hesitant revs on Cbr400rr

[Ice Burger]

Hi,

I've got an old cbr400, I bought it from a dealership who said it'd had a full service and 50 part inspection etc...

However it took me 2 miles for me to find issues with the bike, speedo cable not working, sticky throttle, petrol tank rusted on the inside (had to send back), fuel filter clogged, dark brown brake fluid. I've fixed all these problems myself and had the tank treated under their 3 month warranty. - I had the bike delivered cross country, i never got to test ride.

Unfortunately I've still got problems, I'm beginning to think their service was complete bollocks. If I go for a hard pull on the bike, the power is smooth up to 7,000rpm, but between 7,000-10,000 the engines stutters a bit and stops pulling as hard. Once past 10k the bike will sing to the top of the revs just fine. I'm out of warranty and will have to solve myself.

I initially dismissed the idea of fouled plugs because I assume they would have been changed during the 'service' but I'm now open to the idea they are 20 years old. I've also heard someone talk about sticky diaphragms being the issue.

Any clues on how to fix would be massively appreciated,

Thanks.

[Fisty]

Start with the plugs then look at the carbs.

By now they probably need a clean and balance.
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[spottedtango]

As has been stated.

From what you’ve wrote it would appear that there’s a lot of crud in the fuel tank and lines.

This has probably worked it’s way into the carbs partially blocked jets and other orifices. I’d clean and rebuild them.

[redeem ouzzer]

New plugs and get carbs stripped and rebuilt including ultrasonic cleaning of the bodies.

The dealer sounds like a complete autist.
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Be a REAL MAN!

[Ice Burger]

Changed out my plugs and it's made the world's difference, the bike is running super now.

The carbs where nice and clean inside so i left them alone because I'm not very good with them.