One cylinder way too rich at idle?

I have been trying to fix a problem with my bikes pilot circuit for the past few weeks and am out of ideas as to what could be wrong. I have cleaned the carbs, switched jets, needle positions, adjust float height via clear tube method, checked compression, synced the carbs via drill bit method, checked valve clearances, timing, voltage to the coils, and have narrowed the problem down to the #3 cylinder carburetor.

I am running VM 26 carbs, 120 mains, needle raised one notch, 15 pilots, accelerator pump disabled, stock airbox with a 4 into 1 header. I recently replaced all the pilot screw o-rings with new ones, as the old ones were tore up. With all the pilot screws adjusted out 1 1/2 turns, the bike runs poorly at low speeds, but mid and top end range are great. #3 cylinder exhaust is only about 250* while the rest of the cylinders are close to 600. If I adjust the pilot screw all the way IN on cylinder #3, it runs close to perfect. I can't figure out why it is running so rich compared to the other three cylinders. I don't want to take the carbs off again until I have an idea of what the problem could be. Please help! This is driving me nuts!

toadson wrote: I have been trying to fix a problem with my bikes pilot circuit for the past few weeks and am out of ideas as to what could be wrong. I have cleaned the carbs, switched jets, needle positions, adjust float height via clear tube method, checked compression, synced the carbs via drill bit method, checked valve clearances, timing, voltage to the coils, and have narrowed the problem down to the #3 cylinder carburetor.

I am running VM 26 carbs, 120 mains, needle raised one notch, 15 pilots, accelerator pump disabled, stock airbox with a 4 into 1 header. I recently replaced all the pilot screw o-rings with new ones, as the old ones were tore up. With all the pilot screws adjusted out 1 1/2 turns, the bike runs poorly at low speeds, but mid and top end range are great. #3 cylinder exhaust is only about 250* while the rest of the cylinders are close to 600. If I adjust the pilot screw all the way IN on cylinder #3, it runs close to perfect. I can't figure out why it is running so rich compared to the other three cylinders. I don't want to take the carbs off again until I have an idea of what the problem could be. Please help! This is driving me nuts!

Question, please --
Do the carbs at hand have side-located pilot air or bottom-located pilot mixture screws?




Good Fortune!

toadson wrote: I have been trying to fix a problem with my bikes pilot circuit for the past few weeks and am out of ideas as to what could be wrong. I have cleaned the carbs, switched jets, needle positions, adjust float height via clear tube method, checked compression, synced the carbs via drill bit method, checked valve clearances, timing, voltage to the coils, and have narrowed the problem down to the #3 cylinder carburetor.

I am running VM 26 carbs, 120 mains, needle raised one notch, 15 pilots, accelerator pump disabled, stock airbox with a 4 into 1 header. I recently replaced all the pilot screw o-rings with new ones, as the old ones were tore up. With all the pilot screws adjusted out 1 1/2 turns, the bike runs poorly at low speeds, but mid and top end range are great. #3 cylinder exhaust is only about 250* while the rest of the cylinders are close to 600. If I adjust the pilot screw all the way IN on cylinder #3, it runs close to perfect. I can't figure out why it is running so rich compared to the other three cylinders. I don't want to take the carbs off again until I have an idea of what the problem could be. Please help! This is driving me nuts!

when u cleaned ur carbs did u remove pilot screw and blast carb cleaner into the circuit making sure the passages themselves were clear ????

Are you running a vacuum petcock, and if so is the vacuum port for it on carb #3. Again if so check to see if the diaphragm in the petcock is leaking and fuel is being pulled through the vacuum line to # 3.

Hey Patton, they are the bottom screw type. They have an o-ring that seals them off and I made sure they all went it to place correctly and didn't tear when I had the carbs off.

Jonny, I did make sure those passages were clear. I would think if they were clogged, I would see no effect from turning the screw in and out, but I could be wrong.

gd4now, I do not have a vacuum style petcock, but that is a really good idea I wouldn't have thought about!

Ok you indicate you have disabled the accel pump, how have you done this? Did you block off the ability for fuel to be pulled up through the accel pump circuit on all the carbs? Is it possible that fuel is still be pulled via the accel pump circuit on carb #3?

Would assure when choke is OFF that the plunger on #3 carb is fully closed down.

The following illustration shows the choke ON with the plunger raised.

Would assure that the pad on bottom of the plunger isn't missing or damaged, whereby fuel from the enrichener circuit might contribute to an over-rich mixture




Good Fortune!

Hopefully the #1 pilot air jet is undamaged and that it's allowing sufficient air into the pilot circuit.

Hopefully also that the #2 pilot jet hasn't been over-zealously cleaned whereby it's been unintentionally reamed larger.

Good Fortune!

To disable the accelerator pump, I merely unhooked the lever for it. I don't think fuel could get into the carbs from it without the lever on the bottom of carb 2 being pumped with the throttle.

One thing I forgot to mention is that I removed all the choke plungers a few weeks ago. I never noticed anything wrong with them, but I wasn't looking too closely. I think the problem was happening before I removed them though, so I don't think removing them could have caused the problem.

Last night I actually did notice the #3 plunger wasnt closing all the way, while the rest of them did. After fixing the problem, I noticed no improvement. I was really hoping this was the solution I needed, but it did not fix the problem.

I never have cleaned jets with a wire. I've always used compressed air, so I hope the jet isnt somehow reamed out. It may be worth checking if the #1 pilot air jet passage has something obstructing it though.

toadson wrote: ...checked compression...checked valve clearances...narrowed the problem down to the #3 cylinder carburetor...bike runs poorly at low speeds, but mid and top end range are great. #3 cylinder exhaust is only about 250* while the rest of the cylinders are close to 600. If I adjust the pilot screw all the way IN on cylinder #3, it runs close to perfect...

Won't hurt to re-check compression and valve clearances in #3 cylinder.

And would also visually assure fat blue sparks on #3 spark plug (using a brand new NGKB8ES spark plug). Sometimes a spark plug may appear to fire perfectly well while removed, but fails to provide adequate spark when reinstalled. For example, a new plug may become fuel-fouled and quit firing. Removed at road-side, and spark looks okay and plug still looks new, but won't fire when reinstalled. WTH? Then install a brand new plug which works just fine. Go figure.

Is there any damage to the pointed tip of the pilot mixture screw in #3 carb?
Could swap it with one of the other carbs to see if it makes any difference.

With carbs on the work-bench and the pilot mixture screw in #3 carb is turned all the way in, should be unable to see any carb spray exiting the pilot outlet port into the bore.
When the pilot outlet port is capable of being completely closed by turning the pilot mixture screw all the way in, it helps rule out damage to the port and/or damage to the pointed screw tip.

The over-rich condition in #3 is likely caused by one or more of the following:
Insufficient compression (for whatever reason);
Inadequate spark (for whatever reason);
Over-rich fuel mixture (for whatever reason).

Good Fortune!

i second that you might have to adjust your valves

i guess for a compression test the engine has to be warmed up or something?