CBF 500 Mid Range ( 5K-6K RPM ) Power Loss

[Aibisi]

Hey there guys, I recently bought myself a 2005 CBF 500, bike is in pretty good condition, ( 37.000 KM / 23.000 Miles ) previous owner said all services were done by her mechanic and last one was done pretty recently, which would have been only 1000km / 600 miles ago.
Before I bought the bike I checked the oil, it was pretty fresh and on level, coolant was spot on as well, overall the bike is in mint condition.
However there's one thing thats been bothering, bike accelerates perfectly fine but every time you hit the 5-6k RPM range the bike stutters a little bit and there's a clear loss of power almost like mild fuel cut and after going above 6.000rpm it just picks up power again like nothing ever happened. I am unsure if this is some sort of carburator maintenance they might have missed? The bike idle is fine and stable, the fuel economy is around 38mpg which is a bit higher than I expected but it might be that im just used to my little old 125cc.
Is anyone else having this issue and any advice on what I should do?

[Robby]

The fuel burn is a bit high. That combined with the stutter at 6k suggests it's running rich somewhere.

However, there is good chance it has hardly been ridden for the last year and just needs some fresh fuel a good clear out. Run the tank to almost empty, fill up, and then put 100 miles on it. See if the problem takes care of itself.

[Aibisi]

[jeffyjeff]

A quick fuelly search shows your fuel consumption is indeed on the high side. https://www.fuelly.com/motorcycle/honda/cbf500
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History shows again and again how nature points out the folly of men - BOC

[Robby]

In that case it is possible there is something amiss inside the carbs - either something blocked with varnish, or something else loose or broken. Unlikely to be a misguided attempt at tuning, because people don't generally bother trying to make a CBF500 go faster.

Make sure the air filter is clear first. I do know from the earlier CB500 that a dirty air filter would really choke them up, as would any kind of mods to the air box. The CB500 used to pick up quite significantly around 6-8k rpm, which I assume was down to some clever matching between airbox and cam. Was a bit flat from 5-6k, but not to the point of feeling broken.

Check air filter. If that is all clean and properly fitted, and no-one comes along with a better idea, I would be getting the carbs off and checking them over for trouble.

[Aibisi]

[Nobby the Bastard]

There is an obvious cause which hasn't been mentioned yet.

Does it have the original airbox and filter? In line fours seem to be susseptable to a hole in the power at about 5-6k when the standard induction, which was incidentally developed at a massive cost by the original manufcacturer, gets fucked about with.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[Aibisi]

[jeffyjeff]

[Robby]

I am wondering about something float related, because the CB500 always had a problem with sticking floats. Usually if the carbs had been drained for any reason, they would piss fuel when they got filled up again and needed to be tapped with a screwdriver handle until they freed off and started working.

If it was my bike, I would be going through the carbs and looking for something being set wrong - needle height would be my first place to look, then float height. Also a possibility of shite clogging the emulsion tubes, seeing as it is happening right about where that bike is 40-60% power output. Torn diaphragm is always a possibility.

Or just something silly, like a jet has unscrewed itself or a needle clip has come off.

One thing I do know for sure of the Honda commuter range is that they can still run surprisingly well with major defects. My CB250RS ran fine with a seriously split inlet rubber, just popped a bit more on the overrun.

[Easy-X]

Yes, linked twin diaphragm carbs by the looks of it, quite intimidating to a novice I suppose. Would balancing them be of help?
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Aibisi]

[Courier265]

38MPG? that's terrible, mine does between 50 and 60 MPG depending on the riding conditions.

New plugs and a bottle of RedEx might help, perhaps getting the shims checked, after I had the shims checked the bike did perform that much better. Shims only need to be checked every 50K.
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CBF500 - 2 x Inazuma 250

[Robby]

The only solution is to work through it until you find the problem.

Whilst valve clearances can have a small impact on how it runs, they don't tend to go that far out of spec and are one of those that people just suggest because it's routine servicing that gets ignored, its difficult, and expensive if someone else is doing it. The valves should be in spec, but I doubt they would cause this problem.

I think it's related to fuelling. Logically it should be down to running rich, but logic doesn't always work. An air leak can also cause problems like this, illogically. Good places to look are any vacuum hoses - they fall off, get pinched, get a hole bitten in them. Vacuum ports for carburettor balancing can also be a problem, if the plugs or caps for the ports are loose or missing.

It's even possible that this is an electrical problem, if 6k rpm is a spot where everything is just wrong to make a good spark and you have a bit of corrosion in the system making the job a little harder.

In short, small problems are hard to track down.

[Britabroad]

I hope this helps, l do know that type of problem is not only a pain in the rectum, but quite a worry, especially if you thought you were buying a good 'bike. l'm going with a fuel delivery problem, but, there is a slight possibility it could be the plugs/leads!
https://www.cb500x.com/index.php?topic=8849.0