I've seen this problem on bikes that were only 5 years old, so unless you're certain of how old they are, you need a new set.
Sometimes, this component cracks, probably from thermal expansion & contraction that it can't handle anymore, because it's no longer flexible. This generates a gaping intake leak that's impossible to see, and the effects are numerous.
Either way, you wind up with a temperamental engine; it'll run fine under some conditions, but it'll be crabby & lethargic other times. A classic symptom is an unstable idle speed. If your bike's idle runs way up after it's warm, this is the likely cause. It can be hard-starting and will run hot. It leans out the mixture, can cause your valves to burn, and generally causes annoyance. It is an absolute waste of time trying to tune a motorcycle with an intake leak, as you're aiming at a moving target.
Worst of all, it mimics a host of other problems that you could
spent money trying to correct until hell freezes over, while simply
detracting from the otherwise enjoyable experience of owning a
motorcycle. Soon, out come the whiskey and firearms, and there's
a real mess to clean up.
Aha! Look at the escape path at around 11:00. Three of the four in my engine were cracked like this. Note also that the O-ring is completely flat; these should have a perfectly round cross-section. It's taken a poster-child 'compression set'.
09280-32006 (32mm = 'Small') for '79 and
older 750 & larger
09280-38004 (38mm = 'Large') for most '80 and later bikes.
Usually available at your dealership, as well as here.
For the '80 - '82 450's, I have carb kits and intake parts here:
(No other model years. Sorry.)
the larger or smaller HERE:
Because these screws are difficult to
remove, and because they usually wind up badly mangled, it's become
S.O.P. to replace them with stainless steel allen head (cap)
screws. They also look a whole lot nicer on the bike. I
decided to sell these as well, with matching stainless steel washers,
for one-stop intake shopping. I
strongly recommend using anti-seize compound on these.
Here's the M6 x 1 x 20mm:
See the table above for sizes.
Some bikes use 16mm screws. These include
everything pre-1980, the '80 & '81 GS1000 E & G, and the
'82 & '83 1100G's and 650's. The 16mm screws aren't quite as
chrome-looking. The 80 & later 850's, the '83 750's, and the
'80 and '81 450 all use the 20mm. So does the '85 700. If I
didn't list your bike, check with an online parts retailer like Flatout. Get the
part number for the screw and email me. Some bikes ('80 & '81
1100's & 750's and others) use gaskets instead of intake boot
O-rings, so check the exploded diagrams carefully.
You can order these HERE. These
are priced at $4.85 for the set of eight screws and
Postage is $2.25,
mainly because I have to use a padded envelope.