HUUUGE flat spot, F3 Carbs

WhitsRC · 09-03-2008, 5:51 PM

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

Rickracer · 09-03-2008, 7:08 PM

Got aftermarket exhaust or air filter? Probably needs a jet kit. Jetting can make a huge difference.

WhitsRC · 09-03-2008, 7:59 PM

yes, it has exhaust, a k&n and a jet kit. think someone screwed up??

WhitsRC · 09-03-2008, 8:15 PM

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.

Rickracer · 09-03-2008, 9:18 PM

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.

SoCal94F2 · 09-03-2008, 9:53 PM

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).

WhitsRC · 09-04-2008, 10:44 AM

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

SoCal94F2 · 09-04-2008, 11:51 AM

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.

WhitsRC · 09-04-2008, 12:18 PM

it revs great in neutral. thanks for the info, i'll give a good look and see what happens!

WhitsRC · 09-04-2008, 1:13 PM

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.

SoCal94F2 · 09-04-2008, 3:50 PM

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.

WhitsRC · 09-04-2008, 4:07 PM

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?

SoCal94F2 · 09-04-2008, 6:41 PM

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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.

WhitsRC · 09-04-2008, 6:52 PM

can't wait to check it out, thanks so much for your help! i'll post results.

SoCal94F2 · 09-04-2008, 7:04 PM

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

WhitsRC · 09-04-2008, 9:42 PM

still stumped SoCal. i have all of the tubing from the holes on the ram air but no solenoid, just straight tubing hooked to all of the pictured ports (with the air filters there too). i tried a few more things on the test ride and with the bike around 4k i can hit the gas hard and there is an immediate "dip" then it starts to stutter and bog, eventually it comes out of it. after letting it come out of it and running it up pretty hard, i would let off the throttle and the bike would die, then it was very hard to restart. sometimes it feels as if it "acts up" worse than other times but it still acts up every time i try to give it gas. it revs great in neutral and it screams once its past around 7k.

SoCal94F2 · 09-04-2008, 10:08 PM

My memory is returning to me a little on this. Just to be sure, there are two vent tubes, one between each outer pair of carbs. You have to take off the airbox to get to them. They route directly under the airbox and narrowly exits toward the rear of the bike, sometimes getting in the way of the lower airbox installation. Make sure these are plumbed in to the ram air.

Not impossible, more strangly probable... This sounds a lot like a fuel pump issue I was having on my F3 track bike. I found the positive power to the fuel pump was cutting out when the bike hit that same rpm range and would result in those exact same symptoms. You might try hotwiring the fuel pump and taking it for a spin just to eliminate that as the cause.

WhitsRC · 09-04-2008, 10:21 PM

you correct on the hose routing, there are two, one down each side, the left splits into 2 smaller ones and go in between the outside carbs, the right is larger and goes stright in between the inside carbs. i will try the fuel pump trick next. would switching it to gravity feed help eliminate anything?

SoCal94F2 · 09-04-2008, 10:31 PM

I did run gravity feed when trying to diagnose it but lost my top end. I hope its the pump wiring, sounds just like it...

WhitsRC · 09-04-2008, 10:39 PM

i went out hooked the pump straight to battery to hear it run, it actually makes a clicking sound and i can of course feel these clicks by touching the pump. i hooked it back to the bike and started it, it idled for a few seconds then slowly dies out, now it won't crank at all, the spins over and sputters a bit. should the pump be noisy like that? i could here it clearly while the bike was running.

SoCal94F2 · 09-04-2008, 10:47 PM

Did you disconnect the fuel hoses from the pump or power it connected to the bike? If you disconnected the hoses, make sure you didnt cross them putting them back on. When it died, was it running off its stock power again? If so, you need to run both a ground and power wire to pump (measure which pin needs positive voltage) from the battery to the pump and leave it unplugged from the stock socket... and take it for a ride.

I recall there being a fuel relay somewhere near the battery that may be the culprit for all this, or the wiring to it. I ended up taking 12 volts from the relays socket and hotwiring the fuel pump as a temporary fix.

