Topic: CB400f - Hanging Idle, Running Rich

[.RJ]

SOHC hive mind, I'm stumped here.  I bought a CB400F on ebay that wasnt exactly as advertised - it was nice and clean and original but it ran like #$%*.  So into the bike we go... started here:

http://forums.sohc4.net/index.php/topic,176634.msg2053772.html#msg2053772 - once I got a new clutch cover after several dead ends and months, I did the following:

- Rebuilt carbs with carb kit from 4into1.  Floats set properly (I want to say it was 25 or 26mm, I went with the spec in the manual I have here), all jets stock sized, bench sync'd them.
- New air filter
- Dyna Ignition, New Plugs - it timed up without any fuss
- New manifold & airbox rubbers, new manifold o-rings
- New pattern replacement muffler & gasket from DSS - the old one did not fit properly and the gasket was leaking
- Oil & Filter
- (Edit) Set the valve lash, they were all a little tight

And the result is the bike hangs at 3,000 rpm once warmed up.  Moving the idle mix screws dont seem to have any effect, the manual I have says 2 +/- 1/2 turn, and anywhere from 1-2 turns doesnt seem to matter.  The carbs do balance nicely (using a morgan carbtune) and it seems to run smoothly with no load at 3000 rpm, but when the idle does drop down its a little rough and the idle speed screw doesnt seem to have any effect.  Engaging the choke at all kills the motor.  Taking the bike out on the road it is running rich and it bogs down rolling on the throttle, so, rich again.

I thought maybe the ignition advance was sticking, but it springs back nicely and its clean and the arms move without any fuss.  I swapped back to the stock points as an experiment, set and timed those and no effect on the hanging idle.  I cant find any air leaks spraying carb cleaner around the boots or carb tops, either, but a leak would send it lean so it doesnt match the pattern.  Slides close shut and dont hang, throttle cables are new and not binding. 

I'm stumped.  Given that a lot of the adjustments arent responding as they should, the only thing I can think of at this point is that the 4into1 carb kit isnt really correct - is there any source for new OEM emulsion tubes & needles?  As much as I hate the idea of pulling the carb rack off again, I'll happily do it if I can get this thing on the road again.

Many thanks for any ideas here....

[innovativems]

How’s it run right now?


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[robvangulik]

Did you use new rubbers between inlet and head?

[.RJ]

new rubbers between airbox and carbs, between carb and head, and new o-rings as well.  Spraying brake cleaner around all the joints didnt bring the idle down so I dont think its an air leak.

[.RJ]

How’s it run right now?


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Like #$%* below 2000 rpm no load.  The idle seems rough.  Throttle tip in bogs, well, bogs more than it should with a slide carb.  Above that when you get on the main it seems down on power and smells rich. 

I've got a set of used OEM Honda emulsion tubes, needles, and idle mix screws coming from across the pond, as well as a set of Keihin mains & pilots in the correct stock sizes.  I'll compare those to what I have in there now.  This is more than I wanted to spend on the bike but I'm tired of chasing my tail.

[.RJ]

My OEM needles & needle jets arrived today from the UK.  The needle profile is different, and the OEM emulsion tubes appear to have slightly larger holes.  The idle mixture screw also has a different profile.  I'm going to make sure the pilot & idle circuits are clear and reassemble, fingers crossed.

[.RJ]

Culprit: Aftermarket needles & needle jets.  Why do these things exist in the world?

[.RJ]

[HondaMan]

Culprit: Aftermarket needles & needle jets.  Why do these things exist in the world?

+10...

I answer about 4 e-mails every week, year-round, about aftermarket needles and jets...and they are never even close to being right.
 

[.RJ]

Can anyone confirm the correct idle mix screw setting for the 400F?  The manual I have shows 2 +/- 1/2 turn for the 400F

I started at 2 turns and felt like I had a hair of hesitation on rolling on the throttle and went in 1/4 turn and seems OK.  I still need to balance the carbs though.

[innovativems]

Can anyone confirm the correct idle mix screw setting for the 400F?  The manual I have shows 2 +/- 1/2 turn for the 400F

I started at 2 turns and felt like I had a hair of hesitation on rolling on the throttle and went in 1/4 turn and seems OK.  I still need to balance the carbs though.

Sync the carbs and that will make a world of difference


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[.RJ]

Balanced the carbs and we were pretty close with just the bench sync, it does run smoother.  It seemed like my slight hesitation (in neutral, unloaded) of throttle roll on was gone but once it warms up its back.  It doesnt effect how the bike rides too much, but more of an annoyance and its ever so slight when everything else is going really good. 

Any suggestions?  Everything is stock save for a dyna ignition plate.  Stock needles (3rd clip from top), stock jets, stock airbox/air filter, pipe, idle mix screws 2.5 turns out.  Going from 1.5 to 2.5 turns and back didnt seem to have any effect on that although I have not thrown a fresh set of plugs in and just let it idle to see whether we're on the right setting.  I also only set the timing with a static measurement and a test light, I havent checked it with my timing light - worthwhile to do?

In the mean time, I took my wife out for ice cream on the bike.

[.RJ]

Some further musings while I have to work and cant keep riding this thing.  When I replaced the needles & needle jets with OEM, and with new mains/pilots with original Keihin, the jets were new but the needles & needle jets were OEM honda but cleaned used pieces.  Could it be that they are worn and causing a slightly rich condition off of idle, hence the hesitation?  I'm fairly certain there's no blocked passages in the carb bodies. 

Looks like new needle/jet sets are not easily or cheaply available new here in the US, CMSNL is showing them @ $40/set (ouch!) and DSS only shows 2 in stock at similar pricing. 

[Bodi]

The needles and emulsion tubes - the metering orifice for the needle is in the tube - wear very very slowly and many high mileage engines run perfectly well with original brass.
Ignition timing is best done with a strobe. Cam timing off a tooth will cause odd effects, if the cam chain was ever run super loose it may have jumped a tooth.
Assuming all carb passages, jets, and emulsion tubes are clean and clear, float levels per spec... not too sure what to suggest.
I have seen rich running and lack of power from restricted mufflers but I don't think the OEM muffler is prone to that, it happens with glaspack types fairly often.
Check that the pilot screws and seats are undamaged, and their o-rings are ok. It is very easy to damage the screw tip and seat by overtightenig these.

[.RJ]

I think a jumped cam tooth would cause issues beyond a slight off-idle hesitation - it pulls clean and feels like it should all the way through the rev range.  I'll dig out my timing light.... I'll reiterate that its a pretty slight issue, and doesnt really affect the riding of the bike, but I would of course like to get it perfect

[BRG-BIRD]

It is possible the emulsion tubes are slightly worn but your economy would suffer as well. My 550 had severely worn emulsion tubes but I don’t remember it stumbling off idle much, just very poor economy and black sooted spark plugs.

[.RJ]

I havent run enough fuel through it yet to get a sense of the fuel economy.

Are there cheap standalone wideband probes available?  I think that would be much more insightful than reading plugs.