Hey fellas,
not sure if im in the right section for posting this but im having a major issue with a flat spot on my 98 F3 in between 4k and 7k rpm. I have done anything and everything from carb cleaning (8 times) to syncing to removing and by passing the solenoid (reattaching hoses the right way) to inspecting the TPS sensor and everything i could possibly imagine and still nothing. Starts right up and revs up like a beast but as soon as i get on it i'd be lucky to get to third gear without it almost stalling every time. I even checked the fuel petcock and flow is good. I eliminated the fuel pump and still nothing. No matter what i do there is not one bit of difference. Im getting ready to throw in the towel and part it out since everything is in great shape on it. I also tried the stock exhaust and i think it even made it worse. It's almost like as soon as i take off someone turned off the fuel supply, leave it at idle for 30 seconds and everything is back to normal until i crank it up again. Im lost. Help guys! I had another f3 before this one and had basically the same set up (no fuel pump and no solenoid) and the thing ran like like it was tuned by pros. This one is driving me nuts i just dont get it. I would also mention that engine sounds great with no ticking noises or smoke or anything.

 

Do you get a vacuum in the tank? Blocked vent maybe. Try running it up with the cap off

 

I blew inside the little hole with the compressor and it was flowing good.

 

We had this on a F4 recently - some idiot had jetted it for a freeflow filter and a race exhaust - then sold the bike on with standard air filter and exhaust fitted.

Felt like hitting a wall at around 4k revs.

If you backed off and eased it through the revs on part throttle it would get passed the wall.

Have a look at the main jet sizes compared to standard

 

^^ Do you mean that you blew out the vent in the gas cap or the vents through the tank under the gas cap?

What are you running for intake and exhaust? Is she jetted and piped w/ aftermarket air filter or all stock?

I know how you feel about the cleaning over and over and over again but one little hunk of funk in there messes up the whole works.

What you describe definitely sounds like a fuel delivery problem. Either from the tank, from the pump, or from the carbs. Take a quick spin with the gas cap open. If it still stumbles then it's not the fuel tank. Bypass the fuel pump and feed the carbs directly from the tank. That will eliminate the fuel pump as part of the problem if it's having any issues. If it still stumbles after eliminating the tank and pump it's got to be the air/fuel coming from the carbs. I had a similar problem on my YZF last summer, bad stumble between 7-9k. Drove me crazy trying to figure it out but it came down to just being way too rich a mixture. The bike would almost die/stall and then come back on like a raped ape. Are you experiencing anything like that or does yours just keep bogging down till it dies?

 

That's exactly how it feels man, play with the throttle a bit to get pass the wall and it will take off like a lunatic. I guess i will have to look at the jetting but why is it still doing it with the stock exhaust? It has a stock air filter also. Only thing bike had when i bought was a D&D pipe. From what im reading what others are saying with this issue it goes back to the air duct system and the hoses around the air box.

 

It's stock right now, Yes i blew thru the little hole on top, i have not tried to ride it with the cap open i will tho but the fuel flow is great i see it pouring out the petcock. You are right it is fuel problem cause as soon as i hit that wall at 3500 rpm it wants to die unless i pull the choke down and it will take off again but only if i keep it at idle is i try to throttle up it will jerk back and forth. My mixture is 2.5 turns out not sure if thats good or not. But if im running rich that means im not having fuel delivery issue? I ran it without the fuel pump same thing. My other cbr had no fuel pump and no air solenoid and ran perfect. ???

 

Your basic problem is too much fuel for the air passing through the motor.

Two ways to fix it

1. More air - IE free flow filter and free flow exhaust

2. Less fuel - IE smaller main jets

Basically the carb has three areas of operation

idle to around 2k is low speed jets

2k to around 4k is needle shape

4k and up is main jets

Your issue is 4k and above - it is fixed by throttling back and easing it through the flat spot.

Therefore the mains are too big.

remember that the middle two carbs run one size larger than the outer two because they run hotter.

 

The idle mixture screws are set and sealed at the factory, if the caps that cover them are gone that would lead me to believe it has been jetted at some point in it's life. Are you sure it's stock?

 

They are not sealed on the f3 they are D shaped and it needs its own tool I have that tool and I set them at 2.5 turns out.