WhitsRC · 09-04-2008, 10:52 PM

did not disconnect them, actually tried just now to run the bike with the pump hooked to 12v and the only way it would crank is with the choke all the way out. it didnt want to take any throttle either, as soon as i would let off the choke, it would start to die. the idle screw is still set where it always was and i dont smell gas leaking anywhere. it would of course not run at all with the pump disconnected so i'm pretty sure the pump is not the problem. right now theres no way i could ride it, and an hour or so ago it was ridable. am i looking at taking these carbs apart again?

SoCal94F2 · 09-04-2008, 11:06 PM

Can you give me a history on this bike? Did you inherit this bike like this, did it happen suddenly or did it onset slowly over time.

Just reread your first post, sorry - you picked it up like this. You may have fouled your plugs if the bike went overly rich at some point due to bad jetting/needle position. What a pain pulling the plugs on the F3 is though.

WhitsRC · 09-04-2008, 11:11 PM

first of all, thank you so much for continuing to answer my questions! i bought the bike a few days ago and to my knowledge it has been a track bike for a few years at least. the guy raced it up until a few months ago. he told me there was a funny sport right around 4500 rpm but he didnt worry about as he was mainly running high rpm. i dont know if it got worse when i rode it or not, but the spot is something that would causee you to lose a race if you had to drop the revs! also, after it died on me on the test ride tonight, once started it kept backfiring even as it "felt" like it was doing ok. now i still can not get it to crank without full choke.

SoCal94F2 · 09-04-2008, 11:21 PM

No problem, this board helped me quite a bit on geting to know my F2 and F3, very valuable resource. Its good to pay it forward. Does it have a jet kit? A few ways to tell would be if it had adjustable needles, a plugged hole in the bottom of the slide or DJ designastions on the main jets. Your description of the current state of the bike sounds like fouled plugs - too rich of a needle position would foul the plugs in this manner. You can remove them and clean them on a wire wheel. You may have to do this few times if the jetting is that bad. If it has adjustable needles then then raising the clip, effectively dropping the needle will lean it out. Take a close look at those links I sent you where tuning is concerned. If the clip on the needle is already at the top, the mains might be set overly rich to compensate. This too could foul the plugs. Plugs are very sensative on the F3 for some reason.

WhitsRC · 09-04-2008, 11:28 PM

would it even want to crank at all with fouled plugs? i'll check them too though, why not. the seller told me it was jetted but i do not know details and i did'nt pay attention when i had it all apart... and i do not recall seeing any blocked holes on the sliders. if it was jetted rich, shouldn't it have messed up long ago and not just in the last 3 days? i've got my eye on a set of carbs on ebay just in case...

SoCal94F2 · 09-04-2008, 11:48 PM

Fouled plugs range from wont start at all and only backfire, to starting when choked and not having much power. There are three popular race jet kits on the market. The Factory Pro which uses stock Keihin jets and a tapered needle, the DynoJet kit which uses DJ marked jets (that do not relate to stock jets numericly) along with a stepped tapered needle and Yoshimura Multiple Jet Nozzle which had hollow needles (and was what my F3 came with). I later converted to the Dynojet kit. Read through the carb cleaning link and make sure you covered all those bases - it will be important as any F3 let sit for a while will usually clog in all those places. Dont worry about new carbs, the chances of you needing replacements are slim. They would have really haved to jack up the slides or bodies to require a replacement. This is one of those frustrating areas of F3 ownership, but once you get it worked through they are generally great bikes!!!

WhitsRC · 09-05-2008, 9:32 AM

sounds good, i pulled one of the plugs the AM, an easy one to get to and it looked fine. i'll pull the carbs tonight and follow the cleaning guid AND make not of the jets. thanks

SoCal94F2 · 09-05-2008, 12:56 PM

If the plug is black at all, then it might be fouled. Mine were just slightly tinted black and the bike wouldnt run... cleaned them and it started right up. Like I said, these bikes are finicky. Do make a note of both jet sizes in each carb, the smaller pilot and larger main jets as well as the needle position. I can help guide you to a baseline if its way off before you put it all back together.

WhitsRC · 09-05-2008, 11:52 PM

ok, 134 on the outside, 136 on the inside, the needle clips are all in the center position. i see no other markings or modifications to any of the jets or slides. whats next?