HUUUGE flat spot, F3 Carbs

SoCal94F2 · 09-06-2008, 4:36 AM

That sounds like a Factory Pro kit at the installed baseline with the right jet sizes and the right needle position for your F3 (with aftermarket exhaust). Make sure they are all clean, including the pilot jets and the non removeable idle jets (always clogged if set for a while). Make sure the plugs are cleaned of any carbon. Using the tuning guide and shape of the plugs you can dial the flatspout out or in by lowering or raising the needle position. Typically a big flat spot in mid throttle is needle slightly too low (lean) but since your setup stalled out and was hard to start afterward, theres a possibility it is overly rich and the clip needs to be raised a notch (lowering the needle). It's hard to say over the web which one of those ways it needs to go, the tuning guide and some trial and error should get you there.

WhitsRC · 09-06-2008, 10:47 AM

good to know the jetting is about right. i brought them to work with me today to clean the piss out of them! i'll re-install and try it out. also, is there some trick to getting to the clamps on the rubber collars at the base of the carbs, i can find a way to get to about 3 of them, then i just start wiggling the carbs to get them off.

SoCal94F2 · 09-06-2008, 3:58 PM

The absurdly long screwdriver trick of removing the boot clamps at the head side of the carbs is the easiest way. Then you can rock the airbox and carbs with the boots still on them off as one piece. Soooo much easier. Make sure the clamps are good and loose and they pop right back on the head.

2. Easier carb removal. Go to your favorite tool mart and buy a #2 20" phillips screwdriver. Instead of coming from the transmission side of the carbs to loosen the carb boots, use this driver to insert to the boot clamp at the carb on the top side. You access this from the left side of the bike for all 4 boots. Use a flashlight to guide the drive in. Now when you remove the carbs, the boots stay on the engine and you didnt burn your arm on the trans. You may need to set this up in advance if others have been in there ahead of you and disturbed the stock orientation of the screws. See my pic below as to how all the screw should point the same way.

http://www.musclecross.com/forum/upl..._image_398.jpg

WhitsRC · 09-06-2008, 9:42 PM

tore the carbs down as far as i possibly could today and cleaned them spotless through and through. also cleaned the plugs with a wire brush. fired right up and actually runs better, but not quite perfect. i think now the flat spot is just that, a flat spot due to jetting. it is actually rideable now and pulls very hard up top. down the road i may pull it all back apart and fine tune. very short test run (night time with no headlight), but deffinate progress has been made. thanks so much for your help!

SoCal94F2 · 09-07-2008, 3:16 AM

Hey, good to see you got it back to running status. It's such a chore to do that but after a while it goes quick. Pulling the entire airbox and carbs off in one shot saves a ton of time!!!

Two things to take away from this.

1. A deep carb cleaning is a frequent part of an F3 maintenance. Be prepared to do it whenever the bike sits for more than 4 weeks. (2 weeks for an F2 with vacuum petcock)
2. The F3 has very sensative plugs to fouling so be cautious of running rich for too long. Now you know the symptoms (pretty sure that was causing the backfires and hard start).

Since the needles were in the default installation postion and running the default jetting, you may be just shy of tuning that flat spot out. Since the top end feels good, your most likely very close with the main jets. Adjusting the needle clip position down to richen, or up to lean out the midrange should give you a clean and progressive power band.

An F3 will barely pull the front off the ground in 1st gear with a roll on throttle when its tunes as best as it will get... with -1 tooth on the front sprocket it will pull hard through first gear and end in an unprovoked wheelie though. Thats a good repeatable guage to the max amount of power you can get on these bikes. Call that a seat of the pants dyno method for tuning for repeatability, when it wont throttle wheelie without help you've crossed the tuning apex of max power. Dont try that on a liter bike though!!!

.

Good luck, I'll keep an eye out for progress.

WhitsRC · 09-09-2008, 1:54 PM

just found on another forum where some folks were having bogging issues similar to mine and the fix was to ground out the speed sensor switch that went to the air solenoid. does this sound feasable? another suggestion was to block off the ram air and force the bike to suck in its own air. either of these make sense? ever heard of them? thx

SoCal94F2 · 09-09-2008, 2:29 PM

The wire that goes to the air solenoid from the guages is a ground wire which activates the air solenoid over 30 mph. People running the air solenoid without this wire connected bog down because the secondary (high flow) ram air never opens. In your case, it doesnt exist so thats not your problem. Running without ram air could be a tool for you to identify whether the flat spot is ram air related or carb tuning related (or both), but you would be better to tune using the ram air for best accelleration. Everything you have described supports a bad needle position more than anything else...

Something else to consider is needle shape. For instance, a stock or Factory Pro needles are an even taper from one end to the other while the Dynojet needles are stepped while tapering. They did this to combat flat spots associated with the stock airbox.

Ahh, the airbox. Many 5k rpm dips on bikes are directly associated with the airbox shape and design (thank you EPA!). This is one of the reasons I moved to the F4i airbox which is larger and supports a less restrictive ram air flow.

