Here it is again:
As an update that might be of interest to forum members...Two years ago I resurrected a 380 B that had been sitting in the neighbor's garage for over 30 years. Less than 10,000 miles. Runs like a dream now...except that I have a noisy 5th gear. It could be described as a 'whirring' - sounds like there are a whole bunch of extra gears turning is the only way I can describe it. And it is more than a subtle noise.
No problem shifting in/out of any gear. No noise in any other gears. At first I went "what the heck" and kept my hand over the clutch lever. Now I just go 4th to 6th to effect silent running. But it is like trying to keep your tongue off a chip in your tooth-have to run it in 5th just to see what happens. Nothing HAS happened but I'm wondering if this is just a character of these bikes. But since a search of this site does not yield anything, perhaps not.
I obtained a parts bike-another GT380B- and pulled the transmission with the intent of swapping out the one with the noise issue. When I looked at the gears I noticed a couple of interesting things. First, the 5th drive gear was a different color than all the other gears. Seems there are some that have a brown cast to them, and some are what I would consider the normal gray steel. And then there was the 5th drive gear, that had a color that looked to me like someone had applied significant heat to the steel. Darker, with a bit of a bluish tint. Second, the 5th drive gear has noticeable edge wear. Hard to describe, so I've attached a few photos. Interestingly, there is no apparent wear on the 5th driven gear.
5th drive gear on left, 5th driven on right
5th drive gear
5th drive gear coloration
I've concluded that the 5th gear in the restored bike probably has the same issues as the 5th gear in the potential replacement transmission (pics shown here). So I'm not going to tear into it.
I've also noticed that all drive and driven gears for this transmission are available NOS from various sources - except the 5th drive gear.
Also, the 5th gear part numbers changed with the M (1975) model, so there must have been some issues that needed to be addressed. This seems to coincide with the issuance of Suzuki Service Bulletin GT-10 from May, 1975 that contained a method of increasing the transmission oil capacity from 1.4 to 1.5 liters to "effectively increase the durability of the GT380 drive gear". Hmmm.
So this leaves me with no real solutions to the noisy gear. I suppose figuring out what is going on gives me a little assurance that the tranny is not likely to cannon itself. However, my shift pattern will likely remain 1-2-3-4--6. Kind of a bummer, since the bike runs so well otherwise. And I've increased the rear sprocket by a couple of teeth so I don't absolutely need 5th gear. But it would be nice to have.
I'm surprised that this problem isn't reported more widely and I am looking for any advice, suggestions, commiseration on this.
Paul