Common Sense Is Not a World Made of Trivia

 

 
It’s cold today! With the new year underway, I want to manage my data properly this year, so I’m diligently hunched over my computer every day
doing my best.
The mysterious Yamaha YZF750SP episode
A certain man bought a Yamaha YZF750SP, but something was wrong with it. ① He said the clutch was slipping and wanted to replace it, so we decided to swap out the clutch plates. After the replacement and a test ride, it was still slipping.
Something didn’t add up! A bad assembly? We decided to inspect it again. Structurally, there was absolutely nothing wrong.
The question arose: is it really the clutch?
The clutch is pushed by hydraulic pressure. Let’s inspect the release mechanism. This has a large piston diameter and is surprisingly often overlooked during inspection.
And since high hydraulic pressure isn’t constantly applied, leaks and abnormalities are hard to notice.
Alright, let’s take it apart. Hmm, there doesn’t seem to be any particular problem, but it won’t return! It won’t return by hand alone!
It won’t return! That’s definitely abnormal—the master cylinder smells bad! We had a used master cylinder on hand, so we swapped it in and
it was fixed perfectly. The return side probably had an issue, and we solved it.
Next, it wouldn’t rev past 8,000 RPM. Since he could handle most of it himself, I just gave instructions and had him do the work.
The question was whether it was an electrical issue or a carburetor issue, so we used process of elimination starting with the simple stuff.
Tank cap air vent abnormality and water drain hole inspection → no problems at all
Fuel cock and electric fuel pump inspection → no issues
Hmm? Could it be the carburetor?
Then he found online that a broken pickup coil can cause it not to rev, so we ordered a new one.
→ No change, still broken!
It’s the carburetor! So we took it apart. Since it’s an OEM FCR, the jet shape is the same as the CVK model. And
this bike has been modified for full power with a direct-mount carburetor! Let’s try making the main jet ridiculously large?
Test ride → no change, still won’t rev
Hmm? I’ve seen this before. I opened up all the drawers of my troubleshooting history.
It’s an OEM vacuum carburetor adaptation. I’d had it not rev when adapted to an older bike. In that case, I was struck by the importance of the air hole from the tank for fuel to drain (to apply atmospheric pressure)!
And the float valve diameter.
Normal carburetor tuning doesn’t usually involve changing the float valve diameter, but this is the tricky part. If the diameter is too small,
fuel doesn’t flow in well—I had that experience in the past, so
I said, “Should we change the float valve?” He said he had a used one, so we swapped it right away and it was fixed!
Excellent! But once it’s fixed, it feels kind of boring even though it’s not my problem.
I kind of want to encounter some really occult-level trouble, but also don’t want to.