Topic: CB400f Idle too high after rejet

[kfleschner]

Short history: Replaced old, rusted exhaust with MAC 4 into 1 aftermarket. Looks nice, fits well, couldn't idle. I had to have the choke on at least 1/2 all the time or it would die. Previous post on this suggested upjetting the carbs. I put 42 slows and 80 mains on all four carbs.

Now: Idles at ~4k and blows smoke signals out the pipe without choke. I've read posts on here saying they use similar setups with 40 slows, and I've seen posts saying you should downjet the slows to 36s.

Should I try to find 36s and give them a shot, or is there some other adjustment I need to make?

Thanks!

[chewbacca5000]

Had similar problem on a cb500, the length of the replacement was much shorter than stock.  Aftermarket pilot jets that are the correct size are hard to come by.

[Kevin]

The stock exhaust actually flows quite well, it's just heavy and acts as a skid plate when your dragging your knee....
I'm surprised that just changing the exhaust made such a big difference in your idle. I suspect you've got other things going on.

When switching over to pod filters (not that you did) it's usually a good starting point to go with #38 slows and #80 mains.
I don't think I would drop down to the #36's....

[HondaMan]

Like Kevin said...I have never seen an exhaust change on these bikes that required a jetting change: the pipes are simply too short to make that much difference. That's not to say people don't change the jets, though...

Things to check:
1. The O-rings in the castings between the head and the carb hoses. They are old and hard, and leak.
2. The hose clamps on the carb hoses. They must be tight.
3. The idle air screws' settings. I think the 400F is 2 turns out?

[toolguy]

I chased a 'too rich' at idle issue for 3 months . . my symptoms were a slow to return to idle after some throttle - hanging at 2000 for 15-20 seconds, and the idle mixture screws made almost no difference, all the way in to 5 turns out. .
I finally noticed the emulsion holes in the aftermarket idle jets were much smaller than what a stock Honda jet has. . . so no matter where the mixture screws were, the small emulsion holes in the jets were hindering the air flow to mix with the gas. .  also the aftermarket jets were about 1/4 inch shorter in length, meaning there were not very far down in the fuel in the float bowls. .
Installed a set of real factory jets and all the issues were fixed and mixture screws best at 1 turn out. .

[kfleschner]

So after checking the air screws again, checking tappet gaps, etc. it was still idling too fast. I switched both jets for new ones in stock sizes and tried again. Still idled too fast. Then I remembered that I had torn the whole thing apart to check for any pieces that needed replacement and had the carb adjuster screws turned out WAY too much. I screwed them down and the idle speed came down with it. Sync'd nicely after that with a little bit of idle adjust. I did notice that the vacuum range was greater in some carbs than others, and some were more 'touchy'. Is that a sign of something off, or just normal behavior?

TL:DR - carb adjust screws turned out too much = high idle speed regardless of jets.

[HondaMan]

So after checking the air screws again, checking tappet gaps, etc. it was still idling too fast. I switched both jets for new ones in stock sizes and tried again. Still idled too fast. Then I remembered that I had torn the whole thing apart to check for any pieces that needed replacement and had the carb adjuster screws turned out WAY too much. I screwed them down and the idle speed came down with it. Sync'd nicely after that with a little bit of idle adjust. I did notice that the vacuum range was greater in some carbs than others, and some were more 'touchy'. Is that a sign of something off, or just normal behavior?

TL:DR - carb adjust screws turned out too much = high idle speed regardless of jets.

The little O-rings in those castings that hold the carb hoses to the head are probably leaking. They are old and hard, now, and almost every 500/550/400F/350F I have touched in the last 8 years had these leaking to a lesser or greater amount. The one(s) that leak will give you a darker sparkplug than the others, and lower vacuum readings. And, a tendency to hand ing higher idle speeds.

[kfleschner]

The little O-rings in those castings that hold the carb hoses to the head

Do you mean the O-ring gaskets in the intake insulators? Between the rubber bits connecting the outflow side of the carbs to the engine head, right? I had a shop replace the insulators about 6-7 years ago, but I suppose I could switch them out. Any chance you know the dimensions off-hand?

[Herr Supersport]

I measure:
inner dia = 28mm
outer dia = 33mm
Thickness of the O-Ring = 2.5 mm [at about]