Hi, wondered if someone could understand my carburetor problem that makes my bike so hard to start up cold. It's a 1996 CBR600 F3.

The choke on my bike (starting enrichment circuit) doesn't seem to be working right. Pulling down the choke lever will make the engine fire instantly and rev high for a second or two, then the rpm plummets just as fast and it dies. The only way I'm able to start the engine cold is to turn the idle speed up very high with the choke off, and let the engine warm up for a few minutes before driving.

I have taken the carburetors apart a couple of times now and cleaned them thoroughly as I could and removed lots of varnish. I even got a vacuum synchronizer and sync'ed the carbs. That made startup a little easier, but still the SE doesn't have its intended effect. I really can't find anything that looks wrong in the carburetors.

On the idea that maybe the fuel supply wasn't as good as it should be right at startup I watched the float level on the #1 carb, and it drops while I turn on the choke. That shouldn't happen right? I have a video to demonstrate what I mean: Float Level Changes with Choke on. - YouTube

The engine was warmed up just enough to stay running at idle there. I pulled the choke lever on and the engine revs up and fuel level drops, and the engine slows back down to idle with the choke still engaged. I turn on the choke twice in the video, then two revs with the throttle at the end to confirm that the fuel level doesn't change with the rev.

What's going on there? I'm stumped by this. The choke does not seem to richen the mixture at startup for more than a second, but I did once turn on the choke with the engine warm, revved it up, and it shot out black exhaust smoke. It definitely appeared to get plenty of fuel that time.

Hmm I realized I never took apart the air cutoff valves, didn't know what they were until recently. Maybe it's related?

 

i hope you can figure that out soon. im suprized no one replayed yet. try in general tech.

 

I know that you've cleaned your carburetors a couple of time, but sometimes the 3rd time is the charm... right, LOL.

Anyway, the enrichment circuit has to be cleaned and verified to be clean and clear if it's going to work properly. The one jet that you can't remove is for the enrichment circuit (naturally). During the cleaning process you want to be able to verify that you can get your carb cleaner to flow through these passageways. In the following thread, I show you where the fuel flows for that circuit. Other things that affect it are going to be the idle air mixture adjustment, carburetor synchronization, and the air jets being cleaned.

I wouldn't worry so much about the fluctuation you're getting on the float level. It is going to move up and down a little, that's how the floats work to regulate fuel flow into the bowls. If the movement was drastic, then there would be room for concern. Take a look at this thread and we'll go from there.

https://cbrforum.com/forum/stickies-...cation-124026/

Something else to realize is that the enrichment circuit only works properly if you're not opening the throttle during starting. It relies on the higher vacuum levels in the intake manifold to draw in that extra fuel. If you're opening the throttle, then you loose that vacuum. Remember that these carburetors don't have accelerator pumps in them like a car does. The throttle only operates the butterfly. Everything else operates automatically based on vacuum levels, engine load, and engine RPM.

 

Thanks for the help, I think I might have narrowed down the problem last night. Sorry for double post, didn't seem that I would get any reply in the F3 forum.

Was that identification thread revised? I think I saw it before with the unremovable fuel jet labelled the idle jet. Anyway I took them apart again last night, and used a hose to blow into all the jets to see what happened. The air jet for the SE systems are all working. The pilot jet gets its fuel from the small slotted removable fuel jet.

I did however find that the fixed brass jet supplied fuel to the SE circuit on one carb, and is totally blocked on my other three. Hmm this could be a hard one since they've got the most long torturous passages! I'm gonna start with carb spray and compressed air, maybe try jabbing pipe cleaners in next...

 

Don't jab anything in, to clean a jet. They are sized very precisely and you could damage them. Soak them and use air till you get the blockage cleared.

Ern