\n
![](\"https:\/\/www.decoboco.info\/wp-content\/uploads\/imp\/674393_22577055_1.jpg\")
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
\ndoing my best.
\nThe mysterious Yamaha YZF750SP episode
\nA certain man bought a Yamaha YZF750SP, but something was wrong with it. \u2460 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.
\nSomething didn’t add up! A bad assembly? We decided to inspect it again. Structurally, there was absolutely nothing wrong.
\nThe question arose: is it really the clutch?
\nThe 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.
\nAnd since high hydraulic pressure isn’t constantly applied, leaks and abnormalities are hard to notice.
\nAlright, 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!
\nIt won’t return! That’s definitely abnormal\u2014the master cylinder smells bad! We had a used master cylinder on hand, so we swapped it in and
\nit was fixed perfectly. The return side probably had an issue, and we solved it.
\nNext, 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.
\nThe question was whether it was an electrical issue or a carburetor issue, so we used process of elimination starting with the simple stuff.
\nTank cap air vent abnormality and water drain hole inspection \u2192 no problems at all
\nFuel cock and electric fuel pump inspection \u2192 no issues
\nHmm? Could it be the carburetor?
\nThen he found online that a broken pickup coil can cause it not to rev, so we ordered a new one.
\n\u2192 No change, still broken!
\nIt’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
\nthis bike has been modified for full power with a direct-mount carburetor! Let’s try making the main jet ridiculously large?
\nTest ride \u2192 no change, still won’t rev
\nHmm? I’ve seen this before. I opened up all the drawers of my troubleshooting history.
\nIt’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)!
\nAnd the float valve diameter.
\nNormal carburetor tuning doesn’t usually involve changing the float valve diameter, but this is the tricky part. If the diameter is too small,
\nfuel doesn’t flow in well\u2014I had that experience in the past, so
\nI said, “Should we change the float valve?” He said he had a used one, so we swapped it right away and it was fixed!
\nExcellent! But once it’s fixed, it feels kind of boring even though it’s not my problem.
\nI kind of want to encounter some really occult-level trouble, but also don’t want to.<\/p>\n","protected":false},"excerpt":{"rendered":"