HUUUGE flat spot, F3 Carbs - Honda Motorcycles - FireBlades.org
Honda CBR 600 Discussion of the Honda CBR 600F1, Honda CBR 600F2, Honda CBR 600F3, Honda CBR 600F4, Honda CBR 600F4i, and Honda CBR 600RR Motorcycles.

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post #1 of 44 Old 09-03-2008, 5:51 PM Thread Starter
 
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HUUUGE flat spot, F3 Carbs

picked up a 96 f3 and the guy told me there was a flat "spot" right around 4500 rpm, I pulled the carbs off and went through them, a little tarninshed here and there but nothing major. put it back together and its idling better but the flat "spot" is a mile wide!! it starts around 4k and doesn't pick up until maybe 7k, completely flat, slow as christmas! what do check now? this is my first f3 also. thanks
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post #2 of 44 Old 09-03-2008, 7:08 PM
 
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Re: HUUUGE flat spot, F3 Carbs

Got aftermarket exhaust or air filter? Probably needs a jet kit. Jetting can make a huge difference.
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post #3 of 44 Old 09-03-2008, 7:59 PM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

yes, it has exhaust, a k&n and a jet kit. think someone screwed up??
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post #4 of 44 Old 09-03-2008, 8:15 PM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

and when i say slow as christmas i mean there is absolutely no change in going from 1/4 throttle to wide open, until it eventualy gets up to about 7k and then it runs like it should.
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post #5 of 44 Old 09-03-2008, 9:18 PM
 
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Re: HUUUGE flat spot, F3 Carbs

What main jets are you running? And where is your needle setting? I've got a D&D slip on and a K&N filter, 134 main jets, and the clip in the middle groove. My pilot screw settings made a huge difference in part throttle response. But getting the right combo of main jet and needle setting made my 3.5~5K flat spot go away. Now it pulls hard from 2.5 all the way to redline and beyond. Those F3 carbs may be a bit different though, seems I've read they are actually bigger than the F2 carbs. Whatever you have in there now, invest in a set that are 2 numbers bigger, and if you can afford to experiment a little, 4 sizes up from what you have now too. Put in the 2 sizes bigger, adjust your pilot screws and take it for a ride. You can read the plugs to get a clue as to whats going on, but with the unleaded gas these days, they can be a bit misleading. If I can help in any way, just let me know. I'm relatively new to motorcycle carbs specifically, but I am an automotive carb pro, and have messed with a bunch of outboard and small engine carbs, (even an airplane carb or two), so Ive at least got a clue as to which way to go, as a rule.

Last edited by Rickracer; 09-03-2008 at 9:25 PM.
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post #6 of 44 Old 09-03-2008, 9:53 PM
 
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Re: HUUUGE flat spot, F3 Carbs

Sounds like a the air solenoid is stuck or problems in the routing of the low/high speed air system. Check the hoses coming off the carb and how (if) they are oriented into the solenoid. Try bypassing the solendoid all together and only use the high speed (large plumbing).
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post #7 of 44 Old 09-04-2008, 10:44 AM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

i have zero knowledge of the high/low speed air systems. is there a simple way for me to check them? or somewhere online to find info? thanks
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post #8 of 44 Old 09-04-2008, 11:51 AM
 
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Re: HUUUGE flat spot, F3 Carbs

Take a look at these two diagrams. I thought I had something else a bit more specific but I cant find it and no longer have my bike to reference:

1998 Honda CBR 600 F3 partsfiche
1998 Honda CBR 600 F3 partsfiche

The basics of the system operate as follows. The smaller tubes are routed from your ram air to the carb vents at low speed, then at around 15mph the system switches over to the big tubes. Most people experiencing huge flat spots on the road but not when free revving the bike have a problem with this system. If your just revving it and have a flat spot, look into carb issues.
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post #9 of 44 Old 09-04-2008, 12:18 PM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

it revs great in neutral. thanks for the info, i'll give a good look and see what happens!
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post #10 of 44 Old 09-04-2008, 1:13 PM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

after looking at the microfiche for the air suction valve, i can not remember seeing any of that on the bike, i could be totally wrong though. this was someones track bike, is it possible it was all ripped out as part of some miss-guided modification? will these bikes not run with just carbs sitting on the motor and no other tubes or hoses hooked to them (other than fuel). i have more experience with automotive carbs than bikes so this a little alien to me.
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post #11 of 44 Old 09-04-2008, 3:50 PM
 
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Re: HUUUGE flat spot, F3 Carbs

Ahh, ok that narrows it down. Mine was also a track bike and I had all of that removed too. From memory here is how mine was routed to solve this. I looked all over for pics but couldnt find them.

Both sides of the F3 airbox have tubes that run down the frame toward the back of the carbs. Both these tubes were joined to a T that plumbed to the carb float vents as well as the large main tube running in between the two banks of carbs. What is killing the bike is the ram air which is forcing fuel back through the jets to equalize with the float bowls. the float bowls have to have equal pressure, hence hte lines to the airbox.
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post #12 of 44 Old 09-04-2008, 4:07 PM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

do i need to remove some lines? or make sure they are where they are suposed to be? i remember when i had it all apart, there were small hoses comming off of each bowl, each pair's hose was joined to make one hose for 2 carbs. i dont recall being able to blow air through these hoses. what are they for?
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post #13 of 44 Old 09-04-2008, 6:41 PM
 
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Re: HUUUGE flat spot, F3 Carbs

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Check out the picture above, it was the one I was looking for. Make sure that if pieces are missing that they all plumb back to those ports on the carb. You can do that with a single hose off one side of the ram air. Off the other side, plumb in the remaining vent tubes (much smaller tubes) to the carb bowls. The only hoses/ports remaining are the fuel lines.
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post #14 of 44 Old 09-04-2008, 6:52 PM Thread Starter
 
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Re: HUUUGE flat spot, F3 Carbs

can't wait to check it out, thanks so much for your help! i'll post results.
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post #15 of 44 Old 09-04-2008, 7:04 PM
 
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Re: HUUUGE flat spot, F3 Carbs

Definately, this was a little frustrating for me as well when I picked up my track bike. It took me a while to straighten out and in the end was relatively simple. The only unused ports you should have when you are done are the fuel bowl drains at the bottom of the bowl (has flathead screw to open close) and 1 to 4 capped ports on the intakes at the head for syncronising. Also, you might find these writeups useful for tuning when you are done.

Cleaning and tuning the carbs:
Musclecross Forum / Carb Cleaning and Tuning Guide - CBR 600 F2 / F3

Home made carb sync tool:
Musclecross Forum / Home Made Carb Sync Tool

Troubleshooting carb tuning:
Musclecross Forum / Carb Tuning Guide - Symptoms and Troubleshooting

And if you are really ambitious, valve adjustment:
Musclecross Forum / Valve Adjustment Guide- CBR 600 F2/F3
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