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Firstly, I have checked a similarly titled post (Honda CB400 Super-Four - Power Issue) and their symptoms are different so it's not relevant to me. My bike is a '97 CB400SF, all stock except for instrument cluster and exhaust (i.e. original airbox, filters, carbs, jetting, engine). No change in sprockets or gearing, either.

I got it modified from original and it started behaving differently shortly after but due to work and family commitments I had left it unaddressed for abot a year. It starts and runs fine, but appears to hit redline very early, requiring premature gear changing (when compared with any other CB400). Because there's no tacho installed I can't estimate rpms, but it definitely sounds like redline because I can bounce off redline before changing gears. No spluttering, struggling or smell of unburnt fuel, either.

Not sure where to start with this, but want to have a better understanding before I inevitably send it to the mechanic for better diagnosis and fix (I don't want to leave the mechanic to work it all out himself, it may cost me more than the bike is worth). Plus it's a good opportunity to learn more myself.

At the moment, top speed in 6th is about 65mph. To bumble about city streets it's 3rd/4th gear.

It recently went in for a regular service, so had a carb clean, jets checked, new oils, etc. Partly because I wanted a clean slate to try diagnose this, and I didn't ask the mechanic to specifically address this because I'm in a new town so don't have a trusted mechanic just yet.

Any recommendations where I can start to at least help diagnose the problem? It's likely out of my skill set but at least knowing how to diagnose more problems is a skill I'd like to develop.

One thing I want to check is whether all 4 cylinders are firing. I believe so but I haven't specifically checked. Easiest way I can think of is just checking if all 4 exhaust outlets are hot. Presumably so if the mechanic's aforementioned service was done properly!

Thanks!

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  • What were the modifcations that were made before this started occuring?
    – SteveRacer
    Jan 30, 2018 at 6:01
  • Have you had the carbs balance tested? What was the difference in running before/after the service? Did you put clean gas in the bike? Is the clutch slipping? Tires up to pressure? Brakes dragging? How hard to fit a tacho?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 30, 2018 at 12:10
  • @SteveRacer only the instrument cluster and exhaust. Not sure if it started at exactly that time, though, due to not riding as much. So it may be complete coincidence or the situation degraded over time and I didn't really notice.@Paulster2 - your answers - "not unless the mechanic did it as part of his service", "yes", "yes", "no", "yes", "no", "and surely not hard by a half-decent mechanic", respectively :)
    – chopper24
    Jan 31, 2018 at 2:15
  • If a performance exhaust system replaced a highly restrictive one without a retune, that could be an issue. Probably not likely, but as @Pᴀᴜʟsᴛᴇʀ2 mentioned, carb balance and tune is useful anyway, and might uncover some other issue.
    – SteveRacer
    Jan 31, 2018 at 3:06
  • thanks @SteveRacer. I had also suspected the exhaust wouldn't cause this much of a performance drop (approx 50% top speed reduction) but I have little mechanical experience to run on. I guess I'll get it checked out for a carb balance, but beyond that I'm really scratching my head and my google skills are coming up short on providing any leads on potential explanations
    – chopper24
    Jan 31, 2018 at 6:40

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