CBR F3 Running Lean: Symptoms, Diagnosis, and Repair

A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw. The pilot air screw meters air, so turning it clockwise/in will richen the fuel/air mix, while counter-clockwise/out will lean the fuel/air mix.

cbr f3 running lean definition

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yes indeed. its strange. last night i was try using 17.5 again and get good idle on 2 counterclockwise screw. and if I screw all in, the engine will dies. but same, i feel the engine so rich while running low. if there any possible the pilot jet was broken or the size is incorrect due imitation product? or maybe i need to setup the float more lean? thank you for the reply Matthew. Very appreciated

my idle jumps up and downat pull away i stragle have to rev it up to pull awayi have read this blog but cant say if its lean or rich because if cold and i rev the rev wil drop lower than idle but when the bike is hot and i come to a stop the bike wil rev 3000 and if i drop clutch a bit it wil drop to idle again

Both carbs should be tuned to the same number of turns out on the pilot fuel or air screw. This will result in a smooth running engine. Obviously, when tuning, you can only work on one side at a time.

If it will only run on starting fluid, then the carb is not delivering fuel. When you had carb apart, did you spray carb clean through all passages and verify they are clear? Did you hold all jets up to a light source to see if you can see through them?

Hello, i have a kawasaki mule 2002 with the 620 motor. it never has run right at idle so after some time replaced carb with a new one from kawasaki. the new carb seemed a little better it gave me some idle adjustmant for a while but still seems to be quite lean at tickover. now it has the idle screw right in and strugles to idle and stalls. the top end revs seem good and there is power. also when punching the throtle it bogs. i have looked on various sites and they say there is only one size of pilot jet and there non adjustable due to some regulations?

Now that your fuel screws are nearing 1.25 out, you can get a leaner cruise (1/16-1/8) throttle by dropping pilot jet size. You will have to readjust fuel screws and they will end up about 1/2 further out. If you decide to do this, your will be at the same fuel ratio and the bike will be getting a little less fuel when 100% on pilot.

John, it sounds lean. Go ahead and lightly seat your fuel screws noting how many turns it took. Get the bike HOT and tune the fuel screws for highest idle. Report back how many turn out you end up with and whether it helped.

Hello Matthew,I have a Virago 535 2002 with popping from front cylinder, Bike has Highway Hawk Fat Pipes ( think they are highway hawk, no marking on them)the exhaust had very small baffles in which I have replaced with 8inch baffles and baffle wrap ( makes chuffing sound now instead of loud bang) apparently the previous owner had a Dyna jet carb kit fitted at same time, carb sync looks near enough, neither mixture screw has much effect, front is 3 turns out and rear is 2 2/3 out popping seems to ease a little if front mixture screw screwed in until it is 1 1/5 turns out but neither effect engine revs, when the front mixture screw is fully seated with engine running then loosen it engine revs rise slightly but only momentarily then even out again. spark plugs good colour not sooted up or white. thought I had a vacuume leak sprayed carb cleaner on rubber carb mount to engine on front cylinder, engine revs rise dramatically,, stripped it down twice now and reseated/sealed all I could,, I`m now at a bit of a loss as to what to try next.any suggestions.How it sounds below:-

Yes i did do a pressure test and all was good after that one little hiccup. Has been starting fine ever since. My float hight is factory which is 7mm from the gasket surface with the float just touching but not compressing the needle spring. As far as drilling the slide i know i havent done it but thats not saying someone else didnt. It was jetted but not with a jet kit by a shop i do know that. Im not looking to get rid of all the pop on decel but it was excessive in my book. And the tip of the pipe being white/grey was another lean indication i was thinking. Thanks again for everything. You have been a huge help btw. I tryed for leaks again like you told me. I sprayed everywhere and it is leak free. Ive actually done that like 5 or 6 times before just to make sure i wasnt missing something. Because to me it dont make any sense to need 20 pilot jets when stock is 12.5 but all tests i do come back negative. And compression is good and everything keeps pointing me to pilot jetting. I guess this bike just came mega lean from the get go but how didnt it hessitate when new? Or maybe it did who knows. But its only got 13000 miles on it so it should be able to run and ride smooth i would think.

