Hello everyone,
I’m having ongoing issues with my 2003 Honda CBR600 F4i (80,000 km) and I’d like to share the full story for advice.

The problems began after a ride where I barely made it to a gas station, running on fumes.
After refueling, the bike barely started — the gauges went completely dark, and it took a long time to get it running again. Eventually, it started, but on the way home I could already feel something was wrong with the acceleration.
Between about 1,500 and 4,000 rpm, when accelerating with full throttle, the bike wouldn’t pick up at all — it takes around 5–8 seconds to climb through that range. However, with only about 20% throttle, it goes through those revs much quicker.
It turned out the voltage regulator had burned out. The charging problem has been fixed, but the acceleration problem remains.

Current symptoms:
Poor acceleration, especially 2k–5.5k rpm, and noticeably less power in higher rpm. Above 6k RPM its going pretty well to 17k RPM.
Acceleration is better at partial throttle than at full throttle in low rpm.
Strong fuel smell in the oil, oil level slowly rising.
Fuel smell around the fuel tank area.
High fuel consumption (~8-9 L/100 km).
Idle is stable both cold and hot.
No ECU fault codes at the moment (previously had IAT, TPS, O2 sensor codes).

Repairs and checks performed:
Replaced Fuel Pressure Regulator (FPR) – no change.
Replaced spark plugs.
Checked TPS – readings within spec.
Cleared ECU fault codes – after a week of riding, no new codes appeared.
Checked for vacuum leaks – none found.
Changed oil and filter (due to fuel contamination).
I’m wondering if anyone has had a similar issue, as I’m running out of ideas on what might be causing this.
Thank you for help

 

Hya, first post here or any forum really . Have you made any progress on your issues?

I also have a 2003 F4i with practically the same milleage (75k) and similar symptoms. Got it for cheap with a sloppy camchain, but now that is fixed I also notice some lack of power in the midrange.
Granted, some friends labeled this as "skill issue" riding a 4 cylinder. I'm used to having some torque down low on my 2001 CB500.

But after today's gymkhana session a friend also remarked on bit weak midrange (He owns a CB600 hornet, same engine but torque-ier cams), and now I'm reading up.
I noticed around ~3k coming out of a corner it is faster when rolling on instead of directly going wide open.
No codes (besides speedo), bike smells a bit of gas too. I'm not that sure of my gas mileage since I only use it for gymkhana / parking lot shenanigans and short bits, but I figure around 10L/100

I wonder if it could have something to do with the enrichment / choke. I recently removed and cleaned my throttle bodies and my "cold" RPM went from a steady 2K to 3 to 3.5.
-> on my carbureted CB I spend days hunting a hesitation / lack of power in the midrange, and that ended up being a badly adjusted choke cable, that barely engaged the choke. Was enough to notice when riding. Granted that's a carb not injection, but perhaps it's similar since it's "old fashioned" injection. I'm gonna read up a bit on how the auto choke works on our CBRs, and see from there.

 

note that mine is nearly not as bad as this. However, when I was done working on my camchain tensioner I did forget to plug in the MAP sensor and that resulted in the bike really bogging down like you describe, taking forever to climb from idle to ~ 4k, then bit more life to 7kish, and managing to power wheelie higher up in the revs fine.

 

update: I took a proper look at my vacuum lines and they are pretty cracked / stiff and old. Ordered some replacement (3.5mm ID 7.5mm OD fuel lines apparently), and will replace and probably re-sync my throttle bodies once everything is in.

 

Thank you for your reply. I still haven’t been able to identify the issue, and I’m getting a bit tired of troubleshooting. I’m going to bring the F4i to a motorcycle mechanic and hope they can resolve it.
If you’re having mid-range problems, it could be the TPS, so try measuring it. Someone also told me to check whether it’s firing on all cylinders—especially the 4th coil—and whether the engine behaves the same when you disconnect it. Another thing I would check is the fuel rail and whether the injectors are clogged. Professional injector cleaning, however, costs almost as much as buying a used complete throttle body, so I don’t want to waste money unnecessarily.

 

Well, my cold start is now around 800–1000 RPM. I think it started happening when I changed the fuel pressure regulator from the original 3.0 bar to a 3.5 bar one, but I’m not completely sure.When the engine reaches about 60°C (140°F), the idle goes up to 1300–1400 RPM.
Another thing is that my F4i no longer raises the RPM automatically during cold start. Instead, the RPM slowly increases only as the engine warms up, almost as if the IAC wasn’t working properly—even though it shows values that match the service manual

 

Well, with fresh vacuum lines I _think_ it's a bit snappier? But I still very clearly feel the "dip" between 6k and 8k, when rolling on the throttle slow and steady in 1st.

Or perhaps it's totally normal and just a stock fuel map thing, perhaps to do with emissions? But yeah, I don't really feel like spending all the money for a power commander on this beater either.

That idle issue sounds like something to do with the wax unit, I know people sometimes remove it cus it behaves weirdly. Or could be a coolant bubble or something.

Found a very informative vid on vfr800 throttle bodies , they look to be practically identical to what's on the f4i. Also learned that the thing doesn't have any (mechanical) enrichment, just high idle by that IAC mechanism + ECU enricher based on coolant temp.


Do you happen to have already found reference values for the sensors? All of them "do something" , but perhaps they're just old and giving values that are off.

 

Come think of it it could be briefly dropping a cylinder. But it doesn't burble or pop or bang at all, so that just leaves fuel possibly being the problem.

How thoroughly did you check the vacuum on yours? About half my vacuum lines looked fine but were cracked on the inside, and I think all it takes is one of 5 hoses between manifold and MAP to throw it off.

 

https://www.manualslib.com/manual/12...0f4i-2001.html here you can find all spec. Most of the hoses on my F4i are really, really worn out, but I haven’t noticed any actual leaks. Maybe it really would be a good idea to replace everything as a precaution. I haven’t measured the pressures — my working conditions are a bit limited. I only have access to an underground parking garage, which makes everything a lot more difficult “Mine is backfiring all the time, which is why I think it’s related to incorrect fuel proportions and the custom exhaust. I have the whole winter to find the issue myself; it will take some time before I can get to a good mechanic anyway.

 

Oh yours is a slightly more timid stealthy paint, but looks like the exact same bike


Drove it to my association garage without fairings to not deal with all the plastics






Last night changing the vacuum lines


Yeah I think that explains your issue well... This engine is nutorious for being a machine gun without the giant stock muffler. And the "remedy" of the higher fuel pressure regulator probably only makes the mixture even worse down low, where the effect of the flowing exhaust is negligible.

So, in a perfect world you'd probably lean it out with a power commander down low