I found the right cylinder was running rich, but not the left.
So I started from the beginning.
- I checked head torque at 26 Ft/Lbs. Nothing was loose
- I checked the valve lash. Spot on.
- I checked the jet needle position. Correct at #3 just the same as on the left.
- I checked the main jet and needle jet sizes. Correct sizes and matched the ones on the left. O-rings are in good shape and not torn or brittle.
- I checked fuel depth in the bowls. They are the same on both float bowls; between 19 and 20 mm. The fuel level is correct on both sides.
- I checked the timing. It’s spot on and there is no ghost image. The timing advances smoothly.
- I switched the spark plugs and rode the bike for 50 miles. The original left plug, now in the right cylinder, is sooty and the original right plug, now in the left cylinder, is fine. So, there is no issue with the plug, plug cap or plug wire.
If it’s sooty, it’s rich. So, what’s going on that causes only the right side to run rich?
The, valves, timing, jets, needles, spark plugs, wires and caps are fine. There is no obstruction in the air tube feeding the right carburetor. The crank case breather isn’t showing any more oil than normal, so it’s not getting a lot of oil mist into the right carburetor. I mulled this problem over for several days with no new insight as to what to try next.
A couple days later I was at coffee with some friends and was chatting about this problem when an idea hit me. What if the slide isn’t going all the way up at full throttle? If that’s the case, the vacuum will be higher than normal at higher engine speeds. A higher vacuum means the fuel delivery is going to be higher than normal but the air volume will be lower than normal. That certainly would create a too rich condition and sooty plugs on one side, but not the other.
I take the air tubes off both carburetors and push the slide up on the right side. It won’t go all the way up.
The left one goes all the way up.
So, my hypothesis is correct. The rich condition is caused by the right slide sticking.
But what’s the cause of that?
I updated these carburetors with springs (not originally used on the /5 carburetor, but added to later versions). These help smooth out the transition from idle to mid-range.
As I look at the spring, it looks like it can get caught on a pin that protrudes inside the air well the spring fits inside of.
I can get the spring to hang up on that pin. When I do, the spring is cocked and then snags on the split washer under the screw that holds the plate onto the diaphragm. That stops the slide from moving.
I think this “cussedness of inanimate objects” is what stopped the slide from going all the way up.
I install the spring and make sure it isn’t hung up on the pin. I also loosen the screws holding the plate that secures the diaphragm and push the split washers outward so they aren’t protruding into the air well.
After I put the carburetor back together, the slide moves freely. I’ll take the bike for a ride when it warms up and see if I have fixed the problem. I’ve grown accustomed to not concluding I’ve fixed something until I take a longer ride and the problem doesn’t reappear. 🙂