Ok now this one has got me scratching my head. Since my last post it has been to cold to ride until today so i went out and went a half turn more fuel. Which puts me at 2 turns out. I put alittle bit of sea foam in the tank and topped it off with vp110 and went for a ride. It will no longer accept full throttle. It breaks up really bad at wide open throttle. Almost like its lean on the main jets. But the only thing i have changed is the mixture. Im stumped. But plan on trying 2 1/2 but cant figure out how that would make any difference at wide open throttle.

Just as i had thought the mixture chnage made no difference. Still will not except full throttle. Seems like its running out of fuel or something. I did find something that may be throwing a wrench in the gears tho. I pulled off the vacuum line to the fuel valve and it was wet and had alittle bit of fuel in it. Im wondering if that diaphragm has a hole in it. And maybe thats why it seems to need so much fuel at idle when the vacuum is low. Just a thought. Let me know what yours are. Thanks

Well that did not go as smoothly as planned. I tested the new tps and it tested good and way different then my old one. So i installed it per the manual. I put the 15 pilot jets in and also put the needle back in the middle as a base line starting point. I took it around the block and it ran terrible. Bog off the line so bad i had to rev it and slip the clutch to get it to even move. Once i got above 1/4 throttle it ran smooth and clean tho. And the jerky throttle response is gone now. But this bog is what has me confused. My mixture is at 2 3/4 and if i go leaner it seems like the idle drops and isnt as smooth. I do know for sure that the tps not working made the bike run way rich because i pulled the plugs just grasping at straws and wow were they fouled. Black covered in soot. But i changed them with new ones and it still didnt completely fix it. It almost sounds like the tone of the exhaust has changed also. Way deeper almost v-twin sounding until i get past 1/4 throttle then it hits a turbo and most of the time blows the rear tire off. Kinda scary the couple times it did that lol.

If you go back to the link I provided on tuning cutaway, there is a graph from Mikuni that shows each size changes fuel flow by 10%. On that particular bike, I stated with a 3.0 (lean) and went to 2.0 (rich) and it made a drastic difference. I ended up going to the 2.5. So I experimented with a 20% difference.

The main jet is responsible for 3/4 to full throttle position running. You mentioned hard starting and idling issues? Well, that is all pilot jet/fuel screw. Get that sorted out and move on to the needle and main.

Hi Mathew, I have a Yamaha FZ 750. K&N filter in standard air box and straight through muffler.Bike run great after 3000rpm but up until that is hesitates under acceleration. While not under load it revs up well.Im running standard jet but after reading some other comments it looks like i need to up the pilot jet a size or two.Would that be the thing to try.Thanks.

Hi Matthew, I have been watching your videos and admiring your work on carburetors. I have learned a lot from your videos on pilot circuit adjustments. I have worked on a few carbs myself with good results. I recently purchased a 2000 Honda Sabre VT1100C2 with 16000 miles. The bike had not been ridden much the last few years and I expected to have to disassemble and clean the carbs. When the previous owner first started the bike for me it popped through the carb which I believed would be a lean condition due to a mucked up pilot circuit. I removed and disassembled the carbs completely and cleaned them. The pilot jets were plugged. The rebuild kits I bought include jets and the pilot jets were the same on the orifice side but the air holes in the tube side were smaller. I installed the new ones just the same. Fast forward and after all back together I started the bike and it ran much better, but, it still seemed a bit boggy/skippy at and just off idle. When I attempted to adjust the pilot (fuel side) screws backing it out made little difference in RPM. So I put a tach on it and the idle was way high. When I brought the idle down to the 1000 spec she wanted to die and did stall after 30 seconds or so. I can get a so so idle at 1200 RPM with the Pilot at 3-1/2 turns out. I test drove it and it is still boggy/skippy when taking off from a stop. After it gets above 2K RPM it takes off like a banshee! So, I feel I should take the carb back out and make sure the pilot circuit is cleaned properly. I plan to put the (clean) stock pilot jet back in and then see how it runs. I would really appreciate your feedback and recommendations. Does no movement in RPM when I back out the pilot mean it is too lean or dirty?

Trying to get my 2001 Sportster 1200XL to run, I cleaned the carb out and replaced all gaskets and o rings as well as new float pin(may be issue need to know whats correct setting of pin), I can crank the bike over with choke open once I release it kills out and fuel begins to overflow from carb. I also replaced vaccum lines and fuel/res selector. 2ff7e9595c