Project CB350F cafe brat

stubsryan

Been Around the Block
IMG_1981.JPG
IMG_1982.JPG


Bike runs but idles way too high when warm. Dropping the needle height two spots to see if that helps. Hopefully I don’t need smaller jets. I’m thinking when I did the carbs I lifted the needles for some reason and that’s why it’s running flat out. Usually it’s the other way around when pods and shorter exhausts are installed I think


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trek97

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Slides are closed at idle, needle height won't have any effect.
Sounds like fast idle needs adjusted.
 

stubsryan

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Slides are closed at idle, needle height won't have any effect.
Sounds like fast idle needs adjusted.
Needles are tied directly to the slide. I figured in the lowest position of the common throttle position, if the needle can drop any further then less fuel can go through the primary jet. I’ll start it tomorrow and find out I guess. If that doesn’t work then I’ll change the idle and primary jets as it can’t be anything else. Also should be noted that I have no throttle cable connected to the carbs yet so that isn’t a factor. Not sure what you mean by the fast idle? The idle screw adjustments? They don’t change the rpm much if anything noticeable at 4000 rpm. I’ve tried all the way in and all the way out with no real change


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trek97

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Bike runs but idles way too high when warm.
I was just going off of your earlier statement.
Seems you got it under control. Enjoy.

Needles are tied directly to the slide.
Thanks for the solid info. You got this my man.

Not sure what you mean by the fast idle?
I mean "fast idle." The screw adjustment designed to lift the slides and make it idle at a faster rate (3500 - 4500rpm) rather than 1200rpm, actuated by the choke lever...Is it adjusted properly? 0.3mm gap
 
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stubsryan

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Hey yeah cheers I’m all good. Haven’t even tried to start the bike since I moved the needle heights. Maybe tomorrow I’ll get some stuff done. Cheers m8


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teazer

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Trek is correct that at idle the needle position should have no effect at all.

In your case with a very high idle speed, it is possible that fuel is being drawn in through the main jet circuit, but dropping the needles isn't the cure. At idle, the needles are already into the needle jets as far as they need to go - they should be on the parallel section of the needle leaving almost zero gap. That's how they are designed. I that's not the case, something is wrong with one or more of the components.

Did you check that all 4 slides are fully closed at idle and are all at exactly the same height? If not, obviously that needs to be corrected. If they are at the same height, the next most likely issue is an air leak - probably around one or more of the inlet rubbers.

It is also possible that one or more needle jets are missing or oversized or the needles themselves are not the correct shape for that bike.
 

trek97

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Teazer, In one of the pics above he has the pipes installed for sync.
During sync, on these carbs, it is very easy to "get into" the fast idle adjuster without realizing it, even with choke fully open.
the factory fast idle is set anywhere from 3500-4500rpm.
His bike is idling at 4000...Im sure its not a coincidence. As once its on "fast idle set screw" the idle speed control has zero effect...at any rpm below.

The idle screw adjustments? They don’t change the rpm much if anything noticeable at 4000 rpm. I’ve tried all the way in and all the way out with no real change
Alos Stubsryan, this reads like you may have been mistakenly adjusting the Air/Fuel mix. A good starting point for these carbs on the 400four is 1-1/2 turns out from fully seated. Though I have zero 350four tune up experience, I imagine its close to the same. Dont need your machine to overheat during idle. :cool:
 
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trek97

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Pods and pipes will have virtually zero effect at idle. The only thing that may need a bit of tweaking is the A/F adjustors.
I wrote earlier the stock 400four will be set around 1-1/2 turns out from fully seated.

You may want to crank yours IN to 1 or 1.25 turns from fully seated. Closing these will give it a bit more fuel to run cooler at idle.
A/F mix will always be fine tuned later after its running and riding nice.
 
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stubsryan

Been Around the Block
Air leaks don’t increase rpm. It’s getting too much fuel. I’m more worried about the electrical at the moment but I’ll get to the tuning soon enough so you can work out if your right of not. Also how does more fuel make an engine run cooler? Fuel adds heat when it combusts? Idle isn’t meant to be so high and if the needle is 100% closed off the primary jet would have close to zero flow. So in that case it can only be the idle jet which is unlikely. That’s shown by the idle screw adjustment doing nothing to change the rpm.
Anyway, I’ll get to it later.


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teazer

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Air leaks do cause high idle speeds.
Too much fuel is not your problem.
At the rich end of the spectrum all of the available oxygen is used so the excess fuel is not burned. It will cool the motor though because it evaporates and that draws heat from the surrounding metal parts. It will tend to increase exhaust pipe temperature if it burns in the pipe. So lower CHT, higher EGT potentially.
Needle and needle jet are basically not in play at idle as previously discussed.

So the most likely issues are that one or more slides are further open than they should be. That may be a fast idle if your bike has such a thing or the slides not closing properly. I would take them off again and check to see how the idle stop and fast idle are set.
 

trek97

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Sounds like you got an awesome understanding how carbs work my man.
I’ll just take a step back and leave you to it. Enjoy
I’m out.
 

teazer

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Thanks for that bit of info. I'm OK with Aussies more direct way of communicating. After all, I lived there for 20 years.
 

stubsryan

Been Around the Block
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Adding a little mechanical protection to the cables. Also used some black heat shrink for the visible cable run up to the seat just to keep the gaps black as well. Bit of form and function


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