Also, if you are running a K&N filter... remove it and take it for a run. I can't tell you how many times flat spots ended up being a restrictive filter, especially K&N because the user can accidentally over oil it when cleaning which inhibits flow.

WhitsRC · 09-09-2008, 3:43 PM

it does have a k&n filter, so i will try removing it for a run. also, my bike has no guages, i dont know of that would have some sort of effect on the way the bike runs (i have read removing them can cause running issues, but probly only if air valve is still present). the needles are even tapered and I have raised the clip one notch, lowering the needle, and the bike feels exactly the same, "maybe" a little softer up top. i have not checked the plugs since adjusting the needles. i will punch myslef in the stomache if this is all from an air filter!! but I'll be happy to have it figered out!

SoCal94F2 · 09-09-2008, 4:04 PM

I once spent 2 days fighting a jetting issue that ended up being my freshly cleaned K&N on a newly purchased bike. Words can't describe how pissed I was that I overlooked the simplest solution. Also spent a day adjusting suspension for a speed wobble that ended up being rear tire pressure - another DOH! moment...

SoCal94F2 · 09-09-2008, 4:35 PM

Try using this tuning method by writing down your findings. What I tend to do is take a piece of paper and draw a 4x4 grid on it. This breaks throttle positon up one side in 1/4 increments and rpm down the bottom in 1/4 increments. Make sure to document your jetting and needle position on the paper as well, and use a new paper every time you change it. This will give you a history to reference rather than trying to keep it all in your head. Small changes for the better will become immediately obvious. Now take the bike for a spin in 1st gear only. Pay special attention to your throttle position. You can mark it with 1/4 increments using tape if need be so you can visually double check where it is while riding. Run the RPM band and document the exact specifics of the flat spot.

Take a much closer look at the flat spot on paper. Try and figure out the relationship between rpm and throttle position. For instance, is the flat spot strictly RPM specific, or is it strictly throttle position specific. If the two are interelated, then try and figure out their interdependencies. If the bike surges or pops around the flat spot, its running lean. If it runs rough around the flat spot with a deepish tone, its more likely rich. Your Throttle Position is a great guage as to where your jetting/needle postion is off. From 0-1/4 throttle you are using the pilot jets and needle position, from 1/4 to 3/4 you are strictly needle position and 3/4 to full throttle pretty much only main jets. This is a guide more than anything since an overly high or low needle with effect both 0-1/4 and 3/4 to full throttle as well.

Now start a new paper and do the same thing in second gear. This is where your ram air will have a greater effect and could introduce new issues.

For reference, I have never needed to change the pilot jets on a stock engine with jetted carbs. Main jets and needle position constitutes the majority of changes. You may also want to confirm if the bike has had an ignition advancer added to it - it may be the source of a low end flat spot as an expense to make more top end.

Jonesy920x · 07-23-2009, 9:54 AM

Quote:

Originally Posted by SoCal94F2

The wire that goes to the air solenoid from the guages is a ground wire which activates the air solenoid over 30 mph. People running the air solenoid without this wire connected bog down because the secondary (high flow) ram air never opens. In your case, it doesnt exist so thats not your problem. Running without ram air could be a tool for you to identify whether the flat spot is ram air related or carb tuning related (or both), but you would be better to tune using the ram air for best accelleration. Everything you have described supports a bad needle position more than anything else...

Something else to consider is needle shape. For instance, a stock or Factory Pro needles are an even taper from one end to the other while the Dynojet needles are stepped while tapering. They did this to combat flat spots associated with the stock airbox.

Ahh, the airbox. Many 5k rpm dips on bikes are directly associated with the airbox shape and design (thank you EPA!). This is one of the reasons I moved to the F4i airbox which is larger and supports a less restrictive ram air flow.

Also, if you are running a K&N filter... remove it and take it for a run. I can't tell you how many times flat spots ended up being a restrictive filter, especially K&N because the user can accidentally over oil it when cleaning which inhibits flow.

I just stumbled on to this forum via google and thank God I did !

I have a 98 F3 that I just stunted out and removed the guage cluster. Guess what it wouldn't run over 30 MPH....I was stumped for a week had the carb off at least 10 times. New mains new pilots etc...Then I read this post. After a trip to the local hardware store for a pvc fitting to bypass the air solenoid and a whopping .58 cents the bike screams ! I've been putting it together for 8 weeks and was dying to ride. THANK YOU !

The only thing I might have to change back is the main jets. It had an all stock set up and the mains were 140. I put in 145's k&n and an aftermarket exhaust with the baffle out. It seems to be running great and I have seen any carbon build up in the exhaust tip. It might be a little fat, but I haven't noticed yet.

Timothy1959 · 08-05-2009, 2:31 AM

I recently baught a CBR600F2 that also had a Dynojet kit installed, with similar problems. I went back to stock (OEM) carb parts & solved all the problems. I replaced 4 jet needles, 4 diaphram springs, 4 main jets and removed all the plugs from the CV pistons. The machine now SCREAMS at all engine speeds with PLENTY of power! :-)