jets today for the BS38 from www.islandracing.net/mikuni.html I was told main in the BS38 is an N100.606 so I got a few different sizes to try out (135,140,145).

That's a cool tip... I just recently got some NOS 130's off ebay myself, but I only run a modified exhaust with stock airbox, and I've yet to get the chance to run it this season. Let us know how those jets work out; this could be an invaluable reference for us twin owners!

...KZ750 twin forum suggested 140 main 50 pilot, but others recommended only swapping out main and adjusting pilot screw. Anyone else have suggestions?

Hmm... I wonder if I'm one of those that made that suggestion... But anyway, my simple philosophy is this: bump up one or so on the mains for the pods, one or so for the exhaust, and tune to taste, as a starting point. So yeah, I'd go right to a 135, and *maybe* consider going up 1mm on the float level, or an extra 1/4 turn out on the pilot screws, if things are drastic, a clip change on the jet needle. Somewhere there's a really good methodical document out on the 'Net for finding harmony with these adjustments... To be scientific about it all, you need to tune it in on a Dyno, but of course nobody has a Dyno in the garage, so you might end up doing alot of "plug chops" and reading the plugs to get it there...

Addition for future reference.
Yes BS38 carb choke is "backwards", off down, on up.

If its a 77 750B motor then it probably has brass floats. If you use Ethanol in your fuel it will probably phase separate in the float bowls if it gets cold then warm then cold again. This causes the bowls to fill with water. then it freezes. These brass floats then implode on themselves because of the pressure. you can see it fairly easily. But they don't always just implode. Sometimes they just get leaks. Then when the fuel level gets to high the float doesn't float and shut of the fuel flow. Just set them in a bucket of water overnight so they are under the water. The next day shake them. You'll hear water in them if they are leaking. Either of these things could cause the problems that your having even if you have the float level set properly. I would of course check the float level first.

Oh, and if you replaced the float seats, did you replace the little waher under it?? If you reuse the old one it can leak. Just a thought.

You guys are good. In the "rich" cylinder the float had liquid in it. In a bucket of water, you could see one side dropping and the other side floating besides being able to hear it inside. I completely submerged the float and no air bubbles form though. It obviously got in there some how, but doesn't seem to be doing so quickly. I drilled a small hole in the top and drained it, then soldered the hole and the seams as a temporary fix. The float valve needle tip was clean and shiny and moves easily. It seals nicely with just a tad of pressure so long as the float is functioning.

The "lean" cylinder float was fine, but the main jet was almost entirely plugged. I got the float bowls and jets completely cleaned out in both carbs.

Interesting things I'm finding... the overflow tube of the left, rich cylinder had been tied off. This was causing fuel to overflow out of the inlet from the two small holes toward the bottom of the opening. Should gas be able to come out this way? Carb rebuild kits should be in tomorrow anyways. Also on the "rich" cylinder: there is a screw plugging what appears to be a vacuum tube from the carb. This is not the case in the lean cylinder. What is this tube? Should the "lean" cylinder have this tube capped as well?

Jet needles are both set to factory "4" level and diaphragms are good.

Am I correct in saying that fuel should never be overflowing if the float is set right?

There is no breather tube filter like the picture above shows. Do I need one?

What did I get myself into....

You are correct, the hoses should not overflow if the float valve is seating correctly. They also serve as the "vent" for the float bowls as the fuel level rises and falls as the fuel is being used.
They also should not be plugged as this can create a vacuum in the float bowl affecting fuel flow to the engine.

It would appear that the problem of the "rich" cylinder is the result of a previous owners band-aid fix (plug the dripping overflow tube)of a symptom of the real problem, no fuel flow shut off due to the leaking float.

The vent in the above picture is for the crank case breather (round can under carbs with the nipple on the top)that allows air to blow out (and be sucked in)as the pistons in the cylinders move up and down. Blowing out oil vapor only makes a mess of the outside of your engine but sucking in dirt and grit over time will make a mess of the inside of your engine and trany.

It wont hurt to run the engine without it while you are getting the carbs sorted out. Once you do, (and you are well on your way)it would be better to have one than not.

What did you get yourself into... when you are riding around on YOUR bike that YOU fixed, YOU made better YOU won't be able to wipe the grin off YOUR face.

Good Luck.

Yeah, it's one of those things where I had it running, found a problem, and now it isn't running at all...

I "rebuilt the carbs" and had to adjust the float level a few times to prevent overflow in the "rich" cylinder. I also changed the spark plugs again, because the ones I bought a few months ago were for the 4 cylinder and were fouled. Not sure if it was the plugs fault or the rich fuel, but for $4, might as well start fresh.

Anyway, I'm still not no running. I get an occasional firing "choke" doesn't help at all, but I'm thinking I may just be low on the float level now. With all the adjusting to prevent an overflow, I may have over adjusted out of frustration.

Anyways, like I mentioned before, I have these 2 hoses coming off of each carburetor, one I know is the overflow, but I still don't know what the other is for. As I mentioned one of them was capped off and the other was not. Still, with or without these capped there is no help starting it up.

Electrical is showing a good spark in each plug, and timing is correct per the manual static settings (and it was running before). Good compression, clean oil...Only thing left is fuel, right?