Hi all

After around six months of fruitless work I'm coming here to see if anyone can help.

My HONDA CBR600 FM (1991) is cutting out (revs going down until it dies, unless I let go of the throttle) when under load, past 45-50 mph (can't tell the rev range as I haven't got the dial).
The problem started much worse, as I couldn't even touch the throttle because it would bog down and die, until I've disassembled and blew and cleaned all four carbs.

What's happening:

• Right off the bat, when idling, the sound is definitely not as smooth as it should be - it sounds a little bit like a v-engine instead of inline four, the purring sound is constant (all cylinders/exhausts hot and there is spark so I'm assuming all cylinders run)
• When riding, getting into higher revs, trying to ride over 45 it bogs/revs down and dies unless you let go of the throttle
• After couple of minutes of riding like that it actually worsens to the point that you can barely ride it over 5mph, it revs down and dies as you give it throttle

What's been done:

1. All four carbs disassembled and thoroughly cleaned, there is petrol in the float bowl and coming out of the petcock out of tank, (after cleaning, she could get up to 45-50mph instead of 0, like when the problem started), checked if carbs synced using dedicated vacuum gauges, not starved for petrol due to any negative vacuum from the tank, tried riding with cap open,
2. Air access clear, air filter cleaned and tried it without filter on,
3. New spark plugs, nice blue/white spark is present,
4. Coils low and high voltage checked with multimeter and in spec from Haynes manual,
5. All wiring (LITTERALLY ALL) and connectors checked for continuity and resistance. Even put on a whole new (well, second hand) wiring loom to exclude the connections/electrical issues, all fuses OK, continuity at grounds on the engine and rear of the frame OK,
6. Even checked the timing advance with the strobe - that's in spec,
7. ICU (or whatever that black ignition timing box is called) swapped with another CBR's and no change,
8. Coolant level OK, no boiling or overflowing coolant,
9. Even a new ignition barrel and wiring (because I'm desperate :'D),
10. Battery charged and showing sufficient voltage both, engine off and running,
11. I might be forgetting something

I know it's a long post but I've spent countless hours on this issue without a solution and I have a strong bond with this bike and I'm desperate to make her run again. Any ideas or tips welcome.

Thanks


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Hello Dante,
I would start with taking off the filterhouse and then start the engine.
take a can of breakcleaner and open up the throttle.
If it revs up easily you can exclude about everything except fuel supply.
If you warm up the engine properly you can even experiment how hight the engine revs.

However if the engine has the same trouble with brake cleaner..... well.... you might be in for a real treat.
But first start with the breakcleaner and I will gladly think along with you.

 

Man I know it’s frustrating not being able to figure out an issue, or multiple issues. I finally found a legit shop that was super budget friendly. Got my 98 600 running great! I wanted to do it myself but wanted my bike running more. Got any good shops in your area?

 

That's a good idea. With the brake cleaner you mean for it combust, right? Because for that case I even have the starter fluid. Thanks, checking that this weekend.

 

Yeah, I'm that desperate too but most shops charge heaps for things as simple as brake pads change. I am not aware of any 'friendly' local garages. I might start asking and looking for one but I've never taken her to any before, always did everything myself and it's like an ego thing now, where I'd rather do it myself 😁

 

I can so understand that "ego" thing. I have a bachelor in automotive technology and then it should not be possible that someone else knows more than me about my bike

 

Yes indeed for combustion. Starter fluid can be quite agressive, but should work too. If you are a bit handy you could devide the the spray into 4 hoses to make sure all 4 cylinders get fluid. And you give yourself the opportunity to play with the gas a bit more.

 

Yes, I thought about splitting it into four carbs at once. The only worry is that it seems to run fine when revving while on centerstand in the garage. The problem appears when actually riding so not sure what this test will show me yet but still going to see tomorrow. Unless I find a way to ride 45mph with the tank off, spraying starter fluid into carbs 😁

 

How is it going? Are you any wiser?

If you have a proper center stand you could put the bike in gear and see if problems apear.
If the problems apear you can use the starterfluid to potentially get it going again.

 

Iv got a 99 f4, my bike ran great until it got stolen. Ever since I got it back iv had problems. I had to replace the ignition and once I done that it started back fireing and I took it to a mechanic an he fixed it sayin it was a wiring issue and that I needed a new fuel pump as well, i put a new fuel pump on it myself and now when I reach speeds up to 40mph or above the bike runs awful. It won’t reach 60 an if it does it sounds horrible. I have no idea what could
Be the issue, does anybody else know? It bogs down real bad once I reach 40 or above but it’ll cut in an out like it’s trying to run. It’ll start just fine, I can even ride just fine from 0-25 or 30 but that’s it.