Choking the first second when cranking the throttle
 

Hi everyone :)

This is my first post at this forum. I own a CBR 600F F4 1999 model, bought from a friend that was tired of it not running properly for 50$. I actually took my drivers license on a F4 somewhere around 17 years ago, so I guess the circle is closed.

Anyway, I have spent a lot of time on it making it run properly and I feel I'm nearly there. However, I still have this issue when cranking the throttle wide open from idle. It seems to be choking from around 1200 RPM and to 2500 RPM, before it takes off as normal. For me, it sounds like its getting too much gas the first second until it catches up. Anyone familiar with this? Any suggestions?

I've uploaded a short video of it at YouTube. The first second is when I crank it wide open, the next seconds when i roll the throttle:

So far I've changed the spark plugs, all fluids, brake pads, tires, sprockets, chain, air filters, gas filter (and all gas cables from the pump and to the carburetor). I have also adjusted the valve clearances, cleaned the carbs and blown them clean with high pressure air, changed the gaskets, pilot jets and checked the floaters - twice, before syncronized the carburetors. I have tried all kinds of different settings for the pilot screw, but it doesnt seem to make a difference on this issue. I'm all out of ideas and I figured someone here may have any ideas. Help? :icon_goofygrin:

Did you separate the carburetors from each other and soak them in a carburetor cleaning solution? Blowing with high pressure air is not going to break lose tiny amounts of varnish from old dried up gasoline. Did you use the spray carburetor cleaner and verify that fuel is coming out of the 3 tiny ports next to the butterfly?

Thanks for the reply! I've actually taken them a part three times, the first time I just observed that they looked clean, so I put them together again without doing much. After reading on this forum I took everything a part and cleaned it. I focused on the main jet and the pilot jet, by leaving them in a engine washer component for a few hours, then brushing and blowing through with high compressed air. The rest i cleaned with a soaked rag. I didn't spray carburetor cleaner in the holes, but air. I guess I can go get a dedicated carburetor cleaner and give it a new go.

(I've also changed to new gas and cleaned the filter in the tank btw)

What items did you replace as far as O rings and gaskets? I don't have any specific experience with the carburetors on the F4, only the F3. It's very easy to miss the holes that are right next to the butterfly. These add fuel to the mixture during that initial opening of the throttle. This is an F3 carb, but suspect the F4 is similar. You want to verify that you get flow out of each one. There are 3 holes. The one closest to the camera is the idle mixture screw opening. The other 2 are fed by the slow jet.

https://cimg4.ibsrv.net/gimg/cbrforu...02b1eb5419.jpg

Thank you both for really good answers. I have now cleaned the three ports thoroughly on all four carbs and the carb cleaner is running through them without any issue. I havent got the time to put the bike back together in a few days, but i will update you both when its done.

I used a rebuild kit with gaskets and o rings from Hendler. Should be equivalent to the gasket set a from Honda.

https://www.hendler.co.uk/product/he...-92-95-350515h

If you have the carbs off the bike, I'd recommend item #7 for the O rings between the carbs. They are prone to leaking.
https://www.motorcyclespareparts.eu/...arburetor-assy

Thanks again! I will order those o rings, but it probably takes a couple of weeks to get them. Anything else I should order while I’m at it?

Whenever I'm rebuilding the carburetors, this is where I go for the O rings. I'll get kit A and B which covers all soft parts for all 4 carburetors. Shipping takes a while to get here to the US, but for me it's worth the wait.

https://litetek.co/Carb_Kit_Honda_CBR600_1995-2000.html

A big thank you for your input! :)

I cleaned the three holes on all carbs with carb cleaner and a really soft toothbrush. I also changed one pilot screw just to be sure (it felt a little tight and i were unable to clean it) and it took a while to get the part, thats why it took some time to get back to this thread Meanwhile I also took apart all brake calipers and cleaned them thoroughly as well.

Anyway, after putting it all back together again it now runs perfect. I took it for a 75km spin two days ago and the throttle is so smooth. I’m used to drive new bikes and didnt expect that a 22 year old bike could feel so good. What it lacks in features it takes back in smoothness, simplicity and low weight/easy handling. This weekend I have plans to go on a 1000km trip and I hope that the bike is as ready as I am.

I bought all the o ring/gaskets kits from Litek and I also bought a manual cam chain tensioner to replace the stock one (cam chain is ratling just a little bit). If there are other improvements or services I should perform, please feel free to give advice. The season here is only until october though, so I will wait until then to take a part the bike again:)

Thanks again:icon_bowrofl:

What a result, you must be over the moon.
It is remarkable that these bikes seem to perform so well after a little TLC.
Enjoy your trip, i'm sure you will have some nice piccies to show us when you get back.