Carburetor Boot FAQ
by JP Honeywell and Mike Saar
The rubber parts between the engine and the carburetors are cracked. Should I replace them?
It may not be necessary to replace them. I'll be the first to admit that they look bad but even if it appears that the cracks are deep they may not go all the way through.
There is a simple test you can perform to check to see if they are leaking. First, start and warm up your bike. Then with the engine at idle, (and a fire extinguisher VERY near) spray WD-40 or some other flammable aerosol (such as an ether based, cold weather starting spray will work but be very careful - this stuff is extremely flammable) onto each of the boots from both sides of the bike. If the engine RPM's elevate while you are spraying and return to normal when you stop then you have a leaking carburetor boot.
NOTE: some people prefer the starting fluid over WD-40 as the latter can attract dirt or leave a gummy residue in hard to clean places.
If you find no leaks but the cracks bother you then get some high temperature RTV sealant and smear a coat onto the boots. This will make them more attractive and probably less likely to develop any leaks in the future. Do not use silicone sealant or other gasoline soluble sealant.
What causes the cracks in the first place?
Rubber starts breaking down when exposed to air, sunlight (UV) and heat. If you examine any rubber on your car or bike you will more than likely notice this breakdown occurring if the part is over a year or two old. Your carburetor boots are more than likely 15+ years old.
I've decided to replace the boots. Now what?
If your dealer can find the right carburetor boots for your bike (we are talking about bikes that were made from 1978 until 1982 here) then order them. Get ready for sticker shock. You'll probably pay about $100 USD. The actual replacement of the boots is straight forward.
You will need to remove the seat and gas tank. Then unbolt the air box and slide it back from the carburetors. Remove the throttle cable(s) from where they attach to the carburetors and remove the bank of carburetors. Removing the carbs is much easier said than done and may require much swearing and/or a band-aid. After that just swap out the boots - making sure that the vacuum hose connectors point up.. (Save the old boots - you may be able to help some poor XS-ive on the mailing list with one of those spares :-).
Put the carburetors back on (this too requires much swearing and probably another band-aid or two) and reattach the throttle cable(s).
You will need to adjust the throttle cable for free play. Re-attach the fuel lines and gas tank. At this point you should fire the bike up and run the leak test again to make sure all the boots are sealed to the carburetors.
For me, the worst part of this job getting the airbox boots/flanges to seat correctly onto the carburetors. The rubber used to make these is, compared to the boots, thin and pliable. They sometimes fold in instead of wrapping around the intake lip on the carburetor. This said, it might not be a bad idea to do the same leakage test to the boots between the carburetors and the airbox after you get it re-installed. If one of these is leaking then it is letting in unfiltered air which can be detrimental to your carburetors and engine.
Anything else I should know?
Yes. The carb boots actually serve two purposes. One is to get the fuel/air mixture to the cylinders. The other is to hold the carburetors in place. The carbs are actually suspended in air between the air filter and the cylinders. Only the 8 hose clamps hold them in place. Most of the weight is borne by the front carb boots. With all the engine vibration, heat, and stress from the weight of the carbs, it's no wonder that the boots crack.
Starting now and as a part of routine maintenance (at least twice a year), use a rubber protectant such as Armorall or Son-of-a-Gun to protect the carb boots from cracking again. In another 15+ years, it's doubtful you'll be able to get another set of boots.