Topic: Cb400f stops on down gearing

[ripamarcone]

My bike stops running when going back down through gears and coming to rest. There is little throttle response on down gearing. I'm not sure what jetting is in the carb but I'm sure they are stock idle and main. (My mechanic has changed the jets) Once it stops, it starts right up.
I thought it may have been electrical, and maybe it is, but with the lack of throttle response i think its fuel related.
Hoping to get some heads up before i pull carbs off

[harisuluv]

If you are say in third, and then you pull in the clutch, does it stall/die?  Sounds kind of like an idle adjust issue via idle adjust screw (1) or the air screws (4).

[ripamarcone]

No, there doesnt seem to be any probs between gears. It seems to be only when you are just about to pull up at an intersection or traffic lights.

[calj737]

I'd bet it is the float level of the carbs. The fuel is too low while running, so the bike starves. Check for good flow from the tank (do you have an inline filter by chance?) and verify there are no "upward" bends in the rule in to the carbs.

[Tim2005]

That does sound like fuel starvation. As well as Calj's excellent advice I'd also check that the little breather hole in the fuel cap isn't blocked.

[DaveBarbier]

I seemed to have the same issue while coming to a quick stop or stopping normally downhill. I had to LOWER the fuel level because it was getting too rich as the fuel sloshed towards the pilot jet. I guess it depends where the pilot jets are located in your carbs, if they are engine side then it seems as though you have my same problem. If they are airbox side then you'd have the issue Cal is describing.

Use the clear tube method to determine exactly where the fuel in the bowls are.

Here's the play by play in my build thread: http://forums.sohc4.net/index.php/topic,140833.msg1686947.html#msg1686947

[ripamarcone]

Thanks for your replies... okay. The floats are set at 23 by what i have been told and all the same. Checked with clear line. Yes running a fuel filter. ( I'll remove that tonight when i get home) I'll check the the tank cap as well..ipulled the plugs last night,only to find number 2 to be fouled , the others look great.

[calj737]

If 2 is dry fouled, too much fuel. Wet fouled is poor spark.

[ripamarcone]

Fuel filter removed, gas cap vented, waiting on finding out what jets are in the carbs. Dont want to open them until have this info.
Im thinking maybe 23 mm for floats is a tad high. I read somewhere that air screws should be 2 turns out on these. Can anyone varify?

[harisuluv]

Thanks for your replies... okay. The floats are set at 23 by what i have been told and all the same. Checked with clear line. Yes running a fuel filter. ( I'll remove that tonight when i get home) I'll check the the tank cap as well..ipulled the plugs last night,only to find number 2 to be fouled , the others look great.

The clear line isn't going to necessarily tell you what the float level is set at.  The clear tube method tells you what the fuel level IS.  If you used the clear tube method, how far below the bowl line are the fuel levels?

I'm still not really clear what you mean by poor throttle response through downshifting.  Downshifting itself isn't really related to the fuel system.  What it sounds like is you start to brake/slow and then pull the clutch in, and then you basically go immediately to idle circuit and die.  You can be downshifting while the clutch is pulled in but this is only a correlation not causation. 

Really it sounds like you need to just A)  Raise idle stop set so that it is high enough to sustain idle (like when you pull in the clutch).  or B)  Adjust your mixture so that it is correct enough to sustain idle properly.

2 out is excessive and maybe not even in the range of doing anything.  1-1.5 ballpark is probably where  you want to be.  In your case you would probably be turning them in if you're at 2 now.

Also, what is your bike even like, pod filters, exhaust, modifications?

[ripamarcone]

The floats were set up correctly and the clear line used after to check only.  The air screws were 1.5 out, just read somewhere about being 2 turns out.in regard "I'm still not really clear what you mean by poor throttle response through downshifting" i mean that if you open the throttle a little between going down through the gears, there is little response unless you open the throttle right up.
The engine has just been rebuilt. All standard .5 oversize, standard air box filter covers etc. Running 4 into 2 system with mac pipes on the system

[Tim2005]

You need to resolve the cause of the fouled one first, but if the problem still remains after that I'd change the float heights as they should be 21mm on a 400f. 

[harisuluv]

I still am not really following, I think a video would help immensely. 

[przjohn]

I still am not really following, I think a video would help immensely.

Same here

[Redline it]

You need to resolve the cause of the fouled one first, but if the problem still remains after that I'd change the float heights as they should be 21mm on a 400f.

I tried to set the floats on a cb400f 20 times by the manual's specs of 21 mm. It in one or 2 various carbs would either over flow or not get any fuel at all. Well 19 times, then I took everything appart floats needle jet (fuel inlet valves) and float pins and bowls and put them in a pile. Something I do not ever do. As the same screw goes back into the same threaded hole. This time I was out of patience. Picked a part at a time without care. I eyeballed the float heigth to look level. And the little devil, worked. I bench synced the carbs to the manual's instructions, the bike ran like crap. Instead of using the top of travel slide measurement and the 1/8th in drill bit on low end. I eyeballed the slides at a closed position. Winged it. And it ran better than ever before. I have no idea why.

[ripamarcone]

Hi All. just want to thank everyone that commented on this post.
things were not adding up and so I decided to go with a new needle set, new slow 40, new main 75. bench synced.
installed everything a week ago and got the carbs back onto the bike and then synced them.
well its been  running like a dream. id say the needle sets were worn. I purchased them from Yamiya in Japan.
once again thank you