Cylinders 2 and 3 running rich

So I finally got the bike together and took it for a 1-2 mile test ride. Got her home, let the engine cool and pulled the plugs. Cylinders 2 and 3 are very rich as they are black and sooty. They were not wet. They were dry. Plugs were clean before the ride. Cylinders 1 and 4 look good. Floats are set correctly. Carbs were pulled (26's with fuel s screw behind the bowls) off during the build and the only thing I touched other than cleaning the bowls and jets, were the fuel mixture screws behind the bowls. They were all over the place so I set them at 2 turns out. Should I turn the screws in a turn and see what they look like after a ride? Please help as I am stumped.

Which bike? Ed

Assuming VM type carbs, it will help to understand if that particular model has the fuel screw on the bottom of the carbs, and/or the air screw on the side. The fuel screw typically should be set for 1 turn open, and the air screw at about 1.5-1.75. More specific info is needed before making too many recommendations though.

Your manual should tell you this. The two plugs suggest a weak coil or bad connection.
Steve

I just went through this with a 550 where the middle two cylinders looked rich the outer looked fine. Next time you have it running for a few seconds or a minute shut it down and spray some water on the exhaust pipes. Some folks use an ice cube. I have a non contact thermometer but find the sizzle test works just as well. If your middle two don't sizzle then open the drain screws on those carbs to make sure you have fuel in them. That is exactly what I found. Middle two had no fuel in them at all but the plugs looked dark and carbon but not wet. I pulled the carbs and found that since the fuel inlet is in the middle of the rack, between 2 and 3, that those two had the float valves clogged with gunk.

Not saying that is what your problem is but I point it out to let you know that just reading the plugs is not always an accurate test. Always do more than one test before making a decision on a course of action. Next thing I did was run a compression test and found all 4 with good compression and all very close to 150 PSI. Then I checked for strong spark. Only then did I try to drain the bowls to measure the qty in them.

1976 kz900 Ltd
Stock air box with k&n filter. Air intake enlarged
Vance and Hines 4-1 with baffle
Air screw on bottom of carbs behind bowls
Dyna 3 ohm coils
Dyna electronic ignition

Swest, what connection do you think is bad?Coils are good.

Compression is good, carbs are clean

Fuel is getting to the cylinders as all pipes are hot.

Dirty connection going to the coil, bad connections in the HT leads, dirty connection coming from the pick up.
Steve

You might want to try to pull the primary connections off of the coils and clean them then see if that helps. How does it seem to run? Pull strong and smooth all the way to redline? The reason I ask is my sons KZ1000 developed a high speed miss and I fixed it by swapping out the coils with a known good set I pulled off of a CB900.

Just for a sanity check make sure all of the boots are on the carbs good and the insulator bands are tight. I'm really reaching for straws here.

This is a tough one .. I wish you good luck.

I'm tracking with JDVORCHAK and SWEST, meaning how do you know you have solid and consistent spark? Do you have one the laser temperature readers? How do #2 and #3 look for temp compared to #1 and #4 at idle and when you come in hot after driving? If they are cold (relatively)because you aren't getting consistent firing, then you can work backwards to find out why, per their comments. I haven't seen you directly address the suggestions they made.

JDRAKE4200 wrote: So I finally got the bike together and took it for a 1-2 mile test ride. Got her home, let the engine cool and pulled the plugs. Cylinders 2 and 3 are very rich as they are black and sooty. They were not wet. They were dry. Plugs were clean before the ride. Cylinders 1 and 4 look good. Floats are set correctly. Carbs were pulled (26's with fuel s screw behind the bowls) off during the build and the only thing I touched other than cleaning the bowls and jets, were the fuel mixture screws behind the bowls. They were all over the place so I set them at 2 turns out. Should I turn the screws in a turn and see what they look like after a ride? Please help as I am stumped.

If it's a side-located pilot air screw, turning it in clockwise reduces air which further enrichens the mixture.



Two turns out is probably far enough out when carbs are in perfect condition.

Would use brand new B8ES plugs and visibly inspect the spark quality on removed plugs while cranking the engine.
Compare with spark quality produced by the other plugs.

Fat blue sparks are evidence of a strong ignition system.

If the plugs produce fat blue sparks, but are getting dry-sooty anyhow, would then assure that valve clearances are within specs on both of the "too rich" cylinders. Compression test alone is insufficient, because too tight valve clearances may not cause loss of compression until the engine attains normal operating temperatures.

If spark quality and valve clearances and compression are all acceptable, would then address possible carburetion issues.

Floats are set correctly -- is this evidenced by clear tube testing?

Good Fortune!