Hi,

I have a 95' CB400 that won't go faster than 90mph/150kph.

I have been messing about with the bike since I got it about a year ago. I finally have some time to try and get to the bottom of it.

Any advice is greatly appreciated as this has been bugging me for so long.

From 1st gear to 4th the bike is a beast, it's super smooth, surprisingly fast and everything seems to work as it should. 4th gear is where the acceleration ends. 4th gear gets me to 90mph at the redline, 5th gear will sit at 10,500rpm/90mph and 6th gear can't sustain that speed so the revs drop.

The bike/setup was a mess when I got it. Valves needed done, don't know who messed with the carbs but they shouldn't be allowed to touch anything mechanical.

Anyway, done the clearances and the carbs have had a complete split. In the process replacing orings, replacing jets and adjusting mixtures.

Read everything I can find on the 180kph restriction, which is stressful in itself when my bike can't get close to that. Just in case, fitted the resistor to the clocks and tried speedo convertor with no success.

I've used the colortune to check the mixtures with the engine in neutral, bit more difficult at 90mph.

The plugs come out ok after run but I really need to kill the engine at 90mph in 5th and check them roadside. Which is next on the list.

Anyone experienced this sort of issue?

Cheers

I know it's simple but you never know... have you checked that the air filter is clean and the flow area from the filter to the carbs is clean and not blocked?

Yeah, the carbs have been off about 10-15 times to make adjustments. There are no air leaks at carb inlet or outlet rubbers. The airbox has been off cleaned out and filter replaced, also tried running without filter, no noticeable difference.

The manual suggests to check these solutions if you have poor performance at high speed:



From my experince i know that you can have problem( poor performance at high speed)if you have hole or holes in your carburators diaphragms-membrane.Be sure that your 4 diaphragms-membrane is ok.

Thanks for the info.

The diaphragm's were thoroughly inspected previously, all were ok.

Checked pretty much everything on that troubleshooting guide.

Everything seems great up until 5th gear, that's what's throwing me a bit. A lot of these issues should present themselves in other gears. That's not the case as you can easily go past the redline in 4th.

Although it does'nt quite feel like it, I'm starting to think it's some electronic restriction from the way it just seems to hit a wall.

Sounds silly as you've already added the resistor in the speedo, but have you tried riding the bike with the speedo cable disconnected completely, to see if it will pull in 5th as it does in 4th?

Yeah, tried it disconnected and used sat nav for gps speed. Same result.

Had also fitted the mph convertor with no difference.

The bike doesn't pull very well lower down in 5th or 6th but I put that down to it being a 400. Hard to tell without trying a working one.

Hi does the rev counter red line in neutral , the reason I ask is has someone altered the ratio on the sprockets front and back, just a thought

The bike will go past redline from 1st to 4th. It currently has the standard gearing but I'm going to try lower the gearing and see what happens. Need to wait till payday for new sprockets.

Sorry I think Im having the same problem too, my rpm gauge is broken not functioning at all awaiting for replacement part now. I know its a silly question but will it be possible that this causing it? Some malfunction rpm gauge short circuited the bike high end speed?


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