Honda CBF600 N4 Won't Pull Under Load

[dannymassive]

I'm writing this post in hope that someone may have an idea of what is causing this problem. My friend has a 2004 Honda CBF600 that he uses to commute circa 60 miles per day. He's owned the bike since 2008 when it had just 4k miles on the clock and it has been well maintained throughout his ownership, never presenting any problems..... until now!

So the problem started a while back when the bike started using slightly more fuel than it usually had and the bike was blowing dark smoke out the exhaust upon revving. This was resolved by changing the carb diaphragms and things went back to normal business, until a few months back when it stopped performing in a normal manor and ceased to pull under any load. The usual suspects have been changed on a trial and error basis in hoping to resolve the problem. Firstly, the diaphragms in the carbs had been changed and the carbs cleaned, followed by throttle position sensor, and most recently the CDI unit. All to no avail. The Haynes manual has been throughly studied and we are now at a loose end as to what the cause of the problem is. Very frustrating!

Yesterday we took the Airbox off and ran the bike, which does rev without any load. This is when we spotted that the carb gates do not lift when the bike is revved. This we belive is the culprit! But why?!? Why would they not function? Assuming that they are vacuum operated, what would cause the vacuum to not be sufficient? Could there be another cause??

Thanks in advance guys.......

We don't want to be beaten by this and it would be great to get his commuting machine back on the road.

Oh - and the bike has done just over 100k miles now.
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Previous Ride's - '01 Aprilia SR125 (Sold) '08 HONDA CBF600n (Sold) '96 CB500 (Sold) '01 FZS 1000 (Sold) '06 CB1300SA (Sold) '07 CB600F Hornet (Sold) - '93 Suzuki Savage LS400 (Sold) '98 Honda VTR1000 Firestorm (Sold) '05 CB600F Hornet (Sold) '01 FZS1000 Fazer (Sold) 2016 Harley Davidson Street Bob SE (Sold) '02 Kawasaki ZX9R (Sold) '98 CB600F Hornet (Sold) '04 Suzuki Bandit 1200s (Sold) '13 Yamaha FZ1-N

[jaffa90]

Have you done a compression test.

[dannymassive]

[sickpup]

Are the nipples from the carb tops for balancing missing?

If the carb slides don't rise then there is a carb vacuum problem.

[dannymassive]

[Pete.]

With the airbox lid off, if you lift the carb slides with your finger do they lift easily and spring back down again smoothly? If so, then they are not lifting due to lack of vacuum as said.

First thought is - has a previous owner put a set of intake restrictor washers in the inlet stubs from back when you could restrict any size bike to 33hp? OR, (and this is pure guessing) have heavier springs been fitted above the slides for the same purpose.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[dannymassive]

[sickpup]

[dannymassive]

I think we need to strip the carbs and check for leaks on the diaphragms again. I guess there is the possibility that he may have missed something.
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Previous Ride's - '01 Aprilia SR125 (Sold) '08 HONDA CBF600n (Sold) '96 CB500 (Sold) '01 FZS 1000 (Sold) '06 CB1300SA (Sold) '07 CB600F Hornet (Sold) - '93 Suzuki Savage LS400 (Sold) '98 Honda VTR1000 Firestorm (Sold) '05 CB600F Hornet (Sold) '01 FZS1000 Fazer (Sold) 2016 Harley Davidson Street Bob SE (Sold) '02 Kawasaki ZX9R (Sold) '98 CB600F Hornet (Sold) '04 Suzuki Bandit 1200s (Sold) '13 Yamaha FZ1-N

[Pete.]

Do the slides not lift at all? You can get to full revs with no load with them barely lifting by just reving it gently. If you want to see them lift you have to put a load on the engine, so a hard rev is required.

If you strip out the slides to check the diaphragms you should hold them up to the light and gently pull the diaphragm out and look for pinholes.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[dannymassive]

They only lift slightly, maybe 1cm at most and that is with a full wrist full.
____________________
Previous Ride's - '01 Aprilia SR125 (Sold) '08 HONDA CBF600n (Sold) '96 CB500 (Sold) '01 FZS 1000 (Sold) '06 CB1300SA (Sold) '07 CB600F Hornet (Sold) - '93 Suzuki Savage LS400 (Sold) '98 Honda VTR1000 Firestorm (Sold) '05 CB600F Hornet (Sold) '01 FZS1000 Fazer (Sold) 2016 Harley Davidson Street Bob SE (Sold) '02 Kawasaki ZX9R (Sold) '98 CB600F Hornet (Sold) '04 Suzuki Bandit 1200s (Sold) '13 Yamaha FZ1-N

[Pete.]

Do they all lift the same?
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[dannymassive]

[Pete.]

Then it's not the diaphragms.

It's very hard trying to guess stuff like this over the internet but my guess is the bike is not breathing right. That means it's either not getting enough air in or not getting enough exhaust out.
Dirty air filter (I know a guy who had a 900 hornet that wouldn't rev because it was blocked up solid). Blocked exhaust is a possibility though not likely. Cam timing out can be a cause of not working under load but that's a long way off being likely.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[lingeringstin...]

The symptoms sound exactly like what happens to a two stroke bike when the exhaust clogs from shite buildup. I suppose a four stroke bike could get a clogged exhaust by any number of means too.

I remember once trying for some time to start a VFR750 that had stood for a while, and when finally inspecting the airbox there was evidence that mice had got in and made a nest. God knows how many of them or how much of their mess got sucked into the engine before I looked at the airbox.

I did once put like a whole bunch of grapes down somebody's CBR600 exhaust at a rally, one at a time, in hopes that upon starting the bike it would fire grapes out the exhaust but nothing happened. I followed him all the way back to London and not a single grape was ever seen to exit the exhaust.

I've hear of birds or wasps making nests in bikes, Seriously, check that the exhaust blows normally. Look for spuds, etc.

[Hong Kong Phooey]

[bellerose]

This is a long shot, but has this ever been resolved? I bought a Hornet 2006 from Italy 3 years ago, exact same issue, haven't found the reason.