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 high, 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-cut valves, didn't know what they were until recently. Maybe it's related?

 

Did you set the idle mixture per the manual after you assembled the carbs? It has to be pretty spot on for everything to work properly.

 

I responded to this thread here.

https://cbrforum.com/forum/cbr-600f3...roblem-132611/

 

When you rev an engine, the air pressure on the inlet side of the carb drops. because there are jets and open passageways in the carb, I would expect to see surging in the bowl level. That's most likely normal. If the level is low, you may not be getting enough raw fuel to richen the mixture for cold starts.

What grade fuel are you using? High octane fuel is slow burning. Low octane fuel is faster burning and will help in cold weather starts.

 

Thanks guys, I think I might get it working tonight. I figured out the fuel supply to the starting enrichment circuit is the long brass jet that can't be removed, and they appear to be clogged on 3 of my carbs!

Good suggestion about the fuel TimBucTwo, I've been using 93 the whole time.

I just got a little electronic tach that's made for lawnmowers and dirtbikes etc. now that I'm going to use to set the idle mixture just right this time. I think there might have been a leak in a cracked air-cut hose, so I replace them with new vacuum lines. I guess I gotta sync them all again now too!

 

It think it's all fixed finally. Whooo! I got it to fire up cold and the SE all the way on holds it around 4,000rpm.

I had some clear vinyl tubing in the garage that fit snugly over the jet, the other end inside the tire valve clamp on my 12v air compressor, and I blasted them all out. That's how McGuyver taught me to do it. lol It's 29 degrees here today, was really hard to handle those little parts in an unheated garage!

 

If you don't need it, you are wasting $$. High compression engines need it to stop pre detonates. Low compression engines need a faster burning fuel as they don't have the advantage of the compression to ignite the fuel. The ethanol in the fuel today acts like an octane booster and makes for bad cold starting also.

Unless you hear knocking or pinging, you don't need high octane fuel.

Glad to hear all is well and she is starting now.

 

sorry to kinda ask another question but i got a manual and i can't find any Starting Enrichment Circuit. and a bike im about to buy is having a really hard time starting when it's cold out. we spent like half hour trying to get it started and finally after giving up with trying we got it started from compression starting it and starting fluid. i thought it was just hard because it hadn't been started in a month or two but the next day i guess it was refusing to start again. any ideas?

 

Start a new thread.

 

HI, Sorry for wakening the dead post here, but this post gave me some hope of not loosing my mind, i've been struggeling with the same issue on my 95 cbr600f3, and i see you mention your putting a tube over the jet, is it the the jet with the red ring in my attached photo?



(not my carb, mine are extremly clean after multible cleanings and assemblings)

I used almost the same procedure, but filled the tube with carb cleaner and blasted it thru with air. But the air also comes out somewhere else, inside the float chamber somewhere, is this normal?

Sorry for my incorrect English (Norway)