Intake Boot/Carb Issue

I noticed a couple intake boots on my '81 XJ 650 were cracked. I recently replaced the boots and decided to check the spark plugs. All of the plugs look great except for one. One spark plug looks like the cylinder is on the rich side (see picture) it's very black and sooty. But, that associated intake book was definitely the worst and was badly cracked. Could it be possible that a bad intake boot could cause an air leak that would darken a spark plug and cause that cylinder to run on the rich side? If not, would it mostly be a carb/jetting issue otherwise? Thanks!

 

An air leak in the boot would cause an lean condition, and the plug would be white or almost white.....dry sooty black is rich fuel, wet soot black is oil

 

Black soot is a sign of incomplete combustion, and can show up under lean conditions.

Though I'm not convinced that is the case here.

The intake boots usually don't crack all the way through.


OP:
Have you ever done any work to the carbs on your bike?
If so, when and what.

 

I bought it a couple years ago and it didn’t run at all. I actually took it somewhere and I believe they ended up cleaning the carbs, but didn’t have to rebuild them or anything. So the last time the carbs have been cleaned or adjusted was about 2 years ago.

I had it running one day a couple weeks ago before I repaired the boots and I swear I could actually see a spark through the crack in the boot, that’s when I decided it was time to fix them. I didn’t think they were too bad either, but once I took a closer look they were pretty cracked.

 

Should not be able to see spark from the intake boots they are before the valves and cylinder. Once you are able to take some pics that would really help explain what is going on as well. When you got the carbs back 2 yrs ago, did you put them on the bike and then fill them with gas? If it has been sitting with gas in them for 2 yrs they should be gone through again. Depending on the type of gas you are using as that time, ethanol biased or not?

As for the boots, it y0ou are having idle issues then you are likely to have a air leak some place. There are cheap ways to fix them temporarily or you can get new ones from Len.

 

When I got them back I put them back on the bike and have been running it in the warmer weather ever since. I also try to not use ethanol either when I fill up. I also have a temporary fix for the boots atm, but will be buying a new set and replacing them since I will be taking the carbs off anyway.

I personally have never tuned carbs on this bike. Do you have any tips on how to get started?

 

IN THE CHURCH OF CLEAN

Why you should replace butterfly (throttle shaft) seals.

Replacing your Hitachi throttle shaft seals

Setting the fuel levels

Cleaning and degreasing the boots (and the cracks) then coating them with oil-resistant RTV (worked into the cracks), and smoothed with a wet finger is a good (if ugly) permanent repair.
When you go to fit new boots you need to be extremely cautious. The screws holding the boots to the head like to corrode in the bores, so use plenty of penetrating oil (not WD-40) over a week or so to help free them up, then back them out slowly (turning them back in a bit to help reduce the buildup of schmoo in the treads).

The new boots require gaskets (as do the originals). Be sure to clean the old gaskets off, and dress the new ones with a spray-on gasket dressing to ensure a good seal.