I am going to finally do a full carb cleaning here soon and replacing a couple boots, but my bike is running lean. Mostly white plugs, with a little spot of brown, consistent with all of them. Is there any harm to running my bike like this for another week and a half? Anything I should do to temporarily get it to run a bit richer?

Turn the mixture screws out a bit.
Maybe half to a full turn, or whaterver you're happy with when you do a plug chop. This will make the bike run richer.

Check the spark plug gap.

I'd go a quarter of a turn and do another plug chop to see how the look. A quarter turn is a major adjustment. A half turn is a very major adjustment.

If they are still white then go another quarter turn. You want to do it in small increments.

Vacuum leaks will cause a lean condition. Check these things:
Carb manifolds and gaskets
Vacuum port plugs
Throttle shaft seals
When carbs apart --
Re-jet to Canada specs- USA specs were lean to begin with ( emission standards of the time)
Wet set float levels ( make a jig to clamp carb to so you can do this on the work bench vs on the bike )

To do a proper plug chop, all that matters is the RPM, not the speed, right?

If I had a boot leak of any size, driving it in the rain would show it running poorly, right? (Water would get into the mix).

How does the Canada jetting affect the bike's performance overall? I have never heard anyone mention that idea, how neat.

Before you move your pilot (mixture) screws any you need to know where they are now. Either gently turn them in until they bottom and record, or record their current position (relative to the carb body) and adjust them in VERY small increments.

A plug chop needs to be 'under load' more than anything, above 5k rpms and pulling when you hit the kill switch.

It only takes a very small leak to let in enough air to cause leanness, so riding in the rain wouldn't necessarily show that. Riding long distances or for extended periods can lead to damage if your lean. Your pipes will start to blue and your oil will break down pretty quickly, but you might not notice until you've melted your mill.

I fought a lean problem with my 550 for a while. It turned out to be dirty carbs and in particular in the pilot circuit on three out of four-plus my float levels were at the low end of the spec.

I was looking at chacals parts at the carb section. He shows the proper jets for all of the bikes. Look at it closely and he has Canada bikes jet sizes in there. I saw they were slightly bigger so I bumped my pilots up to that size and it runs very well. Plugs are nice and paper bag brown and my pilot screws are very sensitive when you adjust. Width of a dime can be a lot of difference.
I spent hours on those carbs and put as much new in as possible. My 650 only has 7000 miles on it and runs perfect. I don't ride it very much as it is in show condition. I also installed chacals special bowl drain screws so when it's in storage it's easy to drain the carbs. The drain screws are stainless steel and have an allen head so a T handle Allen wrench works nice.
MN

Well, incidentally, one of my exhaust pipes cracked recently (rusted through). I have access to some pipes that will fit, but they're straight pipes, loud. Should one rejet to compensate for those, in theory? (I'd like to spend as little as possible, limited budget). Never understood why exhaust should cause a need to rejet... I can only imagine it affecting compression against the exhaust valve to help it close faster.

under load = accelerating

How does damage occur if it's lean, though? Just an overheating engine?

Lean condition = no lubrication for the upper cylinder = very hot burn = burnt valve or scored piston/cylinder wall....if you're lucky.

It could also lead to a seized engine , leading to a locking rear tire, leading to a painful 75mph crash.

I've had a 2-stroke seize on me before. Not enough oil in the mix. It's not the most confidence inspiring feeling at 50mph, let alone expressway speeds.

The Cheapest Insurance is adding some Marvell Mystery Oil to the Gas.

Enough to make the Mixture "Burn slower" and Cooler.

Keep the Heat down some.

1.82 Oz. Marvel -to- 1 Gal. Fuel = a Ratio of 70 : 1 .
1.9 = 65:1
1.7 = 75:1
1.6 = 80:1

Air cooled engines have very little tolerance for heat buildup.

Your valves dissipate heat when they close into their seats, and as tskaz said lean creates a very hot burn as near 100% of the fuel is consumed-so that extra heat will build up in the upper cylinder, valves, and raises oil temp <---very bad thing.

Greater exhaust flow means more air coming through your engine (it's basically an air pump) so your mixture is + on air and same on fuel-rejet and you restore that balance.

Yes-accelerating is under load. That's a bummer about your pipes-I wish someone would start manufacturing XJ parts like those-it sucks that they are obsolete.

MOTAD Enterprises manufactures a 4 into 1 System for Yamaha XJ Series Bikes with Oil Filter Access and Center Stand Retention with Stock Back-pressure.

Chrome or Stainless.

300 Pounds Sterling
$480.00

Sad though, the dual pipes is nice.

And that's hella expensive for me.

I'll go buy some marvel, and go find my mixture screw and try a quarter turn. I did some plug chops at 3k and 5k under load, found about the same at every spot--little bit of tan, mostly white.

I still don't understand though: How does less restrictive exhaust affect my air intake?

How can I tell if my engine has had any damage from running hot for the last 2500 miles? (I had a friend that you would really think you could trust tell me he thought my plugs looked brand new--which is exactly the problem, they look almost brand new, mostly white--and had ignored it until I learned enough to realize he just hadn't ever had an engine running lean or understood anything about plug reading). From what I've read and understood, speckles would be the sign that the engine is getting so hot that bits of aluminum are being dislodged and damage is actually occurring, right?

Your oil would be a good place to start. Change it and look through it for any debris. Squeeze it between your thumb and finger and feel for any bits and for viscosity. Your pipes will turn a bluish tint when they are getting too hot. Your plugs electrodes will be disintegrating if they have been overheating. You should also check your compression numbers in each cylinder and record them, any dropping off and you are having issues.

Generally when you ride a good distance/time like at least 30 minutes of average speeds your pipes should be 'pinging' just a little when you stop. If they are 'pinging' repeatedly and at a fast rate then you are running too hot. You might also notice changes in how your clutch is working, since the oil is dramatically changing in viscosity when overheating occurs.

The lean condition from less restrictive exhaust is easy to understand if you don't think of the process as being compartmentalized-like intake, then compression, then combustion, then exhaust. Because although it seems like it's that simple, it's not. Keep in mind that it's all happening at a very fast rate and various degrees of vacuum are occurring in each cylinder. If your exhaust flows faster/better then it's easier for your intake stroke to suck more air in to the greater volume left behind, and your jets are designed to reach a limit of how much fuel they will allow to flow. So at a certain point you can increase the vacuum and the fuel won't increase with it-it's reached max fuel flow.

speckles as you call them of brown can also indicate incomplete burn, or leakage past your rings. Plug reading is not an exact science and there is considerable disagreement about where to look and what exactly it means. But safe to say your plugs should not look brand new or lilly white all over.

Ok done proselytizing for now..