I recently picked up a 2000 CBR600F4 with less than 2K original miles. The bike was never crashed or abused. It was in storage for a long time. It came with a K&N filter, Two Bros slip on exhaust, and stage 1 DynoJet kit installed. I freshened the bike up with a complete and thorough top-to-bottom tune-up including a new battery, testing the charging system, new NGK plugs, new fuel filter, all fresh fluids, I cleaned out the tank and fuel pump, and meticulously went thru the carbs and balanced them. I added block off plates to the top of the motor and removed all associated plumbing. I am running premium pump gas.

The bike starts, idles, and runs perfectly. Throttle response is crisp, plenty of low and mid range power, no hesitating, missing, or flat spots, and no backfiring or smoke. The bike runs well at highway speeds, pulls thru the rev range, and will rev as high as I want to. Until 5th gear. No matter how high the rpm in 4th gear, when I shift to 5th (or 6th) the revs drop and will not climb past 9K rpm. Even if I quickly down shift from 5th, rev up, and grab 5th again, the revs will not pass 9K. I can hold the throttle wide open (even pump the throttle a few time) but cannot get above 9K rpm. At 9K rpm the bike tops out at a steady 110mph. I am running stock gearing with the original chain and sprockets which are in excellent shape and the bike has new Continental rubber in the correct sizes.

All the lights and switches on the bike operate as they should. I have no other symptoms or faults. Is anyone familiar with this condition?

 

Some suggestions for most likely too-rich mixtures:


1. replace K&N with factory OEM filter


2. replace all DynoJet kit parts with factory OEM jets & needles. I've found I can get +5% hp with just downsizing factory main jets.


3. run through some PEA-based fuel-system cleaner

- Red Line SI-1
- Techron Concentrate Plus
- 3M Max Strength Fuel System Cleaner #08814
- Royal Purple Max Atomizer 18000
- Amsoil Performance Enhancer
- Gumout All-in-One (might be off list soon due to lowering amounts of PEA)

 

1. I tried swapping filters and that did not change anything.

2. I am not a fan of DynoJet products. Unfortunately, on this application there is a plug that gets pressed into the main air jet in the top of each carb. I cannot remove the plugs. Figured it was better to tune the carbs to the max with the jet kit installed than to remove the kit and leave the plugs installed. Believe me, these things are clean and balanced. I went thru the dynojet install and verified that everything was installed to their spec. I tried raising the needle to prevent possible fuel starvation at WOT. No difference.

3. I ran several cleaners through the entire fuel system, including Techron and SeaFoam.

 

Looks good, go to smaller main jets and lower needle to lowest setting.

 

Hey there, did you ever get to the bottom.of this?
adie

 

Unfortunately, not yet. It is on my list of winter projects and I hope to get to it in the next 2 weeks. The challenge is removing the tiny plugs in the air jets from the DynoJet kit that was previously installed. (Photos in a prior post.)

Symptoms are the same. Bike starts, idles, and rides beautifully. It is an exceptional machine under 110 mph. At 110 mph it will not go any faster and does not rev higher than about 8K. There is no hesitation or needle bouncing - it just stays at 110 (WOT) with the revs steady like it's being governed. Below 110 mph I can take the revs all the way to the redline in any gear. The carbs are clean and balanced. New NGK spark plugs. Throttle response is crisp. Stock chain/gearing.

 

Tis interesting, my 99 cbr600 has an interesting thing where between 8 and 9k there's a flat spot then it seems fine again after, I can't confirm what the stock UK jetting should be, currently cyl 1+4 are #132 mains 2+3 are #135 mains, pilots are #40s, cannot get to the bottom of it at all, everything is stock as far as I know apart from an aftermarket downpipe of unknown origin, I suspect an early delkavic but can't confirm. In first gear the flat spot isn't evident!
Adie

 

The stock jetting in the US is the same as you described. That flat spot is a sign of a fuel delivery issue or a problem with the air/fuel mixture at a given load. There are a lot of air and fuel passages in the carbs for this bike. Make sure they are all thoroughly clean. Also, be sure to synchronize (balance) the carburetors. While you are at it, check for vacuum leaks. Once the carbs are balanced, let the bike idle and remove and replace the vacuum line from each carb, one at a time. The engine should immediately start to run rough or stall with the vacuum line detached. That is a good thing. Replace it and try the next one. If there is no change when you remove the vacuum line, that means there is already a vacuum leak on that cylinder. Find the leak, correct it, and re-balance the carbs. If that does not solve the problem you can try raising the jet needles by adding a shim (or more than one shim) on each one. I am including a document on carburetor service that you may find useful.

 

Awesome I'll have a look through the document, much appreciated! Funnily enough someone suggested the fuel pump could be a little flaky and unable to keep up with demand momentarily, I'll try another washer on each needle and see if there's a change, so far I've blocked the ram air ducts and tried turning on the enricher to see if they make a difference and they don't, perhaps I'll try with no air filter too and see if that changes anything! The bike rides 100 percent perfect everywhere else and I'm presuming at 8-9k WOT Im surely on the mains or atleast on the transition from needle? Either way I have some slightly leaner jets already and some richer jets on the way so I'll be ready at some point to dig properly!
Thanks for the help and carb documents man again it's much appreciated!
Kind regards,
Adie

 

Following since I'm getting ready to rebuild the carbs on my 99 F4 track bike.