Greetings fellas,
I recently reassembled my cbr600 f4 after replacing a number of hoses and gaskets as its age had taken a toll on the rubber. I opted to remove the engine and do some painting on the frame and other parts because why not…

The bike starts great and idles relatively smooth( I have yet to sinc the carbs but they are close enough for the moment).

the header temperatures are very similar at idle

It will rev somewhat smoothly In neutral with slight hiccups in the middle of the rpm range usually around 6-7000

Once in gear under load, the bike will accelerate through first almost without issue but as soon as it’s shifted to second or third gear it hits a wall at 5-6000 rpm, no acceleration as if it is hitting a limiter, wot has no impact, there is no change, no bogging or acceleration, just a rough 5000 rpm stumble.

i have tested the fuel pump volume and verified that the tank is clean and free of debris. I got a new fuel filter for good measure. I am in the process of rebuilding the carbs at the moment because I assume it’s a fuel issue. However the jets/bowls are all spotless.

Any recommendations are appreciated and thank you for your time.

 

Just wanted to include some images of the hoses and where they are going to, I’m thinking it could be related to hose routing since the carbs are looking really clean internally.

The carb bowl vent is just being vented to the atmosphere, this bike has no Pair system, or DAI system.


The hoses that normally go here are for the atmospheric pressure under the cv diaphragm. I found that the hoses were cracked and one of the elbows were cracked so I figured I would give these a shot, they are a 3D printed nipple with a filter fitted into the center. I have a feeling this could be my issue or the hose for the carb bowl vents could be my issue.

 

Issue ended up being the carb bowl vent hose, it evidently had to be on the air box, I assume the rate at which the bowls refill has to be related to the amount of air flow through the ram air ducts. Runs better than ever after cleaning carbs and synchronizing.

 

Great find, thanks for keeping us up to date.

 

nice one! Do you have any photos?

When looking at my carbs there were tubes going from the carbs to the airbox but they seemed to connect to four extrusions on the airbox plastic but they didn’t seem to have any holes if that makes sense. One of these was broken off and the tube has a bolt into it to block vaccum so wondering if this is a similar issue.

 

Those four small vacuum lines that normally get blocked off by the air box come from the intake under the carbs. They are for synchronizing the carburetors using a carb sync tool.

they commonly come off/crack and cause mixture issues. I replaced mine and made them a little extra long in case they crack so I can trim them…

for your case having a bolt in one is no issue, mine used to all have bolts

 

Ah sugar, thanks for confirming. I am having the same issue intermittently with the bogging at 5000- 6000 rpm. I found not using e10 helped and riding more aggressively above these rpm alleviates it but city centre riding is a bit of a pain as it is really jerky. I have seen people identify multiple issues regarding wiring and perished rubbers but I always worry its just the nature of that generation of 600’s!

 

I run high octane fuel with no ethanol, it’s possible that your carburetors need a good cleaning. A bog at that rpm is likely a clogged jet, after you clean them, pick up a motion pro carb sync tool and find a YouTube video on synchronizing carbs, I had never done it before and it only took me 30 minutes or so to let the bike warm up and learn how.

when chasing this issue I had I cleaned my carbs and synced them, it runs better than ever, I’ve never cleaned them before and now it has a great smooth acceleration, it even sounds healthier at idle.

yesterday was my first highway test, it accelerated flawlessly to 70+ mph but I still need to test the rest of the speedometer. I just have a few things to double check first.

 

It could be, I was hoping it isn’t as I had a mechanic overhall the carbs and replace the fuel pump before dyno and still have issues. I have seen people identify tank fuel filters and wiring connections under the fairing as problem spots also. I was wondering wether the system for adjusting the ram system via the speedo would be a potential troublemaker but wasn’t aure how to loom into it.

 

I truly don’t believe that the ram air system is adjustable, the amount of air entering is proportional to your speed. I would start simple, fuel filter in tank, fuel filter under tank, check and make sure the hoses are are connected correctly especially the ones for the carb bowl vents (this was my issue) check for cracks in hoses or splits near clamps.

check for vacuum leaks near intake boots with a carb cleaner or starting fluid

maybe see what the plugs look like, I’m sure they probably look fine given you only have a big in a small portion of the rpm range.